path: root/doc/rfc/rfc9421.txt

Internet Engineering Task Force (IETF)                   A. Backman, Ed.
Request for Comments: 9421                                        Amazon
Category: Standards Track                                 J. Richer, Ed.
ISSN: 2070-1721                                      Bespoke Engineering
                                                               M. Sporny
                                                          Digital Bazaar
                                                           February 2024


                        HTTP Message Signatures

Abstract

   This document describes a mechanism for creating, encoding, and
   verifying digital signatures or message authentication codes over
   components of an HTTP message.  This mechanism supports use cases
   where the full HTTP message may not be known to the signer and where
   the message may be transformed (e.g., by intermediaries) before
   reaching the verifier.  This document also describes a means for
   requesting that a signature be applied to a subsequent HTTP message
   in an ongoing HTTP exchange.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9421.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Conventions and Terminology
     1.2.  Requirements
     1.3.  HTTP Message Transformations
     1.4.  Application of HTTP Message Signatures
   2.  HTTP Message Components
     2.1.  HTTP Fields
       2.1.1.  Strict Serialization of HTTP Structured Fields
       2.1.2.  Dictionary Structured Field Members
       2.1.3.  Binary-Wrapped HTTP Fields
       2.1.4.  Trailer Fields
     2.2.  Derived Components
       2.2.1.  Method
       2.2.2.  Target URI
       2.2.3.  Authority
       2.2.4.  Scheme
       2.2.5.  Request Target
       2.2.6.  Path
       2.2.7.  Query
       2.2.8.  Query Parameters
       2.2.9.  Status Code
     2.3.  Signature Parameters
     2.4.  Signing Request Components in a Response Message
     2.5.  Creating the Signature Base
   3.  HTTP Message Signatures
     3.1.  Creating a Signature
     3.2.  Verifying a Signature
       3.2.1.  Enforcing Application Requirements
     3.3.  Signature Algorithms
       3.3.1.  RSASSA-PSS Using SHA-512
       3.3.2.  RSASSA-PKCS1-v1_5 Using SHA-256
       3.3.3.  HMAC Using SHA-256
       3.3.4.  ECDSA Using Curve P-256 DSS and SHA-256
       3.3.5.  ECDSA Using Curve P-384 DSS and SHA-384
       3.3.6.  EdDSA Using Curve edwards25519
       3.3.7.  JSON Web Signature (JWS) Algorithms
   4.  Including a Message Signature in a Message
     4.1.  The Signature-Input HTTP Field
     4.2.  The Signature HTTP Field
     4.3.  Multiple Signatures
   5.  Requesting Signatures
     5.1.  The Accept-Signature Field
     5.2.  Processing an Accept-Signature
   6.  IANA Considerations
     6.1.  HTTP Field Name Registration
     6.2.  HTTP Signature Algorithms Registry
       6.2.1.  Registration Template
       6.2.2.  Initial Contents
     6.3.  HTTP Signature Metadata Parameters Registry
       6.3.1.  Registration Template
       6.3.2.  Initial Contents
     6.4.  HTTP Signature Derived Component Names Registry
       6.4.1.  Registration Template
       6.4.2.  Initial Contents
     6.5.  HTTP Signature Component Parameters Registry
       6.5.1.  Registration Template
       6.5.2.  Initial Contents
   7.  Security Considerations
     7.1.  General Considerations
       7.1.1.  Skipping Signature Verification
       7.1.2.  Use of TLS
     7.2.  Message Processing and Selection
       7.2.1.  Insufficient Coverage
       7.2.2.  Signature Replay
       7.2.3.  Choosing Message Components
       7.2.4.  Choosing Signature Parameters and Derived Components
               over HTTP Fields
       7.2.5.  Signature Labels
       7.2.6.  Multiple Signature Confusion
       7.2.7.  Collision of Application-Specific Signature Tag
       7.2.8.  Message Content
     7.3.  Cryptographic Considerations
       7.3.1.  Cryptography and Signature Collision
       7.3.2.  Key Theft
       7.3.3.  Symmetric Cryptography
       7.3.4.  Key Specification Mixup
       7.3.5.  Non-deterministic Signature Primitives
       7.3.6.  Key and Algorithm Specification Downgrades
       7.3.7.  Signing Signature Values
     7.4.  Matching Signature Parameters to the Target Message
       7.4.1.  Modification of Required Message Parameters
       7.4.2.  Matching Values of Covered Components to Values in the
               Target Message
       7.4.3.  Message Component Source and Context
       7.4.4.  Multiple Message Component Contexts
     7.5.  HTTP Processing
       7.5.1.  Processing Invalid HTTP Field Names as Derived
               Component Names
       7.5.2.  Semantically Equivalent Field Values
       7.5.3.  Parsing Structured Field Values
       7.5.4.  HTTP Versions and Component Ambiguity
       7.5.5.  Canonicalization Attacks
       7.5.6.  Non-List Field Values
       7.5.7.  Padding Attacks with Multiple Field Values
       7.5.8.  Ambiguous Handling of Query Elements
   8.  Privacy Considerations
     8.1.  Identification through Keys
     8.2.  Signatures do not provide confidentiality
     8.3.  Oracles
     8.4.  Required Content
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Detecting HTTP Message Signatures
   Appendix B.  Examples
     B.1.  Example Keys
       B.1.1.  Example RSA Key
       B.1.2.  Example RSA-PSS Key
       B.1.3.  Example ECC P-256 Test Key
       B.1.4.  Example Ed25519 Test Key
       B.1.5.  Example Shared Secret
     B.2.  Test Cases
       B.2.1.  Minimal Signature Using rsa-pss-sha512
       B.2.2.  Selective Covered Components Using rsa-pss-sha512
       B.2.3.  Full Coverage Using rsa-pss-sha512
       B.2.4.  Signing a Response Using ecdsa-p256-sha256
       B.2.5.  Signing a Request Using hmac-sha256
       B.2.6.  Signing a Request Using ed25519
     B.3.  TLS-Terminating Proxies
     B.4.  HTTP Message Transformations
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Message integrity and authenticity are security properties that are
   critical to the secure operation of many HTTP applications.
   Application developers typically rely on the transport layer to
   provide these properties, by operating their application over TLS
   [TLS].  However, TLS only guarantees these properties over a single
   TLS connection, and the path between the client and application may
   be composed of multiple independent TLS connections (for example, if
   the application is hosted behind a TLS-terminating gateway or if the
   client is behind a TLS Inspection appliance).  In such cases, TLS
   cannot guarantee end-to-end message integrity or authenticity between
   the client and application.  Additionally, some operating
   environments present obstacles that make it impractical to use TLS
   (such as the presentation of client certificates from a browser) or
   to use features necessary to provide message authenticity.
   Furthermore, some applications require the binding of a higher-level
   application-specific key to the HTTP message, separate from any TLS
   certificates in use.  Consequently, while TLS can meet message
   integrity and authenticity needs for many HTTP-based applications, it
   is not a universal solution.

   Additionally, many applications need to be able to generate and
   verify signatures despite incomplete knowledge of the HTTP message as
   seen on the wire, due to the use of libraries, proxies, or
   application frameworks that alter or hide portions of the message
   from the application at the time of signing or verification.  These
   applications need a means to protect the parts of the message that
   are most relevant to the application without having to violate
   layering and abstraction.

   Finally, object-based signature mechanisms such as JSON Web Signature
   [JWS] require the intact conveyance of the exact information that was
   signed.  When applying such technologies to an HTTP message, elements
   of the HTTP message need to be duplicated in the object payload
   either directly or through the inclusion of a hash.  This practice
   introduces complexity, since the repeated information needs to be
   carefully checked for consistency when the signature is verified.

   This document defines a mechanism for providing end-to-end integrity
   and authenticity for components of an HTTP message by using a
   detached signature on HTTP messages.  The mechanism allows
   applications to create digital signatures or message authentication
   codes (MACs) over only the components of the message that are
   meaningful and appropriate for the application.  Strict
   canonicalization rules ensure that the verifier can verify the
   signature even if the message has been transformed in many of the
   ways permitted by HTTP.

   The signing mechanism described in this document consists of three
   parts:

   *  A common nomenclature and canonicalization rule set for the
      different protocol elements and other components of HTTP messages,
      used to create the signature base (Section 2).

   *  Algorithms for generating and verifying signatures over HTTP
      message components using this signature base through the
      application of cryptographic primitives (Section 3).

   *  A mechanism for attaching a signature and related metadata to an
      HTTP message and for parsing attached signatures and metadata from
      HTTP messages.  To facilitate this, this document defines the
      "Signature-Input" and "Signature" fields (Section 4).

   This document also provides a mechanism for negotiating the use of
   signatures in one or more subsequent messages via the "Accept-
   Signature" field (Section 5).  This optional negotiation mechanism
   can be used along with opportunistic or application-driven message
   signatures by either party.

   The mechanisms defined in this document are important tools that can
   be used to build an overall security mechanism for an application.
   This toolkit provides some powerful capabilities but is not
   sufficient in creating an overall security story.  In particular, the
   requirements listed in Section 1.4 and the security considerations
   discussed in Section 7 are of high importance to all implementors of
   this specification.  For example, this specification does not define
   a means to directly cover HTTP message content (defined in
   Section 6.4 of [HTTP]); rather, it relies on the Digest specification
   [DIGEST] to provide a hash of the message content, as discussed in
   Section 7.2.8.

1.1.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The terms "HTTP message", "HTTP request", "HTTP response", "target
   URI", "gateway", "header field", "intermediary", "request target",
   "trailer field", "sender", "method", and "recipient" are used as
   defined in [HTTP].

   For brevity, the term "signature" on its own is used in this document
   to refer to both digital signatures (which use asymmetric
   cryptography) and keyed MACs (which use symmetric cryptography).
   Similarly, the verb "sign" refers to the generation of either a
   digital signature or keyed MAC over a given signature base.  The
   qualified term "digital signature" refers specifically to the output
   of an asymmetric cryptographic signing operation.

   This document uses the following terminology from Section 3 of
   [STRUCTURED-FIELDS] to specify data types: List, Inner List,
   Dictionary, Item, String, Integer, Byte Sequence, and Boolean.

   This document defines several string constructions using ABNF [ABNF]
   and uses the following ABNF rules: VCHAR, SP, DQUOTE, and LF.  This
   document uses the following ABNF rules from [STRUCTURED-FIELDS]: sf-
   string, inner-list, and parameters.  This document uses the following
   ABNF rules from [HTTP] and [HTTP/1.1]: field-content, obs-fold, and
   obs-text.

   In addition to those listed above, this document uses the following
   terms:

   HTTP Message Signature:
      A digital signature or keyed MAC that covers one or more portions
      of an HTTP message.  Note that a given HTTP message can contain
      multiple HTTP message signatures.

   Signer:
      The entity that is generating or has generated an HTTP message
      signature.  Note that multiple entities can act as signers and
      apply separate HTTP message signatures to a given HTTP message.

   Verifier:
      An entity that is verifying or has verified an HTTP message
      signature against an HTTP message.  Note that an HTTP message
      signature may be verified multiple times, potentially by different
      entities.

   HTTP Message Component:
      A portion of an HTTP message that is capable of being covered by
      an HTTP message signature.

   Derived Component:
      An HTTP message component derived from the HTTP message through
      the use of a specified algorithm or process.  See Section 2.2.

   HTTP Message Component Name:
      A String that identifies an HTTP message component's source, such
      as a field name or derived component name.

   HTTP Message Component Identifier:
      The combination of an HTTP message component name and any
      parameters.  This combination uniquely identifies a specific HTTP
      message component with respect to a particular HTTP message
      signature and the HTTP message it applies to.

   HTTP Message Component Value:
      The value associated with a given component identifier within the
      context of a particular HTTP message.  Component values are
      derived from the HTTP message and are usually subject to a
      canonicalization process.

   Covered Components:
      An ordered set of HTTP message component identifiers for fields
      (Section 2.1) and derived components (Section 2.2) that indicates
      the set of message components covered by the signature, never
      including the @signature-params identifier itself.  The order of
      this set is preserved and communicated between the signer and
      verifier to facilitate reconstruction of the signature base.

   Signature Base:
      The sequence of bytes generated by the signer and verifier using
      the covered components set and the HTTP message.  The signature
      base is processed by the cryptographic algorithm to produce or
      verify the HTTP message signature.

   HTTP Message Signature Algorithm:
      A cryptographic algorithm that describes the signing and
      verification process for the signature, defined in terms of the
      HTTP_SIGN and HTTP_VERIFY primitives described in Section 3.3.

   Key Material:
      The key material required to create or verify the signature.  The
      key material is often identified with an explicit key identifier,
      allowing the signer to indicate to the verifier which key was
      used.

   Creation Time:
      A timestamp representing the point in time that the signature was
      generated, as asserted by the signer.

   Expiration Time:
      A timestamp representing the point in time after which the
      signature should no longer be accepted by the verifier, as
      asserted by the signer.

   Target Message:
      The HTTP message to which an HTTP message signature is applied.

   Signature Context:
      The data source from which the HTTP message component values are
      drawn.  The context includes the target message and any additional
      information the signer or verifier might have, such as the full
      target URI of a request or the related request message for a
      response.

   The term "UNIX timestamp" refers to what Section 4.16 of [POSIX.1]
   calls "seconds since the Epoch".

   This document contains non-normative examples of partial and complete
   HTTP messages.  Some examples use a single trailing backslash (\) to
   indicate line wrapping for long values, as per [RFC8792].  The \
   character and leading spaces on wrapped lines are not part of the
   value.

1.2.  Requirements

   HTTP permits, and sometimes requires, intermediaries to transform
   messages in a variety of ways.  This can result in a recipient
   receiving a message that is not bitwise-equivalent to the message
   that was originally sent.  In such a case, the recipient will be
   unable to verify integrity protections over the raw bytes of the
   sender's HTTP message, as verifying digital signatures or MACs
   requires both signer and verifier to have the exact same signature
   base.  Since the exact raw bytes of the message cannot be relied upon
   as a reliable source for a signature base, the signer and verifier
   have to independently create the signature base from their respective
   versions of the message, via a mechanism that is resilient to safe
   changes that do not alter the meaning of the message.

   For a variety of reasons, it is impractical to strictly define what
   constitutes a safe change versus an unsafe one.  Applications use
   HTTP in a wide variety of ways and may disagree on whether a
   particular piece of information in a message (e.g., the message
   content, the method, or a particular header field) is relevant.
   Thus, a general-purpose solution needs to provide signers with some
   degree of control over which message components are signed.

   HTTP applications may be running in environments that do not provide
   complete access to or control over HTTP messages (such as a web
   browser's JavaScript environment) or may be using libraries that
   abstract away the details of the protocol (such as the Java HTTP
   Client (HttpClient) library
   (https://openjdk.java.net/groups/net/httpclient/intro.html)).  These
   applications need to be able to generate and verify signatures
   despite incomplete knowledge of the HTTP message.

1.3.  HTTP Message Transformations

   As mentioned earlier, HTTP explicitly permits, and in some cases
   requires, implementations to transform messages in a variety of ways.
   Implementations are required to tolerate many of these
   transformations.  What follows is a non-normative and non-exhaustive
   list of transformations that could occur under HTTP, provided as
   context:

   *  Reordering of fields with different field names (Section 5.3 of
      [HTTP]).

   *  Combination of fields with the same field name (Section 5.2 of
      [HTTP]).

   *  Removal of fields listed in the Connection header field
      (Section 7.6.1 of [HTTP]).

   *  Addition of fields that indicate control options (Section 7.6.1 of
      [HTTP]).

   *  Addition or removal of a transfer coding (Section 7.7 of [HTTP]).

   *  Addition of fields such as Via (Section 7.6.3 of [HTTP]) and
      Forwarded (Section 4 of [RFC7239]).

   *  Conversion between different versions of HTTP (e.g., HTTP/1.x to
      HTTP/2, or vice versa).

   *  Changes in case (e.g., "Origin" to "origin") of any case-
      insensitive components such as field names, request URI scheme, or
      host.

   *  Changes to the request target and authority that, when applied
      together, do not result in a change to the message's target URI,
      as defined in Section 7.1 of [HTTP].

   Additionally, there are some transformations that are either
   deprecated or otherwise not allowed but that could still occur in the
   wild.  These transformations can still be handled without breaking
   the signature; they include such actions as:

   *  Use, addition, or removal of leading or trailing whitespace in a
      field value.

   *  Use, addition, or removal of obs-fold in field values (Section 5.2
      of [HTTP/1.1]).

   We can identify these types of transformations as transformations
   that should not prevent signature verification, even when performed
   on message components covered by the signature.  Additionally, all
   changes to components not covered by the signature should not prevent
   signature verification.

   Some examples of these kinds of transformations, and the effect they
   have on the message signature, are found in Appendix B.4.

   Other transformations, such as parsing and reserializing the field
   values of a covered component or changing the value of a derived
   component, can cause a signature to no longer validate against a
   target message.  Applications of this specification need to take care
   to ensure that the transformations expected by the application are
   adequately handled by the choice of covered components.

1.4.  Application of HTTP Message Signatures

   HTTP message signatures are designed to be a general-purpose tool
   applicable in a wide variety of circumstances and applications.  In
   order to properly and safely apply HTTP message signatures, an
   application or profile of this specification MUST specify, at a
   minimum, all of the following items:

   *  The set of component identifiers (Section 2) and signature
      parameters (Section 2.3) that are expected and required to be
      included in the covered components list.  For example, an
      authorization protocol could mandate that the Authorization field
      be covered to protect the authorization credentials and mandate
      that the signature parameters contain a created parameter
      (Section 2.3), while an API expecting semantically relevant HTTP
      message content could require the Content-Digest field defined in
      [DIGEST] to be present and covered as well as mandate a value for
      the tag parameter (Section 2.3) that is specific to the API being
      protected.

   *  The expected Structured Field types [STRUCTURED-FIELDS] of any
      required or expected covered component fields or parameters.

   *  A means of retrieving the key material used to verify the
      signature.  An application will usually use the keyid parameter of
      the signature parameters (Section 2.3) and define rules for
      resolving a key from there, though the appropriate key could be
      known from other means such as preregistration of a signer's key.

   *  The set of allowable signature algorithms to be used by signers
      and accepted by verifiers.

   *  A means of determining that the signature algorithm used to verify
      the signature is appropriate for the key material and context of
      the message.  For example, the process could use the alg parameter
      of the signature parameters (Section 2.3) to state the algorithm
      explicitly, derive the algorithm from the key material, or use
      some preconfigured algorithm agreed upon by the signer and
      verifier.

   *  A means of determining that a given key and algorithm used for a
      signature are appropriate for the context of the message.  For
      example, a server expecting only ECDSA signatures should know to
      reject any RSA signatures, or a server expecting asymmetric
      cryptography should know to reject any symmetric cryptography.

   *  A means of determining the context for derivation of message
      components from an HTTP message and its application context.
      While this is normally the target HTTP message itself, the context
      could include additional information known to the application
      through configuration, such as an external hostname.

   *  If binding between a request and response is needed using the
      mechanism provided in Section 2.4, all elements of the request
      message and the response message that would be required to provide
      properties of such a binding.

   *  The error messages and codes that are returned from the verifier
      to the signer when the signature is invalid, the key material is
      inappropriate, the validity time window is out of specification, a
      component value cannot be calculated, or any other errors occur
      during the signature verification process.  For example, if a
      signature is being used as an authentication mechanism, an HTTP
      status code of 401 (Unauthorized) or 403 (Forbidden) could be
      appropriate.  If the response is from an HTTP API, a response with
      an HTTP status code such as 400 (Bad Request) could include more
      details [RFC7807] [RFC9457], such as an indicator that the wrong
      key material was used.

   When choosing these parameters, an application of HTTP message
   signatures has to ensure that the verifier will have access to all
   required information needed to recreate the signature base.  For
   example, a server behind a reverse proxy would need to know the
   original request URI to make use of the derived component @target-
   uri, even though the apparent target URI would be changed by the
   reverse proxy (see also Section 7.4.3).  Additionally, an application
   using signatures in responses would need to ensure that clients
   receiving signed responses have access to all the signed portions of
   the message, including any portions of the request that were signed
   by the server using the req ("request-response") parameter
   (Section 2.4).

   Details regarding this kind of profiling are within the purview of
   the application and outside the scope of this specification; however,
   some additional considerations are discussed in Section 7.  In
   particular, when choosing the required set of component identifiers,
   care has to be taken to make sure that the coverage is sufficient for
   the application, as discussed in Sections 7.2.1 and 7.2.8.  This
   specification defines only part of a full security system for an
   application.  When building a complete security system based on this
   tool, it is important to perform a security analysis of the entire
   system, of which HTTP message signatures is a part.  Historical
   systems, such as AWS Signature Version 4 [AWS-SIGv4], can provide
   inspiration and examples of how to apply similar mechanisms to an
   application, though review of such historical systems does not negate
   the need for a security analysis of an application of HTTP message
   signatures.

2.  HTTP Message Components

   In order to allow signers and verifiers to establish which components
   are covered by a signature, this document defines component
   identifiers for components covered by an HTTP message signature, a
   set of rules for deriving and canonicalizing the values associated
   with these component identifiers from the HTTP message, and the means
   for combining these canonicalized values into a signature base.

   The signature context for deriving these values MUST be accessible to
   both the signer and the verifier of the message.  The context MUST be
   the same across all components in a given signature.  For example, it
   would be an error to use the raw query string for the @query derived
   component but combined query and form parameters for the @query-param
   derived component.  For more considerations regarding the message
   component context, see Section 7.4.3.

   A component identifier is composed of a component name and any
   parameters associated with that name.  Each component name is either
   an HTTP field name (Section 2.1) or a registered derived component
   name (Section 2.2).  The possible parameters for a component
   identifier are dependent on the component identifier.  The "HTTP
   Signature Component Parameters" registry, which catalogs all possible
   parameters, is defined in Section 6.5.

   Within a single list of covered components, each component identifier
   MUST occur only once.  One component identifier is distinct from
   another if the component name differs or if any of the parameters
   differ for the same component name.  Multiple component identifiers
   having the same component name MAY be included if they have
   parameters that make them distinct, such as "foo";bar and "foo";baz.
   The order of parameters MUST be preserved when processing a component
   identifier (such as when parsing during verification), but the order
   of parameters is not significant when comparing two component
   identifiers for equality checks.  That is to say, "foo";bar;baz
   cannot be in the same message as "foo";baz;bar, since these two
   component identifiers are equivalent, but a system processing one
   form is not allowed to transform it into the other form.

   The component value associated with a component identifier is defined
   by the identifier itself.  Component values MUST NOT contain newline
   (\n) characters.  Some HTTP message components can undergo
   transformations that change the bitwise value without altering the
   meaning of the component's value (for example, when combining field
   values).  Message component values therefore need to be canonicalized
   before they are signed, to ensure that a signature can be verified
   despite such intermediary transformations.  This document defines
   rules for each component identifier that transform the identifier's
   associated component value into such a canonical form.

   The following sections define component identifier names, their
   parameters, their associated values, and the canonicalization rules
   for their values.  The method for combining message components into
   the signature base is defined in Section 2.5.

2.1.  HTTP Fields

   The component name for an HTTP field is the lowercased form of its
   field name as defined in Section 5.1 of [HTTP].  While HTTP field
   names are case insensitive, implementations MUST use lowercased field
   names (e.g., content-type, date, etag) when using them as component
   names.

   The component value for an HTTP field is the field value for the
   named field as defined in Section 5.5 of [HTTP].  The field value
   MUST be taken from the named header field of the target message
   unless this behavior is overridden by additional parameters and
   rules, such as the req and tr flags, below.  For most fields, the
   field value is an ASCII string as recommended by [HTTP], and the
   component value is exactly that string.  Other encodings could exist
   in some implementations, and all non-ASCII field values MUST be
   encoded to ASCII before being added to the signature base.  The bs
   parameter, as described in Section 2.1.3, provides a method for
   wrapping such problematic field values.

   Unless overridden by additional parameters and rules, HTTP field
   values MUST be combined into a single value as defined in Section 5.2
   of [HTTP] to create the component value.  Specifically, HTTP fields
   sent as multiple fields MUST be combined by concatenating the values
   using a single comma and a single space as a separator ("," + " ").
   Note that intermediaries are allowed to combine values of HTTP fields
   with any amount of whitespace between the commas, and if this
   behavior is not accounted for by the verifier, the signature can
   fail, since the signer and verifier will see a different component
   value in their respective signature bases.  For robustness, it is
   RECOMMENDED that signed messages include only a single instance of
   any field covered under the signature, particularly with the value
   for any list-based fields serialized using the algorithm below.  This
   approach increases the chances of the field value remaining untouched
   through intermediaries.  Where that approach is not possible and
   multiple instances of a field need to be sent separately, it is
   RECOMMENDED that signers and verifiers process any list-based fields
   taking all individual field values and combining them based on the
   strict algorithm below, to counter possible intermediary behavior.
   When the field in question is a Structured Field of type List or
   Dictionary, this effect can be accomplished more directly by
   requiring the strict Structured Field serialization of the field
   value, as described in Section 2.1.1.

   Note that some HTTP fields, such as Set-Cookie [COOKIE], do not
   follow a syntax that allows for the combination of field values in
   this manner (such that the combined output is unambiguous from
   multiple inputs).  Even though the component value is never parsed by
   the message signature process and is used only as part of the
   signature base (Section 2.5), caution needs to be taken when
   including such fields in signatures, since the combined value could
   be ambiguous.  The bs parameter, as described in Section 2.1.3,
   provides a method for wrapping such problematic fields.  See
   Section 7.5.6 for more discussion regarding this issue.

   If the correctly combined value is not directly available for a given
   field by an implementation, the following algorithm will produce
   canonicalized results for list-based fields:

   1.  Create an ordered list of the field values of each instance of
       the field in the message, in the order they occur (or will occur)
       in the message.

   2.  Strip leading and trailing whitespace from each item in the list.
       Note that since HTTP field values are not allowed to contain
       leading and trailing whitespace, this would be a no-op in a
       compliant implementation.

   3.  Remove any obsolete line folding within the line, and replace it
       with a single space (" "), as discussed in Section 5.2 of
       [HTTP/1.1].  Note that this behavior is specific to HTTP/1.1 and
       does not apply to other versions of the HTTP specification, which
       do not allow internal line folding.

   4.  Concatenate the list of values with a single comma (",") and a
       single space (" ") between each item.

   The resulting string is the component value for the field.

   Note that some HTTP fields have values with multiple valid
   serializations that have equivalent semantics, such as allowing case-
   insensitive values that intermediaries could change.  Applications
   signing and processing such fields MUST consider how to handle the
   values of such fields to ensure that the signer and verifier can
   derive the same value, as discussed in Section 7.5.2.

   The following are non-normative examples of component values for
   header fields, given the following example HTTP message fragment:

   Host: www.example.com
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   X-OWS-Header:   Leading and trailing whitespace.
   X-Obs-Fold-Header: Obsolete
       line folding.
   Cache-Control: max-age=60
   Cache-Control:    must-revalidate
   Example-Dict:  a=1,    b=2;x=1;y=2,   c=(a   b   c)

   The following example shows the component values for these example
   header fields, presented using the signature base format defined in
   Section 2.5:

   "host": www.example.com
   "date": Tue, 20 Apr 2021 02:07:56 GMT
   "x-ows-header": Leading and trailing whitespace.
   "x-obs-fold-header": Obsolete line folding.
   "cache-control": max-age=60, must-revalidate
   "example-dict": a=1,    b=2;x=1;y=2,   c=(a   b   c)

   Empty HTTP fields can also be signed when present in a message.  The
   canonicalized value is the empty string.  This means that the
   following empty header field, with (SP) indicating a single trailing
   space character before the empty field value:

   X-Empty-Header:(SP)

   is serialized by the signature base generation algorithm
   (Section 2.5) with an empty string value following the colon and
   space added after the component identifier.

   "x-empty-header":(SP)

   Any HTTP field component identifiers MAY have the following
   parameters in specific circumstances, each described in detail in
   their own sections:

   sf  A Boolean flag indicating that the component value is serialized
      using strict encoding of the Structured Field value
      (Section 2.1.1).

   key  A String parameter used to select a single member value from a
      Dictionary Structured Field (Section 2.1.2).

   bs  A Boolean flag indicating that individual field values are
      encoded using Byte Sequence data structures before being combined
      into the component value (Section 2.1.3).

   req  A Boolean flag for signed responses indicating that the
      component value is derived from the request that triggered this
      response message and not from the response message directly.  Note
      that this parameter can also be applied to any derived component
      identifiers that target the request (Section 2.4).

   tr  A Boolean flag indicating that the field value is taken from the
      trailers of the message as defined in Section 6.5 of [HTTP].  If
      this flag is absent, the field value is taken from the header
      fields of the message as defined in Section 6.3 of [HTTP]
      (Section 2.1.4).

   Multiple parameters MAY be specified together, though some
   combinations are redundant or incompatible.  For example, the sf
   parameter's functionality is already covered when the key parameter
   is used on a Dictionary item, since key requires strict serialization
   of the value.  The bs parameter, which requires the raw bytes of the
   field values from the message, is not compatible with the use of the
   sf or key parameters, which require the parsed data structures of the
   field values after combination.

   Additional parameters can be defined in the "HTTP Signature Component
   Parameters" registry established in Section 6.5.

2.1.1.  Strict Serialization of HTTP Structured Fields

   If the value of an HTTP field is known by the application to be a
   Structured Field type (as defined in [STRUCTURED-FIELDS] or its
   extensions or updates) and the expected type of the Structured Field
   is known, the signer MAY include the sf parameter in the component
   identifier.  If this parameter is included with a component
   identifier, the HTTP field value MUST be serialized using the formal
   serialization rules specified in Section 4 of [STRUCTURED-FIELDS] (or
   the applicable formal serialization section of its extensions or
   updates) applicable to the type of the HTTP field.  Note that this
   process will replace any optional internal whitespace with a single
   space character, among other potential transformations of the value.

   If multiple field values occur within a message, these values MUST be
   combined into a single List or Dictionary structure before
   serialization.

   If the application does not know the type of the field or does not
   know how to serialize the type of the field, the use of this flag
   will produce an error.  As a consequence, the signer can only
   reliably sign fields using this flag when the verifier's system knows
   the type as well.

   For example, the following Dictionary field is a valid serialization:

   Example-Dict:  a=1,    b=2;x=1;y=2,   c=(a   b   c)

   If included in the signature base without parameters, its value would
   be:

   "example-dict": a=1,    b=2;x=1;y=2,   c=(a   b   c)

   However, if the sf parameter is added, the value is reserialized as
   follows:

   "example-dict";sf: a=1, b=2;x=1;y=2, c=(a b c)

   The resulting string is used as the component value; see Section 2.1.

2.1.2.  Dictionary Structured Field Members

   If a given field is known by the application to be a Dictionary
   Structured Field, an individual member in the value of that
   Dictionary is identified by using the parameter key and the
   Dictionary member key as a String value.

   If multiple field values occur within a message, these values MUST be
   combined into a single Dictionary structure before serialization.

   An individual member value of a Dictionary Structured Field is
   canonicalized by applying the serialization algorithm described in
   Section 4.1.2 of [STRUCTURED-FIELDS] on the member_value and its
   parameters, not including the Dictionary key itself.  Specifically,
   the value is serialized as an Item or Inner List (the two possible
   values of a Dictionary member), with all parameters and possible
   subfields serialized using the strict serialization rules defined in
   Section 4 of [STRUCTURED-FIELDS] (or the applicable section of its
   extensions or updates).

   Each parameterized key for a given field MUST NOT appear more than
   once in the signature base.  Parameterized keys MAY appear in any
   order in the signature base, regardless of the order they occur in
   the source Dictionary.

   If a Dictionary key is named as a covered component but it does not
   occur in the Dictionary, this MUST cause an error in the signature
   base generation.

   The following are non-normative examples of canonicalized values for
   Dictionary Structured Field members, given the following example
   header field, whose value is known by the application to be a
   Dictionary:

   Example-Dict:  a=1, b=2;x=1;y=2, c=(a   b    c), d

   The following example shows canonicalized values for different
   component identifiers of this field, presented using the signature
   base format discussed in Section 2.5:

   "example-dict";key="a": 1
   "example-dict";key="d": ?1
   "example-dict";key="b": 2;x=1;y=2
   "example-dict";key="c": (a b c)

   Note that the value for key="c" has been reserialized according to
   the strict member_value algorithm, and the value for key="d" has been
   serialized as a Boolean value.

2.1.3.  Binary-Wrapped HTTP Fields

   If the value of the HTTP field in question is known by the
   application to cause problems with serialization, particularly with
   the combination of multiple values into a single line as discussed in
   Section 7.5.6, the signer SHOULD include the bs parameter in a
   component identifier to indicate that the values of the field need to
   be wrapped as binary structures before being combined.

   If this parameter is included with a component identifier, the
   component value MUST be calculated using the following algorithm:

   1.  Let the input be the ordered set of values for a field, in the
       order they appear in the message.

   2.  Create an empty List for accumulating processed field values.

   3.  For each field value in the set:

       3.1.  Strip leading and trailing whitespace from the field value.
             Note that since HTTP field values are not allowed to
             contain leading and trailing whitespace, this would be a
             no-op in a compliant implementation.

       3.2.  Remove any obsolete line folding within the line, and
             replace it with a single space (" "), as discussed in
             Section 5.2 of [HTTP/1.1].  Note that this behavior is
             specific to [HTTP/1.1] and does not apply to other versions
             of the HTTP specification.

       3.3.  Encode the bytes of the resulting field value as a Byte
             Sequence.  Note that most fields are restricted to ASCII
             characters, but other octets could be included in the value
             in some implementations.

       3.4.  Add the Byte Sequence to the List accumulator.

   4.  The intermediate result is a List of Byte Sequence values.

   5.  Follow the strict serialization of a List as described in
       Section 4.1.1 of [STRUCTURED-FIELDS], and return this output.

   For example, the following field with internal commas prevents the
   distinct field values from being safely combined:

   Example-Header: value, with, lots
   Example-Header: of, commas

   In our example, the same field can be sent with a semantically
   different single value:

   Example-Header: value, with, lots, of, commas

   Both of these versions are treated differently by the application.
   However, if included in the signature base without parameters, the
   component value would be the same in both cases:

   "example-header": value, with, lots, of, commas

   However, if the bs parameter is added, the two separate instances are
   encoded and serialized as follows:

   "example-header";bs: :dmFsdWUsIHdpdGgsIGxvdHM=:, :b2YsIGNvbW1hcw==:

   For the single-instance field above, the encoding with the bs
   parameter is:

   "example-header";bs: :dmFsdWUsIHdpdGgsIGxvdHMsIG9mLCBjb21tYXM=:

   This component value is distinct from the multiple-instance field
   above, preventing a collision that could potentially be exploited.

2.1.4.  Trailer Fields

   If the signer wants to include a trailer field in the signature, the
   signer MUST include the tr Boolean parameter to indicate that the
   value MUST be taken from the trailer fields and not from the header
   fields.

   For example, given the following message:

   HTTP/1.1 200 OK
   Content-Type: text/plain
   Transfer-Encoding: chunked
   Trailer: Expires

   4
   HTTP
   7
   Message
   a
   Signatures
   0
   Expires: Wed, 9 Nov 2022 07:28:00 GMT

   The signer decides to add both the Trailer header field and the
   Expires trailer field to the signature base, along with the status
   code derived component:

   "@status": 200
   "trailer": Expires
   "expires";tr: Wed, 9 Nov 2022 07:28:00 GMT

   If a field is available as both a header and a trailer in a message,
   both values MAY be signed, but the values MUST be signed separately.
   The values of header fields and trailer fields of the same name MUST
   NOT be combined for purposes of the signature.

   Since trailer fields could be merged into the header fields or
   dropped entirely by intermediaries as per Section 6.5.1 of [HTTP], it
   is NOT RECOMMENDED to include trailers in the signature unless the
   signer knows that the verifier will have access to the values of the
   trailers as sent.

2.2.  Derived Components

   In addition to HTTP fields, there are a number of different
   components that can be derived from the control data, signature
   context, or other aspects of the HTTP message being signed.  Such
   derived components can be included in the signature base by defining
   a component name, possible parameters, message targets, and the
   derivation method for its component value.

   Derived component names MUST start with the "at" (@) character.  This
   differentiates derived component names from HTTP field names, which
   cannot contain the @ character as per Section 5.1 of [HTTP].
   Processors of HTTP message signatures MUST treat derived component
   names separately from field names, as discussed in Section 7.5.1.

   This specification defines the following derived components:

   @method  The method used for a request (Section 2.2.1).

   @target-uri  The full target URI for a request (Section 2.2.2).

   @authority  The authority of the target URI for a request
      (Section 2.2.3).

   @scheme  The scheme of the target URI for a request (Section 2.2.4).

   @request-target  The request target (Section 2.2.5).

   @path  The absolute path portion of the target URI for a request
      (Section 2.2.6).

   @query  The query portion of the target URI for a request
      (Section 2.2.7).

   @query-param  A parsed and encoded query parameter of the target URI
      for a request (Section 2.2.8).

   @status  The status code for a response (Section 2.2.9).

   Additional derived component names are defined in the "HTTP Signature
   Derived Component Names" registry (Section 6.4).

   Derived component values are taken from the context of the target
   message for the signature.  This context includes information about
   the message itself, such as its control data, as well as any
   additional state and context held by the signer or verifier.  In
   particular, when signing a response, the signer can include any
   derived components from the originating request by using the req
   parameter (Section 2.4).

   request:  Values derived from, and results applied to, an HTTP
      request message as described in Section 3.4 of [HTTP].  If the
      target message of the signature is a response, derived components
      that target request messages can be included by using the req
      parameter as defined in Section 2.4.

   response:  Values derived from, and results applied to, an HTTP
      response message as described in Section 3.4 of [HTTP].

   request, response:  Values derived from, and results applied to,
      either a request message or a response message.

   A derived component definition MUST define all target message types
   to which it can be applied.

   Derived component values MUST be limited to printable characters and
   spaces and MUST NOT contain any newline characters.  Derived
   component values MUST NOT start or end with whitespace characters.

2.2.1.  Method

   The @method derived component refers to the HTTP method of a request
   message.  The component value is canonicalized by taking the value of
   the method as a string.  Note that the method name is case sensitive
   as per [HTTP], Section 9.1.  While conventionally standardized method
   names are uppercase [ASCII], no transformation to the input method
   value's case is performed.

   For example, the following request message:

   POST /path?param=value HTTP/1.1
   Host: www.example.com

   would result in the following @method component value:

   POST

   and the following signature base line:

   "@method": POST

2.2.2.  Target URI

   The @target-uri derived component refers to the target URI of a
   request message.  The component value is the target URI of the
   request ([HTTP], Section 7.1), assembled from all available URI
   components, including the authority.

   For example, the following message sent over HTTPS:

   POST /path?param=value HTTP/1.1
   Host: www.example.com

   would result in the following @target-uri component value:

   https://www.example.com/path?param=value

   and the following signature base line:

   "@target-uri": https://www.example.com/path?param=value

2.2.3.  Authority

   The @authority derived component refers to the authority component of
   the target URI of the HTTP request message, as defined in [HTTP],
   Section 7.2.  In HTTP/1.1, this is usually conveyed using the Host
   header field, while in HTTP/2 and HTTP/3 it is conveyed using the
   :authority pseudo-header.  The value is the fully qualified authority
   component of the request, comprised of the host and, optionally, port
   of the request target, as a string.  The component value MUST be
   normalized according to the rules provided in [HTTP], Section 4.2.3.
   Namely, the hostname is normalized to lowercase, and the default port
   is omitted.

   For example, the following request message:

   POST /path?param=value HTTP/1.1
   Host: www.example.com

   would result in the following @authority component value:

   www.example.com

   and the following signature base line:

   "@authority": www.example.com

   The @authority derived component SHOULD be used instead of signing
   the Host header field directly.  See Section 7.2.4.

2.2.4.  Scheme

   The @scheme derived component refers to the scheme of the target URL
   of the HTTP request message.  The component value is the scheme as a
   lowercase string as defined in [HTTP], Section 4.2.  While the scheme
   itself is case insensitive, it MUST be normalized to lowercase for
   inclusion in the signature base.

   For example, the following request message sent over plain HTTP:

   POST /path?param=value HTTP/1.1
   Host: www.example.com

   would result in the following @scheme component value:

   http

   and the following signature base line:

   "@scheme": http

2.2.5.  Request Target

   The @request-target derived component refers to the full request
   target of the HTTP request message, as defined in [HTTP],
   Section 7.1.  The component value of the request target can take
   different forms, depending on the type of request, as described
   below.

   For HTTP/1.1, the component value is equivalent to the request target
   portion of the request line.  However, this value is more difficult
   to reliably construct in other versions of HTTP.  Therefore, it is
   NOT RECOMMENDED that this component be used when versions of HTTP
   other than 1.1 might be in use.

   The origin form value is a combination of the absolute path and query
   components of the request URL.

   For example, the following request message:

   POST /path?param=value HTTP/1.1
   Host: www.example.com

   would result in the following @request-target component value:

   /path?param=value

   and the following signature base line:

   "@request-target": /path?param=value

   The following request to an HTTP proxy with the absolute-form value,
   containing the fully qualified target URI:

   GET https://www.example.com/path?param=value HTTP/1.1

   would result in the following @request-target component value:

   https://www.example.com/path?param=value

   and the following signature base line:

   "@request-target": https://www.example.com/path?param=value

   The following CONNECT request with an authority-form value,
   containing the host and port of the target:

   CONNECT www.example.com:80 HTTP/1.1
   Host: www.example.com

   would result in the following @request-target component value:

   www.example.com:80

   and the following signature base line:

   "@request-target": www.example.com:80

   The following OPTIONS request message with the asterisk-form value,
   containing a single asterisk (*) character:

   OPTIONS * HTTP/1.1
   Host: www.example.com

   would result in the following @request-target component value:

   *

   and the following signature base line:

   "@request-target": *

2.2.6.  Path

   The @path derived component refers to the target path of the HTTP
   request message.  The component value is the absolute path of the
   request target defined by [URI], with no query component and no
   trailing question mark (?) character.  The value is normalized
   according to the rules provided in [HTTP], Section 4.2.3.  Namely, an
   empty path string is normalized as a single slash (/) character.
   Path components are represented by their values before decoding any
   percent-encoded octets, as described in the simple string comparison
   rules provided in Section 6.2.1 of [URI].

   For example, the following request message:

   GET /path?param=value HTTP/1.1
   Host: www.example.com

   would result in the following @path component value:

   /path

   and the following signature base line:

   "@path": /path

2.2.7.  Query

   The @query derived component refers to the query component of the
   HTTP request message.  The component value is the entire normalized
   query string defined by [URI], including the leading ? character.
   The value is read using the simple string comparison rules provided
   in Section 6.2.1 of [URI].  Namely, percent-encoded octets are not
   decoded.

   For example, the following request message:

   GET /path?param=value&foo=bar&baz=bat%2Dman HTTP/1.1
   Host: www.example.com

   would result in the following @query component value:

   ?param=value&foo=bar&baz=bat%2Dman

   and the following signature base line:

   "@query": ?param=value&foo=bar&baz=bat%2Dman

   The following request message:

   POST /path?queryString HTTP/1.1
   Host: www.example.com

   would result in the following @query component value:

   ?queryString

   and the following signature base line:

   "@query": ?queryString

   Just like including an empty path component, the signer can include
   an empty query component to indicate that this component is not used
   in the message.  If the query string is absent from the request
   message, the component value is the leading ? character alone:

   ?

   resulting in the following signature base line:

   "@query": ?

2.2.8.  Query Parameters

   If the query portion of a request target URI uses HTML form
   parameters in the format defined in Section 5 ("application/
   x-www-form-urlencoded") of [HTMLURL], the @query-param derived
   component allows addressing of these individual query parameters.
   The query parameters MUST be parsed according to Section 5.1
   ("application/x-www-form-urlencoded parsing") of [HTMLURL], resulting
   in a list of (nameString, valueString) tuples.  The REQUIRED name
   parameter of each component identifier contains the encoded
   nameString of a single query parameter as a String value.  The
   component value of a single named parameter is the encoded
   valueString of that single query parameter.  Several different named
   query parameters MAY be included in the covered components.  Single
   named parameters MAY occur in any order in the covered components,
   regardless of the order they occur in the query string.

   The value of the name parameter and the component value of a single
   named parameter are calculated via the following process:

   1.  Parse the nameString or valueString of the named query parameter
       defined by Section 5.1 ("application/x-www-form-urlencoded
       parsing") of [HTMLURL]; this is the value after percent-encoded
       octets are decoded.

   2.  Encode the nameString or valueString using the "percent-encode
       after encoding" process defined by Section 5.2 ("application/
       x-www-form-urlencoded serializing") of [HTMLURL]; this results in
       an ASCII string [ASCII].

   3.  Output the ASCII string.

   Note that the component value does not include any leading question
   mark (?) characters, equals sign (=) characters, or separating
   ampersand (&) characters.  Named query parameters with an empty
   valueString have an empty string as the component value.  Note that
   due to inconsistencies in implementations, some query parameter
   parsing libraries drop such empty values.

   If a query parameter is named as a covered component but it does not
   occur in the query parameters, this MUST cause an error in the
   signature base generation.

   For example, for the following request:

   GET /path?param=value&foo=bar&baz=batman&qux= HTTP/1.1
   Host: www.example.com

   Indicating the baz, qux, and param named query parameters would
   result in the following @query-param component values:

   _baz_: batman

   _qux_: an empty string

   _param_: value

   and the following signature base lines, with (SP) indicating a single
   trailing space character before the empty component value:

   "@query-param";name="baz": batman
   "@query-param";name="qux":(SP)
   "@query-param";name="param": value

   This derived component has some limitations.  Specifically, the
   algorithms provided in Section 5 ("application/
   x-www-form-urlencoded") of [HTMLURL] only support query parameters
   using percent-escaped UTF-8 encoding.  Other encodings are not
   supported.  Additionally, multiple instances of a named parameter are
   not reliably supported in the wild.  If a parameter name occurs
   multiple times in a request, the named query parameter MUST NOT be
   included.  If multiple parameters are common within an application,
   it is RECOMMENDED to sign the entire query string using the @query
   component identifier defined in Section 2.2.7.

   The encoding process allows query parameters that include newlines or
   other problematic characters in their values, or with alternative
   encodings such as using the plus (+) character to represent spaces.
   For the query parameters in this message:

   NOTE: '\' line wrapping per RFC 8792

   GET /parameters?var=this%20is%20a%20big%0Amultiline%20value&\
     bar=with+plus+whitespace&fa%C3%A7ade%22%3A%20=something HTTP/1.1
   Host: www.example.com
   Date: Tue, 20 Apr 2021 02:07:56 GMT

   The resulting values are encoded as follows:

   "@query-param";name="var": this%20is%20a%20big%0Amultiline%20value
   "@query-param";name="bar": with%20plus%20whitespace
   "@query-param";name="fa%C3%A7ade%22%3A%20": something

   If the encoding were not applied, the resultant values would be:

   "@query-param";name="var": this is a big
   multiline value
   "@query-param";name="bar": with plus whitespace
   "@query-param";name="façade\": ": something

   This base string contains characters that violate the constraints on
   component names and values and is therefore invalid.

2.2.9.  Status Code

   The @status derived component refers to the three-digit numeric HTTP
   status code of a response message as defined in [HTTP], Section 15.
   The component value is the serialized three-digit integer of the HTTP
   status code, with no descriptive text.

   For example, the following response message:

   HTTP/1.1 200 OK
   Date: Fri, 26 Mar 2010 00:05:00 GMT

   would result in the following @status component value:

   200

   and the following signature base line:

   "@status": 200

   The @status component identifier MUST NOT be used in a request
   message.

2.3.  Signature Parameters

   HTTP message signatures have metadata properties that provide
   information regarding the signature's generation and verification,
   consisting of the ordered set of covered components and the ordered
   set of parameters, where the parameters include a timestamp of
   signature creation, identifiers for verification key material, and
   other utilities.  This metadata is represented by a special message
   component in the signature base for signature parameters; this
   special message component is treated slightly differently from other
   message components.  Specifically, the signature parameters message
   component is REQUIRED as the last line of the signature base
   (Section 2.5), and the component identifier MUST NOT be enumerated
   within the set of covered components for any signature, including
   itself.

   The signature parameters component name is @signature-params.

   The signature parameters component value is the serialization of the
   signature parameters for this signature, including the covered
   components ordered set with all associated parameters.  These
   parameters include any of the following:

   created:  Creation time as a UNIX timestamp value of type Integer.
      Sub-second precision is not supported.  The inclusion of this
      parameter is RECOMMENDED.

   expires:  Expiration time as a UNIX timestamp value of type Integer.
      Sub-second precision is not supported.

   nonce:  A random unique value generated for this signature as a
      String value.

   alg:  The HTTP message signature algorithm from the "HTTP Signature
      Algorithms" registry, as a String value.

   keyid:  The identifier for the key material as a String value.

   tag:  An application-specific tag for the signature as a String
      value.  This value is used by applications to help identify
      signatures relevant for specific applications or protocols.

   Additional parameters can be defined in the "HTTP Signature Metadata
   Parameters" registry (Section 6.3).  Note that the parameters are not
   in any general order, but once an ordering is chosen for a given set
   of parameters, it cannot be changed without altering the signature
   parameters value.

   The signature parameters component value is serialized as a
   parameterized Inner List using the rules provided in Section 4 of
   [STRUCTURED-FIELDS] as follows:

   1.  Let the output be an empty string.

   2.  Determine an order for the component identifiers of the covered
       components, not including the @signature-params component
       identifier itself.  Once this order is chosen, it cannot be
       changed.  This order MUST be the same order as that used in
       creating the signature base (Section 2.5).

   3.  Serialize the component identifiers of the covered components,
       including all parameters, as an ordered Inner List of String
       values according to Section 4.1.1.1 of [STRUCTURED-FIELDS]; then,
       append this to the output.  Note that the component identifiers
       can include their own parameters, and these parameters are
       ordered sets.  Once an order is chosen for a component's
       parameters, the order cannot be changed.

   4.  Determine an order for any signature parameters.  Once this order
       is chosen, it cannot be changed.

   5.  Append the parameters to the Inner List in order according to
       Section 4.1.1.2 of [STRUCTURED-FIELDS], skipping parameters that
       are not available or not used for this message signature.

   6.  The output contains the signature parameters component value.

   Note that the Inner List serialization from Section 4.1.1.1 of
   [STRUCTURED-FIELDS] is used for the covered component value instead
   of the List serialization from Section 4.1.1 of [STRUCTURED-FIELDS]
   in order to facilitate parallelism with this value's inclusion in the
   Signature-Input field, as discussed in Section 4.1.

   This example shows the serialized component value for the parameters
   of an example message signature:

   NOTE: '\' line wrapping per RFC 8792

   ("@target-uri" "@authority" "date" "cache-control")\
     ;keyid="test-key-rsa-pss";alg="rsa-pss-sha512";\
     created=1618884475;expires=1618884775

   Note that an HTTP message could contain multiple signatures
   (Section 4.3), but only the signature parameters used for a single
   signature are included in a given signature parameters entry.

2.4.  Signing Request Components in a Response Message

   When a request message results in a signed response message, the
   signer can include portions of the request message in the signature
   base by adding the req parameter to the component identifier.

   req  A Boolean flag indicating that the component value is derived
      from the request that triggered this response message and not from
      the response message directly.

   This parameter can be applied to both HTTP fields and derived
   components that target the request, with the same semantics.  The
   component value for a message component using this parameter is
   calculated in the same manner as it is normally, but data is pulled
   from the request message instead of the target response message to
   which the signature is applied.

   Note that the same component name MAY be included with and without
   the req parameter in a single signature base, indicating the same
   named component from both the request message and the response
   message.

   The req parameter MAY be combined with other parameters as
   appropriate for the component identifier, such as the key parameter
   for a Dictionary field.

   For example, when serving a response for this request:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Content-Type: application/json
   Content-Length: 18

   {"hello": "world"}

   This would result in the following unsigned response message:

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 503 Service Unavailable
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Content-Length: 62
   Content-Digest: sha-512=:0Y6iCBzGg5rZtoXS95Ijz03mslf6KAMCloESHObfwn\
     HJDbkkWWQz6PhhU9kxsTbARtY2PTBOzq24uJFpHsMuAg==:

   {"busy": true, "message": "Your call is very important to us"}

   The server signs the response with its own key, including the @status
   code and several header fields in the covered components.  While this
   covers a reasonable amount of the response for this application, the
   server additionally includes several components derived from the
   original request message that triggered this response.  In this
   example, the server includes the method, authority, path, and content
   digest from the request in the covered components of the response.
   The Content-Digest for both the request and the response is included
   under the response signature.  For the application in this example,
   the query is deemed not to be relevant to the response and is
   therefore not covered.  Other applications would make different
   decisions based on application needs, as discussed in Section 1.4.

   The signature base for this example is:

   NOTE: '\' line wrapping per RFC 8792

   "@status": 503
   "content-digest": sha-512=:0Y6iCBzGg5rZtoXS95Ijz03mslf6KAMCloESHObf\
     wnHJDbkkWWQz6PhhU9kxsTbARtY2PTBOzq24uJFpHsMuAg==:
   "content-type": application/json
   "@authority";req: example.com
   "@method";req: POST
   "@path";req: /foo
   "content-digest";req: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A\
     2svX+TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   "@signature-params": ("@status" "content-digest" "content-type" \
     "@authority";req "@method";req "@path";req "content-digest";req)\
     ;created=1618884479;keyid="test-key-ecc-p256"

   The signed response message is:

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 503 Service Unavailable
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Content-Length: 62
   Content-Digest: sha-512=:0Y6iCBzGg5rZtoXS95Ijz03mslf6KAMCloESHObfwn\
     HJDbkkWWQz6PhhU9kxsTbARtY2PTBOzq24uJFpHsMuAg==:
   Signature-Input: reqres=("@status" "content-digest" "content-type" \
     "@authority";req "@method";req "@path";req "content-digest";req)\
     ;created=1618884479;keyid="test-key-ecc-p256"
   Signature: reqres=:dMT/A/76ehrdBTD/2Xx8QuKV6FoyzEP/I9hdzKN8LQJLNgzU\
     4W767HK05rx1i8meNQQgQPgQp8wq2ive3tV5Ag==:

   {"busy": true, "message": "Your call is very important to us"}

   Note that the ECDSA signature algorithm in use here is non-
   deterministic, meaning that a different signature value will be
   created every time the algorithm is run.  The signature value
   provided here can be validated against the given keys, but newly
   generated signature values are not expected to match the example.
   See Section 7.3.5.

   Since the component values from the request are not repeated in the
   response message, the requester MUST keep the original message
   component values around long enough to validate the signature of the
   response that uses this component identifier parameter.  In most
   cases, this means the requester needs to keep the original request
   message around, since the signer could choose to include any portions
   of the request in its response, according to the needs of the
   application.  Since it is possible for an intermediary to alter a
   request message before it is processed by the server, applications
   need to take care not to sign such altered values, as the client
   would not be able to validate the resulting signature.

   It is also possible for a server to create a signed response in
   response to a signed request.  For this example of a signed request:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Content-Type: application/json
   Content-Length: 18
   Signature-Input: sig1=("@method" "@authority" "@path" "@query" \
     "content-digest" "content-type" "content-length")\
     ;created=1618884475;keyid="test-key-rsa-pss"
   Signature: sig1=:e8UJ5wMiRaonlth5ERtE8GIiEH7Akcr493nQ07VPNo6y3qvjdK\
     t0fo8VHO8xXDjmtYoatGYBGJVlMfIp06eVMEyNW2I4vN7XDAz7m5v1108vGzaDljr\
     d0H8+SJ28g7bzn6h2xeL/8q+qUwahWA/JmC8aOC9iVnwbOKCc0WSrLgWQwTY6VLp4\
     2Qt7jjhYT5W7/wCvfK9A1VmHH1lJXsV873Z6hpxesd50PSmO+xaNeYvDLvVdZlhtw\
     5PCtUYzKjHqwmaQ6DEuM8udRjYsoNqp2xZKcuCO1nKc0V3RjpqMZLuuyVbHDAbCzr\
     0pg2d2VM/OC33JAU7meEjjaNz+d7LWPg==:

   {"hello": "world"}

   The server could choose to sign portions of this response, including
   several portions of the request, resulting in this signature base:

   NOTE: '\' line wrapping per RFC 8792

   "@status": 503
   "content-digest": sha-512=:0Y6iCBzGg5rZtoXS95Ijz03mslf6KAMCloESHObf\
     wnHJDbkkWWQz6PhhU9kxsTbARtY2PTBOzq24uJFpHsMuAg==:
   "content-type": application/json
   "@authority";req: example.com
   "@method";req: POST
   "@path";req: /foo
   "@query";req: ?param=Value&Pet=dog
   "content-digest";req: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A\
     2svX+TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   "content-type";req: application/json
   "content-length";req: 18
   "@signature-params": ("@status" "content-digest" "content-type" \
     "@authority";req "@method";req "@path";req "@query";req \
     "content-digest";req "content-type";req "content-length";req)\
     ;created=1618884479;keyid="test-key-ecc-p256"

   and the following signed response:

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 503 Service Unavailable
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Content-Length: 62
   Content-Digest: sha-512=:0Y6iCBzGg5rZtoXS95Ijz03mslf6KAMCloESHObfwn\
     HJDbkkWWQz6PhhU9kxsTbARtY2PTBOzq24uJFpHsMuAg==:
   Signature-Input: reqres=("@status" "content-digest" "content-type" \
     "@authority";req "@method";req "@path";req "@query";req \
     "content-digest";req "content-type";req "content-length";req)\
     ;created=1618884479;keyid="test-key-ecc-p256"
   Signature: reqres=:C73J41GVKc+TYXbSobvZf0CmNcptRiWN+NY1Or0A36ISg6ym\
     dRN6ZgR2QfrtopFNzqAyv+CeWrMsNbcV2Ojsgg==:

   {"busy": true, "message": "Your call is very important to us"}

   Note that the ECDSA signature algorithm in use here is non-
   deterministic, meaning that a different signature value will be
   created every time the algorithm is run.  The signature value
   provided here can be validated against the given keys, but newly
   generated signature values are not expected to match the example.
   See Section 7.3.5.

   Applications signing a response to a signed request SHOULD sign all
   of the components of the request signature value to provide
   sufficient coverage and protection against a class of collision
   attacks, as discussed in Section 7.3.7.  The server in this example
   has included all components listed in the Signature-Input field of
   the client's signature on the request in the response signature, in
   addition to components of the response.

   While it is syntactically possible to include the Signature and
   Signature-Input fields of the request message in the signature
   components of a response to a message using this mechanism, this
   practice is NOT RECOMMENDED.  This is because signatures of
   signatures do not provide transitive coverage of covered components
   as one might expect, and the practice is susceptible to several
   attacks as discussed in Section 7.3.7.  An application that needs to
   signal successful processing or receipt of a signature would need to
   carefully specify alternative mechanisms for sending such a signal
   securely.

   The response signature can only ever cover what is included in the
   request message when using this flag.  Consequently, if an
   application needs to include the message content of the request under
   the signature of its response, the client needs to include a means
   for covering that content, such as a Content-Digest field.  See the
   discussion in Section 7.2.8 for more information.

   The req parameter MUST NOT be used for any component in a signature
   that targets a request message.

2.5.  Creating the Signature Base

   The signature base is an ASCII string [ASCII] containing the
   canonicalized HTTP message components covered by the signature.  The
   input to the signature base creation algorithm is the ordered set of
   covered component identifiers and their associated values, along with
   any additional signature parameters discussed in Section 2.3.

   Component identifiers are serialized using the strict serialization
   rules defined by [STRUCTURED-FIELDS], Section 4.  The component
   identifier has a component name, which is a String Item value
   serialized using the sf-string ABNF rule.  The component identifier
   MAY also include defined parameters that are serialized using the
   parameters ABNF rule.  The signature parameters line defined in
   Section 2.3 follows this same pattern, but the component identifier
   is a String Item with a fixed value and no parameters, and the
   component value is always an Inner List with optional parameters.

   Note that this means the serialization of the component name itself
   is encased in double quotes, with parameters following as a
   semicolon-separated list, such as "cache-control", "@authority",
   "@signature-params", or "example-dictionary";key="foo".

   The output is the ordered set of bytes that form the signature base,
   which conforms to the following ABNF:

   signature-base = *( signature-base-line LF ) signature-params-line
   signature-base-line = component-identifier ":" SP
       ( derived-component-value / *field-content )
       ; no obs-fold nor obs-text
   component-identifier = component-name parameters
   component-name = sf-string
   derived-component-value = *( VCHAR / SP )
   signature-params-line = DQUOTE "@signature-params" DQUOTE
        ":" SP inner-list

   To create the signature base, the signer or verifier concatenates
   entries for each component identifier in the signature's covered
   components (including their parameters) using the following
   algorithm.  All errors produced as described MUST fail the algorithm
   immediately, without outputting a signature base.

   1.  Let the output be an empty string.

   2.  For each message component item in the covered components set (in
       order):

       2.1.  If the component identifier (including its parameters) has
             already been added to the signature base, produce an error.

       2.2.  Append the component identifier for the covered component
             serialized according to the component-identifier ABNF rule.
             Note that this serialization places the component name in
             double quotes and appends any parameters outside of the
             quotes.

       2.3.  Append a single colon (:).

       2.4.  Append a single space (" ").

       2.5.  Determine the component value for the component identifier.

             *  If the component identifier has a parameter that is not
                understood, produce an error.

             *  If the component identifier has parameters that are
                mutually incompatible with one another, such as bs and
                sf, produce an error.

             *  If the component identifier contains the req parameter
                and the target message is a request, produce an error.

             *  If the component identifier contains the req parameter
                and the target message is a response, the context for
                the component value is the related request message of
                the target response message.  Otherwise, the context for
                the component value is the target message.

             *  If the component name starts with an "at" (@) character,
                derive the component's value from the message according
                to the specific rules defined for the derived component,
                as provided in Section 2.2, including processing of any
                known valid parameters.  If the derived component name
                is unknown or the value cannot be derived, produce an
                error.

             *  If the component name does not start with an "at" (@)
                character, canonicalize the HTTP field value as
                described in Section 2.1, including processing of any
                known valid parameters.  If the field cannot be found in
                the message or the value cannot be obtained in the
                context, produce an error.

       2.6.  Append the covered component's canonicalized component
             value.

       2.7.  Append a single newline (\n).

   3.  Append the signature parameters component (Section 2.3) according
       to the signature-params-line rule as follows:

       3.1.  Append the component identifier for the signature
             parameters serialized according to the component-identifier
             rule, i.e., the exact value "@signature-params" (including
             double quotes).

       3.2.  Append a single colon (:).

       3.3.  Append a single space (" ").

       3.4.  Append the signature parameters' canonicalized component
             values as defined in Section 2.3, i.e., Inner List
             Structured Field values with parameters.

   4.  Produce an error if the output string contains any non-ASCII
       characters [ASCII].

   5.  Return the output string.

   If covered components reference a component identifier that cannot be
   resolved to a component value in the message, the implementation MUST
   produce an error and not create a signature base.  Such situations
   include, but are not limited to, the following:

   *  The signer or verifier does not understand the derived component
      name.

   *  The component name identifies a field that is not present in the
      message or whose value is malformed.

   *  The component identifier includes a parameter that is unknown or
      does not apply to the component identifier to which it is
      attached.

   *  The component identifier indicates that a Structured Field
      serialization is used (via the sf parameter), but the field in
      question is known to not be a Structured Field or the type of
      Structured Field is not known to the implementation.

   *  The component identifier is a Dictionary member identifier that
      references a field that is not present in the message, that is not
      a Dictionary Structured Field, or whose value is malformed.

   *  The component identifier is a Dictionary member identifier or a
      named query parameter identifier that references a member that is
      not present in the component value or whose value is malformed.
      For example, the identifier is "example-dict";key="c", and the
      value of the Example-Dict header field is a=1, b=2, which does not
      have the c value.

   In the following non-normative example, the HTTP message being signed
   is the following request:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Content-Length: 18

   {"hello": "world"}

   The covered components consist of the @method, @authority, and @path
   derived components followed by the Content-Digest, Content-Length,
   and Content-Type HTTP header fields, in order.  The signature
   parameters consist of a creation timestamp of 1618884473 and a key
   identifier of test-key-rsa-pss.  Note that no explicit alg parameter
   is given here, since the verifier is known by the application to use
   the RSA-PSS algorithm based on the identified key.  The signature
   base for this message with these parameters is:

   NOTE: '\' line wrapping per RFC 8792

   "@method": POST
   "@authority": example.com
   "@path": /foo
   "content-digest": sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX\
     +TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   "content-length": 18
   "content-type": application/json
   "@signature-params": ("@method" "@authority" "@path" \
     "content-digest" "content-length" "content-type")\
     ;created=1618884473;keyid="test-key-rsa-pss"

               Figure 1: Non-normative Example Signature Base

   Note that the example signature base above does not include the final
   newline that ends the displayed example, nor do other example
   signature bases displayed elsewhere in this specification.

3.  HTTP Message Signatures

   An HTTP message signature is a signature over a string generated from
   a subset of the components of an HTTP message in addition to metadata
   about the signature itself.  When successfully verified against an
   HTTP message, an HTTP message signature provides cryptographic proof
   that the message is semantically equivalent to the message for which
   the signature was generated, with respect to the subset of message
   components that was signed.

3.1.  Creating a Signature

   Creation of an HTTP message signature is a process that takes as its
   input the signature context (including the target message) and the
   requirements for the application.  The output is a signature value
   and set of signature parameters that can be communicated to the
   verifier by adding them to the message.

   In order to create a signature, a signer MUST apply the following
   algorithm:

   1.  The signer chooses an HTTP signature algorithm and key material
       for signing from the set of potential signing algorithms.  The
       set of potential algorithms is determined by the application and
       is out of scope for this document.  The signer MUST choose key
       material that is appropriate for the signature's algorithm and
       that conforms to any requirements defined by the algorithm, such
       as key size or format.  The mechanism by which the signer chooses
       the algorithm and key material is out of scope for this document.

   2.  The signer sets the signature's creation time to the current
       time.

   3.  If applicable, the signer sets the signature's expiration time
       property to the time at which the signature is to expire.  The
       expiration is a hint to the verifier, expressing the time at
       which the signer is no longer willing to vouch for the signature.
       An appropriate expiration length, and the processing requirements
       of this parameter, are application specific.

   4.  The signer creates an ordered set of component identifiers
       representing the message components to be covered by the
       signature and attaches signature metadata parameters to this set.
       The serialized value of this set is later used as the value of
       the Signature-Input field as described in Section 4.1.

       *  Once an order of covered components is chosen, the order MUST
          NOT change for the life of the signature.

       *  Each covered component identifier MUST be either (1) an HTTP
          field (Section 2.1) in the signature context or (2) a derived
          component listed in Section 2.2 or in the "HTTP Signature
          Derived Component Names" registry.

       *  Signers of a request SHOULD include some or all of the message
          control data in the covered components, such as the @method,
          @authority, @target-uri, or some combination thereof.

       *  Signers SHOULD include the created signature metadata
          parameter to indicate when the signature was created.

       *  The @signature-params derived component identifier MUST NOT be
          present in the list of covered component identifiers.  The
          derived component is required to always be the last line in
          the signature base, ensuring that a signature always covers
          its own metadata and the metadata cannot be substituted.

       *  Further guidance on what to include in this set and in what
          order is out of scope for this document.

   5.  The signer creates the signature base using these parameters and
       the signature base creation algorithm (Section 2.5).

   6.  The signer uses the HTTP_SIGN primitive function to sign the
       signature base with the chosen signing algorithm using the key
       material chosen by the signer.  The HTTP_SIGN primitive and
       several concrete applications of signing algorithms are defined
       in Section 3.3.

   7.  The byte array output of the signature function is the HTTP
       message signature output value to be included in the Signature
       field as defined in Section 4.2.

   For example, given the HTTP message and signature parameters in the
   example in Section 2.5, the example signature base is signed with the
   test-key-rsa-pss key (see Appendix B.1.2) and the RSASSA-PSS
   algorithm described in Section 3.3.1, giving the following message
   signature output value, encoded in Base64:

   NOTE: '\' line wrapping per RFC 8792

   HIbjHC5rS0BYaa9v4QfD4193TORw7u9edguPh0AW3dMq9WImrlFrCGUDih47vAxi4L2\
   YRZ3XMJc1uOKk/J0ZmZ+wcta4nKIgBkKq0rM9hs3CQyxXGxHLMCy8uqK488o+9jrptQ\
   +xFPHK7a9sRL1IXNaagCNN3ZxJsYapFj+JXbmaI5rtAdSfSvzPuBCh+ARHBmWuNo1Uz\
   VVdHXrl8ePL4cccqlazIJdC4QEjrF+Sn4IxBQzTZsL9y9TP5FsZYzHvDqbInkTNigBc\
   E9cKOYNFCn4D/WM7F6TNuZO9EgtzepLWcjTymlHzK7aXq6Am6sfOrpIC49yXjj3ae6H\
   RalVc/g==

              Figure 2: Non-normative Example Signature Value

   Note that the RSA-PSS algorithm in use here is non-deterministic,
   meaning that a different signature value will be created every time
   the algorithm is run.  The signature value provided here can be
   validated against the given keys, but newly generated signature
   values are not expected to match the example.  See Section 7.3.5.

3.2.  Verifying a Signature

   Verification of an HTTP message signature is a process that takes as
   its input the signature context (including the target message,
   particularly its Signature and Signature-Input fields) and the
   requirements for the application.  The output of the verification is
   either a positive verification or an error.

   In order to verify a signature, a verifier MUST apply the following
   algorithm:

   1.  Parse the Signature and Signature-Input fields as described in
       Sections 4.1 and 4.2, and extract the signatures to be verified
       and their labels.

       1.1.  If there is more than one signature value present,
             determine which signature should be processed for this
             message based on the policy and configuration of the
             verifier.  If an applicable signature is not found, produce
             an error.

       1.2.  If the chosen Signature field value does not have a
             corresponding Signature-Input field value (i.e., one with
             the same label), produce an error.

   2.  Parse the values of the chosen Signature-Input field as a
       parameterized Inner List to get the ordered list of covered
       components and the signature parameters for the signature to be
       verified.

   3.  Parse the value of the corresponding Signature field to get the
       byte array value of the signature to be verified.

   4.  Examine the signature parameters to confirm that the signature
       meets the requirements described in this document, as well as any
       additional requirements defined by the application such as which
       message components are required to be covered by the signature
       (Section 3.2.1).

   5.  Determine the verification key material for this signature.  If
       the key material is known through external means such as static
       configuration or external protocol negotiation, the verifier will
       use the applicable technique to obtain the key material from this
       external knowledge.  If the key is identified in the signature
       parameters, the verifier will dereference the key identifier to
       appropriate key material to use with the signature.  The verifier
       has to determine the trustworthiness of the key material for the
       context in which the signature is presented.  If a key is
       identified that the verifier does not know or trust for this
       request or that does not match something preconfigured, the
       verification MUST fail.

   6.  Determine the algorithm to apply for verification:

       6.1.  Start with the set of allowable algorithms known to the
             application.  If any of the following steps select an
             algorithm that is not in this set, the signature validation
             fails.

       6.2.  If the algorithm is known through external means such as
             static configuration or external protocol negotiation, the
             verifier will use that algorithm.

       6.3.  If the algorithm can be determined from the keying
             material, such as through an algorithm field on the key
             value itself, the verifier will use that algorithm.

       6.4.  If the algorithm is explicitly stated in the signature
             parameters using a value from the "HTTP Signature
             Algorithms" registry, the verifier will use the referenced
             algorithm.

       6.5.  If the algorithm is specified in more than one location
             (e.g., a combination of static configuration, the algorithm
             signature parameter, and the key material itself), the
             resolved algorithms MUST be the same.  If the algorithms
             are not the same, the verifier MUST fail the verification.

   7.  Use the received HTTP message and the parsed signature parameters
       to recreate the signature base, using the algorithm defined in
       Section 2.5.  The value of the @signature-params input is the
       value of the Signature-Input field for this signature serialized
       according to the rules described in Section 2.3.  Note that this
       does not include the signature's label from the Signature-Input
       field.

   8.  If the key material is appropriate for the algorithm, apply the
       appropriate HTTP_VERIFY cryptographic verification algorithm to
       the signature, recalculated signature base, key material, and
       signature value.  The HTTP_VERIFY primitive and several concrete
       algorithms are defined in Section 3.3.

   9.  The results of the verification algorithm function are the final
       results of the cryptographic verification function.

   If any of the above steps fail or produce an error, the signature
   validation fails.

   For example, verifying the signature with the label sig1 of the
   following message with the test-key-rsa-pss key (see Appendix B.1.2)
   and the RSASSA-PSS algorithm described in Section 3.3.1:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Content-Length: 18
   Signature-Input: sig1=("@method" "@authority" "@path" \
     "content-digest" "content-length" "content-type")\
     ;created=1618884473;keyid="test-key-rsa-pss"
   Signature: sig1=:HIbjHC5rS0BYaa9v4QfD4193TORw7u9edguPh0AW3dMq9WImrl\
     FrCGUDih47vAxi4L2YRZ3XMJc1uOKk/J0ZmZ+wcta4nKIgBkKq0rM9hs3CQyxXGxH\
     LMCy8uqK488o+9jrptQ+xFPHK7a9sRL1IXNaagCNN3ZxJsYapFj+JXbmaI5rtAdSf\
     SvzPuBCh+ARHBmWuNo1UzVVdHXrl8ePL4cccqlazIJdC4QEjrF+Sn4IxBQzTZsL9y\
     9TP5FsZYzHvDqbInkTNigBcE9cKOYNFCn4D/WM7F6TNuZO9EgtzepLWcjTymlHzK7\
     aXq6Am6sfOrpIC49yXjj3ae6HRalVc/g==:

   {"hello": "world"}

   With the additional requirements that at least the method, authority,
   path, content-digest, content-length, and content-type entries be
   signed, and that the signature creation timestamp be recent enough at
   the time of verification, the verification passes.

3.2.1.  Enforcing Application Requirements

   The verification requirements specified in this document are intended
   as a baseline set of restrictions that are generally applicable to
   all use cases.  Applications using HTTP message signatures MAY impose
   requirements above and beyond those specified by this document, as
   appropriate for their use case.

   Some non-normative examples of additional requirements an application
   might define are:

   *  Requiring a specific set of header fields to be signed (e.g.,
      Authorization, Content-Digest).

   *  Enforcing a maximum signature age from the time of the created
      timestamp.

   *  Rejecting signatures past the expiration time in the expires
      timestamp.  Note that the expiration time is a hint from the
      signer and that a verifier can always reject a signature ahead of
      its expiration time.

   *  Prohibiting certain signature metadata parameters, such as runtime
      algorithm signaling with the alg parameter when the algorithm is
      determined from the key information.

   *  Ensuring successful dereferencing of the keyid parameter to valid
      and appropriate key material.

   *  Prohibiting the use of certain algorithms or mandating the use of
      a specific algorithm.

   *  Requiring keys to be of a certain size (e.g., 2048 bits vs. 1024
      bits).

   *  Enforcing uniqueness of the nonce parameter.

   *  Requiring an application-specific value for the tag parameter.

   Application-specific requirements are expected and encouraged.  When
   an application defines additional requirements, it MUST enforce them
   during the signature verification process, and signature verification
   MUST fail if the signature does not conform to the application's
   requirements.

   Applications MUST enforce the requirements defined in this document.
   Regardless of use case, applications MUST NOT accept signatures that
   do not conform to these requirements.

3.3.  Signature Algorithms

   An HTTP message signature MUST use a cryptographic digital signature
   or MAC method that is appropriate for the key material, environment,
   and needs of the signer and verifier.  This specification does not
   strictly limit the available signature algorithms, and any signature
   algorithm that meets these basic requirements MAY be used by an
   application of HTTP message signatures.

   For each signing method, HTTP_SIGN takes as its input the signature
   base defined in Section 2.5 as a byte array (M) and the signing key
   material (Ks), and outputs the resultant signature as a byte array
   (S):

   HTTP_SIGN (M, Ks)  ->  S

   For each verification method, HTTP_VERIFY takes as its input the
   regenerated signature base defined in Section 2.5 as a byte array
   (M), the verification key material (Kv), and the presented signature
   to be verified as a byte array (S), and outputs the verification
   result (V) as a Boolean:

   HTTP_VERIFY (M, Kv, S) -> V

   The following sections contain several common signature algorithms
   and demonstrate how these cryptographic primitives map to the
   HTTP_SIGN and HTTP_VERIFY definitions above.  Which method to use can
   be communicated through the explicit algorithm (alg) signature
   parameter (Section 2.3), by reference to the key material, or through
   mutual agreement between the signer and verifier.  Signature
   algorithms selected using the alg parameter MUST use values from the
   "HTTP Signature Algorithms" registry (Section 6.2).

3.3.1.  RSASSA-PSS Using SHA-512

   To sign using this algorithm, the signer applies the RSASSA-PSS-SIGN
   (K, M) function defined in [RFC8017] with the signer's private
   signing key (K) and the signature base (M) (Section 2.5).  The mask
   generation function is MGF1 as specified in [RFC8017] with a hash
   function of SHA-512 [RFC6234].  The salt length (sLen) is 64 bytes.
   The hash function (Hash) SHA-512 [RFC6234] is applied to the
   signature base to create the digest content to which the digital
   signature is applied.  The resulting signed content byte array (S) is
   the HTTP message signature output used in Section 3.1.

   To verify using this algorithm, the verifier applies the RSASSA-PSS-
   VERIFY ((n, e), M, S) function [RFC8017] using the public key portion
   of the verification key material (n, e) and the signature base (M)
   recreated as described in Section 3.2.  The mask generation function
   is MGF1 as specified in [RFC8017] with a hash function of SHA-512
   [RFC6234].  The salt length (sLen) is 64 bytes.  The hash function
   (Hash) SHA-512 [RFC6234] is applied to the signature base to create
   the digest content to which the verification function is applied.
   The verifier extracts the HTTP message signature to be verified (S)
   as described in Section 3.2.  The results of the verification
   function indicate whether the signature presented is valid.

   Note that the output of the RSASSA-PSS algorithm is non-
   deterministic; therefore, it is not correct to recalculate a new
   signature on the signature base and compare the results to an
   existing signature.  Instead, the verification algorithm defined here
   needs to be used.  See Section 7.3.5.

   The use of this algorithm can be indicated at runtime using the rsa-
   pss-sha512 value for the alg signature parameter.

3.3.2.  RSASSA-PKCS1-v1_5 Using SHA-256

   To sign using this algorithm, the signer applies the RSASSA-
   PKCS1-V1_5-SIGN (K, M) function defined in [RFC8017] with the
   signer's private signing key (K) and the signature base (M)
   (Section 2.5).  The hash SHA-256 [RFC6234] is applied to the
   signature base to create the digest content to which the digital
   signature is applied.  The resulting signed content byte array (S) is
   the HTTP message signature output used in Section 3.1.

   To verify using this algorithm, the verifier applies the RSASSA-
   PKCS1-V1_5-VERIFY ((n, e), M, S) function [RFC8017] using the public
   key portion of the verification key material (n, e) and the signature
   base (M) recreated as described in Section 3.2.  The hash function
   SHA-256 [RFC6234] is applied to the signature base to create the
   digest content to which the verification function is applied.  The
   verifier extracts the HTTP message signature to be verified (S) as
   described in Section 3.2.  The results of the verification function
   indicate whether the signature presented is valid.

   The use of this algorithm can be indicated at runtime using the rsa-
   v1_5-sha256 value for the alg signature parameter.

3.3.3.  HMAC Using SHA-256

   To sign and verify using this algorithm, the signer applies the HMAC
   function [RFC2104] with the shared signing key (K) and the signature
   base (text) (Section 2.5).  The hash function SHA-256 [RFC6234] is
   applied to the signature base to create the digest content to which
   the HMAC is applied, giving the signature result.

   For signing, the resulting value is the HTTP message signature output
   used in Section 3.1.

   For verification, the verifier extracts the HTTP message signature to
   be verified (S) as described in Section 3.2.  The output of the HMAC
   function is compared bytewise to the value of the HTTP message
   signature, and the results of the comparison determine the validity
   of the signature presented.

   The use of this algorithm can be indicated at runtime using the hmac-
   sha256 value for the alg signature parameter.

3.3.4.  ECDSA Using Curve P-256 DSS and SHA-256

   To sign using this algorithm, the signer applies the ECDSA signature
   algorithm defined in [FIPS186-5] using curve P-256 with the signer's
   private signing key and the signature base (Section 2.5).  The hash
   SHA-256 [RFC6234] is applied to the signature base to create the
   digest content to which the digital signature is applied (M).  The
   signature algorithm returns two integer values: r and s.  These are
   both encoded as big-endian unsigned integers, zero-padded to 32
   octets each.  These encoded values are concatenated into a single
   64-octet array consisting of the encoded value of r followed by the
   encoded value of s.  The resulting concatenation of (r, s) is a byte
   array of the HTTP message signature output used in Section 3.1.

   To verify using this algorithm, the verifier applies the ECDSA
   signature algorithm defined in [FIPS186-5] using the public key
   portion of the verification key material and the signature base
   recreated as described in Section 3.2.  The hash function SHA-256
   [RFC6234] is applied to the signature base to create the digest
   content to which the signature verification function is applied (M).
   The verifier extracts the HTTP message signature to be verified (S)
   as described in Section 3.2.  This value is a 64-octet array
   consisting of the encoded values of r and s concatenated in order.
   These are both encoded as big-endian unsigned integers, zero-padded
   to 32 octets each.  The resulting signature value (r, s) is used as
   input to the signature verification function.  The results of the
   verification function indicate whether the signature presented is
   valid.

   Note that the output of ECDSA signature algorithms is non-
   deterministic; therefore, it is not correct to recalculate a new
   signature on the signature base and compare the results to an
   existing signature.  Instead, the verification algorithm defined here
   needs to be used.  See Section 7.3.5.

   The use of this algorithm can be indicated at runtime using the
   ecdsa-p256-sha256 value for the alg signature parameter.

3.3.5.  ECDSA Using Curve P-384 DSS and SHA-384

   To sign using this algorithm, the signer applies the ECDSA signature
   algorithm defined in [FIPS186-5] using curve P-384 with the signer's
   private signing key and the signature base (Section 2.5).  The hash
   SHA-384 [RFC6234] is applied to the signature base to create the
   digest content to which the digital signature is applied (M).  The
   signature algorithm returns two integer values: r and s.  These are
   both encoded as big-endian unsigned integers, zero-padded to 48
   octets each.  These encoded values are concatenated into a single
   96-octet array consisting of the encoded value of r followed by the
   encoded value of s.  The resulting concatenation of (r, s) is a byte
   array of the HTTP message signature output used in Section 3.1.

   To verify using this algorithm, the verifier applies the ECDSA
   signature algorithm defined in [FIPS186-5] using the public key
   portion of the verification key material and the signature base
   recreated as described in Section 3.2.  The hash function SHA-384
   [RFC6234] is applied to the signature base to create the digest
   content to which the signature verification function is applied (M).
   The verifier extracts the HTTP message signature to be verified (S)
   as described in Section 3.2.  This value is a 96-octet array
   consisting of the encoded values of r and s concatenated in order.
   These are both encoded as big-endian unsigned integers, zero-padded
   to 48 octets each.  The resulting signature value (r, s) is used as
   input to the signature verification function.  The results of the
   verification function indicate whether the signature presented is
   valid.

   Note that the output of ECDSA signature algorithms is non-
   deterministic; therefore, it is not correct to recalculate a new
   signature on the signature base and compare the results to an
   existing signature.  Instead, the verification algorithm defined here
   needs to be used.  See Section 7.3.5.

   The use of this algorithm can be indicated at runtime using the
   ecdsa-p384-sha384 value for the alg signature parameter.

3.3.6.  EdDSA Using Curve edwards25519

   To sign using this algorithm, the signer applies the Ed25519
   algorithm defined in Section 5.1.6 of [RFC8032] with the signer's
   private signing key and the signature base (Section 2.5).  The
   signature base is taken as the input message (M) with no prehash
   function.  The signature is a 64-octet concatenation of R and S as
   specified in Section 5.1.6 of [RFC8032], and this is taken as a byte
   array for the HTTP message signature output used in Section 3.1.

   To verify using this algorithm, the signer applies the Ed25519
   algorithm defined in Section 5.1.7 of [RFC8032] using the public key
   portion of the verification key material (A) and the signature base
   recreated as described in Section 3.2.  The signature base is taken
   as the input message (M) with no prehash function.  The signature to
   be verified is processed as the 64-octet concatenation of R and S as
   specified in Section 5.1.7 of [RFC8032].  The results of the
   verification function indicate whether the signature presented is
   valid.

   The use of this algorithm can be indicated at runtime using the
   ed25519 value for the alg signature parameter.

3.3.7.  JSON Web Signature (JWS) Algorithms

   If the signing algorithm is a JSON Object Signing and Encryption
   (JOSE) signing algorithm from the "JSON Web Signature and Encryption
   Algorithms" registry established by [RFC7518], the JWS algorithm
   definition determines the signature and hashing algorithms to apply
   for both signing and verification.

   For both signing and verification, the HTTP message's signature base
   (Section 2.5) is used as the entire "JWS Signing Input".  The JOSE
   Header [JWS] [RFC7517] is not used, and the signature base is not
   first encoded in Base64 before applying the algorithm.  The output of
   the JWS Signature is taken as a byte array prior to the Base64url
   encoding used in JOSE.

   The JWS algorithm MUST NOT be "none" and MUST NOT be any algorithm
   with a JOSE Implementation Requirement of "Prohibited".

   JSON Web Algorithm (JWA) values from the "JSON Web Signature and
   Encryption Algorithms" registry are not included as signature
   parameters.  Typically, the JWS algorithm can be signaled using JSON
   Web Keys (JWKs) or other mechanisms common to JOSE implementations.
   In fact, JWA values are not registered in the "HTTP Signature
   Algorithms" registry (Section 6.2), and so the explicit alg signature
   parameter is not used at all when using JOSE signing algorithms.

4.  Including a Message Signature in a Message

   HTTP message signatures can be included within an HTTP message via
   the Signature-Input and Signature fields, both defined within this
   specification.

   The Signature-Input field identifies the covered components and
   parameters that describe how the signature was generated, while the
   Signature field contains the signature value.  Each field MAY contain
   multiple labeled values.

   An HTTP message signature is identified by a label within an HTTP
   message.  This label MUST be unique within a given HTTP message and
   MUST be used in both the Signature-Input field and the Signature
   field.  The label is chosen by the signer, except where a specific
   label is dictated by protocol negotiations such as those described in
   Section 5.

   An HTTP message signature MUST use both the Signature-Input field and
   the Signature field, and each field MUST contain the same labels.
   The presence of a label in one field but not the other is an error.

4.1.  The Signature-Input HTTP Field

   The Signature-Input field is a Dictionary Structured Field (defined
   in Section 3.2 of [STRUCTURED-FIELDS]) containing the metadata for
   one or more message signatures generated from components within the
   HTTP message.  Each member describes a single message signature.  The
   member's key is the label that uniquely identifies the message
   signature within the HTTP message.  The member's value is the covered
   components ordered set serialized as an Inner List, including all
   signature metadata parameters identified by the label:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig1=("@method" "@target-uri" "@authority" \
     "content-digest" "cache-control");\
     created=1618884475;keyid="test-key-rsa-pss"

   To facilitate signature validation, the Signature-Input field value
   MUST contain the same serialized value used in generating the
   signature base's @signature-params value defined in Section 2.3.
   Note that in a Structured Field value, list order and parameter order
   have to be preserved.

   The signer MAY include the Signature-Input field as a trailer to
   facilitate signing a message after its content has been processed by
   the signer.  However, since intermediaries are allowed to drop
   trailers as per [HTTP], it is RECOMMENDED that the Signature-Input
   field be included only as a header field to avoid signatures being
   inadvertently stripped from a message.

   Multiple Signature-Input fields MAY be included in a single HTTP
   message.  The signature labels MUST be unique across all field
   values.

4.2.  The Signature HTTP Field

   The Signature field is a Dictionary Structured Field (defined in
   Section 3.2 of [STRUCTURED-FIELDS]) containing one or more message
   signatures generated from the signature context of the target
   message.  The member's key is the label that uniquely identifies the
   message signature within the HTTP message.  The member's value is a
   Byte Sequence containing the signature value for the message
   signature identified by the label:

   NOTE: '\' line wrapping per RFC 8792

   Signature: sig1=:P0wLUszWQjoi54udOtydf9IWTfNhy+r53jGFj9XZuP4uKwxyJo\
     1RSHi+oEF1FuX6O29d+lbxwwBao1BAgadijW+7O/PyezlTnqAOVPWx9GlyntiCiHz\
     C87qmSQjvu1CFyFuWSjdGa3qLYYlNm7pVaJFalQiKWnUaqfT4LyttaXyoyZW84jS8\
     gyarxAiWI97mPXU+OVM64+HVBHmnEsS+lTeIsEQo36T3NFf2CujWARPQg53r58Rmp\
     Z+J9eKR2CD6IJQvacn5A4Ix5BUAVGqlyp8JYm+S/CWJi31PNUjRRCusCVRj05NrxA\
     BNFv3r5S9IXf2fYJK+eyW4AiGVMvMcOg==:

   The signer MAY include the Signature field as a trailer to facilitate
   signing a message after its content has been processed by the signer.
   However, since intermediaries are allowed to drop trailers as per
   [HTTP], it is RECOMMENDED that the Signature field be included only
   as a header field to avoid signatures being inadvertently stripped
   from a message.

   Multiple Signature fields MAY be included in a single HTTP message.
   The signature labels MUST be unique across all field values.

4.3.  Multiple Signatures

   Multiple distinct signatures MAY be included in a single message.
   Each distinct signature MUST have a unique label.  These multiple
   signatures could all be added by the same signer, or they could come
   from several different signers.  For example, a signer may include
   multiple signatures signing the same message components with
   different keys or algorithms to support verifiers with different
   capabilities, or a reverse proxy may include information about the
   client in fields when forwarding the request to a service host,
   including a signature over the client's original signature values.

   The following non-normative example starts with a signed request from
   the client.  A reverse proxy takes this request and validates the
   client's signature:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Length: 18
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Signature-Input: sig1=("@method" "@authority" "@path" \
     "content-digest" "content-type" "content-length")\
     ;created=1618884475;keyid="test-key-ecc-p256"
   Signature: sig1=:X5spyd6CFnAG5QnDyHfqoSNICd+BUP4LYMz2Q0JXlb//4Ijpzp\
     +kve2w4NIyqeAuM7jTDX+sNalzA8ESSaHD3A==:

   {"hello": "world"}

   The proxy then alters the message before forwarding it on to the
   origin server, changing the target host and adding the Forwarded
   header field defined in [RFC7239]:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: origin.host.internal.example
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Content-Length: 18
   Forwarded: for=192.0.2.123;host=example.com;proto=https
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Signature-Input: sig1=("@method" "@authority" "@path" \
     "content-digest" "content-type" "content-length")\
     ;created=1618884475;keyid="test-key-ecc-p256"
   Signature: sig1=:X5spyd6CFnAG5QnDyHfqoSNICd+BUP4LYMz2Q0JXlb//4Ijpzp\
     +kve2w4NIyqeAuM7jTDX+sNalzA8ESSaHD3A==:

   {"hello": "world"}

   The proxy is in a position to validate the incoming client's
   signature and make its own statement to the origin server about the
   nature of the request that it is forwarding by adding its own
   signature over the new message before passing it along to the origin
   server.  The proxy also includes all the elements from the original
   message that are relevant to the origin server's processing.  In many
   cases, the proxy will want to cover all the same components that were
   covered by the client's signature, which is the case in the following
   example.  Note that in this example, the proxy is signing over the
   new authority value, which it has changed.  The proxy also adds the
   Forwarded header field to its own signature value.  The proxy
   identifies its own key and algorithm and, in this example, includes
   an expiration for the signature to indicate to downstream systems
   that the proxy will not vouch for this signed message past this short
   time window.  This results in a signature base of:

   NOTE: '\' line wrapping per RFC 8792

   "@method": POST
   "@authority": origin.host.internal.example
   "@path": /foo
   "content-digest": sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX\
     +TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   "content-type": application/json
   "content-length": 18
   "forwarded": for=192.0.2.123;host=example.com;proto=https
   "@signature-params": ("@method" "@authority" "@path" \
     "content-digest" "content-type" "content-length" "forwarded")\
     ;created=1618884480;keyid="test-key-rsa";alg="rsa-v1_5-sha256"\
     ;expires=1618884540

   and a signature output value of:

   NOTE: '\' line wrapping per RFC 8792

   S6ZzPXSdAMOPjN/6KXfXWNO/f7V6cHm7BXYUh3YD/fRad4BCaRZxP+JH+8XY1I6+8Cy\
   +CM5g92iHgxtRPz+MjniOaYmdkDcnL9cCpXJleXsOckpURl49GwiyUpZ10KHgOEe11s\
   x3G2gxI8S0jnxQB+Pu68U9vVcasqOWAEObtNKKZd8tSFu7LB5YAv0RAGhB8tmpv7sFn\
   Im9y+7X5kXQfi8NMaZaA8i2ZHwpBdg7a6CMfwnnrtflzvZdXAsD3LH2TwevU+/PBPv0\
   B6NMNk93wUs/vfJvye+YuI87HU38lZHowtznbLVdp770I6VHR6WfgS9ddzirrswsE1w\
   5o0LV/g==

   These values are added to the HTTP request message by the proxy.  The
   original signature is included under the label sig1, and the reverse
   proxy's signature is included under the label proxy_sig.  The proxy
   uses the key test-key-rsa to create its signature using the rsa-
   v1_5-sha256 signature algorithm, while the client's original
   signature was made using the key test-key-rsa-pss and an RSA-PSS
   signature algorithm:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: origin.host.internal.example
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Content-Length: 18
   Forwarded: for=192.0.2.123;host=example.com;proto=https
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Signature-Input: sig1=("@method" "@authority" "@path" \
       "content-digest" "content-type" "content-length")\
       ;created=1618884475;keyid="test-key-ecc-p256", \
     proxy_sig=("@method" "@authority" "@path" "content-digest" \
       "content-type" "content-length" "forwarded")\
       ;created=1618884480;keyid="test-key-rsa";alg="rsa-v1_5-sha256"\
       ;expires=1618884540
   Signature: sig1=:X5spyd6CFnAG5QnDyHfqoSNICd+BUP4LYMz2Q0JXlb//4Ijpzp\
       +kve2w4NIyqeAuM7jTDX+sNalzA8ESSaHD3A==:, \
     proxy_sig=:S6ZzPXSdAMOPjN/6KXfXWNO/f7V6cHm7BXYUh3YD/fRad4BCaRZxP+\
       JH+8XY1I6+8Cy+CM5g92iHgxtRPz+MjniOaYmdkDcnL9cCpXJleXsOckpURl49G\
       wiyUpZ10KHgOEe11sx3G2gxI8S0jnxQB+Pu68U9vVcasqOWAEObtNKKZd8tSFu7\
       LB5YAv0RAGhB8tmpv7sFnIm9y+7X5kXQfi8NMaZaA8i2ZHwpBdg7a6CMfwnnrtf\
       lzvZdXAsD3LH2TwevU+/PBPv0B6NMNk93wUs/vfJvye+YuI87HU38lZHowtznbL\
       Vdp770I6VHR6WfgS9ddzirrswsE1w5o0LV/g==:

   {"hello": "world"}

   While the proxy could additionally include the client's Signature
   field value and Signature-Input fields from the original message in
   the new signature's covered components, this practice is NOT
   RECOMMENDED due to known weaknesses in signing signature values as
   discussed in Section 7.3.7.  The proxy is in a position to validate
   the client's signature; the changes the proxy makes to the message
   will invalidate the existing signature when the message is seen by
   the origin server.  In this example, it is possible for the origin
   server to have additional information in its signature context to
   account for the change in authority, though this practice requires
   additional configuration and extra care as discussed in
   Section 7.4.4.  In other applications, the origin server will not be
   able to verify the original signature itself but will still want to
   verify that the proxy has done the appropriate validation of the
   client's signature.  An application that needs to signal successful
   processing or receipt of a signature would need to carefully specify
   alternative mechanisms for sending such a signal securely.

5.  Requesting Signatures

   While a signer is free to attach a signature to a request or response
   without prompting, it is often desirable for a potential verifier to
   signal that it expects a signature from a potential signer using the
   Accept-Signature field.

   When the Accept-Signature field is sent in an HTTP request message,
   the field indicates that the client desires the server to sign the
   response using the identified parameters, and the target message is
   the response to this request.  All responses from resources that
   support such signature negotiation SHOULD either be uncacheable or
   contain a Vary header field that lists Accept-Signature, in order to
   prevent a cache from returning a response with a signature intended
   for a different request.

   When the Accept-Signature field is used in an HTTP response message,
   the field indicates that the server desires the client to sign its
   next request to the server with the identified parameters, and the
   target message is the client's next request.  The client can choose
   to also continue signing future requests to the same server in the
   same way.

   The target message of an Accept-Signature field MUST include all
   labeled signatures indicated in the Accept-Signature field, each
   covering the same identified components of the Accept-Signature
   field.

   The sender of an Accept-Signature field MUST include only identifiers
   that are appropriate for the type of the target message.  For
   example, if the target message is a request, the covered components
   cannot include the @status component identifier.

5.1.  The Accept-Signature Field

   The Accept-Signature field is a Dictionary Structured Field (defined
   in Section 3.2 of [STRUCTURED-FIELDS]) containing the metadata for
   one or more requested message signatures to be generated from message
   components of the target HTTP message.  Each member describes a
   single message signature.  The member's key is the label that
   uniquely identifies the requested message signature within the
   context of the target HTTP message.

   The member's value is the serialization of the desired covered
   components of the target message, including any allowed component
   metadata parameters, using the serialization process defined in
   Section 2.3:

   NOTE: '\' line wrapping per RFC 8792

   Accept-Signature: sig1=("@method" "@target-uri" "@authority" \
     "content-digest" "cache-control");\
     keyid="test-key-rsa-pss";created;tag="app-123"

   The list of component identifiers indicates the exact set of
   component identifiers to be included in the requested signature,
   including all applicable component parameters.

   The signature request MAY include signature metadata parameters that
   indicate desired behavior for the signer.  The following behavior is
   defined by this specification:

   created:  The signer is requested to generate and include a creation
      time.  This parameter has no associated value when sent as a
      signature request.

   expires:  The signer is requested to generate and include an
      expiration time.  This parameter has no associated value when sent
      as a signature request.

   nonce:  The signer is requested to include the value of this
      parameter as the signature nonce in the target signature.

   alg:  The signer is requested to use the indicated signature
      algorithm from the "HTTP Signature Algorithms" registry to create
      the target signature.

   keyid:  The signer is requested to use the indicated key material to
      create the target signature.

   tag:  The signer is requested to include the value of this parameter
      as the signature tag in the target signature.

5.2.  Processing an Accept-Signature

   The receiver of an Accept-Signature field fulfills that header as
   follows:

   1.  Parse the field value as a Dictionary.

   2.  For each member of the Dictionary:

       2.1.  The key is taken as the label of the output signature as
             specified in Section 4.1.

       2.2.  Parse the value of the member to obtain the set of covered
             component identifiers.

       2.3.  Determine that the covered components are applicable to the
             target message.  If not, the process fails and returns an
             error.

       2.4.  Process the requested parameters, such as the signing
             algorithm and key material.  If any requested parameters
             cannot be fulfilled or if the requested parameters conflict
             with those deemed appropriate to the target message, the
             process fails and returns an error.

       2.5.  Select and generate any additional parameters necessary for
             completing the signature.

       2.6.  Create the HTTP message signature over the target message.

       2.7.  Create the Signature-Input and Signature field values, and
             associate them with the label.

   3.  Optionally create any additional Signature-Input and Signature
       field values, with unique labels not found in the Accept-
       Signature field.

   4.  Combine all labeled Signature-Input and Signature field values,
       and attach both fields to the target message.

   By this process, a signature applied to a target message MUST have
   the same label, MUST include the same set of covered components, MUST
   process all requested parameters, and MAY have additional parameters.

   The receiver of an Accept-Signature field MAY ignore any signature
   request that does not fit application parameters.

   The target message MAY include additional signatures not specified by
   the Accept-Signature field.  For example, to cover additional message
   components, the signer can create a second signature that includes
   the additional components as well as the signature output of the
   requested signature.

6.  IANA Considerations

   IANA has updated one registry and created four new registries,
   according to the following sections.

6.1.  HTTP Field Name Registration

   IANA has updated the entries in the "Hypertext Transfer Protocol
   (HTTP) Field Name Registry" as follows:

        +==================+===========+=========================+
        | Field Name       | Status    | Reference               |
        +==================+===========+=========================+
        | Signature-Input  | permanent | Section 4.1 of RFC 9421 |
        +------------------+-----------+-------------------------+
        | Signature        | permanent | Section 4.2 of RFC 9421 |
        +------------------+-----------+-------------------------+
        | Accept-Signature | permanent | Section 5.1 of RFC 9421 |
        +------------------+-----------+-------------------------+

           Table 1: Updates to the Hypertext Transfer Protocol
                        (HTTP) Field Name Registry

6.2.  HTTP Signature Algorithms Registry

   This document defines HTTP signature algorithms, for which IANA has
   created and now maintains a new registry titled "HTTP Signature
   Algorithms".  Initial values for this registry are given in
   Section 6.2.2.  Future assignments and modifications to existing
   assignments are to be made through the Specification Required
   registration policy [RFC8126].

   The algorithms listed in this registry identify some possible
   cryptographic algorithms for applications to use with this
   specification, but the entries neither represent an exhaustive list
   of possible algorithms nor indicate fitness for purpose with any
   particular application of this specification.  An application is free
   to implement any algorithm that suits its needs, provided the signer
   and verifier can agree to the parameters of that algorithm in a
   secure and deterministic fashion.  When an application needs to
   signal the use of a particular algorithm at runtime using the alg
   signature parameter, this registry provides a mapping between the
   value of that parameter and a particular algorithm.  However, the use
   of the alg parameter needs to be treated with caution to avoid
   various forms of algorithm confusion and substitution attacks, as
   discussed in Section 7.

   The Status value should reflect standardization status and the broad
   opinion of relevant interest groups such as the IETF or security-
   related Standards Development Organizations (SDOs).  When an
   algorithm is first registered, the designated expert (DE) should set
   the Status field to "Active" if there is consensus for the algorithm
   to be generally recommended as secure or "Provisional" if the
   algorithm has not reached that consensus, e.g., for an experimental
   algorithm.  A status of "Provisional" does not mean that the
   algorithm is known to be insecure but instead indicates that the
   algorithm has not reached consensus regarding its properties.  If at
   a future time the algorithm as registered is found to have flaws, the
   registry entry can be updated and the algorithm can be marked as
   "Deprecated" to indicate that the algorithm has been found to have
   problems.  This status does not preclude an application from using a
   particular algorithm; rather, it serves to provide a warning
   regarding possible known issues with an algorithm that need to be
   considered by the application.  The DE can further ensure that the
   registration includes an explanation and reference for the Status
   value; this is particularly important for deprecated algorithms.

   The DE is expected to do the following:

   *  Ensure that the algorithms referenced by a registered algorithm
      identifier are fully defined with all parameters (e.g., salt,
      hash, required key length) fixed by the defining text.

   *  Ensure that the algorithm definition fully specifies the HTTP_SIGN
      and HTTP_VERIFY primitive functions, including how all defined
      inputs and outputs map to the underlying cryptographic algorithm.

   *  Reject any registrations that are aliases of existing
      registrations.

   *  Ensure that all registrations follow the template presented in
      Section 6.2.1; this includes ensuring that the length of the name
      is not excessive while still being unique and recognizable.

   This specification creates algorithm identifiers by including major
   parameters in the identifier String in order to make the algorithm
   name unique and recognizable by developers.  However, algorithm
   identifiers in this registry are to be interpreted as whole String
   values and not as a combination of parts.  That is to say, it is
   expected that implementors understand rsa-pss-sha512 as referring to
   one specific algorithm with its hash, mask, and salt values set as
   defined in the defining text that establishes the identifier in
   question.  Implementors do not parse out the rsa, pss, and sha512
   portions of the identifier to determine parameters of the signing
   algorithm from the String, and the registration of one combination of
   parameters does not imply the registration of other combinations.

6.2.1.  Registration Template

   Algorithm Name:
      An identifier for the HTTP signature algorithm.  The name MUST be
      an ASCII string that conforms to the sf-string ABNF rule in
      Section 3.3.3 of [STRUCTURED-FIELDS] and SHOULD NOT exceed 20
      characters in length.  The identifier MUST be unique within the
      context of the registry.

   Description:
      A brief description of the algorithm used to sign the signature
      base.

   Status:
      The status of the algorithm.  MUST start with one of the following
      values and MAY contain additional explanatory text.  The options
      are:

      "Active":  For algorithms without known problems.  The signature
         algorithm is fully specified, and its security properties are
         understood.

      "Provisional":  For unproven algorithms.  The signature algorithm
         is fully specified, but its security properties are not known
         or proven.

      "Deprecated":  For algorithms with known security issues.  The
         signature algorithm is no longer recommended for general use
         and might be insecure or unsafe in some known circumstances.

   Reference:
      Reference to the document or documents that specify the algorithm,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

6.2.2.  Initial Contents

   The table below contains the initial contents of the "HTTP Signature
   Algorithms" registry.

    +===================+===================+========+===============+
    | Algorithm Name    | Description       | Status | Reference     |
    +===================+===================+========+===============+
    | rsa-pss-sha512    | RSASSA-PSS using  | Active | Section 3.3.1 |
    |                   | SHA-512           |        | of RFC 9421   |
    +-------------------+-------------------+--------+---------------+
    | rsa-v1_5-sha256   | RSASSA-PKCS1-v1_5 | Active | Section 3.3.2 |
    |                   | using SHA-256     |        | of RFC 9421   |
    +-------------------+-------------------+--------+---------------+
    | hmac-sha256       | HMAC using        | Active | Section 3.3.3 |
    |                   | SHA-256           |        | of RFC 9421   |
    +-------------------+-------------------+--------+---------------+
    | ecdsa-p256-sha256 | ECDSA using curve | Active | Section 3.3.4 |
    |                   | P-256 DSS and     |        | of RFC 9421   |
    |                   | SHA-256           |        |               |
    +-------------------+-------------------+--------+---------------+
    | ecdsa-p384-sha384 | ECDSA using curve | Active | Section 3.3.5 |
    |                   | P-384 DSS and     |        | of RFC 9421   |
    |                   | SHA-384           |        |               |
    +-------------------+-------------------+--------+---------------+
    | ed25519           | EdDSA using curve | Active | Section 3.3.6 |
    |                   | edwards25519      |        | of RFC 9421   |
    +-------------------+-------------------+--------+---------------+

        Table 2: Initial Contents of the HTTP Signature Algorithms
                                 Registry

6.3.  HTTP Signature Metadata Parameters Registry

   This document defines the signature parameters structure
   (Section 2.3), which may have parameters containing metadata about a
   message signature.  IANA has created and now maintains a new registry
   titled "HTTP Signature Metadata Parameters" to record and maintain
   the set of parameters defined for use with member values in the
   signature parameters structure.  Initial values for this registry are
   given in Section 6.3.2.  Future assignments and modifications to
   existing assignments are to be made through the Expert Review
   registration policy [RFC8126].

   The DE is expected to do the following:

   *  Ensure that the name follows the template presented in
      Section 6.3.1; this includes ensuring that the length of the name
      is not excessive while still being unique and recognizable for its
      defined function.

   *  Ensure that the defined functionality is clear and does not
      conflict with other registered parameters.

   *  Ensure that the definition of the metadata parameter includes its
      behavior when used as part of the normal signature process as well
      as when used in an Accept-Signature field.

6.3.1.  Registration Template

   Name:
      An identifier for the HTTP signature metadata parameter.  The name
      MUST be an ASCII string that conforms to the key ABNF rule defined
      in Section 3.1.2 of [STRUCTURED-FIELDS] and SHOULD NOT exceed 20
      characters in length.  The identifier MUST be unique within the
      context of the registry.

   Description:
      A brief description of the metadata parameter and what it
      represents.

   Reference:
      Reference to the document or documents that specify the parameter,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

6.3.2.  Initial Contents

   The table below contains the initial contents of the "HTTP Signature
   Metadata Parameters" registry.  Each row in the table represents a
   distinct entry in the registry.

         +=========+===============================+=============+
         | Name    | Description                   | Reference   |
         +=========+===============================+=============+
         | alg     | Explicitly declared signature | Section 2.3 |
         |         | algorithm                     | of RFC 9421 |
         +---------+-------------------------------+-------------+
         | created | Timestamp of signature        | Section 2.3 |
         |         | creation                      | of RFC 9421 |
         +---------+-------------------------------+-------------+
         | expires | Timestamp of proposed         | Section 2.3 |
         |         | signature expiration          | of RFC 9421 |
         +---------+-------------------------------+-------------+
         | keyid   | Key identifier for the        | Section 2.3 |
         |         | signing and verification keys | of RFC 9421 |
         |         | used to create this signature |             |
         +---------+-------------------------------+-------------+
         | nonce   | A single-use nonce value      | Section 2.3 |
         |         |                               | of RFC 9421 |
         +---------+-------------------------------+-------------+
         | tag     | An application-specific tag   | Section 2.3 |
         |         | for a signature               | of RFC 9421 |
         +---------+-------------------------------+-------------+

              Table 3: Initial Contents of the HTTP Signature
                        Metadata Parameters Registry

6.4.  HTTP Signature Derived Component Names Registry

   This document defines a method for canonicalizing HTTP message
   components, including components that can be derived from the context
   of the target message outside of the HTTP fields.  These derived
   components are identified by a unique String, known as the component
   name.  Component names for derived components always start with the
   "at" (@) symbol to distinguish them from HTTP field names.  IANA has
   created and now maintains a new registry titled "HTTP Signature
   Derived Component Names" to record and maintain the set of non-field
   component names and the methods used to produce their associated
   component values.  Initial values for this registry are given in
   Section 6.4.2.  Future assignments and modifications to existing
   assignments are to be made through the Expert Review registration
   policy [RFC8126].

   The DE is expected to do the following:

   *  Ensure that the name follows the template presented in
      Section 6.4.1; this includes ensuring that the length of the name
      is not excessive while still being unique and recognizable for its
      defined function.

   *  Ensure that the component value represented by the registration
      request can be deterministically derived from the target HTTP
      message.

   *  Ensure that any parameters defined for the registration request
      are clearly documented, along with their effects on the component
      value.

   The DE should ensure that a registration is sufficiently distinct
   from existing derived component definitions to warrant its
   registration.

   When setting a registered item's status to "Deprecated", the DE
   should ensure that a reason for the deprecation is documented, along
   with instructions for moving away from the deprecated functionality.

6.4.1.  Registration Template

   Name:
      A name for the HTTP derived component.  The name MUST begin with
      the "at" (@) character followed by an ASCII string consisting only
      of lowercase characters ("a"-"z"), digits ("0"-"9"), and hyphens
      ("-"), and SHOULD NOT exceed 20 characters in length.  The name
      MUST be unique within the context of the registry.

   Description:
      A description of the derived component.

   Status:
      A brief text description of the status of the algorithm.  The
      description MUST begin with one of "Active" or "Deprecated" and
      MAY provide further context or explanation as to the reason for
      the status.  A value of "Deprecated" indicates that the derived
      component name is no longer recommended for use.

   Target:
      The valid message targets for the derived parameter.  MUST be one
      of the values "Request", "Response", or "Request, Response".  The
      semantics of these entries are defined in Section 2.2.

   Reference:
      Reference to the document or documents that specify the derived
      component, preferably including a URI that can be used to retrieve
      a copy of the document(s).  An indication of the relevant sections
      may also be included but is not required.

6.4.2.  Initial Contents

   The table below contains the initial contents of the "HTTP Signature
   Derived Component Names" registry.

   +===================+==============+========+==========+===========+
   | Name              | Description  | Status | Target   | Reference |
   +===================+==============+========+==========+===========+
   | @signature-params | Reserved for | Active | Request, | Section   |
   |                   | signature    |        | Response | 2.3 of    |
   |                   | parameters   |        |          | RFC 9421  |
   |                   | line in      |        |          |           |
   |                   | signature    |        |          |           |
   |                   | base         |        |          |           |
   +-------------------+--------------+--------+----------+-----------+
   | @method           | The HTTP     | Active | Request  | Section   |
   |                   | request      |        |          | 2.2.1 of  |
   |                   | method       |        |          | RFC 9421  |
   +-------------------+--------------+--------+----------+-----------+
   | @authority        | The HTTP     | Active | Request  | Section   |
   |                   | authority,   |        |          | 2.2.3 of  |
   |                   | or target    |        |          | RFC 9421  |
   |                   | host         |        |          |           |
   +-------------------+--------------+--------+----------+-----------+
   | @scheme           | The URI      | Active | Request  | Section   |
   |                   | scheme of    |        |          | 2.2.4 of  |
   |                   | the request  |        |          | RFC 9421  |
   |                   | URI          |        |          |           |
   +-------------------+--------------+--------+----------+-----------+
   | @target-uri       | The full     | Active | Request  | Section   |
   |                   | target URI   |        |          | 2.2.2 of  |
   |                   | of the       |        |          | RFC 9421  |
   |                   | request      |        |          |           |
   +-------------------+--------------+--------+----------+-----------+
   | @request-target   | The request  | Active | Request  | Section   |
   |                   | target of    |        |          | 2.2.5 of  |
   |                   | the request  |        |          | RFC 9421  |
   +-------------------+--------------+--------+----------+-----------+
   | @path             | The full     | Active | Request  | Section   |
   |                   | path of the  |        |          | 2.2.6 of  |
   |                   | request URI  |        |          | RFC 9421  |
   +-------------------+--------------+--------+----------+-----------+
   | @query            | The full     | Active | Request  | Section   |
   |                   | query of the |        |          | 2.2.7 of  |
   |                   | request URI  |        |          | RFC 9421  |
   +-------------------+--------------+--------+----------+-----------+
   | @query-param      | A single     | Active | Request  | Section   |
   |                   | named query  |        |          | 2.2.8 of  |
   |                   | parameter    |        |          | RFC 9421  |
   +-------------------+--------------+--------+----------+-----------+
   | @status           | The status   | Active | Response | Section   |
   |                   | code of the  |        |          | 2.2.9 of  |
   |                   | response     |        |          | RFC 9421  |
   +-------------------+--------------+--------+----------+-----------+

         Table 4: Initial Contents of the HTTP Signature Derived
                         Component Names Registry

6.5.  HTTP Signature Component Parameters Registry

   This document defines several kinds of component identifiers, some of
   which can be parameterized in specific circumstances to provide
   unique modified behavior.  IANA has created and now maintains a new
   registry titled "HTTP Signature Component Parameters" to record and
   maintain the set of parameter names, the component identifiers they
   are associated with, and the modifications these parameters make to
   the component value.  Definitions of parameters MUST define the
   targets to which they apply (such as specific field types, derived
   components, or contexts).  Initial values for this registry are given
   in Section 6.5.2.  Future assignments and modifications to existing
   assignments are to be made through the Expert Review registration
   policy [RFC8126].

   The DE is expected to do the following:

   *  Ensure that the name follows the template presented in
      Section 6.5.1; this includes ensuring that the length of the name
      is not excessive while still being unique and recognizable for its
      defined function.

   *  Ensure that the definition of the field sufficiently defines any
      interactions or incompatibilities with other existing parameters
      known at the time of the registration request.

   *  Ensure that the component value defined by the component
      identifier with the parameter applied can be deterministically
      derived from the target HTTP message in cases where the parameter
      changes the component value.

6.5.1.  Registration Template

   Name:
      A name for the parameter.  The name MUST be an ASCII string that
      conforms to the key ABNF rule defined in Section 3.1.2 of
      [STRUCTURED-FIELDS] and SHOULD NOT exceed 20 characters in length.
      The name MUST be unique within the context of the registry.

   Description:
      A description of the parameter's function.

   Reference:
      Reference to the document or documents that specify the derived
      component, preferably including a URI that can be used to retrieve
      a copy of the document(s).  An indication of the relevant sections
      may also be included but is not required.

6.5.2.  Initial Contents

   The table below contains the initial contents of the "HTTP Signature
   Component Parameters" registry.

     +======+=======================================+===============+
     | Name | Description                           | Reference     |
     +======+=======================================+===============+
     | sf   | Strict Structured Field serialization | Section 2.1.1 |
     |      |                                       | of RFC 9421   |
     +------+---------------------------------------+---------------+
     | key  | Single key value of Dictionary        | Section 2.1.2 |
     |      | Structured Fields                     | of RFC 9421   |
     +------+---------------------------------------+---------------+
     | bs   | Byte Sequence wrapping indicator      | Section 2.1.3 |
     |      |                                       | of RFC 9421   |
     +------+---------------------------------------+---------------+
     | tr   | Trailer                               | Section 2.1.4 |
     |      |                                       | of RFC 9421   |
     +------+---------------------------------------+---------------+
     | req  | Related request indicator             | Section 2.4   |
     |      |                                       | of RFC 9421   |
     +------+---------------------------------------+---------------+
     | name | Single named query parameter          | Section 2.2.8 |
     |      |                                       | of RFC 9421   |
     +------+---------------------------------------+---------------+

        Table 5: Initial Contents of the HTTP Signature Component
                           Parameters Registry

7.  Security Considerations

   In order for an HTTP message to be considered _covered_ by a
   signature, all of the following conditions have to be true:

   *  A signature is expected or allowed on the message by the verifier.

   *  The signature exists on the message.

   *  The signature is verified against the identified key material and
      algorithm.

   *  The key material and algorithm are appropriate for the context of
      the message.

   *  The signature is within expected time boundaries.

   *  The signature covers the expected content, including any critical
      components.

   *  The list of covered components is applicable to the context of the
      message.

   In addition to the application requirement definitions listed in
   Section 1.4, the following security considerations provide discussion
   and context regarding the requirements of creating and verifying
   signatures on HTTP messages.

7.1.  General Considerations

7.1.1.  Skipping Signature Verification

   HTTP message signatures only provide security if the signature is
   verified by the verifier.  Since the message to which the signature
   is attached remains a valid HTTP message without the Signature or
   Signature-Input fields, it is possible for a verifier to ignore the
   output of the verification function and still process the message.
   Common reasons for this could be relaxed requirements in a
   development environment or a temporary suspension of enforcing
   verification while debugging an overall system.  Such temporary
   suspensions are difficult to detect under positive-example testing,
   since a good signature will always trigger a valid response whether
   or not it has been checked.

   To detect this, verifiers should be tested using both valid and
   invalid signatures, ensuring that an invalid signature fails as
   expected.

7.1.2.  Use of TLS

   The use of HTTP message signatures does not negate the need for TLS
   or its equivalent to protect information in transit.  Message
   signatures provide message integrity over the covered message
   components but do not provide any confidentiality for communication
   between parties.

   TLS provides such confidentiality between the TLS endpoints.  As part
   of this, TLS also protects the signature data itself from being
   captured by an attacker.  This is an important step in preventing
   signature replay (Section 7.2.2).

   When TLS is used, it needs to be deployed according to the
   recommendations provided in [BCP195].

7.2.  Message Processing and Selection

7.2.1.  Insufficient Coverage

   Any portions of the message not covered by the signature are
   susceptible to modification by an attacker without affecting the
   signature.  An attacker can take advantage of this by introducing or
   modifying a header field or other message component that will change
   the processing of the message but will not be covered by the
   signature.  Such an altered message would still pass signature
   verification, but when the verifier processes the message as a whole,
   the unsigned content injected by the attacker would subvert the trust
   conveyed by the valid signature and change the outcome of processing
   the message.

   To combat this, an application of this specification should require
   as much of the message as possible to be signed, within the limits of
   the application and deployment.  The verifier should only trust
   message components that have been signed.  Verifiers could also strip
   out any sensitive unsigned portions of the message before processing
   of the message continues.

7.2.2.  Signature Replay

   Since HTTP message signatures allow sub-portions of the HTTP message
   to be signed, it is possible for two different HTTP messages to
   validate against the same signature.  The most extreme form of this
   would be a signature over no message components.  If such a signature
   were intercepted, it could be replayed at will by an attacker,
   attached to any HTTP message.  Even with sufficient component
   coverage, a given signature could be applied to two similar HTTP
   messages, allowing a message to be replayed by an attacker with the
   signature intact.

   To counteract these kinds of attacks, it's first important for the
   signer to cover sufficient portions of the message to differentiate
   it from other messages.  In addition, the signature can use the nonce
   signature parameter to provide a per-message unique value to allow
   the verifier to detect replay of the signature itself if a nonce
   value is repeated.  Furthermore, the signer can provide a timestamp
   for when the signature was created and a time at which the signer
   considers the signature to be expired, limiting the utility of a
   captured signature value.

   If a verifier wants to trigger a new signature from a signer, it can
   send the Accept-Signature header field with a new nonce parameter.
   An attacker that is simply replaying a signature would not be able to
   generate a new signature with the chosen nonce value.

7.2.3.  Choosing Message Components

   Applications of HTTP message signatures need to decide which message
   components will be covered by the signature.  Depending on the
   application, some components could be expected to be changed by
   intermediaries prior to the signature's verification.  If these
   components are covered, such changes would, by design, break the
   signature.

   However, this document allows for flexibility in determining which
   components are signed precisely so that a given application can
   choose the appropriate portions of the message that need to be
   signed, avoiding problematic components.  For example, a web
   application framework that relies on rewriting query parameters might
   avoid using the @query derived component in favor of sub-indexing the
   query value using @query-param derived components instead.

   Some components are expected to be changed by intermediaries and
   ought not to be signed under most circumstances.  The Via and
   Forwarded header fields, for example, are expected to be manipulated
   by proxies and other middleboxes, including replacing or entirely
   dropping existing values.  These fields should not be covered by the
   signature, except in very limited and tightly coupled scenarios.

   Additional considerations for choosing signature aspects are
   discussed in Section 1.4.

7.2.4.  Choosing Signature Parameters and Derived Components over HTTP
        Fields

   Some HTTP fields have values and interpretations that are similar to
   HTTP signature parameters or derived components.  In most cases, it
   is more desirable to sign the non-field alternative.  In particular,
   the following fields should usually not be included in the signature
   unless the application specifically requires it:

   "date"  The Date header field value represents the timestamp of the
      HTTP message.  However, the creation time of the signature itself
      is encoded in the created signature parameter.  These two values
      can be different, depending on how the signature and the HTTP
      message are created and serialized.  Applications processing
      signatures for valid time windows should use the created signature
      parameter for such calculations.  An application could also put
      limits on how much skew there is between the Date field and the
      created signature parameter, in order to limit the application of
      a generated signature to different HTTP messages.  See also
      Sections 7.2.2 and 7.2.1.

   "host"  The Host header field is specific to HTTP/1.1, and its
      functionality is subsumed by the @authority derived component,
      defined in Section 2.2.3.  In order to preserve the value across
      different HTTP versions, applications should always use the
      @authority derived component.  See also Section 7.5.4.

7.2.5.  Signature Labels

   HTTP message signature values are identified in the Signature and
   Signature-Input field values by unique labels.  These labels are
   chosen only when attaching the signature values to the message and
   are not accounted for during the signing process.  An intermediary is
   allowed to relabel an existing signature when processing the message.

   Therefore, applications should not rely on specific labels being
   present, and applications should not put semantic meaning on the
   labels themselves.  Instead, additional signature parameters can be
   used to convey whatever additional meaning is required to be attached
   to, and covered by, the signature.  In particular, the tag parameter
   can be used to define an application-specific value as described in
   Section 7.2.7.

7.2.6.  Multiple Signature Confusion

   Since multiple signatures can be applied to one message
   (Section 4.3), it is possible for an attacker to attach their own
   signature to a captured message without modifying existing
   signatures.  This new signature could be completely valid based on
   the attacker's key, or it could be an invalid signature for any
   number of reasons.  Each of these situations needs to be accounted
   for.

   A verifier processing a set of valid signatures needs to account for
   all of the signers, identified by the signing keys.  Only signatures
   from expected signers should be accepted, regardless of the
   cryptographic validity of the signature itself.

   A verifier processing a set of signatures on a message also needs to
   determine what to do when one or more of the signatures are not
   valid.  If a message is accepted when at least one signature is
   valid, then a verifier could drop all invalid signatures from the
   request before processing the message further.  Alternatively, if the
   verifier rejects a message for a single invalid signature, an
   attacker could use this to deny service to otherwise valid messages
   by injecting invalid signatures alongside the valid signatures.

7.2.7.  Collision of Application-Specific Signature Tag

   Multiple applications and protocols could apply HTTP signatures on
   the same message simultaneously.  In fact, this is a desired feature
   in many circumstances; see Section 4.3.  A naive verifier could
   become confused while processing multiple signatures, either
   accepting or rejecting a message based on an unrelated or irrelevant
   signature.  In order to help an application select which signatures
   apply to its own processing, the application can declare a specific
   value for the tag signature parameter as defined in Section 2.3.  For
   example, a signature targeting an application gateway could require
   tag="app-gateway" as part of the signature parameters for that
   application.

   The use of the tag parameter does not prevent an attacker from also
   using the same value as a target application, since the parameter's
   value is public and otherwise unrestricted.  As a consequence, a
   verifier should only use a value of the tag parameter to limit which
   signatures to check.  Each signature still needs to be examined by
   the verifier to ensure that sufficient coverage is provided, as
   discussed in Section 7.2.1.

7.2.8.  Message Content

   On its own, this specification does not provide coverage for the
   content of an HTTP message under the signature, in either a request
   or a response.  However, [DIGEST] defines a set of fields that allow
   a cryptographic digest of the content to be represented in a field.
   Once this field is created, it can be included just like any other
   field as defined in Section 2.1.

   For example, in the following response message:

   HTTP/1.1 200 OK
   Content-Type: application/json

   {"hello": "world"}

   The digest of the content can be added to the Content-Digest field as
   follows:

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Content-Digest: \
     sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

   {"hello": "world"}

   This field can be included in a signature base just like any other
   field along with the basic signature parameters:

   "@status": 200
   "content-digest": \
     sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:
   "@signature-input": ("@status" "content-digest")

   From here, the signing process proceeds as usual.

   Upon verification, it is important that the verifier validate not
   only the signature but also the value of the Content-Digest field
   itself against the actual received content.  Unless the verifier
   performs this step, it would be possible for an attacker to
   substitute the message content but leave the Content-Digest field
   value untouched to pass the signature.  Since only the field value is
   covered by the signature directly, checking only the signature is not
   sufficient protection against such a substitution attack.

   As discussed in [DIGEST], the value of the Content-Digest field is
   dependent on the content encoding of the message.  If an intermediary
   changes the content encoding, the resulting Content-Digest value
   would change.  This would in turn invalidate the signature.  Any
   intermediary performing such an action would need to apply a new
   signature with the updated Content-Digest field value, similar to the
   reverse proxy use case discussed in Section 4.3.

   Applications that make use of the req parameter (Section 2.4) also
   need to be aware of the limitations of this functionality.
   Specifically, if a client does not include something like a Content-
   Digest header field in the request, the server is unable to include a
   signature that covers the request's content.

7.3.  Cryptographic Considerations

7.3.1.  Cryptography and Signature Collision

   This document does not define any of its own cryptographic primitives
   and instead relies on other specifications to define such elements.
   If the signature algorithm or key used to process the signature base
   is vulnerable to any attacks, the resulting signature will also be
   susceptible to these same attacks.

   A common attack against signature systems is to force a signature
   collision, where the same signature value successfully verifies
   against multiple different inputs.  Since this specification relies
   on reconstruction of the signature base from an HTTP message and the
   list of components signed is fixed in the signature, it is difficult
   but not impossible for an attacker to effect such a collision.  An
   attacker would need to manipulate the HTTP message and its covered
   message components in order to make the collision effective.

   To counter this, only vetted keys and signature algorithms should be
   used to sign HTTP messages.  The "HTTP Signature Algorithms" registry
   is one source of trusted signature algorithms for applications to
   apply to their messages.

   While it is possible for an attacker to substitute the signature
   parameters value or the signature value separately, the signature
   base generation algorithm (Section 2.5) always covers the signature
   parameters as the final value in the signature base using a
   deterministic serialization method.  This step strongly binds the
   signature base with the signature value in a way that makes it much
   more difficult for an attacker to perform a partial substitution on
   the signature base.

7.3.2.  Key Theft

   A foundational assumption of signature-based cryptographic systems is
   that the signing key is not compromised by an attacker.  If the keys
   used to sign the message are exfiltrated or stolen, the attacker will
   be able to generate their own signatures using those keys.  As a
   consequence, signers have to protect any signing key material from
   exfiltration, capture, and use by an attacker.

   To combat this, signers can rotate keys over time to limit the amount
   of time that stolen keys are useful.  Signers can also use key escrow
   and storage systems to limit the attack surface against keys.
   Furthermore, the use of asymmetric signing algorithms exposes key
   material less than the use of symmetric signing algorithms
   (Section 7.3.3).

7.3.3.  Symmetric Cryptography

   This document allows both asymmetric and symmetric cryptography to be
   applied to HTTP messages.  By their nature, symmetric cryptographic
   methods require the same key material to be known by both the signer
   and verifier.  This effectively means that a verifier is capable of
   generating a valid signature, since they have access to the same key
   material.  An attacker that is able to compromise a verifier would be
   able to then impersonate a signer.

   Where possible, asymmetric methods or secure key agreement mechanisms
   should be used in order to avoid this type of attack.  When symmetric
   methods are used, distribution of the key material needs to be
   protected by the overall system.  One technique for this is the use
   of separate cryptographic modules that separate the verification
   process (and therefore the key material) from other code, minimizing
   the vulnerable attack surface.  Another technique is the use of key
   derivation functions that allow the signer and verifier to agree on
   unique keys for each message without having to share the key values
   directly.

   Additionally, if symmetric algorithms are allowed within a system,
   special care must be taken to avoid key downgrade attacks
   (Section 7.3.6).

7.3.4.  Key Specification Mixup

   The existence of a valid signature on an HTTP message is not
   sufficient to prove that the message has been signed by the
   appropriate party.  It is up to the verifier to ensure that a given
   key and algorithm are appropriate for the message in question.  If
   the verifier does not perform such a step, an attacker could
   substitute their own signature using their own key on a message and
   force a verifier to accept and process it.  To combat this, the
   verifier needs to ensure not only that the signature can be validated
   for a message but that the key and algorithm used are appropriate.

7.3.5.  Non-deterministic Signature Primitives

   Some cryptographic primitives, such as RSA-PSS and ECDSA, have non-
   deterministic outputs, which include some amount of entropy within
   the algorithm.  For such algorithms, multiple signatures generated in
   succession will not match.  A lazy implementation of a verifier could
   ignore this distinction and simply check for the same value being
   created by re-signing the signature base.  Such an implementation
   would work for deterministic algorithms such as HMAC and EdDSA but
   fail to verify valid signatures made using non-deterministic
   algorithms.  It is therefore important that a verifier always use the
   correctly defined verification function for the algorithm in question
   and not do a simple comparison.

7.3.6.  Key and Algorithm Specification Downgrades

   Applications of this specification need to protect against key
   specification downgrade attacks.  For example, the same RSA key can
   be used for both RSA-PSS and RSA v1.5 signatures.  If an application
   expects a key to only be used with RSA-PSS, it needs to reject
   signatures for any key that uses the weaker RSA 1.5 specification.

   Another example of a downgrade attack would be when an asymmetric
   algorithm is expected, such as RSA-PSS, but an attacker substitutes a
   signature using a symmetric algorithm, such as HMAC.  A naive
   verifier implementation could use the value of the public RSA key as
   the input to the HMAC verification function.  Since the public key is
   known to the attacker, this would allow the attacker to create a
   valid HMAC signature against this known key.  To prevent this, the
   verifier needs to ensure that both the key material and the algorithm
   are appropriate for the usage in question.  Additionally, while this
   specification does allow runtime specification of the algorithm using
   the alg signature parameter, applications are encouraged to use other
   mechanisms such as static configuration or a higher-protocol-level
   algorithm specification instead, preventing an attacker from
   substituting the algorithm specified.

7.3.7.  Signing Signature Values

   When applying the req parameter (Section 2.4) or multiple signatures
   (Section 4.3) to a message, it is possible to sign the value of an
   existing Signature field, thereby covering the bytes of the existing
   signature output in the new signature's value.  While it would seem
   that this practice would transitively cover the components under the
   original signature in a verifiable fashion, the attacks described in
   [JACKSON2019] can be used to impersonate a signature output value on
   an unrelated message.

   In this example, Alice intends to send a signed request to Bob, and
   Bob wants to provide a signed response to Alice that includes a
   cryptographic proof that Bob is responding to Alice's incoming
   message.  Mallory wants to intercept this traffic and replace Alice's
   message with her own, without Alice being aware that the interception
   has taken place.

   1.   Alice creates a message Req_A and applies a signature Sig_A
        using her private key Key_A_Sign.

   2.   Alice believes she is sending Req_A to Bob.

   3.   Mallory intercepts Req_A and reads the value Sig_A from this
        message.

   4.   Mallory generates a different message Req_M to send to Bob
        instead.

   5.   Mallory crafts a signing key Key_M_Sign such that she can create
        a valid signature Sig_M over her request Req_M using this key,
        but the byte value of Sig_M exactly equals that of Sig_A.

   6.   Mallory sends Req_M with Sig_M to Bob.

   7.   Bob validates Sig_M against Mallory's verification key
        Key_M_Verify.  At no time does Bob think that he's responding to
        Alice.

   8.   Bob responds with response message Res_B to Req_M and creates
        signature Sig_B over this message using his key Key_B_Sign.  Bob
        includes the value of Sig_M under Sig_B's covered components but
        does not include anything else from the request message.

   9.   Mallory receives the response Res_B from Bob, including the
        signature Sig_B value.  Mallory replays this response to Alice.

   10.  Alice reads Res_B from Mallory and verifies Sig_B using Bob's
        verification key Key_B_Verify.  Alice includes the bytes of her
        original signature Sig_A in the signature base, and the
        signature verifies.

   11.  Alice is led to believe that Bob has responded to her message
        and believes she has cryptographic proof of this happening, but
        in fact Bob responded to Mallory's malicious request and Alice
        is none the wiser.

   To mitigate this, Bob can sign more portions of the request message
   than just the Signature field, in order to more fully differentiate
   Alice's message from Mallory's.  Applications using this feature,
   particularly for non-repudiation purposes, can stipulate that any
   components required in the original signature also be covered
   separately in the second signature.  For signed messages, requiring
   coverage of the corresponding Signature-Input field of the first
   signature ensures that unique items such as nonces and timestamps are
   also covered sufficiently by the second signature.

7.4.  Matching Signature Parameters to the Target Message

7.4.1.  Modification of Required Message Parameters

   An attacker could effectively deny a service by modifying an
   otherwise benign signature parameter or signed message component.
   While rejecting a modified message is the desired behavior,
   consistently failing signatures could lead to (1) the verifier
   turning off signature checking in order to make systems work again
   (see Section 7.1.1) or (2) the application minimizing the
   requirements related to the signed component.

   If such failures are common within an application, the signer and
   verifier should compare their generated signature bases with each
   other to determine which part of the message is being modified.  If
   an expected modification is found, the signer and verifier can agree
   on an alternative set of requirements that will pass.  However, the
   signer and verifier should not remove the requirement to sign the
   modified component when it is suspected that an attacker is modifying
   the component.

7.4.2.  Matching Values of Covered Components to Values in the Target
        Message

   The verifier needs to make sure that the signed message components
   match those in the message itself.  For example, the @method derived
   component requires that the value within the signature base be the
   same as the HTTP method used when presenting this message.  This
   specification encourages this by requiring the verifier to derive the
   signature base from the message, but lazy caching or conveyance of a
   raw signature base to a processing subsystem could lead to downstream
   verifiers accepting a message that does not match the presented
   signature.

   To counter this, the component that generates the signature base
   needs to be trusted by both the signer and verifier within a system.

7.4.3.  Message Component Source and Context

   The signature context for deriving message component values includes
   the target HTTP message itself, any associated messages (such as the
   request that triggered a response), and additional information that
   the signer or verifier has access to.  Both signers and verifiers
   need to carefully consider the source of all information when
   creating component values for the signature base and take care not to
   take information from untrusted sources.  Otherwise, an attacker
   could leverage such a loosely defined message context to inject their
   own values into the signature base string, overriding or corrupting
   the intended values.

   For example, in most situations, the target URI of the message is as
   defined in [HTTP], Section 7.1.  However, let's say that there is an
   application that requires signing of the @authority of the incoming
   request, but the application doing the processing is behind a reverse
   proxy.  Such an application would expect a change in the @authority
   value, and it could be configured to know the external target URI as
   seen by the client on the other side of the proxy.  This application
   would use this configured value as its target URI for the purposes of
   deriving message component values such as @authority instead of using
   the target URI of the incoming message.

   This approach is not without problems, as a misconfigured system
   could accept signed requests intended for different components in the
   system.  For this scenario, an intermediary could instead add its own
   signature to be verified by the application directly, as demonstrated
   in Section 4.3.  This alternative approach requires a more active
   intermediary but relies less on the target application knowing
   external configuration values.

   As another example, Section 2.4 defines a method for signing response
   messages and also including portions of the request message that
   triggered the response.  In this case, the context for component
   value calculation is the combination of the response and request
   messages, not just the single message to which the signature is
   applied.  For this feature, the req flag allows both signers to
   explicitly signal which part of the context is being sourced for a
   component identifier's value.  Implementations need to ensure that
   only the intended message is being referred to for each component;
   otherwise, an attacker could attempt to subvert a signature by
   manipulating one side or the other.

7.4.4.  Multiple Message Component Contexts

   It is possible that the context for deriving message component values
   could be distinct for each signature present within a single message.
   This is particularly the case when proxies mutate messages and
   include signatures over the mutated values, in addition to any
   existing signatures.  For example, a reverse proxy can replace a
   public hostname in a request to a service with the hostname for the
   individual service host to which it is forwarding the request.  If
   both the client and the reverse proxy add signatures covering
   @authority, the service host will see two signatures on the request,
   each signing different values for the @authority message component,
   reflecting the change to that component as the message made its way
   from the client to the service host.

   In such a case, it's common for the internal service to verify only
   one of the signatures or to use externally configured information, as
   discussed in Section 7.4.3.  However, a verifier processing both
   signatures has to use a different message component context for each
   signature, since the component value for the @authority component
   will be different for each signature.  Verifiers like this need to be
   aware of both the reverse proxy's context for incoming messages and
   the target service's context for the message coming from the reverse
   proxy.  The verifier needs to take particular care to apply the
   correct context to the correct signature; otherwise, an attacker
   could use knowledge of this complex setup to confuse the inputs to
   the verifier.

   Such verifiers also need to ensure that any differences in message
   component contexts between signatures are expected and permitted.
   For example, in the above scenario, the reverse proxy could include
   the original hostname in a Forwarded header field and could sign
   @authority, forwarded, and the client's entry in the Signature field.
   The verifier can use the hostname from the Forwarded header field to
   confirm that the hostname was transformed as expected.

7.5.  HTTP Processing

7.5.1.  Processing Invalid HTTP Field Names as Derived Component Names

   The definition of HTTP field names does not allow for the use of the
   @ character anywhere in the name.  As such, since all derived
   component names start with the @ character, these namespaces should
   be completely separate.  However, some HTTP implementations are not
   sufficiently strict about the characters accepted in HTTP field
   names.  In such implementations, a sender (or attacker) could inject
   a header field starting with an @ character and have it passed
   through to the application code.  These invalid header fields could
   be used to override a portion of the derived message content and
   substitute an arbitrary value, providing a potential place for an
   attacker to mount a signature collision (Section 7.3.1) attack or
   other functional substitution attack (such as using the signature
   from a GET request on a crafted POST request).

   To combat this, when selecting values for a message component, if the
   component name starts with the @ character, it needs to be processed
   as a derived component and never processed as an HTTP field.  Only if
   the component name does not start with the @ character can it be
   taken from the fields of the message.  The algorithm discussed in
   Section 2.5 provides a safe order of operations.

7.5.2.  Semantically Equivalent Field Values

   The signature base generation algorithm (Section 2.5) uses the value
   of an HTTP field as its component value.  In the common case, this
   amounts to taking the actual bytes of the field value as the
   component value for both the signer and verifier.  However, some
   field values allow for transformation of the values in semantically
   equivalent ways that alter the bytes used in the value itself.  For
   example, a field definition can declare some or all of its values to
   be case insensitive or to have special handling of internal
   whitespace characters.  Other fields have expected transformations
   from intermediaries, such as the removal of comments in the Via
   header field.  In such cases, a verifier could be tripped up by using
   the equivalent transformed field value, which would differ from the
   byte value used by the signer.  The verifier would have a difficult
   time finding this class of errors, since the value of the field is
   still acceptable for the application but the actual bytes required by
   the signature base would not match.

   When processing such fields, the signer and verifier have to agree on
   how to handle such transformations, if at all.  One option is to not
   sign problematic fields, but care must be taken to ensure that there
   is still sufficient signature coverage (Section 7.2.1) for the
   application.  Another option is to define an application-specific
   canonicalization value for the field before it is added to the HTTP
   message, such as to always remove internal comments before signing or
   to always transform values to lowercase.  Since these transformations
   are applied prior to the field being used as input to the signature
   base generation algorithm, the signature base will still simply
   contain the byte value of the field as it appears within the message.
   If the transformations were to be applied after the value is
   extracted from the message but before it is added to the signature
   base, different attack surfaces such as value substitution attacks
   could be launched against the application.  All application-specific
   additional rules are outside the scope of this specification, and by
   their very nature these transformations would harm interoperability
   of the implementation outside of this specific application.  It is
   recommended that applications avoid the use of such additional rules
   wherever possible.

7.5.3.  Parsing Structured Field Values

   Several parts of this specification rely on the parsing of Structured
   Field values [STRUCTURED-FIELDS] -- in particular, strict
   serialization of HTTP Structured Field values (Section 2.1.1),
   referencing members of a Dictionary Structured Field (Section 2.1.2),
   and processing the @signature-input value when verifying a signature
   (Section 3.2).  While Structured Field values are designed to be
   relatively simple to parse, a naive or broken implementation of such
   a parser could lead to subtle attack surfaces being exposed in the
   implementation.

   For example, if a buggy parser of the @signature-input value does not
   enforce proper closing of quotes around string values within the list
   of component identifiers, an attacker could take advantage of this
   and inject additional content into the signature base through
   manipulating the Signature-Input field value on a message.

   To counteract this, implementations should use fully compliant and
   trusted parsers for all Structured Field processing, on both the
   signer side and the verifier side.

7.5.4.  HTTP Versions and Component Ambiguity

   Some message components are expressed in different ways across HTTP
   versions.  For example, the authority of the request target is sent
   using the Host header field in HTTP/1.1 but with the :authority
   pseudo-header in HTTP/2.  If a signer sends an HTTP/1.1 message and
   signs the Host header field but the message is translated to HTTP/2
   before it reaches the verifier, the signature will not validate, as
   the Host header field could be dropped.

   It is for this reason that HTTP message signatures define a set of
   derived components that define a single way to get the value in
   question, such as the @authority derived component (Section 2.2.3) in
   lieu of the Host header field.  Applications should therefore prefer
   derived components for such options where possible.

7.5.5.  Canonicalization Attacks

   Any ambiguity in the generation of the signature base could provide
   an attacker with leverage to substitute or break a signature on a
   message.  Some message component values, particularly HTTP field
   values, are potentially susceptible to broken implementations that
   could lead to unexpected and insecure behavior.  Naive
   implementations of this specification might implement HTTP field
   processing by taking the single value of a field and using it as the
   direct component value without processing it appropriately.

   For example, if the handling of obs-fold field values does not remove
   the internal line folding and whitespace, additional newlines could
   be introduced into the signature base by the signer, providing a
   potential place for an attacker to mount a signature collision
   (Section 7.3.1) attack.  Alternatively, if header fields that appear
   multiple times are not joined into a single string value, as required
   by this specification, similar attacks can be mounted, as a signed
   component value would show up in the signature base more than once
   and could be substituted or otherwise attacked in this way.

   To counter this, the entire field value processing algorithm needs to
   be implemented by all implementations of signers and verifiers.

7.5.6.  Non-List Field Values

   When an HTTP field occurs multiple times in a single message, these
   values need to be combined into a single one-line string value to be
   included in the HTTP signature base, as described in Section 2.5.
   Not all HTTP fields can be combined into a single value in this way
   and still be a valid value for the field.  For the purposes of
   generating the signature base, the message component value is never
   meant to be read back out of the signature base string or used in the
   application.  Therefore, it is considered best practice to treat the
   signature base generation algorithm separately from processing the
   field values by the application, particularly for fields that are
   known to have this property.  If the field values that are being
   signed do not validate, the signed message should also be rejected.

   If an HTTP field allows for unquoted commas within its values,
   combining multiple field values can lead to a situation where two
   semantically different messages produce the same line in a signature
   base.  For example, take the following hypothetical header field with
   an internal comma in its syntax, here used to define two separate
   lists of values:

   Example-Header: value, with, lots
   Example-Header: of, commas

   For this header field, sending all of these values as a single field
   value results in a single list of values:

   Example-Header: value, with, lots, of, commas

   Both of these messages would create the following line in the
   signature base:

   "example-header": value, with, lots, of, commas

   Since two semantically distinct inputs can create the same output in
   the signature base, special care has to be taken when handling such
   values.

   Specifically, the Set-Cookie field [COOKIE] defines an internal
   syntax that does not conform to the List syntax provided in
   [STRUCTURED-FIELDS].  In particular, some portions allow unquoted
   commas, and the field is typically sent as multiple separate field
   lines with distinct values when sending multiple cookies.  When
   multiple Set-Cookie fields are sent in the same message, it is not
   generally possible to combine these into a single line and be able to
   parse and use the results, as discussed in [HTTP], Section 5.3.
   Therefore, all the cookies need to be processed from their separate
   field values, without being combined, while the signature base needs
   to be processed from the special combined value generated solely for
   this purpose.  If the cookie value is invalid, the signed message
   ought to be rejected, as this is a possible padding attack as
   described in Section 7.5.7.

   To deal with this, an application can choose to limit signing of
   problematic fields like Set-Cookie, such as including the field in a
   signature only when a single field value is present and the results
   would be unambiguous.  Similar caution needs to be taken with all
   fields that could have non-deterministic mappings into the signature
   base.  Signers can also make use of the bs parameter to armor such
   fields, as described in Section 2.1.3.

7.5.7.  Padding Attacks with Multiple Field Values

   Since HTTP field values need to be combined into a single string
   value to be included in the HTTP signature base (see Section 2.5), it
   is possible for an attacker to inject an additional value for a given
   field and add this to the signature base of the verifier.

   In most circumstances, this causes the signature validation to fail
   as expected, since the new signature base value will not match the
   one used by the signer to create the signature.  However, it is
   theoretically possible for the attacker to inject both a garbage
   value into a field and a desired value into another field in order to
   force a particular input.  This is a variation of the collision
   attack described in Section 7.3.1, where the attacker accomplishes
   their change in the message by adding to existing field values.

   To counter this, an application needs to validate the content of the
   fields covered in the signature in addition to ensuring that the
   signature itself validates.  With such protections, the attacker's
   padding attack would be rejected by the field value processor, even
   in the case where the attacker could force a signature collision.

7.5.8.  Ambiguous Handling of Query Elements

   The HTML form parameters format defined in Section 5 ("application/
   x-www-form-urlencoded") of [HTMLURL] is widely deployed and supported
   by many application frameworks.  For convenience, some of these
   frameworks in particular combine query parameters that are found in
   the HTTP query and those found in the message content, particularly
   for POST messages with a Content-Type value of "application/x-www-
   form-urlencoded".  The @query-param derived component identifier
   defined in Section 2.2.8 draws its values only from the query section
   of the target URI of the request.  As such, it would be possible for
   an attacker to shadow or replace query parameters in a request by
   overriding a signed query parameter with an unsigned form parameter,
   or vice versa.

   To counter this, an application needs to make sure that values used
   for the signature base and the application are drawn from a
   consistent context, in this case the query component of the target
   URI.  Additionally, when the HTTP request has content, an application
   should sign the message content as well, as discussed in
   Section 7.2.8.

8.  Privacy Considerations

8.1.  Identification through Keys

   If a signer uses the same key with multiple verifiers or uses the
   same key over time with a single verifier, the ongoing use of that
   key can be used to track the signer throughout the set of verifiers
   that messages are sent to.  Since cryptographic keys are meant to be
   functionally unique, the use of the same key over time is a strong
   indicator that it is the same party signing multiple messages.

   In many applications, this is a desirable trait, and it allows HTTP
   message signatures to be used as part of authenticating the signer to
   the verifier.  However, it could also result in unintentional
   tracking that a signer might not be aware of.  To counter this kind
   of tracking, a signer can use a different key for each verifier that
   it is in communication with.  Sometimes, a signer could also rotate
   their key when sending messages to a given verifier.  These
   approaches do not negate the need for other anti-tracking techniques
   to be applied as necessary.

8.2.  Signatures do not provide confidentiality

   HTTP message signatures do not provide confidentiality for any of the
   information protected by the signature.  The content of the HTTP
   message, including the value of all fields and the value of the
   signature itself, is presented in plaintext to any party with access
   to the message.

   To provide confidentiality at the transport level, TLS or its
   equivalent can be used, as discussed in Section 7.1.2.

8.3.  Oracles

   It is important to balance the need for providing useful feedback to
   developers regarding error conditions without providing additional
   information to an attacker.  For example, a naive but helpful server
   implementation might try to indicate the required key identifier
   needed for requesting a resource.  If someone knows who controls that
   key, a correlation can be made between the resource's existence and
   the party identified by the key.  Access to such information could be
   used by an attacker as a means to target the legitimate owner of the
   resource for further attacks.

8.4.  Required Content

   A core design tenet of this specification is that all message
   components covered by the signature need to be available to the
   verifier in order to recreate the signature base and verify the
   signature.  As a consequence, if an application of this specification
   requires that a particular field be signed, the verifier will need
   access to the value of that field.

   For example, in some complex systems with intermediary processors,
   this could cause surprising behavior where, for fear of breaking the
   signature, an intermediary cannot remove privacy-sensitive
   information from a message before forwarding it on for processing.
   One way to mitigate this specific situation would be for the
   intermediary to verify the signature itself and then modify the
   message to remove the privacy-sensitive information.  The
   intermediary can add its own signature at this point to signal to the
   next destination that the incoming signature was validated, as shown
   in the example in Section 4.3.

9.  References

9.1.  Normative References

   [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [ASCII]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/info/rfc20>.

   [FIPS186-5]
              NIST, "Digital Signature Standard (DSS)",
              DOI 10.6028/NIST.FIPS.186-5, February 2023,
              <https://doi.org/10.6028/NIST.FIPS.186-5>.

   [HTMLURL]  WHATWG, "URL (Living Standard)", January 2024,
              <https://url.spec.whatwg.org/>.

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
              June 2022, <https://www.rfc-editor.org/info/rfc9112>.

   [POSIX.1]  IEEE, "The Open Group Base Specifications Issue 7, 2018
              edition", 2018,
              <https://pubs.opengroup.org/onlinepubs/9699919799/>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <https://www.rfc-editor.org/info/rfc7517>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <https://www.rfc-editor.org/info/rfc7518>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [STRUCTURED-FIELDS]
              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
              <https://www.rfc-editor.org/info/rfc8941>.

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

9.2.  Informative References

   [AWS-SIGv4]
              Amazon Simple Storage Service, "Authenticating Requests
              (AWS Signature Version 4)", March 2006,
              <https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-
              authenticating-requests.html>.

   [BCP195]   Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
              1.1", BCP 195, RFC 8996, March 2021.

              Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, November 2022.

              <https://www.rfc-editor.org/info/bcp195>

   [CLIENT-CERT]
              Campbell, B. and M. Bishop, "Client-Cert HTTP Header
              Field", RFC 9440, DOI 10.17487/RFC9440, July 2023,
              <https://www.rfc-editor.org/info/rfc9440>.

   [COOKIE]   Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

   [DIGEST]   Polli, R. and L. Pardue, "Digest Fields", RFC 9530,
              DOI 10.17487/RFC9530, February 2024,
              <https://www.rfc-editor.org/info/rfc9530>.

   [JACKSON2019]
              Jackson, D., Cremers, C., Cohn-Gordon, K., and R. Sasse,
              "Seems Legit: Automated Analysis of Subtle Attacks on
              Protocols that Use Signatures", CCS '19: Proceedings of
              the 2019 ACM SIGSAC Conference on Computer and
              Communications Security, pp. 2165-2180,
              DOI 10.1145/3319535.3339813, November 2019,
              <https://dl.acm.org/doi/10.1145/3319535.3339813>.

   [JWS]      Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7239]  Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
              RFC 7239, DOI 10.17487/RFC7239, June 2014,
              <https://www.rfc-editor.org/info/rfc7239>.

   [RFC7468]  Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
              PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
              April 2015, <https://www.rfc-editor.org/info/rfc7468>.

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
              <https://www.rfc-editor.org/info/rfc7807>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
              <https://www.rfc-editor.org/info/rfc8792>.

   [RFC9457]  Nottingham, M., Wilde, E., and S. Dalal, "Problem Details
              for HTTP APIs", RFC 9457, DOI 10.17487/RFC9457, July 2023,
              <https://www.rfc-editor.org/info/rfc9457>.

   [SIGNING-HTTP-MESSAGES]
              Cavage, M. and M. Sporny, "Signing HTTP Messages", Work in
              Progress, Internet-Draft, draft-cavage-http-signatures-12,
              21 October 2019, <https://datatracker.ietf.org/doc/html/
              draft-cavage-http-signatures-12>.

   [SIGNING-HTTP-REQS-OAUTH]
              Richer, J., Ed., Bradley, J., and H. Tschofenig, "A Method
              for Signing HTTP Requests for OAuth", Work in Progress,
              Internet-Draft, draft-ietf-oauth-signed-http-request-03, 8
              August 2016, <https://datatracker.ietf.org/doc/html/draft-
              ietf-oauth-signed-http-request-03>.

   [TLS]      Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

Appendix A.  Detecting HTTP Message Signatures

   There have been many attempts to create signed HTTP messages in the
   past, including other non-standardized definitions of the Signature
   field that is used within this specification.  It is recommended that
   developers wishing to support this specification, other published
   documents, or other historical drafts do so carefully and
   deliberately, as incompatibilities between this specification and
   other documents or various versions of other drafts could lead to
   unexpected problems.

   It is recommended that implementors first detect and validate the
   Signature-Input field defined in this specification to detect that
   the mechanism described in this document is in use and not an
   alternative.  If the Signature-Input field is present, all Signature
   fields can be parsed and interpreted in the context of this
   specification.

Appendix B.  Examples

   The following non-normative examples are provided as a means of
   testing implementations of HTTP message signatures.  The signed
   messages given can be used to create the signature base with the
   stated parameters, creating signatures using the stated algorithms
   and keys.

   The private keys given can be used to generate signatures, though
   since several of the demonstrated algorithms are non-deterministic,
   the results of a signature are expected to be different from the
   exact bytes of the examples.  The public keys given can be used to
   validate all signed examples.

B.1.  Example Keys

   This section provides cryptographic keys that are referenced in
   example signatures throughout this document.  These keys MUST NOT be
   used for any purpose other than testing.

   The key identifiers for each key are used throughout the examples in
   this specification.  It is assumed for these examples that the signer
   and verifier can unambiguously dereference all key identifiers used
   here and that the keys and algorithms used are appropriate for the
   context in which the signature is presented.

   The components for each private key, in PEM format [RFC7468], can be
   displayed by executing the following OpenSSL command:

   openssl pkey -text

   This command was tested with all the example keys on OpenSSL version
   1.1.1m.  Note that some systems cannot produce or use all of these
   keys directly and may require additional processing.  All keys are
   also made available in JWK format.

B.1.1.  Example RSA Key

   The following key is a 2048-bit RSA public and private key pair,
   referred to in this document as test-key-rsa.  This key is encoded in
   PEM format, with no encryption.

   -----BEGIN RSA PUBLIC KEY-----
   MIIBCgKCAQEAhAKYdtoeoy8zcAcR874L8cnZxKzAGwd7v36APp7Pv6Q2jdsPBRrw
   WEBnez6d0UDKDwGbc6nxfEXAy5mbhgajzrw3MOEt8uA5txSKobBpKDeBLOsdJKFq
   MGmXCQvEG7YemcxDTRPxAleIAgYYRjTSd/QBwVW9OwNFhekro3RtlinV0a75jfZg
   kne/YiktSvLG34lw2zqXBDTC5NHROUqGTlML4PlNZS5Ri2U4aCNx2rUPRcKIlE0P
   uKxI4T+HIaFpv8+rdV6eUgOrB2xeI1dSFFn/nnv5OoZJEIB+VmuKn3DCUcCZSFlQ
   PSXSfBDiUGhwOw76WuSSsf1D4b/vLoJ10wIDAQAB
   -----END RSA PUBLIC KEY-----

   -----BEGIN RSA PRIVATE KEY-----
   MIIEqAIBAAKCAQEAhAKYdtoeoy8zcAcR874L8cnZxKzAGwd7v36APp7Pv6Q2jdsP
   BRrwWEBnez6d0UDKDwGbc6nxfEXAy5mbhgajzrw3MOEt8uA5txSKobBpKDeBLOsd
   JKFqMGmXCQvEG7YemcxDTRPxAleIAgYYRjTSd/QBwVW9OwNFhekro3RtlinV0a75
   jfZgkne/YiktSvLG34lw2zqXBDTC5NHROUqGTlML4PlNZS5Ri2U4aCNx2rUPRcKI
   lE0PuKxI4T+HIaFpv8+rdV6eUgOrB2xeI1dSFFn/nnv5OoZJEIB+VmuKn3DCUcCZ
   SFlQPSXSfBDiUGhwOw76WuSSsf1D4b/vLoJ10wIDAQABAoIBAG/JZuSWdoVHbi56
   vjgCgkjg3lkO1KrO3nrdm6nrgA9P9qaPjxuKoWaKO1cBQlE1pSWp/cKncYgD5WxE
   CpAnRUXG2pG4zdkzCYzAh1i+c34L6oZoHsirK6oNcEnHveydfzJL5934egm6p8DW
   +m1RQ70yUt4uRc0YSor+q1LGJvGQHReF0WmJBZHrhz5e63Pq7lE0gIwuBqL8SMaA
   yRXtK+JGxZpImTq+NHvEWWCu09SCq0r838ceQI55SvzmTkwqtC+8AT2zFviMZkKR
   Qo6SPsrqItxZWRty2izawTF0Bf5S2VAx7O+6t3wBsQ1sLptoSgX3QblELY5asI0J
   YFz7LJECgYkAsqeUJmqXE3LP8tYoIjMIAKiTm9o6psPlc8CrLI9CH0UbuaA2JCOM
   cCNq8SyYbTqgnWlB9ZfcAm/cFpA8tYci9m5vYK8HNxQr+8FS3Qo8N9RJ8d0U5Csw
   DzMYfRghAfUGwmlWj5hp1pQzAuhwbOXFtxKHVsMPhz1IBtF9Y8jvgqgYHLbmyiu1
   mwJ5AL0pYF0G7x81prlARURwHo0Yf52kEw1dxpx+JXER7hQRWQki5/NsUEtv+8RT
   qn2m6qte5DXLyn83b1qRscSdnCCwKtKWUug5q2ZbwVOCJCtmRwmnP131lWRYfj67
   B/xJ1ZA6X3GEf4sNReNAtaucPEelgR2nsN0gKQKBiGoqHWbK1qYvBxX2X3kbPDkv
   9C+celgZd2PW7aGYLCHq7nPbmfDV0yHcWjOhXZ8jRMjmANVR/eLQ2EfsRLdW69bn
   f3ZD7JS1fwGnO3exGmHO3HZG+6AvberKYVYNHahNFEw5TsAcQWDLRpkGybBcxqZo
   81YCqlqidwfeO5YtlO7etx1xLyqa2NsCeG9A86UjG+aeNnXEIDk1PDK+EuiThIUa
   /2IxKzJKWl1BKr2d4xAfR0ZnEYuRrbeDQYgTImOlfW6/GuYIxKYgEKCFHFqJATAG
   IxHrq1PDOiSwXd2GmVVYyEmhZnbcp8CxaEMQoevxAta0ssMK3w6UsDtvUvYvF22m
   qQKBiD5GwESzsFPy3Ga0MvZpn3D6EJQLgsnrtUPZx+z2Ep2x0xc5orneB5fGyF1P
   WtP+fG5Q6Dpdz3LRfm+KwBCWFKQjg7uTxcjerhBWEYPmEMKYwTJF5PBG9/ddvHLQ
   EQeNC8fHGg4UXU8mhHnSBt3EA10qQJfRDs15M38eG2cYwB1PZpDHScDnDA0=
   -----END RSA PRIVATE KEY-----

   The same public and private key pair in JWK format:

   NOTE: '\' line wrapping per RFC 8792

   {
     "kty": "RSA",
     "kid": "test-key-rsa",
     "p": "sqeUJmqXE3LP8tYoIjMIAKiTm9o6psPlc8CrLI9CH0UbuaA2JCOMcCNq8Sy\
     YbTqgnWlB9ZfcAm_cFpA8tYci9m5vYK8HNxQr-8FS3Qo8N9RJ8d0U5CswDzMYfRgh\
     AfUGwmlWj5hp1pQzAuhwbOXFtxKHVsMPhz1IBtF9Y8jvgqgYHLbmyiu1mw",
     "q": "vSlgXQbvHzWmuUBFRHAejRh_naQTDV3GnH4lcRHuFBFZCSLn82xQS2_7xFO\
     qfabqq17kNcvKfzdvWpGxxJ2cILAq0pZS6DmrZlvBU4IkK2ZHCac_XfWVZFh-PrsH\
     _EnVkDpfcYR_iw1F40C1q5w8R6WBHaew3SAp",
     "d": "b8lm5JZ2hUduLnq-OAKCSODeWQ7Uqs7eet2bqeuAD0_2po-PG4qhZoo7VwF\
     CUTWlJan9wqdxiAPlbEQKkCdFRcbakbjN2TMJjMCHWL5zfgvqhmgeyKsrqg1wSce9\
     7J1_Mkvn3fh6CbqnwNb6bVFDvTJS3i5FzRhKiv6rUsYm8ZAdF4XRaYkFkeuHPl7rc\
     -ruUTSAjC4GovxIxoDJFe0r4kbFmkiZOr40e8RZYK7T1IKrSvzfxx5AjnlK_OZOTC\
     q0L7wBPbMW-IxmQpFCjpI-yuoi3FlZG3LaLNrBMXQF_lLZUDHs77q3fAGxDWwum2h\
     KBfdBuUQtjlqwjQlgXPsskQ",
     "e": "AQAB",
     "qi": "PkbARLOwU_LcZrQy9mmfcPoQlAuCyeu1Q9nH7PYSnbHTFzmiud4Hl8bIXU\
     9a0_58blDoOl3PctF-b4rAEJYUpCODu5PFyN6uEFYRg-YQwpjBMkXk8Eb39128ctA\
     RB40Lx8caDhRdTyaEedIG3cQDXSpAl9EOzXkzfx4bZxjAHU9mkMdJwOcMDQ",
     "dp": "aiodZsrWpi8HFfZfeRs8OS_0L5x6WBl3Y9btoZgsIeruc9uZ8NXTIdxaM6\
     FdnyNEyOYA1VH94tDYR-xEt1br1ud_dkPslLV_Aac7d7EaYc7cdkb7oC9t6sphVg0\
     dqE0UTDlOwBxBYMtGmQbJsFzGpmjzVgKqWqJ3B947li2U7t63HXEvKprY2w",
     "dq": "b0DzpSMb5p42dcQgOTU8Mr4S6JOEhRr_YjErMkpaXUEqvZ3jEB9HRmcRi5\
     Gtt4NBiBMiY6V9br8a5gjEpiAQoIUcWokBMAYjEeurU8M6JLBd3YaZVVjISaFmdty\
     nwLFoQxCh6_EC1rSywwrfDpSwO29S9i8Xbaap",
     "n": "hAKYdtoeoy8zcAcR874L8cnZxKzAGwd7v36APp7Pv6Q2jdsPBRrwWEBnez6\
     d0UDKDwGbc6nxfEXAy5mbhgajzrw3MOEt8uA5txSKobBpKDeBLOsdJKFqMGmXCQvE\
     G7YemcxDTRPxAleIAgYYRjTSd_QBwVW9OwNFhekro3RtlinV0a75jfZgkne_YiktS\
     vLG34lw2zqXBDTC5NHROUqGTlML4PlNZS5Ri2U4aCNx2rUPRcKIlE0PuKxI4T-HIa\
     Fpv8-rdV6eUgOrB2xeI1dSFFn_nnv5OoZJEIB-VmuKn3DCUcCZSFlQPSXSfBDiUGh\
     wOw76WuSSsf1D4b_vLoJ10w"
   }

B.1.2.  Example RSA-PSS Key

   The following key is a 2048-bit RSA public and private key pair,
   referred to in this document as test-key-rsa-pss.  This key is PKCS
   #8 encoded in PEM format, with no encryption.

   -----BEGIN PUBLIC KEY-----
   MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAr4tmm3r20Wd/PbqvP1s2
   +QEtvpuRaV8Yq40gjUR8y2Rjxa6dpG2GXHbPfvMs8ct+Lh1GH45x28Rw3Ry53mm+
   oAXjyQ86OnDkZ5N8lYbggD4O3w6M6pAvLkhk95AndTrifbIFPNU8PPMO7OyrFAHq
   gDsznjPFmTOtCEcN2Z1FpWgchwuYLPL+Wokqltd11nqqzi+bJ9cvSKADYdUAAN5W
   Utzdpiy6LbTgSxP7ociU4Tn0g5I6aDZJ7A8Lzo0KSyZYoA485mqcO0GVAdVw9lq4
   aOT9v6d+nb4bnNkQVklLQ3fVAvJm+xdDOp9LCNCN48V2pnDOkFV6+U9nV5oyc6XI
   2wIDAQAB
   -----END PUBLIC KEY-----

   -----BEGIN PRIVATE KEY-----
   MIIEvgIBADALBgkqhkiG9w0BAQoEggSqMIIEpgIBAAKCAQEAr4tmm3r20Wd/Pbqv
   P1s2+QEtvpuRaV8Yq40gjUR8y2Rjxa6dpG2GXHbPfvMs8ct+Lh1GH45x28Rw3Ry5
   3mm+oAXjyQ86OnDkZ5N8lYbggD4O3w6M6pAvLkhk95AndTrifbIFPNU8PPMO7Oyr
   FAHqgDsznjPFmTOtCEcN2Z1FpWgchwuYLPL+Wokqltd11nqqzi+bJ9cvSKADYdUA
   AN5WUtzdpiy6LbTgSxP7ociU4Tn0g5I6aDZJ7A8Lzo0KSyZYoA485mqcO0GVAdVw
   9lq4aOT9v6d+nb4bnNkQVklLQ3fVAvJm+xdDOp9LCNCN48V2pnDOkFV6+U9nV5oy
   c6XI2wIDAQABAoIBAQCUB8ip+kJiiZVKF8AqfB/aUP0jTAqOQewK1kKJ/iQCXBCq
   pbo360gvdt05H5VZ/RDVkEgO2k73VSsbulqezKs8RFs2tEmU+JgTI9MeQJPWcP6X
   aKy6LIYs0E2cWgp8GADgoBs8llBq0UhX0KffglIeek3n7Z6Gt4YFge2TAcW2WbN4
   XfK7lupFyo6HHyWRiYHMMARQXLJeOSdTn5aMBP0PO4bQyk5ORxTUSeOciPJUFktQ
   HkvGbym7KryEfwH8Tks0L7WhzyP60PL3xS9FNOJi9m+zztwYIXGDQuKM2GDsITeD
   2mI2oHoPMyAD0wdI7BwSVW18p1h+jgfc4dlexKYRAoGBAOVfuiEiOchGghV5vn5N
   RDNscAFnpHj1QgMr6/UG05RTgmcLfVsI1I4bSkbrIuVKviGGf7atlkROALOG/xRx
   DLadgBEeNyHL5lz6ihQaFJLVQ0u3U4SB67J0YtVO3R6lXcIjBDHuY8SjYJ7Ci6Z6
   vuDcoaEujnlrtUhaMxvSfcUJAoGBAMPsCHXte1uWNAqYad2WdLjPDlKtQJK1diCm
   rqmB2g8QE99hDOHItjDBEdpyFBKOIP+NpVtM2KLhRajjcL9Ph8jrID6XUqikQuVi
   4J9FV2m42jXMuioTT13idAILanYg8D3idvy/3isDVkON0X3UAVKrgMEne0hJpkPL
   FYqgetvDAoGBAKLQ6JZMbSe0pPIJkSamQhsehgL5Rs51iX4m1z7+sYFAJfhvN3Q/
   OGIHDRp6HjMUcxHpHw7U+S1TETxePwKLnLKj6hw8jnX2/nZRgWHzgVcY+sPsReRx
   NJVf+Cfh6yOtznfX00p+JWOXdSY8glSSHJwRAMog+hFGW1AYdt7w80XBAoGBAImR
   NUugqapgaEA8TrFxkJmngXYaAqpA0iYRA7kv3S4QavPBUGtFJHBNULzitydkNtVZ
   3w6hgce0h9YThTo/nKc+OZDZbgfN9s7cQ75x0PQCAO4fx2P91Q+mDzDUVTeG30mE
   t2m3S0dGe47JiJxifV9P3wNBNrZGSIF3mrORBVNDAoGBAI0QKn2Iv7Sgo4T/XjND
   dl2kZTXqGAk8dOhpUiw/HdM3OGWbhHj2NdCzBliOmPyQtAr770GITWvbAI+IRYyF
   S7Fnk6ZVVVHsxjtaHy1uJGFlaZzKR4AGNaUTOJMs6NadzCmGPAxNQQOCqoUjn4XR
   rOjr9w349JooGXhOxbu8nOxX
   -----END PRIVATE KEY-----

   The same public and private key pair in JWK format:

   NOTE: '\' line wrapping per RFC 8792

   {
     "kty": "RSA",
     "kid": "test-key-rsa-pss",
     "p": "5V-6ISI5yEaCFXm-fk1EM2xwAWekePVCAyvr9QbTlFOCZwt9WwjUjhtKRus\
     i5Uq-IYZ_tq2WRE4As4b_FHEMtp2AER43IcvmXPqKFBoUktVDS7dThIHrsnRi1U7d\
     HqVdwiMEMe5jxKNgnsKLpnq-4NyhoS6OeWu1SFozG9J9xQk",
     "q": "w-wIde17W5Y0Cphp3ZZ0uM8OUq1AkrV2IKauqYHaDxAT32EM4ci2MMER2nI\
     UEo4g_42lW0zYouFFqONwv0-HyOsgPpdSqKRC5WLgn0VXabjaNcy6KhNPXeJ0Agtq\
     diDwPeJ2_L_eKwNWQ43RfdQBUquAwSd7SEmmQ8sViqB628M",
     "d": "lAfIqfpCYomVShfAKnwf2lD9I0wKjkHsCtZCif4kAlwQqqW6N-tIL3bdOR-\
     VWf0Q1ZBIDtpO91UrG7pansyrPERbNrRJlPiYEyPTHkCT1nD-l2isuiyGLNBNnFoK\
     fBgA4KAbPJZQatFIV9Cn34JSHnpN5-2ehreGBYHtkwHFtlmzeF3yu5bqRcqOhx8lk\
     YmBzDAEUFyyXjknU5-WjAT9DzuG0MpOTkcU1EnjnIjyVBZLUB5Lxm8puyq8hH8B_E\
     5LNC-1oc8j-tDy98UvRTTiYvZvs87cGCFxg0LijNhg7CE3g9piNqB6DzMgA9MHSOw\
     cElVtfKdYfo4H3OHZXsSmEQ",
     "e": "AQAB",
     "qi": "jRAqfYi_tKCjhP9eM0N2XaRlNeoYCTx06GlSLD8d0zc4ZZuEePY10LMGWI\
     6Y_JC0CvvvQYhNa9sAj4hFjIVLsWeTplVVUezGO1ofLW4kYWVpnMpHgAY1pRM4kyz\
     o1p3MKYY8DE1BA4KqhSOfhdGs6Ov3Dfj0migZeE7Fu7yc7Fc",
     "dp": "otDolkxtJ7Sk8gmRJqZCGx6GAvlGznWJfibXPv6xgUAl-G83dD84YgcNGn\
     oeMxRzEekfDtT5LVMRPF4_AoucsqPqHDyOdfb-dlGBYfOBVxj6w-xF5HE0lV_4J-H\
     rI63Od9fTSn4lY5d1JjyCVJIcnBEAyiD6EUZbUBh23vDzRcE",
     "dq": "iZE1S6CpqmBoQDxOsXGQmaeBdhoCqkDSJhEDuS_dLhBq88FQa0UkcE1QvO\
     K3J2Q21VnfDqGBx7SH1hOFOj-cpz45kNluB832ztxDvnHQ9AIA7h_HY_3VD6YPMNR\
     VN4bfSYS3abdLR0Z7jsmInGJ9X0_fA0E2tkZIgXeas5EFU0M",
     "n": "r4tmm3r20Wd_PbqvP1s2-QEtvpuRaV8Yq40gjUR8y2Rjxa6dpG2GXHbPfvM\
     s8ct-Lh1GH45x28Rw3Ry53mm-oAXjyQ86OnDkZ5N8lYbggD4O3w6M6pAvLkhk95An\
     dTrifbIFPNU8PPMO7OyrFAHqgDsznjPFmTOtCEcN2Z1FpWgchwuYLPL-Wokqltd11\
     nqqzi-bJ9cvSKADYdUAAN5WUtzdpiy6LbTgSxP7ociU4Tn0g5I6aDZJ7A8Lzo0KSy\
     ZYoA485mqcO0GVAdVw9lq4aOT9v6d-nb4bnNkQVklLQ3fVAvJm-xdDOp9LCNCN48V\
     2pnDOkFV6-U9nV5oyc6XI2w"
   }

B.1.3.  Example ECC P-256 Test Key

   The following key is a public and private elliptical curve key pair
   over the curve P-256, referred to in this document as test-key-ecc-
   p256.  This key is encoded in PEM format, with no encryption.

   -----BEGIN PUBLIC KEY-----
   MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEqIVYZVLCrPZHGHjP17CTW0/+D9Lf
   w0EkjqF7xB4FivAxzic30tMM4GF+hR6Dxh71Z50VGGdldkkDXZCnTNnoXQ==
   -----END PUBLIC KEY-----

   -----BEGIN EC PRIVATE KEY-----
   MHcCAQEEIFKbhfNZfpDsW43+0+JjUr9K+bTeuxopu653+hBaXGA7oAoGCCqGSM49
   AwEHoUQDQgAEqIVYZVLCrPZHGHjP17CTW0/+D9Lfw0EkjqF7xB4FivAxzic30tMM
   4GF+hR6Dxh71Z50VGGdldkkDXZCnTNnoXQ==
   -----END EC PRIVATE KEY-----

   The same public and private key pair in JWK format:

   {
     "kty": "EC",
     "crv": "P-256",
     "kid": "test-key-ecc-p256",
     "d": "UpuF81l-kOxbjf7T4mNSv0r5tN67Gim7rnf6EFpcYDs",
     "x": "qIVYZVLCrPZHGHjP17CTW0_-D9Lfw0EkjqF7xB4FivA",
     "y": "Mc4nN9LTDOBhfoUeg8Ye9WedFRhnZXZJA12Qp0zZ6F0"
   }

B.1.4.  Example Ed25519 Test Key

   The following key is an elliptical curve key over the Edwards curve
   ed25519, referred to in this document as test-key-ed25519.  This key
   is PKCS #8 encoded in PEM format, with no encryption.

   -----BEGIN PUBLIC KEY-----
   MCowBQYDK2VwAyEAJrQLj5P/89iXES9+vFgrIy29clF9CC/oPPsw3c5D0bs=
   -----END PUBLIC KEY-----

   -----BEGIN PRIVATE KEY-----
   MC4CAQAwBQYDK2VwBCIEIJ+DYvh6SEqVTm50DFtMDoQikTmiCqirVv9mWG9qfSnF
   -----END PRIVATE KEY-----

   The same public and private key pair in JWK format:

   {
     "kty": "OKP",
     "crv": "Ed25519",
     "kid": "test-key-ed25519",
     "d": "n4Ni-HpISpVObnQMW0wOhCKROaIKqKtW_2ZYb2p9KcU",
     "x": "JrQLj5P_89iXES9-vFgrIy29clF9CC_oPPsw3c5D0bs"
   }

B.1.5.  Example Shared Secret

   The following shared secret is 64 randomly generated bytes encoded in
   Base64, referred to in this document as test-shared-secret:

   NOTE: '\' line wrapping per RFC 8792

   uzvJfB4u3N0Jy4T7NZ75MDVcr8zSTInedJtkgcu46YW4XByzNJjxBdtjUkdJPBt\
     bmHhIDi6pcl8jsasjlTMtDQ==

B.2.  Test Cases

   This section provides non-normative examples that may be used as test
   cases to validate implementation correctness.  These examples are
   based on the following HTTP messages:

   For requests, this test-request message is used:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Content-Length: 18

   {"hello": "world"}

   For responses, this test-response message is used:

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Content-Digest: sha-512=:mEWXIS7MaLRuGgxOBdODa3xqM1XdEvxoYhvlCFJ41Q\
     JgJc4GTsPp29l5oGX69wWdXymyU0rjJuahq4l5aGgfLQ==:
   Content-Length: 23

   {"message": "good dog"}

B.2.1.  Minimal Signature Using rsa-pss-sha512

   This example presents a minimal signature using the rsa-pss-sha512
   algorithm over test-request, covering none of the components of the
   HTTP message but providing a timestamped signature proof of
   possession of the key with a signer-provided nonce.

   The corresponding signature base is:

   NOTE: '\' line wrapping per RFC 8792

   "@signature-params": ();created=1618884473;keyid="test-key-rsa-pss"\
     ;nonce="b3k2pp5k7z-50gnwp.yemd"

   This results in the following Signature-Input and Signature header
   fields being added to the message under the signature label sig-b21:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig-b21=();created=1618884473\
     ;keyid="test-key-rsa-pss";nonce="b3k2pp5k7z-50gnwp.yemd"
   Signature: sig-b21=:d2pmTvmbncD3xQm8E9ZV2828BjQWGgiwAaw5bAkgibUopem\
     LJcWDy/lkbbHAve4cRAtx31Iq786U7it++wgGxbtRxf8Udx7zFZsckzXaJMkA7ChG\
     52eSkFxykJeNqsrWH5S+oxNFlD4dzVuwe8DhTSja8xxbR/Z2cOGdCbzR72rgFWhzx\
     2VjBqJzsPLMIQKhO4DGezXehhWwE56YCE+O6c0mKZsfxVrogUvA4HELjVKWmAvtl6\
     UnCh8jYzuVG5WSb/QEVPnP5TmcAnLH1g+s++v6d4s8m0gCw1fV5/SITLq9mhho8K3\
     +7EPYTU8IU1bLhdxO5Nyt8C8ssinQ98Xw9Q==:

   Note that since the covered components list is empty, this signature
   could be applied by an attacker to an unrelated HTTP message.  In
   this example, the nonce parameter is included to prevent the same
   signature from being replayed more than once, but if an attacker
   intercepts the signature and prevents its delivery to the verifier,
   the attacker could apply this signature to another message.
   Therefore, the use of an empty covered components set is discouraged.
   See Section 7.2.1 for more discussion.

   Note that the RSA-PSS algorithm in use here is non-deterministic,
   meaning that a different signature value will be created every time
   the algorithm is run.  The signature value provided here can be
   validated against the given keys, but newly generated signature
   values are not expected to match the example.  See Section 7.3.5.

B.2.2.  Selective Covered Components Using rsa-pss-sha512

   This example covers additional components (the authority, the
   Content-Digest header field, and a single named query parameter) in
   test-request using the rsa-pss-sha512 algorithm.  This example also
   adds a tag parameter with the application-specific value of header-
   example.

   The corresponding signature base is:

   NOTE: '\' line wrapping per RFC 8792

   "@authority": example.com
   "content-digest": sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX\
     +TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   "@query-param";name="Pet": dog
   "@signature-params": ("@authority" "content-digest" \
     "@query-param";name="Pet")\
     ;created=1618884473;keyid="test-key-rsa-pss"\
     ;tag="header-example"

   This results in the following Signature-Input and Signature header
   fields being added to the message under the label sig-b22:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig-b22=("@authority" "content-digest" \
     "@query-param";name="Pet");created=1618884473\
     ;keyid="test-key-rsa-pss";tag="header-example"
   Signature: sig-b22=:LjbtqUbfmvjj5C5kr1Ugj4PmLYvx9wVjZvD9GsTT4F7GrcQ\
     EdJzgI9qHxICagShLRiLMlAJjtq6N4CDfKtjvuJyE5qH7KT8UCMkSowOB4+ECxCmT\
     8rtAmj/0PIXxi0A0nxKyB09RNrCQibbUjsLS/2YyFYXEu4TRJQzRw1rLEuEfY17SA\
     RYhpTlaqwZVtR8NV7+4UKkjqpcAoFqWFQh62s7Cl+H2fjBSpqfZUJcsIk4N6wiKYd\
     4je2U/lankenQ99PZfB4jY3I5rSV2DSBVkSFsURIjYErOs0tFTQosMTAoxk//0RoK\
     UqiYY8Bh0aaUEb0rQl3/XaVe4bXTugEjHSw==:

   Note that the RSA-PSS algorithm in use here is non-deterministic,
   meaning that a different signature value will be created every time
   the algorithm is run.  The signature value provided here can be
   validated against the given keys, but newly generated signature
   values are not expected to match the example.  See Section 7.3.5.

B.2.3.  Full Coverage Using rsa-pss-sha512

   This example covers all applicable message components in test-request
   (including the content type and length) plus many derived components,
   again using the rsa-pss-sha512 algorithm.  Note that the Host header
   field is not covered because the @authority derived component is
   included instead.

   The corresponding signature base is:

   NOTE: '\' line wrapping per RFC 8792

   "date": Tue, 20 Apr 2021 02:07:55 GMT
   "@method": POST
   "@path": /foo
   "@query": ?param=Value&Pet=dog
   "@authority": example.com
   "content-type": application/json
   "content-digest": sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX\
     +TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   "content-length": 18
   "@signature-params": ("date" "@method" "@path" "@query" \
     "@authority" "content-type" "content-digest" "content-length")\
     ;created=1618884473;keyid="test-key-rsa-pss"

   This results in the following Signature-Input and Signature header
   fields being added to the message under the label sig-b23:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig-b23=("date" "@method" "@path" "@query" \
     "@authority" "content-type" "content-digest" "content-length")\
     ;created=1618884473;keyid="test-key-rsa-pss"
   Signature: sig-b23=:bbN8oArOxYoyylQQUU6QYwrTuaxLwjAC9fbY2F6SVWvh0yB\
     iMIRGOnMYwZ/5MR6fb0Kh1rIRASVxFkeGt683+qRpRRU5p2voTp768ZrCUb38K0fU\
     xN0O0iC59DzYx8DFll5GmydPxSmme9v6ULbMFkl+V5B1TP/yPViV7KsLNmvKiLJH1\
     pFkh/aYA2HXXZzNBXmIkoQoLd7YfW91kE9o/CCoC1xMy7JA1ipwvKvfrs65ldmlu9\
     bpG6A9BmzhuzF8Eim5f8ui9eH8LZH896+QIF61ka39VBrohr9iyMUJpvRX2Zbhl5Z\
     JzSRxpJyoEZAFL2FUo5fTIztsDZKEgM4cUA==:

   Note in this example that the value of the Date header field and the
   value of the created signature parameter need not be the same.  This
   is due to the fact that the Date header field is added when creating
   the HTTP message and the created parameter is populated when creating
   the signature over that message, and these two times could vary.  If
   the Date header field is covered by the signature, it is up to the
   verifier to determine whether its value has to match that of the
   created parameter or not.  See Section 7.2.4 for more discussion.

   Note that the RSA-PSS algorithm in use here is non-deterministic,
   meaning that a different signature value will be created every time
   the algorithm is run.  The signature value provided here can be
   validated against the given keys, but newly generated signature
   values are not expected to match the example.  See Section 7.3.5.

B.2.4.  Signing a Response Using ecdsa-p256-sha256

   This example covers portions of the test-response message using the
   ecdsa-p256-sha256 algorithm and the key test-key-ecc-p256.

   The corresponding signature base is:

   NOTE: '\' line wrapping per RFC 8792

   "@status": 200
   "content-type": application/json
   "content-digest": sha-512=:mEWXIS7MaLRuGgxOBdODa3xqM1XdEvxoYhvlCFJ4\
     1QJgJc4GTsPp29l5oGX69wWdXymyU0rjJuahq4l5aGgfLQ==:
   "content-length": 23
   "@signature-params": ("@status" "content-type" "content-digest" \
     "content-length");created=1618884473;keyid="test-key-ecc-p256"

   This results in the following Signature-Input and Signature header
   fields being added to the message under the label sig-b24:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig-b24=("@status" "content-type" \
     "content-digest" "content-length");created=1618884473\
     ;keyid="test-key-ecc-p256"
   Signature: sig-b24=:wNmSUAhwb5LxtOtOpNa6W5xj067m5hFrj0XQ4fvpaCLx0NK\
     ocgPquLgyahnzDnDAUy5eCdlYUEkLIj+32oiasw==:

   Note that the ECDSA signature algorithm in use here is non-
   deterministic, meaning that a different signature value will be
   created every time the algorithm is run.  The signature value
   provided here can be validated against the given keys, but newly
   generated signature values are not expected to match the example.
   See Section 7.3.5.

B.2.5.  Signing a Request Using hmac-sha256

   This example covers portions of the test-request message using the
   hmac-sha256 algorithm and the secret test-shared-secret.

   The corresponding signature base is:

   NOTE: '\' line wrapping per RFC 8792

   "date": Tue, 20 Apr 2021 02:07:55 GMT
   "@authority": example.com
   "content-type": application/json
   "@signature-params": ("date" "@authority" "content-type")\
     ;created=1618884473;keyid="test-shared-secret"

   This results in the following Signature-Input and Signature header
   fields being added to the message under the label sig-b25:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig-b25=("date" "@authority" "content-type")\
     ;created=1618884473;keyid="test-shared-secret"
   Signature: sig-b25=:pxcQw6G3AjtMBQjwo8XzkZf/bws5LelbaMk5rGIGtE8=:

   Before using symmetric signatures in practice, see the discussion
   regarding security trade-offs in Section 7.3.3.

B.2.6.  Signing a Request Using ed25519

   This example covers portions of the test-request message using the
   Ed25519 algorithm and the key test-key-ed25519.

   The corresponding signature base is:

   NOTE: '\' line wrapping per RFC 8792

   "date": Tue, 20 Apr 2021 02:07:55 GMT
   "@method": POST
   "@path": /foo
   "@authority": example.com
   "content-type": application/json
   "content-length": 18
   "@signature-params": ("date" "@method" "@path" "@authority" \
     "content-type" "content-length");created=1618884473\
     ;keyid="test-key-ed25519"

   This results in the following Signature-Input and Signature header
   fields being added to the message under the label sig-b26:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig-b26=("date" "@method" "@path" "@authority" \
     "content-type" "content-length");created=1618884473\
     ;keyid="test-key-ed25519"
   Signature: sig-b26=:wqcAqbmYJ2ji2glfAMaRy4gruYYnx2nEFN2HN6jrnDnQCK1\
     u02Gb04v9EDgwUPiu4A0w6vuQv5lIp5WPpBKRCw==:

B.3.  TLS-Terminating Proxies

   In this example, there is a TLS-terminating reverse proxy sitting in
   front of the resource.  The client does not sign the request but
   instead uses mutual TLS to make its call.  The terminating proxy
   validates the TLS stream and injects a Client-Cert header field
   according to [CLIENT-CERT], and then applies a signature to this
   field.  By signing this header field, a reverse proxy not only can
   attest to its own validation of the initial request's TLS parameters
   but can also authenticate itself to the backend system independently
   of the client's actions.

   The client makes the following request to the TLS-terminating proxy
   using mutual TLS:

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Length: 18

   {"hello": "world"}

   The proxy processes the TLS connection and extracts the client's TLS
   certificate to a Client-Cert header field and passes it along to the
   internal service hosted at service.internal.example.  This results in
   the following unsigned request:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: service.internal.example
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Length: 18
   Client-Cert: :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQKD\
     BJMZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQT\
     AeFw0yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDMFk\
     wEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXmck\
     C8vdgJ1p5Be5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQYDV\
     R0TBAIwADAfBgNVHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8BAf\
     8EBAMCBsAwEwYDVR0lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQGV\
     4YW1wbGUuY29tMAoGCCqGSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0Q6\
     bMjeSkC3dFCOOB8TAiEAx/kHSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:

   {"hello": "world"}

   Without a signature, the internal service would need to trust that
   the incoming connection has the right information.  By signing the
   Client-Cert header field and other portions of the internal request,
   the internal service can be assured that the correct party, the
   trusted proxy, has processed the request and presented it to the
   correct service.  The proxy's signature base consists of the
   following:

   NOTE: '\' line wrapping per RFC 8792

   "@path": /foo
   "@query": ?param=Value&Pet=dog
   "@method": POST
   "@authority": service.internal.example
   "client-cert": :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQ\
     KDBJMZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBD\
     QTAeFw0yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDM\
     FkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXm\
     ckC8vdgJ1p5Be5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQY\
     DVR0TBAIwADAfBgNVHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8B\
     Af8EBAMCBsAwEwYDVR0lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQ\
     GV4YW1wbGUuY29tMAoGCCqGSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0\
     Q6bMjeSkC3dFCOOB8TAiEAx/kHSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:
   "@signature-params": ("@path" "@query" "@method" "@authority" \
     "client-cert");created=1618884473;keyid="test-key-ecc-p256"

   This results in the following signature:

   NOTE: '\' line wrapping per RFC 8792

   xVMHVpawaAC/0SbHrKRs9i8I3eOs5RtTMGCWXm/9nvZzoHsIg6Mce9315T6xoklyy0y\
   zhD9ah4JHRwMLOgmizw==

   which results in the following signed request sent from the proxy to
   the internal service with the proxy's signature under the label ttrp:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: service.internal.example
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Length: 18
   Client-Cert: :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQKD\
     BJMZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQT\
     AeFw0yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDMFk\
     wEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXmck\
     C8vdgJ1p5Be5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQYDV\
     R0TBAIwADAfBgNVHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8BAf\
     8EBAMCBsAwEwYDVR0lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQGV\
     4YW1wbGUuY29tMAoGCCqGSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0Q6\
     bMjeSkC3dFCOOB8TAiEAx/kHSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:
   Signature-Input: ttrp=("@path" "@query" "@method" "@authority" \
     "client-cert");created=1618884473;keyid="test-key-ecc-p256"
   Signature: ttrp=:xVMHVpawaAC/0SbHrKRs9i8I3eOs5RtTMGCWXm/9nvZzoHsIg6\
     Mce9315T6xoklyy0yzhD9ah4JHRwMLOgmizw==:

   {"hello": "world"}

   The internal service can validate the proxy's signature and therefore
   be able to trust that the client's certificate has been appropriately
   processed.

B.4.  HTTP Message Transformations

   HTTP allows intermediaries and applications to transform an HTTP
   message without affecting the semantics of the message itself.  HTTP
   message signatures are designed to be robust against many of these
   transformations in different circumstances.

   For example, the following HTTP request message has been signed using
   the Ed25519 algorithm and the key test-key-ed25519:

   NOTE: '\' line wrapping per RFC 8792

   GET /demo?name1=Value1&Name2=value2 HTTP/1.1
   Host: example.org
   Date: Fri, 15 Jul 2022 14:24:55 GMT
   Accept: application/json
   Accept: */*
   Signature-Input: transform=("@method" "@path" "@authority" \
     "accept");created=1618884473;keyid="test-key-ed25519"
   Signature: transform=:ZT1kooQsEHpZ0I1IjCqtQppOmIqlJPeo7DHR3SoMn0s5J\
     Z1eRGS0A+vyYP9t/LXlh5QMFFQ6cpLt2m0pmj3NDA==:

   The signature base string for this message is:

   "@method": GET
   "@path": /demo
   "@authority": example.org
   "accept": application/json, */*
   "@signature-params": ("@method" "@path" "@authority" "accept")\
     ;created=1618884473;keyid="test-key-ed25519"

   The following message has been altered by adding the Accept-Language
   header field as well as adding a query parameter.  However, since
   neither the Accept-Language header field nor the query is covered by
   the signature, the same signature is still valid:

   NOTE: '\' line wrapping per RFC 8792

   GET /demo?name1=Value1&Name2=value2&param=added HTTP/1.1
   Host: example.org
   Date: Fri, 15 Jul 2022 14:24:55 GMT
   Accept: application/json
   Accept: */*
   Accept-Language: en-US,en;q=0.5
   Signature-Input: transform=("@method" "@path" "@authority" \
     "accept");created=1618884473;keyid="test-key-ed25519"
   Signature: transform=:ZT1kooQsEHpZ0I1IjCqtQppOmIqlJPeo7DHR3SoMn0s5J\
     Z1eRGS0A+vyYP9t/LXlh5QMFFQ6cpLt2m0pmj3NDA==:

   The following message has been altered by removing the Date header
   field, adding a Referer header field, and collapsing the Accept
   header field into a single line.  The Date and Referer header fields
   are not covered by the signature, and the collapsing of the Accept
   header field is an allowed transformation that is already accounted
   for by the canonicalization algorithm for HTTP field values.  The
   same signature is still valid:

   NOTE: '\' line wrapping per RFC 8792

   GET /demo?name1=Value1&Name2=value2 HTTP/1.1
   Host: example.org
   Referer: https://developer.example.org/demo
   Accept: application/json, */*
   Signature-Input: transform=("@method" "@path" "@authority" \
     "accept");created=1618884473;keyid="test-key-ed25519"
   Signature: transform=:ZT1kooQsEHpZ0I1IjCqtQppOmIqlJPeo7DHR3SoMn0s5J\
     Z1eRGS0A+vyYP9t/LXlh5QMFFQ6cpLt2m0pmj3NDA==:

   The following message has been altered by reordering the field values
   of the original message but not reordering the individual Accept
   header fields.  The same signature is still valid:

   NOTE: '\' line wrapping per RFC 8792

   GET /demo?name1=Value1&Name2=value2 HTTP/1.1
   Accept: application/json
   Accept: */*
   Date: Fri, 15 Jul 2022 14:24:55 GMT
   Host: example.org
   Signature-Input: transform=("@method" "@path" "@authority" \
     "accept");created=1618884473;keyid="test-key-ed25519"
   Signature: transform=:ZT1kooQsEHpZ0I1IjCqtQppOmIqlJPeo7DHR3SoMn0s5J\
     Z1eRGS0A+vyYP9t/LXlh5QMFFQ6cpLt2m0pmj3NDA==:

   The following message has been altered by changing the method to POST
   and the authority to "example.com" (inside the Host header field).
   Since both the method and authority are covered by the signature, the
   same signature is NOT still valid:

   NOTE: '\' line wrapping per RFC 8792

   POST /demo?name1=Value1&Name2=value2 HTTP/1.1
   Host: example.com
   Date: Fri, 15 Jul 2022 14:24:55 GMT
   Accept: application/json
   Accept: */*
   Signature-Input: transform=("@method" "@path" "@authority" \
     "accept");created=1618884473;keyid="test-key-ed25519"
   Signature: transform=:ZT1kooQsEHpZ0I1IjCqtQppOmIqlJPeo7DHR3SoMn0s5J\
     Z1eRGS0A+vyYP9t/LXlh5QMFFQ6cpLt2m0pmj3NDA==:

   The following message has been altered by changing the order of the
   two instances of the Accept header field.  Since the order of fields
   with the same name is semantically significant in HTTP, this changes
   the value used in the signature base, and the same signature is NOT
   still valid:

   NOTE: '\' line wrapping per RFC 8792

   GET /demo?name1=Value1&Name2=value2 HTTP/1.1
   Host: example.org
   Date: Fri, 15 Jul 2022 14:24:55 GMT
   Accept: */*
   Accept: application/json
   Signature-Input: transform=("@method" "@path" "@authority" \
     "accept");created=1618884473;keyid="test-key-ed25519"
   Signature: transform=:ZT1kooQsEHpZ0I1IjCqtQppOmIqlJPeo7DHR3SoMn0s5J\
     Z1eRGS0A+vyYP9t/LXlh5QMFFQ6cpLt2m0pmj3NDA==:

Acknowledgements

   This specification was initially based on [SIGNING-HTTP-MESSAGES].
   The editors would like to thank the authors of
   [SIGNING-HTTP-MESSAGES] -- Mark Cavage and Manu Sporny -- for their
   work on that Internet-Draft and their continuing contributions.  This
   specification also includes contributions from
   [SIGNING-HTTP-REQS-OAUTH] and other similar efforts.

   The editors would also like to thank the following individuals
   (listed in alphabetical order) for feedback, insight, and
   implementation of this document and its predecessors: Mark Adamcin,
   Mark Allen, Paul Annesley, Karl Böhlmark, Stéphane Bortzmeyer, Sarven
   Capadisli, Liam Dennehy, Stephen Farrell, Phillip Hallam-Baker, Tyler
   Ham, Eric Holmes, Andrey Kislyuk, Adam Knight, Dave Lehn, Ilari
   Liusvaara, Dave Longley, James H. Manger, Kathleen Moriarty, Yoav
   Nir, Mark Nottingham, Adrian Palmer, Lucas Pardue, Roberto Polli,
   Julian Reschke, Michael Richardson, Wojciech Rygielski, Rich Salz,
   Adam Scarr, Cory J. Slep, Dirk Stein, Henry Story, Lukasz Szewc,
   Chris Webber, and Jeffrey Yasskin.

Authors' Addresses

   Annabelle Backman (editor)
   Amazon
   P.O. Box 81226
   Seattle, WA 98108-1226
   United States of America
   Email: richanna@amazon.com
   URI:   https://www.amazon.com/


   Justin Richer (editor)
   Bespoke Engineering
   Email: ietf@justin.richer.org
   URI:   https://bspk.io/


   Manu Sporny
   Digital Bazaar
   203 Roanoke Street W.
   Blacksburg, VA 24060
   United States of America
   Email: msporny@digitalbazaar.com