path: root/doc/rfc/rfc9530.txt

Internet Engineering Task Force (IETF)                          R. Polli
Request for Comments: 9530             Team Digitale, Italian Government
Obsoletes: 3230                                                L. Pardue
Category: Standards Track                                     Cloudflare
ISSN: 2070-1721                                            February 2024


                             Digest Fields

Abstract

   This document defines HTTP fields that support integrity digests.
   The Content-Digest field can be used for the integrity of HTTP
   message content.  The Repr-Digest field can be used for the integrity
   of HTTP representations.  Want-Content-Digest and Want-Repr-Digest
   can be used to indicate a sender's interest and preferences for
   receiving the respective Integrity fields.

   This document obsoletes RFC 3230 and the Digest and Want-Digest HTTP
   fields.

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/rfc9530.

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.  Document Structure
     1.2.  Concept Overview
     1.3.  Obsoleting RFC 3230
     1.4.  Notational Conventions
   2.  The Content-Digest Field
   3.  The Repr-Digest Field
     3.1.  Using Repr-Digest in State-Changing Requests
     3.2.  Repr-Digest and Content-Location in Responses
   4.  Integrity Preference Fields
   5.  Hash Algorithm Considerations and Registration
   6.  Security Considerations
     6.1.  HTTP Messages Are Not Protected in Full
     6.2.  End-to-End Integrity
     6.3.  Usage in Signatures
     6.4.  Usage in Trailer Fields
     6.5.  Variations within Content-Encoding
     6.6.  Algorithm Agility
     6.7.  Resource Exhaustion
   7.  IANA Considerations
     7.1.  HTTP Field Name Registration
     7.2.  Creation of the Hash Algorithms for HTTP Digest Fields
           Registry
     7.3.  Deprecate the Hypertext Transfer Protocol (HTTP) Digest
           Algorithm Values Registry
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  Resource Representation and Representation Data
   Appendix B.  Examples of Unsolicited Digest
     B.1.  Server Returns Full Representation Data
     B.2.  Server Returns No Representation Data
     B.3.  Server Returns Partial Representation Data
     B.4.  Client and Server Provide Full Representation Data
     B.5.  Client Provides Full Representation Data and Server
            Provides No Representation Data
     B.6.  Client and Server Provide Full Representation Data
     B.7.  POST Response Does Not Reference the Request URI
     B.8.  POST Response Describes the Request Status
     B.9.  Digest with PATCH
     B.10. Error Responses
     B.11. Use with Trailer Fields and Transfer Coding
   Appendix C.  Examples of Want-Repr-Digest Solicited Digest
     C.1.  Server Selects Client's Least Preferred Algorithm
     C.2.  Server Selects Algorithm Unsupported by Client
     C.3.  Server Does Not Support Client Algorithm and Returns an
           Error
   Appendix D.  Sample Digest Values
   Appendix E.  Migrating from RFC 3230
   Acknowledgements
   Authors' Addresses

1.  Introduction

   HTTP does not define the means to protect the data integrity of
   content or representations.  When HTTP messages are transferred
   between endpoints, lower-layer features or properties such as TCP
   checksums or TLS records [TLS] can provide some integrity protection.
   However, transport-oriented integrity provides a limited utility
   because it is opaque to the application layer and only covers the
   extent of a single connection.  HTTP messages often travel over a
   chain of separate connections.  In between connections, there is a
   possibility for data corruption.  An HTTP integrity mechanism can
   provide the means for endpoints, or applications using HTTP, to
   detect data corruption and make a choice about how to act on it.  An
   example use case is to aid fault detection and diagnosis across
   system boundaries.

   This document defines two digest integrity mechanisms for HTTP.
   First, content integrity, which acts on conveyed content (Section 6.4
   of [HTTP]).  Second, representation data integrity, which acts on
   representation data (Section 8.1 of [HTTP]).  This supports advanced
   use cases, such as validating the integrity of a resource that was
   reconstructed from parts retrieved using multiple requests or
   connections.

   This document obsoletes [RFC3230] and therefore the Digest and Want-
   Digest HTTP fields; see Section 1.3.

1.1.  Document Structure

   This document is structured as follows:

   *  New request and response header and trailer field definitions.

      -  Section 2 (Content-Digest),

      -  Section 3 (Repr-Digest), and

      -  Section 4 (Want-Content-Digest and Want-Repr-Digest).

   *  Considerations specific to representation data integrity.

      -  Section 3.1 (State-changing requests),

      -  Section 3.2 (Content-Location),

      -  Appendix A contains worked examples of representation data in
         message exchanges, and

      -  Appendixes B and C contain worked examples of Repr-Digest and
         Want-Repr-Digest fields in message exchanges.

   *  Section 5 presents hash algorithm considerations and defines
      registration procedures for future entries.

1.2.  Concept Overview

   The HTTP fields defined in this document can be used for HTTP
   integrity.  Senders choose a hashing algorithm and calculate a digest
   from an input related to the HTTP message.  The algorithm identifier
   and digest are transmitted in an HTTP field.  Receivers can validate
   the digest for integrity purposes.  Hashing algorithms are registered
   in the "Hash Algorithms for HTTP Digest Fields" registry (see
   Section 7.2).

   Selecting the data on which digests are calculated depends on the use
   case of the HTTP messages.  This document provides different fields
   for HTTP representation data and HTTP content.

   There are use cases where a simple digest of the HTTP content bytes
   is required.  The Content-Digest request and response header and
   trailer field is defined to support digests of content (Section 6.4
   of [HTTP]); see Section 2.

   For more advanced use cases, the Repr-Digest request and response
   header and trailer field (Section 3) is defined.  It contains a
   digest value computed by applying a hashing algorithm to selected
   representation data (Section 8.1 of [HTTP]).  Basing Repr-Digest on
   the selected representation makes it straightforward to apply it to
   use cases where the message content requires some sort of
   manipulation to be considered as representation of the resource or
   the content conveys a partial representation of a resource, such as
   range requests (see Section 14 of [HTTP]).

   Content-Digest and Repr-Digest support hashing algorithm agility.
   The Want-Content-Digest and Want-Repr-Digest fields allow endpoints
   to express interest in Content-Digest and Repr-Digest, respectively,
   and to express algorithm preferences in either.

   Content-Digest and Repr-Digest are collectively termed "Integrity
   fields".  Want-Content-Digest and Want-Repr-Digest are collectively
   termed "Integrity preference fields".

   Integrity fields are tied to the Content-Encoding and Content-Type
   header fields.  Therefore, a given resource may have multiple
   different digest values when transferred with HTTP.

   Integrity fields apply to HTTP message content or HTTP
   representations.  They do not apply to HTTP messages or fields.
   However, they can be combined with other mechanisms that protect
   metadata, such as digital signatures, in order to protect the phases
   of an HTTP exchange in whole or in part.  For example, HTTP Message
   Signatures [SIGNATURES] could be used to sign Integrity fields, thus
   providing coverage for HTTP content or representation data.

   This specification does not define means for authentication,
   authorization, or privacy.

1.3.  Obsoleting RFC 3230

   [RFC3230] defined the Digest and Want-Digest HTTP fields for HTTP
   integrity.  It also coined the terms "instance" and "instance
   manipulation" in order to explain concepts, such as selected
   representation data (Section 8.1 of [HTTP]), that are now more
   universally defined and implemented as HTTP semantics.

   Experience has shown that implementations of [RFC3230] have
   interpreted the meaning of "instance" inconsistently, leading to
   interoperability issues.  The most common issue relates to the
   mistake of calculating the digest using (what we now call) message
   content, rather than using (what we now call) representation data as
   was originally intended.  Interestingly, time has also shown that a
   digest of message content can be beneficial for some use cases, so it
   is difficult to detect if non-conformance to [RFC3230] is intentional
   or unintentional.

   In order to address potential inconsistencies and ambiguity across
   implementations of Digest and Want-Digest, this document obsoletes
   [RFC3230].  The Integrity fields (Sections 2 and 3) and Integrity
   preference fields (Section 4) defined in this document are better
   aligned with current HTTP semantics and have names that more clearly
   articulate the intended usages.

1.4.  Notational Conventions

   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.

   This document uses the Augmented BNF defined in [RFC5234] and updated
   by [RFC7405].  This includes the rules CR (carriage return), LF (line
   feed), and CRLF (CR LF).

   This document uses the following terminology from Section 3 of
   [STRUCTURED-FIELDS] to specify syntax and parsing: Boolean, Byte
   Sequence, Dictionary, Integer, and List.

   The definitions "representation", "selected representation",
   "representation data", "representation metadata", "user agent", and
   "content" in this document are to be interpreted as described in
   [HTTP].

   This document uses the line folding strategies described in
   [FOLDING].

   Hashing algorithm names respect the casing used in their definition
   document (e.g., SHA-1, CRC32c).

   HTTP messages indicate hashing algorithms using an Algorithm Key
   (algorithms).  Where the document refers to an Algorithm Key in
   prose, it is quoted (e.g., "sha", "crc32c").

   The term "checksum" describes the output of applying an algorithm to
   a sequence of bytes, whereas "digest" is only used in relation to the
   value contained in the fields.

   "Integrity fields" is the collective term for Content-Digest and
   Repr-Digest.

   "Integrity preference fields" is the collective term for Want-Repr-
   Digest and Want-Content-Digest.

2.  The Content-Digest Field

   The Content-Digest HTTP field can be used in requests and responses
   to communicate digests that are calculated using a hashing algorithm
   applied to the actual message content (see Section 6.4 of [HTTP]).
   It is a Dictionary (see Section 3.2 of [STRUCTURED-FIELDS]), where
   each:

   *  key conveys the hashing algorithm (see Section 5) used to compute
      the digest;

   *  value is a Byte Sequence (Section 3.3.5 of [STRUCTURED-FIELDS])
      that conveys an encoded version of the byte output produced by the
      digest calculation.

   For example:

   NOTE: '\' line wrapping per RFC 8792

   Content-Digest: \
     sha-512=:YMAam51Jz/jOATT6/zvHrLVgOYTGFy1d6GJiOHTohq4yP+pgk4vf2aCs\
     yRZOtw8MjkM7iw7yZ/WkppmM44T3qg==:

   The Dictionary type can be used, for example, to attach multiple
   digests calculated using different hashing algorithms in order to
   support a population of endpoints with different or evolving
   capabilities.  Such an approach could support transitions away from
   weaker algorithms (see Section 6.6).

   NOTE: '\' line wrapping per RFC 8792

   Content-Digest: \
     sha-256=:d435Qo+nKZ+gLcUHn7GQtQ72hiBVAgqoLsZnZPiTGPk=:,\
     sha-512=:YMAam51Jz/jOATT6/zvHrLVgOYTGFy1d6GJiOHTohq4yP+pgk4vf2aCs\
     yRZOtw8MjkM7iw7yZ/WkppmM44T3qg==:

   A recipient MAY ignore any or all digests.  Application-specific
   behavior or local policy MAY set additional constraints on the
   processing and validation practices of the conveyed digests.  The
   security considerations cover some of the issues related to ignoring
   digests (see Section 6.6) and validating multiple digests (see
   Section 6.7).

   A sender MAY send a digest without knowing whether the recipient
   supports a given hashing algorithm.  A sender MAY send a digest if it
   knows the recipient will ignore it.

   Content-Digest can be sent in a trailer section.  In this case,
   Content-Digest MAY be merged into the header section; see
   Section 6.5.1 of [HTTP].

3.  The Repr-Digest Field

   The Repr-Digest HTTP field can be used in requests and responses to
   communicate digests that are calculated using a hashing algorithm
   applied to the entire selected representation data (see Section 8.1
   of [HTTP]).

   Representations take into account the effect of the HTTP semantics on
   messages.  For example, the content can be affected by range requests
   or methods, such as HEAD, while the way the content is transferred
   "on the wire" is dependent on other transformations (e.g., transfer
   codings for HTTP/1.1; see Section 6.1 of [HTTP/1.1]).  To help
   illustrate HTTP representation concepts, several examples are
   provided in Appendix A.

   When a message has no representation data, it is still possible to
   assert that no representation data was sent by computing the digest
   on an empty string (see Section 6.3).

   Repr-Digest is a Dictionary (see Section 3.2 of [STRUCTURED-FIELDS]),
   where each:

   *  key conveys the hashing algorithm (see Section 5) used to compute
      the digest;

   *  value is a Byte Sequence that conveys an encoded version of the
      byte output produced by the digest calculation.

   For example:

   NOTE: '\' line wrapping per RFC 8792

   Repr-Digest: \
     sha-512=:YMAam51Jz/jOATT6/zvHrLVgOYTGFy1d6GJiOHTohq4yP+pgk4vf2aCs\
     yRZOtw8MjkM7iw7yZ/WkppmM44T3qg==:

   The Dictionary type can be used to attach multiple digests calculated
   using different hashing algorithms in order to support a population
   of endpoints with different or evolving capabilities.  Such an
   approach could support transitions away from weaker algorithms (see
   Section 6.6).

   NOTE: '\' line wrapping per RFC 8792

   Repr-Digest: \
     sha-256=:d435Qo+nKZ+gLcUHn7GQtQ72hiBVAgqoLsZnZPiTGPk=:,\
     sha-512=:YMAam51Jz/jOATT6/zvHrLVgOYTGFy1d6GJiOHTohq4yP+pgk4vf2aCs\
     yRZOtw8MjkM7iw7yZ/WkppmM44T3qg==:

   A recipient MAY ignore any or all digests.  Application-specific
   behavior or local policy MAY set additional constraints on the
   processing and validation practices of the conveyed digests.  The
   security considerations cover some of the issues related to ignoring
   digests (see Section 6.6) and validating multiple digests (see
   Section 6.7).

   A sender MAY send a digest without knowing whether the recipient
   supports a given hashing algorithm.  A sender MAY send a digest if it
   knows the recipient will ignore it.

   Repr-Digest can be sent in a trailer section.  In this case, Repr-
   Digest MAY be merged into the header section; see Section 6.5.1 of
   [HTTP].

3.1.  Using Repr-Digest in State-Changing Requests

   When the representation enclosed in a state-changing request does not
   describe the target resource, the representation digest MUST be
   computed on the representation data.  This is the only possible
   choice because representation digest requires complete representation
   metadata (see Section 3).

   In responses,

   *  if the representation describes the status of the request, Repr-
      Digest MUST be computed on the enclosed representation (see
      Appendix B.8);

   *  if there is a referenced resource, Repr-Digest MUST be computed on
      the selected representation of the referenced resource even if
      that is different from the target resource.  This might or might
      not result in computing Repr-Digest on the enclosed
      representation.

   The latter case is done according to the HTTP semantics of the given
   method, for example, using the Content-Location header field (see
   Section 8.7 of [HTTP]).  In contrast, the Location header field does
   not affect Repr-Digest because it is not representation metadata.

   For example, in PATCH requests, the representation digest will be
   computed on the patch document because the representation metadata
   refers to the patch document and not the target resource (see
   Section 2 of [PATCH]).  In responses, instead, the representation
   digest will be computed on the selected representation of the patched
   resource.

3.2.  Repr-Digest and Content-Location in Responses

   When a state-changing method returns the Content-Location header
   field, the enclosed representation refers to the resource identified
   by its value and Repr-Digest is computed accordingly.  An example is
   given in Appendix B.7.

4.  Integrity Preference Fields

   Senders can indicate their interest in Integrity fields and hashing
   algorithm preferences using the Want-Content-Digest or Want-Repr-
   Digest HTTP fields.  These can be used in both requests and
   responses.

   Want-Content-Digest indicates that the sender would like to receive
   (via the Content-Digest field) a content digest on messages
   associated with the request URI and representation metadata.  Want-
   Repr-Digest indicates that the sender would like to receive (via the
   Repr-Digest field) a representation digest on messages associated
   with the request URI and representation metadata.

   If Want-Content-Digest or Want-Repr-Digest are used in a response, it
   indicates that the server would like the client to provide the
   respective Integrity field on future requests.

   Integrity preference fields are only a hint.  The receiver of the
   field can ignore it and send an Integrity field using any algorithm
   or omit the field entirely; for example, see Appendix C.2.  It is not
   a protocol error if preferences are ignored.  Applications that use
   Integrity fields and Integrity preferences can define expectations or
   constraints that operate in addition to this specification.  Ignored
   preferences are an application-specific concern.

   Want-Content-Digest and Want-Repr-Digest are of type Dictionary where
   each:

   *  key conveys the hashing algorithm (see Section 5);

   *  value is an Integer (Section 3.3.1 of [STRUCTURED-FIELDS]) that
      conveys an ascending, relative, weighted preference.  It must be
      in the range 0 to 10 inclusive. 1 is the least preferred, 10 is
      the most preferred, and a value of 0 means "not acceptable".

   Examples:

   Want-Repr-Digest: sha-256=1
   Want-Repr-Digest: sha-512=3, sha-256=10, unixsum=0
   Want-Content-Digest: sha-256=1
   Want-Content-Digest: sha-512=3, sha-256=10, unixsum=0

5.  Hash Algorithm Considerations and Registration

   There are a wide variety of hashing algorithms that can be used for
   the purposes of integrity.  The choice of algorithm depends on
   several factors such as the integrity use case, implementation needs
   or constraints, or application design and workflows.

   An initial set of algorithms will be registered with IANA in the
   "Hash Algorithms for HTTP Digest Fields" registry; see Section 7.2.
   Additional algorithms can be registered in accordance with the
   policies set out in this section.

   Each algorithm has a status field that is intended to provide an aid
   to implementation selection.

   Algorithms with a status value of "Active" are suitable for many
   purposes and it is RECOMMENDED that applications use these
   algorithms.  These can be used in adversarial situations where hash
   functions might need to provide resistance to collision, first-
   preimage, and second-preimage attacks.  For adversarial situations,
   selection of the acceptable "Active" algorithms will depend on the
   level of protection the circumstances demand.  More considerations
   are presented in Section 6.6.

   Algorithms with a status value of "Deprecated" either provide none of
   these properties or are known to be weak (see [NO-MD5] and [NO-SHA]).
   These algorithms MAY be used to preserve integrity against
   corruption, but MUST NOT be used in a potentially adversarial
   setting, for example, when signing Integrity fields' values for
   authenticity.  Permitting the use of these algorithms can help some
   applications (such as those that previously used [RFC3230], are
   migrating to this specification (Appendix E), and have existing
   stored collections of computed digest values) avoid undue operational
   overhead caused by recomputation using other more-secure algorithms.
   Such applications are not exempt from the requirements in this
   section.  Furthermore, applications without such legacy or history
   ought to follow the guidance for using algorithms with the status
   value "Active".

   Discussion of algorithm agility is presented in Section 6.6.

   Registration requests for the "Hash Algorithms for HTTP Digest
   Fields" registry use the Specification Required policy (Section 4.6
   of [RFC8126]).  Requests should use the following template:

   Algorithm Key:  The Structured Fields key value used in Content-
      Digest, Repr-Digest, Want-Content-Digest, or Want-Repr-Digest
      field Dictionary member keys.

   Status:  The status of the algorithm.  The options are:

      "Active":  Algorithms without known problems

      "Provisional":  Unproven algorithms

      "Deprecated":  Deprecated or insecure algorithms

   Description:  A short description of the algorithm.

   Reference(s):  Pointer(s) to the primary document(s) defining the
      Algorithm Key and technical details of the algorithm.

   When reviewing registration requests, the designated expert(s) should
   pay attention to the requested status.  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).  The "Active" status is not
   suitable for an algorithm that is known to be weak, broken, or
   experimental.  If a registration request attempts to register such an
   algorithm as "Active", the designated expert(s) should suggest an
   alternative status of "Deprecated" or "Provisional".

   When reviewing registration requests, the designated expert(s) cannot
   use a status of "Deprecated" or "Provisional" as grounds for
   rejection.

   Requests to update or change the fields in an existing registration
   are permitted.  For example, this could allow for the transition of
   an algorithm status from "Active" to "Deprecated" as the security
   environment evolves.

6.  Security Considerations

6.1.  HTTP Messages Are Not Protected in Full

   This document specifies a data integrity mechanism that protects HTTP
   representation data or content, but not HTTP header and trailer
   fields, from certain kinds of corruption.

   Integrity fields are not intended to be a general protection against
   malicious tampering with HTTP messages.  In the absence of additional
   security mechanisms, an on-path malicious actor can either remove a
   digest value entirely or substitute it with a new digest value
   computed over manipulated representation data or content.  This
   attack can be mitigated by combining mechanisms described in this
   document with other approaches such as Transport Layer Security (TLS)
   or digital signatures (for example, HTTP Message Signatures
   [SIGNATURES]).

6.2.  End-to-End Integrity

   Integrity fields can help detect representation data or content
   modification due to implementation errors, undesired "transforming
   proxies" (see Section 7.7 of [HTTP]), or other actions as the data
   passes across multiple hops or system boundaries.  Even a simple
   mechanism for end-to-end representation data integrity is valuable
   because a user agent can validate that resource retrieval succeeded
   before handing off to an HTML parser, video player, etc., for
   parsing.

   Note that using these mechanisms alone does not provide end-to-end
   integrity of HTTP messages over multiple hops since metadata could be
   manipulated at any stage.  Methods to protect metadata are discussed
   in Section 6.3.

6.3.  Usage in Signatures

   Digital signatures are widely used together with checksums to provide
   the certain identification of the origin of a message [FIPS186-5].
   Such signatures can protect one or more HTTP fields and there are
   additional considerations when Integrity fields are included in this
   set.

   There are no restrictions placed on the type or format of digital
   signature that Integrity fields can be used with.  One possible
   approach is to combine them with HTTP Message Signatures
   [SIGNATURES].

   Digests explicitly depend on the "representation metadata" (e.g., the
   values of Content-Type, Content-Encoding, etc.).  A signature that
   protects Integrity fields but not other "representation metadata" can
   expose the communication to tampering.  For example, an actor could
   manipulate the Content-Type field-value and cause a digest validation
   failure at the recipient, preventing the application from accessing
   the representation.  Such an attack consumes the resources of both
   endpoints.  See also Section 3.2.

   Signatures are likely to be deemed an adversarial setting when
   applying Integrity fields; see Section 5.  Repr-Digest offers an
   interesting possibility when combined with signatures.  In the
   scenario where there is no content to send, the digest of an empty
   string can be included in the message and, if signed, can help the
   recipient detect if content was added either as a result of accident
   or purposeful manipulation.  The opposite scenario is also supported;
   including an Integrity field for content and signing it can help a
   recipient detect where the content was removed.

   Any mangling of Integrity fields might affect signature validation.
   Examples of such mangling include de-duplicating digests or combining
   different field values (see Section 5.2 of [HTTP]).

6.4.  Usage in Trailer Fields

   Before sending Integrity fields in a trailer section, the sender
   should consider that intermediaries are explicitly allowed to drop
   any trailer (see Section 6.5.2 of [HTTP]).

   When Integrity fields are used in a trailer section, the field-values
   are received after the content.  Eager processing of content before
   the trailer section prevents digest validation, possibly leading to
   processing of invalid data.

   One of the benefits of using Integrity fields in a trailer section is
   that it allows hashing of bytes as they are sent.  However, it is
   possible to design a hashing algorithm that requires processing of
   content in such a way that would negate these benefits.  For example,
   Merkle Integrity Content Encoding [MICE] requires content to be
   processed in reverse order.  This means the complete data needs to be
   available, which means there is negligible processing difference in
   sending an Integrity field in a header versus a trailer section.

6.5.  Variations within Content-Encoding

   Content coding mechanisms can support different encoding parameters,
   meaning that the same input content can produce different outputs.
   For example, GZIP supports multiple compression levels.  Such
   encoding parameters are generally not communicated as representation
   metadata.  For instance, different compression levels would all use
   the same "Content-Encoding: gzip" field.  Other examples include
   where encoding relies on nonces or timestamps, such as the aes128gcm
   content coding defined in [RFC8188].

   Since it is possible for there to be variation within content coding,
   the checksum conveyed by the Integrity fields cannot be used to
   provide a proof of integrity "at rest" unless the whole content is
   persisted.

6.6.  Algorithm Agility

   The security properties of hashing algorithms are not fixed.
   Algorithm agility (see [RFC7696]) is achieved by providing
   implementations with flexibility to choose hashing algorithms from
   the IANA Hash Algorithms for HTTP Digest Fields registry; see
   Section 7.2.

   Transition from weak algorithms is supported by negotiation of
   hashing algorithm using Want-Content-Digest or Want-Repr-Digest (see
   Section 4) or by sending multiple digests from which the receiver
   chooses.  A receiver that depends on a digest for security will be
   vulnerable to attacks on the weakest algorithm it is willing to
   accept.  Endpoints are advised that sending multiple values consumes
   resources that may be wasted if the receiver ignores them (see
   Section 3).

   While algorithm agility allows the migration to stronger algorithms,
   it does not prevent the use of weaker algorithms.  Integrity fields
   do not provide any mitigations for downgrade or substitution attacks
   (see Section 1 of [RFC6211]) of the hashing algorithm.  To protect
   against such attacks, endpoints could restrict their set of supported
   algorithms to stronger ones and protect the fields' values by using
   TLS and/or digital signatures.

6.7.  Resource Exhaustion

   Integrity field validation consumes computational resources.  In
   order to avoid resource exhaustion, implementations can restrict
   validation of the algorithm types, the number of validations, or the
   size of content.  In these cases, skipping validation entirely or
   ignoring validation failure of a more-preferred algorithm leaves the
   possibility of a downgrade attack (see Section 6.6).

7.  IANA Considerations

7.1.  HTTP Field Name Registration

   IANA has updated the "Hypertext Transfer Protocol (HTTP) Field Name
   Registry" [HTTP] as shown in the table below:

       +=====================+===========+========================+
       | Field Name          | Status    | Reference              |
       +=====================+===========+========================+
       | Content-Digest      | permanent | Section 2 of RFC 9530  |
       +---------------------+-----------+------------------------+
       | Repr-Digest         | permanent | Section 3 of RFC 9530  |
       +---------------------+-----------+------------------------+
       | Want-Content-Digest | permanent | Section 4 of RFC 9530  |
       +---------------------+-----------+------------------------+
       | Want-Repr-Digest    | permanent | Section 4 of RFC 9530  |
       +---------------------+-----------+------------------------+
       | Digest              | obsoleted | [RFC3230], Section 1.3 |
       |                     |           | of RFC 9530            |
       +---------------------+-----------+------------------------+
       | Want-Digest         | obsoleted | [RFC3230], Section 1.3 |
       |                     |           | of RFC 9530            |
       +---------------------+-----------+------------------------+

          Table 1: Hypertext Transfer Protocol (HTTP) Field Name
                             Registry Update

7.2.  Creation of the Hash Algorithms for HTTP Digest Fields Registry

   IANA has created the new "Hash Algorithms for HTTP Digest Fields"
   registry at <https://www.iana.org/assignments/http-digest-hash-alg/>
   and populated it with the entries in Table 2.  The procedure for new
   registrations is provided in Section 5.

   +===========+============+============================+============+
   | Algorithm | Status     | Description                | Reference  |
   | Key       |            |                            |            |
   +===========+============+============================+============+
   | sha-512   | Active     | The SHA-512 algorithm.     | [RFC6234], |
   |           |            |                            | [RFC4648], |
   |           |            |                            | RFC 9530   |
   +-----------+------------+----------------------------+------------+
   | sha-256   | Active     | The SHA-256 algorithm.     | [RFC6234], |
   |           |            |                            | [RFC4648], |
   |           |            |                            | RFC 9530   |
   +-----------+------------+----------------------------+------------+
   | md5       | Deprecated | The MD5 algorithm.  It is  | [RFC1321], |
   |           |            | vulnerable to collision    | [RFC4648], |
   |           |            | attacks; see [NO-MD5] and  | RFC 9530   |
   |           |            | [CMU-836068]               |            |
   +-----------+------------+----------------------------+------------+
   | sha       | Deprecated | The SHA-1 algorithm.  It   | [RFC3174], |
   |           |            | is vulnerable to collision | [RFC4648], |
   |           |            | attacks; see [NO-SHA] and  | [RFC6234], |
   |           |            | [IACR-2020-014]            | RFC 9530   |
   +-----------+------------+----------------------------+------------+
   | unixsum   | Deprecated | The algorithm used by the  | [RFC4648], |
   |           |            | UNIX "sum" command.        | [RFC6234], |
   |           |            |                            | [UNIX],    |
   |           |            |                            | RFC 9530   |
   +-----------+------------+----------------------------+------------+
   | unixcksum | Deprecated | The algorithm used by the  | [RFC4648], |
   |           |            | UNIX "cksum" command.      | [RFC6234], |
   |           |            |                            | [UNIX],    |
   |           |            |                            | RFC 9530   |
   +-----------+------------+----------------------------+------------+
   | adler     | Deprecated | The ADLER32 algorithm.     | [RFC1950], |
   |           |            |                            | RFC 9530   |
   +-----------+------------+----------------------------+------------+
   | crc32c    | Deprecated | The CRC32c algorithm.      | Appendix A |
   |           |            |                            | of         |
   |           |            |                            | [RFC9260], |
   |           |            |                            | RFC 9530   |
   +-----------+------------+----------------------------+------------+

                     Table 2: Initial Hash Algorithms

7.3.  Deprecate the Hypertext Transfer Protocol (HTTP) Digest Algorithm
      Values Registry

   IANA has deprecated the "Hypertext Transfer Protocol (HTTP) Digest
   Algorithm Values" registry at <https://www.iana.org/assignments/http-
   dig-alg/> and replaced the note on that registry with the following
   text:

   |  This registry is deprecated since it lists the algorithms that can
   |  be used with the Digest and Want-Digest fields defined in
   |  [RFC3230], which has been obsoleted by RFC 9530.  While
   |  registration is not closed, new registrations are encouraged to
   |  use the Hash Algorithms for HTTP Digest Fields
   |  (https://www.iana.org/assignments/http-digest-hash-alg/) registry
   |  instead.

8.  References

8.1.  Normative References

   [FOLDING]  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>.

   [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>.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <https://www.rfc-editor.org/info/rfc1321>.

   [RFC1950]  Deutsch, P. and J. Gailly, "ZLIB Compressed Data Format
              Specification version 3.3", RFC 1950,
              DOI 10.17487/RFC1950, May 1996,
              <https://www.rfc-editor.org/info/rfc1950>.

   [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>.

   [RFC3174]  Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
              <https://www.rfc-editor.org/info/rfc3174>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5234]  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>.

   [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>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [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>.

   [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>.

8.2.  Informative References

   [CMU-836068]
              Carnegie Mellon University, Software Engineering
              Institute, "MD5 vulnerable to collision attacks", December
              2008, <https://www.kb.cert.org/vuls/id/836068/>.

   [FIPS186-5]
              National Institute of Standards and Technology (NIST),
              "Digital Signature Standard (DSS)", FIPS PUB 186-5,
              DOI 10.6028/NIST.FIPS.186-5, February 2023,
              <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.186-5.pdf>.

   [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>.

   [IACR-2020-014]
              Leurent, G. and T. Peyrin, "SHA-1 is a Shambles", January
              2020, <https://eprint.iacr.org/2020/014.pdf>.

   [MICE]     Thomson, M. and J. Yasskin, "Merkle Integrity Content
              Encoding", Work in Progress, Internet-Draft, draft-
              thomson-http-mice-03, 13 August 2018,
              <https://datatracker.ietf.org/doc/html/draft-thomson-http-
              mice-03>.

   [NO-MD5]   Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,
              <https://www.rfc-editor.org/info/rfc6151>.

   [NO-SHA]   Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
              <https://www.rfc-editor.org/info/rfc6194>.

   [PATCH]    Dusseault, L. and J. Snell, "PATCH Method for HTTP",
              RFC 5789, DOI 10.17487/RFC5789, March 2010,
              <https://www.rfc-editor.org/info/rfc5789>.

   [RFC3230]  Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
              RFC 3230, DOI 10.17487/RFC3230, January 2002,
              <https://www.rfc-editor.org/info/rfc3230>.

   [RFC6211]  Schaad, J., "Cryptographic Message Syntax (CMS) Algorithm
              Identifier Protection Attribute", RFC 6211,
              DOI 10.17487/RFC6211, April 2011,
              <https://www.rfc-editor.org/info/rfc6211>.

   [RFC7396]  Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
              DOI 10.17487/RFC7396, October 2014,
              <https://www.rfc-editor.org/info/rfc7396>.

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
              <https://www.rfc-editor.org/info/rfc7696>.

   [RFC8188]  Thomson, M., "Encrypted Content-Encoding for HTTP",
              RFC 8188, DOI 10.17487/RFC8188, June 2017,
              <https://www.rfc-editor.org/info/rfc8188>.

   [RFC9260]  Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control
              Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260,
              June 2022, <https://www.rfc-editor.org/info/rfc9260>.

   [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>.

   [SIGNATURES]
              Backman, A., Ed., Richer, J., Ed., and M. Sporny, "HTTP
              Message Signatures", RFC 9421, DOI 10.17487/RFC9421,
              February 2024, <https://www.rfc-editor.org/info/rfc9421>.

   [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>.

   [UNIX]     The Open Group, "The Single UNIX Specification, Version 2
              - 6 Vol Set for UNIX 98", January 1998.

Appendix A.  Resource Representation and Representation Data

   The following examples show how representation metadata, content
   transformations, and methods impact the message and content.  These
   examples a not exhaustive.

   Unless otherwise indicated, the examples are based on the JSON object
   {"hello": "world"} followed by an LF.  When the content contains non-
   printable characters (e.g., when it is encoded), it is shown as a
   sequence of hex-encoded bytes.

   Consider a client that wishes to upload a JSON object using the PUT
   method.  It could do this using the application/json Content-Type
   without any content coding.

   PUT /entries/1234 HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Content-Length: 19

   {"hello": "world"}

   Figure 1: Request Containing a JSON Object without Any Content Coding

   However, the use of content coding is quite common.  The client could
   also upload the same data with a GZIP coding (Section 8.4.1.3 of
   [HTTP]).  Note that in this case, the Content-Length contains a
   larger value due to the coding overheads.

   PUT /entries/1234 HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Content-Encoding: gzip
   Content-Length: 39

   1F 8B 08 00 88 41 37 64 00 FF
   AB 56 CA 48 CD C9 C9 57 B2 52
   50 2A CF 2F CA 49 51 AA E5 02
   00 D9 E4 31 E7 13 00 00 00

          Figure 2: Request Containing a GZIP-Encoded JSON Object

   Sending the GZIP-coded data without indicating it via Content-
   Encoding means that the content is malformed.  In this case, the
   server can reply with an error.

   PUT /entries/1234 HTTP/1.1
   Host: foo.example
   Content-Type: application/json

   Content-Length: 39

   1F 8B 08 00 88 41 37 64 00 FF
   AB 56 CA 48 CD C9 C9 57 B2 52
   50 2A CF 2F CA 49 51 AA E5 02
   00 D9 E4 31 E7 13 00 00 00

                Figure 3: Request Containing Malformed JSON

   HTTP/1.1 400 Bad Request

             Figure 4: An Error Response for Malformed Content

   A Range-Request affects the transferred message content.  In this
   example, the client is accessing the resource at /entries/1234, which
   is the JSON object {"hello": "world"} followed by an LF.  However,
   the client has indicated a preferred content coding and a specific
   byte range.

   GET /entries/1234 HTTP/1.1
   Host: foo.example
   Accept-Encoding: gzip
   Range: bytes=1-7

                   Figure 5: Request for Partial Content

   The server satisfies the client request by responding with a partial
   representation (equivalent to the first 10 bytes of the JSON object
   displayed in whole in Figure 2).

   HTTP/1.1 206 Partial Content
   Content-Encoding: gzip
   Content-Type: application/json
   Content-Range: bytes 0-9/39

   1F 8B 08 00 A5 B4 BD 62 02 FF

       Figure 6: Partial Response from a GZIP-Encoded Representation

   Aside from content coding or range requests, the method can also
   affect the transferred message content.  For example, the response to
   a HEAD request does not carry content, but this example case includes
   Content-Length; see Section 8.6 of [HTTP].

   HEAD /entries/1234 HTTP/1.1
   Host: foo.example
   Accept: application/json
   Accept-Encoding: gzip

                           Figure 7: HEAD Request

   HTTP/1.1 200 OK
   Content-Type: application/json
   Content-Encoding: gzip
   Content-Length: 39

             Figure 8: Response to HEAD Request (Empty Content)

   Finally, the semantics of a response might decouple the target URI
   from the enclosed representation.  In the example below, the client
   issues a POST request directed to /authors/, but the response
   includes a Content-Location header field indicating that the enclosed
   representation refers to the resource available at /authors/123.
   Note that Content-Length is not sent in this example.

   POST /authors/ HTTP/1.1
   Host: foo.example
   Accept: application/json
   Content-Type: application/json

   {"author": "Camilleri"}

                           Figure 9: POST Request

   HTTP/1.1 201 Created
   Content-Type: application/json
   Content-Location: /authors/123
   Location: /authors/123

   {"id": "123", "author": "Camilleri"}

              Figure 10: Response with Content-Location Header

Appendix B.  Examples of Unsolicited Digest

   The following examples demonstrate interactions where a server
   responds with a Content-Digest or Repr-Digest field, even though the
   client did not solicit one using Want-Content-Digest or Want-Repr-
   Digest.

   Some examples include JSON objects in the content.  For presentation
   purposes, objects that fit completely within the line-length limits
   are presented on a single line using compact notation with no leading
   space.  Objects that would exceed line-length limits are presented
   across multiple lines (one line per key-value pair) with two spaces
   of leading indentation.

   Checksum mechanisms defined in this document are media-type agnostic
   and do not provide canonicalization algorithms for specific formats.
   Examples are calculated inclusive of any space.  While examples can
   include both fields, Content-Digest and Repr-Digest can be returned
   independently.

B.1.  Server Returns Full Representation Data

   In this example, the message content conveys complete representation
   data.  This means that in the response, Content-Digest and Repr-
   Digest are both computed over the JSON object {"hello": "world"}
   followed by an LF; thus, they have the same value.

   GET /items/123 HTTP/1.1
   Host: foo.example

                     Figure 11: GET Request for an Item

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Content-Length: 19
   Content-Digest: \
     sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:
   Repr-Digest: \
     sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:

   {"hello": "world"}

     Figure 12: Response with Identical Repr-Digest and Content-Digest

B.2.  Server Returns No Representation Data

   In this example, a HEAD request is used to retrieve the checksum of a
   resource.

   The response Content-Digest field-value is computed on empty content.
   Repr-Digest is calculated over the JSON object {"hello": "world"}
   followed by an LF, which is not shown because there is no content.

   HEAD /items/123 HTTP/1.1
   Host: foo.example

                    Figure 13: HEAD Request for an Item

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Content-Digest: \
     sha-256=:47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=:
   Repr-Digest: \
     sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:

       Figure 14: Response with Both Content-Digest and Digest (Empty
                                  Content)

B.3.  Server Returns Partial Representation Data

   In this example, the client makes a range request and the server
   responds with partial content.

   GET /items/123 HTTP/1.1
   Host: foo.example
   Range: bytes=10-18

                   Figure 15: Request for Partial Content

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 206 Partial Content
   Content-Type: application/json
   Content-Range: bytes 10-18/19
   Content-Digest: \
     sha-256=:jjcgBDWNAtbYUXI37CVG3gRuGOAjaaDRGpIUFsdyepQ=:
   Repr-Digest: \
     sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:

   "world"}

    Figure 16: Partial Response with Both Content-Digest and Repr-Digest

   In the response message above, note that the Repr-Digest and Content-
   Digests are different.  The Repr-Digest field-value is calculated
   across the entire JSON object {"hello": "world"} followed by an LF,
   and the field appears as follows:

   NOTE: '\' line wrapping per RFC 8792

   Repr-Digest: \
     sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:

   However, since the message content is constrained to bytes 10-18, the
   Content-Digest field-value is calculated over the sequence "world"}
   followed by an LF, thus resulting in the following:

   NOTE: '\' line wrapping per RFC 8792

   Content-Digest: \
     sha-256=:jjcgBDWNAtbYUXI37CVG3gRuGOAjaaDRGpIUFsdyepQ=:

B.4.  Client and Server Provide Full Representation Data

   The request contains a Repr-Digest field-value calculated on the
   enclosed representation.  It also includes an Accept-Encoding: br
   header field that advertises that the client supports Brotli
   encoding.

   The response includes a Content-Encoding: br that indicates the
   selected representation is Brotli-encoded.  The Repr-Digest field-
   value is therefore different compared to the request.

   For presentation purposes, the response body is displayed as a
   sequence of hex-encoded bytes because it contains non-printable
   characters.

   NOTE: '\' line wrapping per RFC 8792

   PUT /items/123 HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Accept-Encoding: br
   Repr-Digest: \
     sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:

   {"hello": "world"}

                     Figure 17: PUT Request with Digest

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Content-Location: /items/123
   Content-Encoding: br
   Content-Length: 23
   Repr-Digest: \
     sha-256=:d435Qo+nKZ+gLcUHn7GQtQ72hiBVAgqoLsZnZPiTGPk=:

   8B 08 80 7B 22 68 65 6C 6C 6F
   22 3A 20 22 77 6F 72 6C 64 22
   7D 0A 03

            Figure 18: Response with Digest of Encoded Response

B.5.  Client Provides Full Representation Data and Server Provides No
      Representation Data

   The request Repr-Digest field-value is calculated on the enclosed
   content, which is the JSON object {"hello": "world"} followed by an
   LF.

   The response Repr-Digest field-value depends on the representation
   metadata header fields, including Content-Encoding: br, even when the
   response does not contain content.

   NOTE: '\' line wrapping per RFC 8792

   PUT /items/123 HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Content-Length: 19
   Accept-Encoding: br
   Repr-Digest: \
     sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg==:

   {"hello": "world"}

   HTTP/1.1 204 No Content
   Content-Type: application/json
   Content-Encoding: br
   Repr-Digest: sha-256=:d435Qo+nKZ+gLcUHn7GQtQ72hiBVAgqoLsZnZPiTGPk=:

                   Figure 19: Empty Response with Digest

B.6.  Client and Server Provide Full Representation Data

   The response contains two digest values using different algorithms.

   For presentation purposes, the response body is displayed as a
   sequence of hex-encoded bytes because it contains non-printable
   characters.

   NOTE: '\' line wrapping per RFC 8792

   PUT /items/123 HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Accept-Encoding: br
   Repr-Digest: \
     sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg==:

   {"hello": "world"}

                     Figure 20: PUT Request with Digest

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Content-Encoding: br
   Content-Location: /items/123
   Repr-Digest: \
     sha-256=:d435Qo+nKZ+gLcUHn7GQtQ72hiBVAgqoLsZnZPiTGPk=:,\
     sha-512=:db7fdBbgZMgX1Wb2MjA8zZj+rSNgfmDCEEXM8qLWfpfoNY0sCpHAzZbj\
     09X1/7HAb7Od5Qfto4QpuBsFbUO3dQ==:

   8B 08 80 7B 22 68 65 6C 6C 6F
   22 3A 20 22 77 6F 72 6C 64 22
   7D 0A 03

             Figure 21: Response with Digest of Encoded Content

B.7.  POST Response Does Not Reference the Request URI

   The request Repr-Digest field-value is computed on the enclosed
   representation (see Section 3.1), which is the JSON object {"title":
   "New Title"} followed by an LF.

   The representation enclosed in the response is a multiline JSON
   object followed by an LF.  It refers to the resource identified by
   Content-Location (see Section 6.4.2 of [HTTP]); thus, an application
   can use Repr-Digest in association with the resource referenced by
   Content-Location.

   POST /books HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Accept: application/json
   Accept-Encoding: identity
   Repr-Digest: sha-256=:mEkdbO7Srd9LIOegftO0aBX+VPTVz7/CSHes2Z27gc4=:

   {"title": "New Title"}

                    Figure 22: POST Request with Digest

   HTTP/1.1 201 Created
   Content-Type: application/json
   Content-Location: /books/123
   Location: /books/123
   Repr-Digest: sha-256=:uVSlinTTdQUwm2On4k8TJUikGN1bf/Ds8WPX4oe0h9I=:

   {
     "id": "123",
     "title": "New Title"
   }

                Figure 23: Response with Digest of Resource

B.8.  POST Response Describes the Request Status

   The request Repr-Digest field-value is computed on the enclosed
   representation (see Section 3.1), which is the JSON object {"title":
   "New Title"} followed by an LF.

   The representation enclosed in the response describes the status of
   the request, so Repr-Digest is computed on that enclosed
   representation.  It is a multiline JSON object followed by an LF.

   Response Repr-Digest has no explicit relation with the resource
   referenced by Location.

   POST /books HTTP/1.1
   Host: foo.example
   Content-Type: application/json
   Accept: application/json
   Accept-Encoding: identity
   Repr-Digest: sha-256=:mEkdbO7Srd9LIOegftO0aBX+VPTVz7/CSHes2Z27gc4=:

   {"title": "New Title"}

                    Figure 24: POST Request with Digest

   HTTP/1.1 201 Created
   Content-Type: application/json
   Repr-Digest: sha-256=:yXIGDTN5VrfoyisKlXgRKUHHMs35SNtyC3szSz1dbO8=:
   Location: /books/123

   {
     "status": "created",
     "id": "123",
     "ts": 1569327729,
     "instance": "/books/123"
   }

             Figure 25: Response with Digest of Representation

B.9.  Digest with PATCH

   This case is analogous to a POST request where the target resource
   reflects the target URI.

   The PATCH request uses the application/merge-patch+json media type
   defined in [RFC7396].  Repr-Digest is calculated on the content that
   corresponds to the patch document and is the JSON object {"title":
   "New Title"} followed by an LF.

   The response Repr-Digest field-value is computed on the complete
   representation of the patched resource.  It is a multiline JSON
   object followed by an LF.

   PATCH /books/123 HTTP/1.1
   Host: foo.example
   Content-Type: application/merge-patch+json
   Accept: application/json
   Accept-Encoding: identity
   Repr-Digest: sha-256=:mEkdbO7Srd9LIOegftO0aBX+VPTVz7/CSHes2Z27gc4=:

   {"title": "New Title"}

                    Figure 26: PATCH Request with Digest

   HTTP/1.1 200 OK
   Content-Type: application/json
   Repr-Digest: sha-256=:uVSlinTTdQUwm2On4k8TJUikGN1bf/Ds8WPX4oe0h9I=:

   {
     "id": "123",
     "title": "New Title"
   }

             Figure 27: Response with Digest of Representation

   Note that a 204 No Content response without content, but with the
   same Repr-Digest field-value, would have been legitimate too.  In
   that case, Content-Digest would have been computed on an empty
   content.

B.10.  Error Responses

   In error responses, the representation data does not necessarily
   refer to the target resource.  Instead, it refers to the
   representation of the error.

   In the following example, a client sends the same request from
   Figure 26 to patch the resource located at /books/123.  However, the
   resource does not exist and the server generates a 404 response with
   a body that describes the error in accordance with [RFC9457].

   The response Repr-Digest field-value is computed on this enclosed
   representation.  It is a multiline JSON object followed by an LF.

   HTTP/1.1 404 Not Found
   Content-Type: application/problem+json
   Repr-Digest: sha-256=:EXB0S2VF2H7ijkAVJkH1Sm0pBho0iDZcvVUHHXTTZSA=:

   {
     "title": "Not Found",
     "detail": "Cannot PATCH a non-existent resource",
     "status": 404
   }

          Figure 28: Response with Digest of Error Representation

B.11.  Use with Trailer Fields and Transfer Coding

   An origin server sends Repr-Digest as trailer field, so it can
   calculate digest-value while streaming content and thus mitigate
   resource consumption.  The Repr-Digest field-value is the same as in
   Appendix B.1 because Repr-Digest is designed to be independent of the
   use of one or more transfer codings (see Section 3).

   In the response content below, the string "\r\n" represents the CRLF
   bytes.

   GET /items/123 HTTP/1.1
   Host: foo.example

                           Figure 29: GET Request

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Transfer-Encoding: chunked
   Trailer: Repr-Digest

   8\r\n
   {"hello"\r\n
   8\r\n
   : "world\r\n
   3\r\n
   "}\n\r\n
   0\r\n
   Repr-Digest: \
     sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg==:\r\n

                  Figure 30: Chunked Response with Digest

Appendix C.  Examples of Want-Repr-Digest Solicited Digest

   The following examples demonstrate interactions where a client
   solicits a Repr-Digest using Want-Repr-Digest.  The behavior of
   Content-Digest and Want-Content-Digest is identical.

   Some examples include JSON objects in the content.  For presentation
   purposes, objects that fit completely within the line-length limits
   are presented on a single line using compact notation with no leading
   space.  Objects that would exceed line-length limits are presented
   across multiple lines (one line per key-value pair) with two spaces
   of leading indentation.

   Checksum mechanisms described in this document are media-type
   agnostic and do not provide canonicalization algorithms for specific
   formats.  Examples are calculated inclusive of any space.

C.1.  Server Selects Client's Least Preferred Algorithm

   The client requests a digest and prefers "sha".  The server is free
   to reply with "sha-256" anyway.

   GET /items/123 HTTP/1.1
   Host: foo.example
   Want-Repr-Digest: sha-256=3, sha=10

                Figure 31: GET Request with Want-Repr-Digest

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Repr-Digest: \
     sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg==:

   {"hello": "world"}

                Figure 32: Response with Different Algorithm

C.2.  Server Selects Algorithm Unsupported by Client

   The client requests a "sha" digest because that is the only algorithm
   it supports.  The server is not obliged to produce a response
   containing a "sha" digest; it instead uses a different algorithm.

   GET /items/123 HTTP/1.1
   Host: foo.example
   Want-Repr-Digest: sha=10

                Figure 33: GET Request with Want-Repr-Digest

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Repr-Digest: \
     sha-512=:YMAam51Jz/jOATT6/zvHrLVgOYTGFy1d6GJiOHTohq4yP+pgk4vf2aCs\
     yRZOtw8MjkM7iw7yZ/WkppmM44T3qg==:

   {"hello": "world"}

               Figure 34: Response with Unsupported Algorithm

C.3.  Server Does Not Support Client Algorithm and Returns an Error

   Appendix C.2 is an example where a server ignores the client's
   preferred digest algorithm.  Alternatively, a server can also reject
   the request and return a response with an error status code such as
   4xx or 5xx.  This specification does not prescribe any requirement on
   status code selection; the following example illustrates one possible
   option.

   In this example, the client requests a "sha" Repr-Digest, and the
   server returns an error with problem details [RFC9457] contained in
   the content.  The problem details contain a list of the hashing
   algorithms that the server supports.  This is purely an example; this
   specification does not define any format or requirements for such
   content.

   GET /items/123 HTTP/1.1
   Host: foo.example
   Want-Repr-Digest: sha=10

                Figure 35: GET Request with Want-Repr-Digest

   HTTP/1.1 400 Bad Request
   Content-Type: application/problem+json

   {
     "title": "Bad Request",
     "detail": "Supported hashing algorithms: sha-256, sha-512",
     "status": 400
   }

          Figure 36: Response Advertising the Supported Algorithms

Appendix D.  Sample Digest Values

   This section shows examples of digest values for different hashing
   algorithms.  The input value is the JSON object {"hello": "world"}.
   The digest values are each produced by running the relevant hashing
   algorithm over the input and running the output bytes through Byte
   Sequence serialization; see Section 4.1.8 of [STRUCTURED-FIELDS].

   NOTE: '\' line wrapping per RFC 8792

   sha-512 -   :WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+\
               AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:

   sha-256 -   :X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

   md5 -       :Sd/dVLAcvNLSq16eXua5uQ==:

   sha -       :07CavjDP4u3/TungoUHJO/Wzr4c=:

   unixsum -   :GQU=:

   unixcksum - :7zsHAA==:

   adler -     :OZkGFw==:

   crc32c -    :Q3lHIA==:

Appendix E.  Migrating from RFC 3230

   HTTP digests are computed by applying a hashing algorithm to input
   data.  [RFC3230] defined the input data as an "instance", a term it
   also defined.  The concept of an instance has since been superseded
   by the HTTP semantic term "representation".  It is understood that
   some implementations of [RFC3230] mistook "instance" to mean HTTP
   content.  Using content for the Digest field is an error that leads
   to interoperability problems between peers that implement [RFC3230].

   [RFC3230] was only ever intended to use what HTTP now defines as
   selected representation data.  The semantic concept of digest and
   representation are explained alongside the definition of the Repr-
   Digest field (Section 3).

   While the syntax of Digest and Repr-Digest are different, the
   considerations and examples this document gives for Repr-Digest apply
   equally to Digest because they operate on the same input data; see
   Sections 3.1, 6 and 6.3.

   [RFC3230] could never communicate the digest of HTTP message content
   in the Digest field; Content-Digest now provides that capability.

   [RFC3230] allowed algorithms to define their output encoding format
   for use with the Digest field.  This resulted in a mix of formats
   such as base64, hex, or decimal.  By virtue of using Structured
   Fields, Content-Digest, and Repr-Digest use only a single encoding
   format.  Further explanation and examples are provided in Appendix D.

Acknowledgements

   This document is based on ideas from [RFC3230], so thanks to Jeff
   Mogul and Arthur Van Hoff for their great work.  The original idea of
   refreshing [RFC3230] arose from an interesting discussion with Mark
   Nottingham, Jeffrey Yasskin, and Martin Thomson when reviewing the
   MICE content coding.

   Thanks to Julian Reschke for his valuable contributions to this
   document, and to the following contributors that have helped improve
   this specification by reporting bugs, asking smart questions,
   drafting or reviewing text, and evaluating open issues: Mike Bishop,
   Brian Campbell, Matthew Kerwin, James Manger, Tommy Pauly, Sean
   Turner, Justin Richer, and Erik Wilde.

Authors' Addresses

   Roberto Polli
   Team Digitale, Italian Government
   Italy
   Email: robipolli@gmail.com


   Lucas Pardue
   Cloudflare
   Email: lucas@lucaspardue.com