path: root/doc/rfc/rfc8555.txt

Internet Engineering Task Force (IETF)                         R. Barnes
Request for Comments: 8555                                         Cisco
Category: Standards Track                             J. Hoffman-Andrews
ISSN: 2070-1721                                                      EFF
                                                             D. McCarney
                                                           Let's Encrypt
                                                               J. Kasten
                                                  University of Michigan
                                                              March 2019


          Automatic Certificate Management Environment (ACME)

Abstract

   Public Key Infrastructure using X.509 (PKIX) certificates are used
   for a number of purposes, the most significant of which is the
   authentication of domain names.  Thus, certification authorities
   (CAs) in the Web PKI are trusted to verify that an applicant for a
   certificate legitimately represents the domain name(s) in the
   certificate.  As of this writing, this verification is done through a
   collection of ad hoc mechanisms.  This document describes a protocol
   that a CA and an applicant can use to automate the process of
   verification and certificate issuance.  The protocol also provides
   facilities for other certificate management functions, such as
   certificate revocation.

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/rfc8555.











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Copyright Notice

   Copyright (c) 2019 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................4
   2. Deployment Model and Operator Experience ........................5
   3. Terminology .....................................................7
   4. Protocol Overview ...............................................7
   5. Character Encoding .............................................10
   6. Message Transport ..............................................10
      6.1. HTTPS Requests ............................................10
      6.2. Request Authentication ....................................11
      6.3. GET and POST-as-GET Requests ..............................13
      6.4. Request URL Integrity .....................................13
           6.4.1. "url" (URL) JWS Header Parameter ...................14
      6.5. Replay Protection .........................................14
           6.5.1. Replay-Nonce .......................................15
           6.5.2. "nonce" (Nonce) JWS Header Parameter ...............16
      6.6. Rate Limits ...............................................16
      6.7. Errors ....................................................16
           6.7.1. Subproblems ........................................18
   7. Certificate Management .........................................20
      7.1. Resources .................................................20
           7.1.1. Directory ..........................................23
           7.1.2. Account Objects ....................................24
           7.1.3. Order Objects ......................................26
           7.1.4. Authorization Objects ..............................28
           7.1.5. Challenge Objects ..................................30
           7.1.6. Status Changes .....................................30
      7.2. Getting a Nonce ...........................................34
      7.3. Account Management ........................................34
           7.3.1. Finding an Account URL Given a Key .................36
           7.3.2. Account Update .....................................37
           7.3.3. Changes of Terms of Service ........................38
           7.3.4. External Account Binding ...........................38



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           7.3.5. Account Key Rollover ...............................40
           7.3.6. Account Deactivation ...............................43
      7.4. Applying for Certificate Issuance .........................44
           7.4.1. Pre-authorization ..................................49
           7.4.2. Downloading the Certificate ........................51
      7.5. Identifier Authorization ..................................53
           7.5.1. Responding to Challenges ...........................54
           7.5.2. Deactivating an Authorization ......................57
      7.6. Certificate Revocation ....................................58
   8. Identifier Validation Challenges ...............................60
      8.1. Key Authorizations ........................................62
      8.2. Retrying Challenges .......................................63
      8.3. HTTP Challenge ............................................63
      8.4. DNS Challenge .............................................66
   9. IANA Considerations ............................................68
      9.1. Media Type: application/pem-certificate-chain .............68
      9.2. Well-Known URI for the HTTP Challenge .....................69
      9.3. Replay-Nonce HTTP Header ..................................69
      9.4. "url" JWS Header Parameter ................................70
      9.5. "nonce" JWS Header Parameter ..............................70
      9.6. URN Sub-namespace for ACME (urn:ietf:params:acme) .........70
      9.7. New Registries ............................................71
           9.7.1. Fields in Account Objects ..........................71
           9.7.2. Fields in Order Objects ............................72
           9.7.3. Fields in Authorization Objects ....................73
           9.7.4. Error Types ........................................74
           9.7.5. Resource Types .....................................74
           9.7.6. Fields in the "meta" Object within a
                  Directory Object ...................................75
           9.7.7. Identifier Types ...................................76
           9.7.8. Validation Methods .................................76
   10. Security Considerations .......................................78
      10.1. Threat Model .............................................78
      10.2. Integrity of Authorizations ..............................80
      10.3. Denial-of-Service Considerations .........................83
      10.4. Server-Side Request Forgery ..............................84
      10.5. CA Policy Considerations .................................84
   11. Operational Considerations ....................................86
      11.1. Key Selection ............................................86
      11.2. DNS Security .............................................87
      11.3. Token Entropy ............................................88
      11.4. Malformed Certificate Chains .............................88
   12. References ....................................................88
      12.1. Normative References .....................................88
      12.2. Informative References ...................................92
   Acknowledgements ..................................................94
   Authors' Addresses ................................................95




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1.  Introduction

   Certificates [RFC5280] in the Web PKI are most commonly used to
   authenticate domain names.  Thus, certification authorities (CAs) in
   the Web PKI are trusted to verify that an applicant for a certificate
   legitimately represents the domain name(s) in the certificate.

   Different types of certificates reflect different kinds of CA
   verification of information about the certificate subject.  "Domain
   Validation" (DV) certificates are by far the most common type.  The
   only validation the CA is required to perform in the DV issuance
   process is to verify that the requester has effective control of the
   domain [CABFBR].  The CA is not required to attempt to verify the
   requester's real-world identity.  (This is as opposed to
   "Organization Validation" (OV) and "Extended Validation" (EV)
   certificates, where the process is intended to also verify the real-
   world identity of the requester.)

   Existing Web PKI certification authorities tend to use a set of ad
   hoc protocols for certificate issuance and identity verification.  In
   the case of DV certificates, a typical user experience is something
   like:

   o  Generate a PKCS#10 [RFC2986] Certificate Signing Request (CSR).

   o  Cut and paste the CSR into a CA's web page.

   o  Prove ownership of the domain(s) in the CSR by one of the
      following methods:

      *  Put a CA-provided challenge at a specific place on the web
         server.

      *  Put a CA-provided challenge in a DNS record corresponding to
         the target domain.

      *  Receive a CA-provided challenge at (hopefully) an
         administrator-controlled email address corresponding to the
         domain, and then respond to it on the CA's web page.

   o  Download the issued certificate and install it on the user's Web
      Server.

   With the exception of the CSR itself and the certificates that are
   issued, these are all completely ad hoc procedures and are
   accomplished by getting the human user to follow interactive natural-
   language instructions from the CA rather than by machine-implemented
   published protocols.  In many cases, the instructions are difficult



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   to follow and cause significant frustration and confusion.  Informal
   usability tests by the authors indicate that webmasters often need
   1-3 hours to obtain and install a certificate for a domain.  Even in
   the best case, the lack of published, standardized mechanisms
   presents an obstacle to the wide deployment of HTTPS and other PKIX-
   dependent systems because it inhibits mechanization of tasks related
   to certificate issuance, deployment, and revocation.

   This document describes an extensible framework for automating the
   issuance and domain validation procedure, thereby allowing servers
   and infrastructure software to obtain certificates without user
   interaction.  Use of this protocol should radically simplify the
   deployment of HTTPS and the practicality of PKIX-based authentication
   for other protocols based on Transport Layer Security (TLS)
   [RFC8446].

   It should be noted that while the focus of this document is on
   validating domain names for purposes of issuing certificates in the
   Web PKI, ACME supports extensions for uses with other identifiers in
   other PKI contexts.  For example, as of this writing, there is
   ongoing work to use ACME for issuance of Web PKI certificates
   attesting to IP addresses [ACME-IP] and Secure Telephone Identity
   Revisited (STIR) certificates attesting to telephone numbers
   [ACME-TELEPHONE].

   ACME can also be used to automate some aspects of certificate
   management even where non-automated processes are still needed.  For
   example, the external account binding feature (see Section 7.3.4) can
   allow an ACME account to use authorizations that have been granted to
   an external, non-ACME account.  This allows ACME to address issuance
   scenarios that cannot yet be fully automated, such as the issuance of
   "Extended Validation" certificates.

2.  Deployment Model and Operator Experience

   The guiding use case for ACME is obtaining certificates for websites
   (HTTPS [RFC2818]).  In this case, a web server is intended to speak
   for one or more domains, and the process of certificate issuance is
   intended to verify that this web server actually speaks for the
   domain(s).

   DV certificate validation commonly checks claims about properties
   related to control of a domain name -- properties that can be
   observed by the certificate issuer in an interactive process that can
   be conducted purely online.  That means that under typical
   circumstances, all steps in the request, verification, and issuance
   process can be represented and performed by Internet protocols with
   no out-of-band human intervention.



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   Prior to ACME, when deploying an HTTPS server, a server operator
   typically gets a prompt to generate a self-signed certificate.  If
   the operator were instead deploying an HTTPS server using ACME, the
   experience would be something like this:

   o  The operator's ACME client prompts the operator for the intended
      domain name(s) that the web server is to stand for.

   o  The ACME client presents the operator with a list of CAs from
      which it could get a certificate.  (This list will change over
      time based on the capabilities of CAs and updates to ACME
      configuration.)  The ACME client might prompt the operator for
      payment information at this point.

   o  The operator selects a CA.

   o  In the background, the ACME client contacts the CA and requests
      that it issue a certificate for the intended domain name(s).

   o  The CA verifies that the client controls the requested domain
      name(s) by having the ACME client perform some action(s) that can
      only be done with control of the domain name(s).  For example, the
      CA might require a client requesting example.com to provision a
      DNS record under example.com or an HTTP resource under
      http://example.com.

   o  Once the CA is satisfied, it issues the certificate and the ACME
      client automatically downloads and installs it, potentially
      notifying the operator via email, SMS, etc.

   o  The ACME client periodically contacts the CA to get updated
      certificates, stapled Online Certificate Status Protocol (OCSP)
      responses [RFC6960], or whatever else would be required to keep
      the web server functional and its credentials up to date.

   In this way, it would be nearly as easy to deploy with a CA-issued
   certificate as with a self-signed certificate.  Furthermore, the
   maintenance of that CA-issued certificate would require minimal
   manual intervention.  Such close integration of ACME with HTTPS
   servers allows the immediate and automated deployment of certificates
   as they are issued, sparing the human administrator from much of the
   time-consuming work described in the previous section.









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3.  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 two main roles in ACME are "client" and "server".  The ACME
   client uses the protocol to request certificate management actions,
   such as issuance or revocation.  An ACME client may run on a web
   server, mail server, or some other server system that requires valid
   X.509 certificates.  Or, it may run on a separate server that does
   not consume the certificate but is authorized to respond to a CA-
   provided challenge.  The ACME server runs at a certification
   authority and responds to client requests, performing the requested
   actions if the client is authorized.

   An ACME client authenticates to the server by means of an "account
   key pair".  The client uses the private key of this key pair to sign
   all messages sent to the server.  The server uses the public key to
   verify the authenticity and integrity of messages from the client.

4.  Protocol Overview

   ACME allows a client to request certificate management actions using
   a set of JavaScript Object Notation (JSON) messages [RFC8259] carried
   over HTTPS [RFC2818].  Issuance using ACME resembles a traditional
   CA's issuance process, in which a user creates an account, requests a
   certificate, and proves control of the domain(s) in that certificate
   in order for the CA to issue the requested certificate.

   The first phase of ACME is for the client to request an account with
   the ACME server.  The client generates an asymmetric key pair and
   requests a new account, optionally providing contact information,
   agreeing to terms of service (ToS), and/or associating the account
   with an existing account in another system.  The creation request is
   signed with the generated private key to prove that the client
   controls it.












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         Client                                                   Server

         [Contact Information]
         [ToS Agreement]
         [Additional Data]
         Signature                     ------->
                                                             Account URL
                                       <-------           Account Object


                   [] Information covered by request signatures

                             Account Creation

   Once an account is registered, there are four major steps the client
   needs to take to get a certificate:

   1.  Submit an order for a certificate to be issued

   2.  Prove control of any identifiers requested in the certificate

   3.  Finalize the order by submitting a CSR

   4.  Await issuance and download the issued certificate

   The client's order for a certificate describes the desired
   identifiers plus a few additional fields that capture semantics that
   are not supported in the CSR format.  If the server is willing to
   consider issuing such a certificate, it responds with a list of
   requirements that the client must satisfy before the certificate will
   be issued.

   For example, in most cases, the server will require the client to
   demonstrate that it controls the identifiers in the requested
   certificate.  Because there are many different ways to validate
   possession of different types of identifiers, the server will choose
   from an extensible set of challenges that are appropriate for the
   identifier being claimed.  The client responds with a set of
   responses that tell the server which challenges the client has
   completed.  The server then validates that the client has completed
   the challenges.

   Once the validation process is complete and the server is satisfied
   that the client has met its requirements, the client finalizes the
   order by submitting a PKCS#10 Certificate Signing Request (CSR).  The
   server will issue the requested certificate and make it available to
   the client.




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         Client                                                   Server

         [Order]
         Signature                     ------->
                                       <-------  Required Authorizations

         [Responses]
         Signature                     ------->

                             <~~~~~~~~Validation~~~~~~~~>

         [CSR]
         Signature                     ------->
                                       <-------          Acknowledgement

                             <~~~~~~Await issuance~~~~~~>

         [POST-as-GET request]
         Signature                     ------->
                                       <-------              Certificate

                   [] Information covered by request signatures

                           Certificate Issuance

   To revoke a certificate, the client sends a signed revocation request
   indicating the certificate to be revoked:

         Client                                                 Server

         [Revocation request]
         Signature                    -------->

                                      <--------                 Result

                   [] Information covered by request signatures

                          Certificate Revocation

   Note that while ACME is defined with enough flexibility to handle
   different types of identifiers in principle, the primary use case
   addressed by this document is the case where domain names are used as
   identifiers.  For example, all of the identifier validation
   challenges described in Section 8 address validation of domain names.
   The use of ACME for other identifiers will require further
   specification in order to describe how these identifiers are encoded
   in the protocol and what types of validation challenges the server
   might require.



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5.  Character Encoding

   All requests and responses sent via HTTP by ACME clients, ACME
   servers, and validation servers as well as any inputs for digest
   computations MUST be encoded using the UTF-8 character set [RFC3629].
   Note that identifiers that appear in certificates may have their own
   encoding considerations (e.g., DNS names containing non-ASCII
   characters are expressed as A-labels rather than U-labels).  Any such
   encoding considerations are to be applied prior to the aforementioned
   UTF-8 encoding.

6.  Message Transport

   Communications between an ACME client and an ACME server are done
   over HTTPS, using JSON Web Signature (JWS) [RFC7515] to provide some
   additional security properties for messages sent from the client to
   the server.  HTTPS provides server authentication and
   confidentiality.  With some ACME-specific extensions, JWS provides
   authentication of the client's request payloads, anti-replay
   protection, and integrity for the HTTPS request URL.

6.1.  HTTPS Requests

   Each ACME function is accomplished by the client sending a sequence
   of HTTPS requests to the server [RFC2818], carrying JSON messages
   [RFC8259].  Use of HTTPS is REQUIRED.  Each subsection of Section 7
   below describes the message formats used by the function and the
   order in which messages are sent.

   In most HTTPS transactions used by ACME, the ACME client is the HTTPS
   client and the ACME server is the HTTPS server.  The ACME server acts
   as a client when validating challenges: an HTTP client when
   validating an 'http-01' challenge, a DNS client with 'dns-01', etc.

   ACME servers SHOULD follow the recommendations of [RFC7525] when
   configuring their TLS implementations.  ACME servers that support TLS
   1.3 MAY allow clients to send early data (0-RTT).  This is safe
   because the ACME protocol itself includes anti-replay protections
   (see Section 6.5) in all cases where they are required.  For this
   reason, there are no restrictions on what ACME data can be carried in
   0-RTT.

   ACME clients MUST send a User-Agent header field, in accordance with
   [RFC7231].  This header field SHOULD include the name and version of
   the ACME software in addition to the name and version of the
   underlying HTTP client software.





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   ACME clients SHOULD send an Accept-Language header field in
   accordance with [RFC7231] to enable localization of error messages.

   ACME servers that are intended to be generally accessible need to use
   Cross-Origin Resource Sharing (CORS) in order to be accessible from
   browser-based clients [W3C.REC-cors-20140116].  Such servers SHOULD
   set the Access-Control-Allow-Origin header field to the value "*".

   Binary fields in the JSON objects used by ACME are encoded using
   base64url encoding described in Section 5 of [RFC4648] according to
   the profile specified in JSON Web Signature in Section 2 of
   [RFC7515].  This encoding uses a URL safe character set.  Trailing
   '=' characters MUST be stripped.  Encoded values that include
   trailing '=' characters MUST be rejected as improperly encoded.

6.2.  Request Authentication

   All ACME requests with a non-empty body MUST encapsulate their
   payload in a JSON Web Signature (JWS) [RFC7515] object, signed using
   the account's private key unless otherwise specified.  The server
   MUST verify the JWS before processing the request.  Encapsulating
   request bodies in JWS provides authentication of requests.

   A JWS object sent as the body of an ACME request MUST meet the
   following additional criteria:

   o  The JWS MUST be in the Flattened JSON Serialization [RFC7515]

   o  The JWS MUST NOT have multiple signatures

   o  The JWS Unencoded Payload Option [RFC7797] MUST NOT be used

   o  The JWS Unprotected Header [RFC7515] MUST NOT be used

   o  The JWS Payload MUST NOT be detached

   o  The JWS Protected Header MUST include the following fields:

      *  "alg" (Algorithm)

         +  This field MUST NOT contain "none" or a Message
            Authentication Code (MAC) algorithm (e.g. one in which the
            algorithm registry description mentions MAC/HMAC).

      *  "nonce" (defined in Section 6.5)

      *  "url" (defined in Section 6.4)




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      *  Either "jwk" (JSON Web Key) or "kid" (Key ID) as specified
         below

   An ACME server MUST implement the "ES256" signature algorithm
   [RFC7518] and SHOULD implement the "EdDSA" signature algorithm using
   the "Ed25519" variant (indicated by "crv") [RFC8037].

   The "jwk" and "kid" fields are mutually exclusive.  Servers MUST
   reject requests that contain both.

   For newAccount requests, and for revokeCert requests authenticated by
   a certificate key, there MUST be a "jwk" field.  This field MUST
   contain the public key corresponding to the private key used to sign
   the JWS.

   For all other requests, the request is signed using an existing
   account, and there MUST be a "kid" field.  This field MUST contain
   the account URL received by POSTing to the newAccount resource.

   If the client sends a JWS signed with an algorithm that the server
   does not support, then the server MUST return an error with status
   code 400 (Bad Request) and type
   "urn:ietf:params:acme:error:badSignatureAlgorithm".  The problem
   document returned with the error MUST include an "algorithms" field
   with an array of supported "alg" values.  See Section 6.7 for more
   details on the structure of error responses.

   If the server supports the signature algorithm "alg" but either does
   not support or chooses to reject the public key "jwk", then the
   server MUST return an error with status code 400 (Bad Request) and
   type "urn:ietf:params:acme:error:badPublicKey".  The problem document
   detail SHOULD describe the reason for rejecting the public key; some
   example reasons are:

   o  "alg" is "RS256" but the modulus "n" is too small (e.g., 512-bit)

   o  "alg" is "ES256" but "jwk" does not contain a valid P-256 public
      key

   o  "alg" is "EdDSA" and "crv" is "Ed448", but the server only
      supports "EdDSA" with "Ed25519"

   o  the corresponding private key is known to have been compromised








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   Because client requests in ACME carry JWS objects in the Flattened
   JSON Serialization, they must have the Content-Type header field set
   to "application/jose+json".  If a request does not meet this
   requirement, then the server MUST return a response with status code
   415 (Unsupported Media Type).

6.3.  GET and POST-as-GET Requests

   Note that authentication via signed JWS request bodies implies that
   requests without an entity body are not authenticated, in particular
   GET requests.  Except for the cases described in this section, if the
   server receives a GET request, it MUST return an error with status
   code 405 (Method Not Allowed) and type "malformed".

   If a client wishes to fetch a resource from the server (which would
   otherwise be done with a GET), then it MUST send a POST request with
   a JWS body as described above, where the payload of the JWS is a
   zero-length octet string.  In other words, the "payload" field of the
   JWS object MUST be present and set to the empty string ("").

   We will refer to these as "POST-as-GET" requests.  On receiving a
   request with a zero-length (and thus non-JSON) payload, the server
   MUST authenticate the sender and verify any access control rules.
   Otherwise, the server MUST treat this request as having the same
   semantics as a GET request for the same resource.

   The server MUST allow GET requests for the directory and newNonce
   resources (see Section 7.1), in addition to POST-as-GET requests for
   these resources.  This enables clients to bootstrap into the ACME
   authentication system.

6.4.  Request URL Integrity

   It is common in deployment for the entity terminating TLS for HTTPS
   to be different from the entity operating the logical HTTPS server,
   with a "request routing" layer in the middle.  For example, an ACME
   CA might have a content delivery network terminate TLS connections
   from clients so that it can inspect client requests for denial-of-
   service (DoS) protection.

   These intermediaries can also change values in the request that are
   not signed in the HTTPS request, e.g., the request URL and header
   fields.  ACME uses JWS to provide an integrity mechanism, which
   protects against an intermediary changing the request URL to another
   ACME URL.






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   As noted in Section 6.2, all ACME request objects carry a "url"
   header parameter in their protected header.  This header parameter
   encodes the URL to which the client is directing the request.  On
   receiving such an object in an HTTP request, the server MUST compare
   the "url" header parameter to the request URL.  If the two do not
   match, then the server MUST reject the request as unauthorized.

   Except for the directory resource, all ACME resources are addressed
   with URLs provided to the client by the server.  In POST requests
   sent to these resources, the client MUST set the "url" header
   parameter to the exact string provided by the server (rather than
   performing any re-encoding on the URL).  The server SHOULD perform
   the corresponding string equality check, configuring each resource
   with the URL string provided to clients and having the resource check
   that requests have the same string in their "url" header parameter.
   The server MUST reject the request as unauthorized if the string
   equality check fails.

6.4.1.  "url" (URL) JWS Header Parameter

   The "url" header parameter specifies the URL [RFC3986] to which this
   JWS object is directed.  The "url" header parameter MUST be carried
   in the protected header of the JWS.  The value of the "url" header
   parameter MUST be a string representing the target URL.

6.5.  Replay Protection

   In order to protect ACME resources from any possible replay attacks,
   ACME POST requests have a mandatory anti-replay mechanism.  This
   mechanism is based on the server maintaining a list of nonces that it
   has issued, and requiring any signed request from the client to carry
   such a nonce.

   An ACME server provides nonces to clients using the HTTP Replay-Nonce
   header field, as specified in Section 6.5.1.  The server MUST include
   a Replay-Nonce header field in every successful response to a POST
   request and SHOULD provide it in error responses as well.

   Every JWS sent by an ACME client MUST include, in its protected
   header, the "nonce" header parameter, with contents as defined in
   Section 6.5.2.  As part of JWS verification, the ACME server MUST
   verify that the value of the "nonce" header is a value that the
   server previously provided in a Replay-Nonce header field.  Once a
   nonce value has appeared in an ACME request, the server MUST consider
   it invalid, in the same way as a value it had never issued.






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RFC 8555                          ACME                        March 2019


   When a server rejects a request because its nonce value was
   unacceptable (or not present), it MUST provide HTTP status code 400
   (Bad Request), and indicate the ACME error type
   "urn:ietf:params:acme:error:badNonce".  An error response with the
   "badNonce" error type MUST include a Replay-Nonce header field with a
   fresh nonce that the server will accept in a retry of the original
   query (and possibly in other requests, according to the server's
   nonce scoping policy).  On receiving such a response, a client SHOULD
   retry the request using the new nonce.

   The precise method used to generate and track nonces is up to the
   server.  For example, the server could generate a random 128-bit
   value for each response, keep a list of issued nonces, and strike
   nonces from this list as they are used.

   Other than the constraint above with regard to nonces issued in
   "badNonce" responses, ACME does not constrain how servers scope
   nonces.  Clients MAY assume that nonces have broad scope, e.g., by
   having a single pool of nonces used for all requests.  However, when
   retrying in response to a "badNonce" error, the client MUST use the
   nonce provided in the error response.  Servers should scope nonces
   broadly enough that retries are not needed very often.

6.5.1.  Replay-Nonce

   The Replay-Nonce HTTP header field includes a server-generated value
   that the server can use to detect unauthorized replay in future
   client requests.  The server MUST generate the values provided in
   Replay-Nonce header fields in such a way that they are unique to each
   message, with high probability, and unpredictable to anyone besides
   the server.  For instance, it is acceptable to generate Replay-Nonces
   randomly.

   The value of the Replay-Nonce header field MUST be an octet string
   encoded according to the base64url encoding described in Section 2 of
   [RFC7515].  Clients MUST ignore invalid Replay-Nonce values.  The
   ABNF [RFC5234] for the Replay-Nonce header field follows:

     base64url = ALPHA / DIGIT / "-" / "_"

     Replay-Nonce = 1*base64url

   The Replay-Nonce header field SHOULD NOT be included in HTTP request
   messages.







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RFC 8555                          ACME                        March 2019


6.5.2.  "nonce" (Nonce) JWS Header Parameter

   The "nonce" header parameter provides a unique value that enables the
   verifier of a JWS to recognize when replay has occurred.  The "nonce"
   header parameter MUST be carried in the protected header of the JWS.

   The value of the "nonce" header parameter MUST be an octet string,
   encoded according to the base64url encoding described in Section 2 of
   [RFC7515].  If the value of a "nonce" header parameter is not valid
   according to this encoding, then the verifier MUST reject the JWS as
   malformed.

6.6.  Rate Limits

   Creation of resources can be rate limited by ACME servers to ensure
   fair usage and prevent abuse.  Once the rate limit is exceeded, the
   server MUST respond with an error with the type
   "urn:ietf:params:acme:error:rateLimited".  Additionally, the server
   SHOULD send a Retry-After header field [RFC7231] indicating when the
   current request may succeed again.  If multiple rate limits are in
   place, that is the time where all rate limits allow access again for
   the current request with exactly the same parameters.

   In addition to the human-readable "detail" field of the error
   response, the server MAY send one or multiple link relations in the
   Link header field [RFC8288] pointing to documentation about the
   specific rate limit that was hit, using the "help" link relation
   type.

6.7.  Errors

   Errors can be reported in ACME both at the HTTP layer and within
   challenge objects as defined in Section 8.  ACME servers can return
   responses with an HTTP error response code (4XX or 5XX).  For
   example, if the client submits a request using a method not allowed
   in this document, then the server MAY return status code 405 (Method
   Not Allowed).

   When the server responds with an error status, it SHOULD provide
   additional information using a problem document [RFC7807].  To
   facilitate automatic response to errors, this document defines the
   following standard tokens for use in the "type" field (within the
   ACME URN namespace "urn:ietf:params:acme:error:"):








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RFC 8555                          ACME                        March 2019


   +-------------------------+-----------------------------------------+
   | Type                    | Description                             |
   +-------------------------+-----------------------------------------+
   | accountDoesNotExist     | The request specified an account that   |
   |                         | does not exist                          |
   |                         |                                         |
   | alreadyRevoked          | The request specified a certificate to  |
   |                         | be revoked that has already been        |
   |                         | revoked                                 |
   |                         |                                         |
   | badCSR                  | The CSR is unacceptable (e.g., due to a |
   |                         | short key)                              |
   |                         |                                         |
   | badNonce                | The client sent an unacceptable anti-   |
   |                         | replay nonce                            |
   |                         |                                         |
   | badPublicKey            | The JWS was signed by a public key the  |
   |                         | server does not support                 |
   |                         |                                         |
   | badRevocationReason     | The revocation reason provided is not   |
   |                         | allowed by the server                   |
   |                         |                                         |
   | badSignatureAlgorithm   | The JWS was signed with an algorithm    |
   |                         | the server does not support             |
   |                         |                                         |
   | caa                     | Certification Authority Authorization   |
   |                         | (CAA) records forbid the CA from        |
   |                         | issuing a certificate                   |
   |                         |                                         |
   | compound                | Specific error conditions are indicated |
   |                         | in the "subproblems" array              |
   |                         |                                         |
   | connection              | The server could not connect to         |
   |                         | validation target                       |
   |                         |                                         |
   | dns                     | There was a problem with a DNS query    |
   |                         | during identifier validation            |
   |                         |                                         |
   | externalAccountRequired | The request must include a value for    |
   |                         | the "externalAccountBinding" field      |
   |                         |                                         |
   | incorrectResponse       | Response received didn't match the      |
   |                         | challenge's requirements                |
   |                         |                                         |
   | invalidContact          | A contact URL for an account was        |
   |                         | invalid                                 |
   |                         |                                         |
   | malformed               | The request message was malformed       |



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   |                         |                                         |
   | orderNotReady           | The request attempted to finalize an    |
   |                         | order that is not ready to be finalized |
   |                         |                                         |
   | rateLimited             | The request exceeds a rate limit        |
   |                         |                                         |
   | rejectedIdentifier      | The server will not issue certificates  |
   |                         | for the identifier                      |
   |                         |                                         |
   | serverInternal          | The server experienced an internal      |
   |                         | error                                   |
   |                         |                                         |
   | tls                     | The server received a TLS error during  |
   |                         | validation                              |
   |                         |                                         |
   | unauthorized            | The client lacks sufficient             |
   |                         | authorization                           |
   |                         |                                         |
   | unsupportedContact      | A contact URL for an account used an    |
   |                         | unsupported protocol scheme             |
   |                         |                                         |
   | unsupportedIdentifier   | An identifier is of an unsupported type |
   |                         |                                         |
   | userActionRequired      | Visit the "instance" URL and take       |
   |                         | actions specified there                 |
   +-------------------------+-----------------------------------------+

   This list is not exhaustive.  The server MAY return errors whose
   "type" field is set to a URI other than those defined above.  Servers
   MUST NOT use the ACME URN namespace for errors not listed in the
   appropriate IANA registry (see Section 9.6).  Clients SHOULD display
   the "detail" field of all errors.

   In the remainder of this document, we use the tokens in the table
   above to refer to error types, rather than the full URNs.  For
   example, an "error of type 'badCSR'" refers to an error document with
   "type" value "urn:ietf:params:acme:error:badCSR".

6.7.1.  Subproblems

   Sometimes a CA may need to return multiple errors in response to a
   request.  Additionally, the CA may need to attribute errors to
   specific identifiers.  For instance, a newOrder request may contain
   multiple identifiers for which the CA cannot issue certificates.  In
   this situation, an ACME problem document MAY contain the
   "subproblems" field, containing a JSON array of problem documents,
   each of which MAY contain an "identifier" field.  If present, the
   "identifier" field MUST contain an ACME identifier (Section 9.7.7).



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RFC 8555                          ACME                        March 2019


   The "identifier" field MUST NOT be present at the top level in ACME
   problem documents.  It can only be present in subproblems.
   Subproblems need not all have the same type, and they do not need to
   match the top level type.

   ACME clients may choose to use the "identifier" field of a subproblem
   as a hint that an operation would succeed if that identifier were
   omitted.  For instance, if an order contains ten DNS identifiers, and
   the newOrder request returns a problem document with two subproblems
   (referencing two of those identifiers), the ACME client may choose to
   submit another order containing only the eight identifiers not listed
   in the problem document.

HTTP/1.1 403 Forbidden
Content-Type: application/problem+json
Link: <https://example.com/acme/directory>;rel="index"

{
    "type": "urn:ietf:params:acme:error:malformed",
    "detail": "Some of the identifiers requested were rejected",
    "subproblems": [
        {
            "type": "urn:ietf:params:acme:error:malformed",
            "detail": "Invalid underscore in DNS name \"_example.org\"",
            "identifier": {
                "type": "dns",
                "value": "_example.org"
            }
        },
        {
            "type": "urn:ietf:params:acme:error:rejectedIdentifier",
            "detail": "This CA will not issue for \"example.net\"",
            "identifier": {
                "type": "dns",
                "value": "example.net"
            }
        }
    ]
}












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RFC 8555                          ACME                        March 2019


7.  Certificate Management

   In this section, we describe the certificate management functions
   that ACME enables:

   o  Account Creation

   o  Ordering a Certificate

   o  Identifier Authorization

   o  Certificate Issuance

   o  Certificate Revocation

7.1.  Resources

   ACME is structured as an HTTP-based application with the following
   types of resources:

   o  Account resources, representing information about an account
      (Section 7.1.2, Section 7.3)

   o  Order resources, representing an account's requests to issue
      certificates (Section 7.1.3)

   o  Authorization resources, representing an account's authorization
      to act for an identifier (Section 7.1.4)

   o  Challenge resources, representing a challenge to prove control of
      an identifier (Section 7.5, Section 8)

   o  Certificate resources, representing issued certificates
      (Section 7.4.2)

   o  A "directory" resource (Section 7.1.1)

   o  A "newNonce" resource (Section 7.2)

   o  A "newAccount" resource (Section 7.3)

   o  A "newOrder" resource (Section 7.4)

   o  A "revokeCert" resource (Section 7.6)

   o  A "keyChange" resource (Section 7.3.5)

   The server MUST provide "directory" and "newNonce" resources.



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   ACME uses different URLs for different management functions.  Each
   function is listed in a directory along with its corresponding URL,
   so clients only need to be configured with the directory URL.  These
   URLs are connected by a few different link relations [RFC8288].

   The "up" link relation is used with challenge resources to indicate
   the authorization resource to which a challenge belongs.  It is also
   used, with some media types, from certificate resources to indicate a
   resource from which the client may fetch a chain of CA certificates
   that could be used to validate the certificate in the original
   resource.

   The "index" link relation is present on all resources other than the
   directory and indicates the URL of the directory.

   The following diagram illustrates the relations between resources on
   an ACME server.  For the most part, these relations are expressed by
   URLs provided as strings in the resources' JSON representations.
   Lines with labels in quotes indicate HTTP link relations.

                                  directory
                                      |
                                      +--> newNonce
                                      |
          +----------+----------+-----+-----+------------+
          |          |          |           |            |
          |          |          |           |            |
          V          V          V           V            V
     newAccount   newAuthz   newOrder   revokeCert   keyChange
          |          |          |
          |          |          |
          V          |          V
       account       |        order --+--> finalize
                     |          |     |
                     |          |     +--> cert
                     |          V
                     +---> authorization
                               | ^
                               | | "up"
                               V |
                             challenge

                     ACME Resources and Relationships








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RFC 8555                          ACME                        March 2019


   The following table illustrates a typical sequence of requests
   required to establish a new account with the server, prove control of
   an identifier, issue a certificate, and fetch an updated certificate
   some time after issuance.  The "->" is a mnemonic for a Location
   header field pointing to a created resource.

   +-------------------+--------------------------------+--------------+
   | Action            | Request                        | Response     |
   +-------------------+--------------------------------+--------------+
   | Get directory     | GET  directory                 | 200          |
   |                   |                                |              |
   | Get nonce         | HEAD newNonce                  | 200          |
   |                   |                                |              |
   | Create account    | POST newAccount                | 201 ->       |
   |                   |                                | account      |
   |                   |                                |              |
   | Submit order      | POST newOrder                  | 201 -> order |
   |                   |                                |              |
   | Fetch challenges  | POST-as-GET order's            | 200          |
   |                   | authorization urls             |              |
   |                   |                                |              |
   | Respond to        | POST authorization challenge   | 200          |
   | challenges        | urls                           |              |
   |                   |                                |              |
   | Poll for status   | POST-as-GET order              | 200          |
   |                   |                                |              |
   | Finalize order    | POST order's finalize url      | 200          |
   |                   |                                |              |
   | Poll for status   | POST-as-GET order              | 200          |
   |                   |                                |              |
   | Download          | POST-as-GET order's            | 200          |
   | certificate       | certificate url                |              |
   +-------------------+--------------------------------+--------------+

   The remainder of this section provides the details of how these
   resources are structured and how the ACME protocol makes use of them.















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7.1.1.  Directory

   In order to help clients configure themselves with the right URLs for
   each ACME operation, ACME servers provide a directory object.  This
   should be the only URL needed to configure clients.  It is a JSON
   object, whose field names are drawn from the resource registry
   (Section 9.7.5) and whose values are the corresponding URLs.

                    +------------+--------------------+
                    | Field      | URL in Value       |
                    +------------+--------------------+
                    | newNonce   | New nonce          |
                    |            |                    |
                    | newAccount | New account        |
                    |            |                    |
                    | newOrder   | New order          |
                    |            |                    |
                    | newAuthz   | New authorization  |
                    |            |                    |
                    | revokeCert | Revoke certificate |
                    |            |                    |
                    | keyChange  | Key change         |
                    +------------+--------------------+

   There is no constraint on the URL of the directory except that it
   should be different from the other ACME server resources' URLs, and
   that it should not clash with other services.  For instance:

   o  a host that functions as both an ACME and a Web server may want to
      keep the root path "/" for an HTML "front page" and place the ACME
      directory under the path "/acme".

   o  a host that only functions as an ACME server could place the
      directory under the path "/".

   If the ACME server does not implement pre-authorization
   (Section 7.4.1), it MUST omit the "newAuthz" field of the directory.

   The object MAY additionally contain a "meta" field.  If present, it
   MUST be a JSON object; each field in the object is an item of
   metadata relating to the service provided by the ACME server.

   The following metadata items are defined (Section 9.7.6), all of
   which are OPTIONAL:

   termsOfService (optional, string):  A URL identifying the current
      terms of service.




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   website (optional, string):  An HTTP or HTTPS URL locating a website
      providing more information about the ACME server.

   caaIdentities (optional, array of string):  The hostnames that the
      ACME server recognizes as referring to itself for the purposes of
      CAA record validation as defined in [RFC6844].  Each string MUST
      represent the same sequence of ASCII code points that the server
      will expect to see as the "Issuer Domain Name" in a CAA issue or
      issuewild property tag.  This allows clients to determine the
      correct issuer domain name to use when configuring CAA records.

   externalAccountRequired (optional, boolean):  If this field is
      present and set to "true", then the CA requires that all
      newAccount requests include an "externalAccountBinding" field
      associating the new account with an external account.

   Clients access the directory by sending a GET request to the
   directory URL.

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

   {
     "newNonce": "https://example.com/acme/new-nonce",
     "newAccount": "https://example.com/acme/new-account",
     "newOrder": "https://example.com/acme/new-order",
     "newAuthz": "https://example.com/acme/new-authz",
     "revokeCert": "https://example.com/acme/revoke-cert",
     "keyChange": "https://example.com/acme/key-change",
     "meta": {
       "termsOfService": "https://example.com/acme/terms/2017-5-30",
       "website": "https://www.example.com/",
       "caaIdentities": ["example.com"],
       "externalAccountRequired": false
     }
   }

7.1.2.  Account Objects

   An ACME account resource represents a set of metadata associated with
   an account.  Account resources have the following structure:

   status (required, string):  The status of this account.  Possible
      values are "valid", "deactivated", and "revoked".  The value
      "deactivated" should be used to indicate client-initiated
      deactivation whereas "revoked" should be used to indicate server-
      initiated deactivation.  See Section 7.1.6.




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   contact (optional, array of string):  An array of URLs that the
      server can use to contact the client for issues related to this
      account.  For example, the server may wish to notify the client
      about server-initiated revocation or certificate expiration.  For
      information on supported URL schemes, see Section 7.3.

   termsOfServiceAgreed (optional, boolean):  Including this field in a
      newAccount request, with a value of true, indicates the client's
      agreement with the terms of service.  This field cannot be updated
      by the client.

   externalAccountBinding (optional, object):  Including this field in a
      newAccount request indicates approval by the holder of an existing
      non-ACME account to bind that account to this ACME account.  This
      field is not updateable by the client (see Section 7.3.4).

   orders (required, string):  A URL from which a list of orders
      submitted by this account can be fetched via a POST-as-GET
      request, as described in Section 7.1.2.1.

   {
     "status": "valid",
     "contact": [
       "mailto:cert-admin@example.org",
       "mailto:admin@example.org"
     ],
     "termsOfServiceAgreed": true,
     "orders": "https://example.com/acme/orders/rzGoeA"
   }

7.1.2.1.  Orders List

   Each account object includes an "orders" URL from which a list of
   orders created by the account can be fetched via POST-as-GET request.
   The result of the request MUST be a JSON object whose "orders" field
   is an array of URLs, each identifying an order belonging to the
   account.  The server SHOULD include pending orders and SHOULD NOT
   include orders that are invalid in the array of URLs.  The server MAY
   return an incomplete list, along with a Link header field with a
   "next" link relation indicating where further entries can be
   acquired.










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RFC 8555                          ACME                        March 2019


   HTTP/1.1 200 OK
   Content-Type: application/json
   Link: <https://example.com/acme/directory>;rel="index"
   Link: <https://example.com/acme/orders/rzGoeA?cursor=2>;rel="next"

   {
     "orders": [
       "https://example.com/acme/order/TOlocE8rfgo",
       "https://example.com/acme/order/4E16bbL5iSw",
       /* more URLs not shown for example brevity */
       "https://example.com/acme/order/neBHYLfw0mg"
     ]
   }

7.1.3.  Order Objects

   An ACME order object represents a client's request for a certificate
   and is used to track the progress of that order through to issuance.
   Thus, the object contains information about the requested
   certificate, the authorizations that the server requires the client
   to complete, and any certificates that have resulted from this order.

   status (required, string):  The status of this order.  Possible
      values are "pending", "ready", "processing", "valid", and
      "invalid".  See Section 7.1.6.

   expires (optional, string):  The timestamp after which the server
      will consider this order invalid, encoded in the format specified
      in [RFC3339].  This field is REQUIRED for objects with "pending"
      or "valid" in the status field.

   identifiers (required, array of object):  An array of identifier
      objects that the order pertains to.

      type (required, string):  The type of identifier.  This document
         defines the "dns" identifier type.  See the registry defined in
         Section 9.7.7 for any others.

      value (required, string):  The identifier itself.

   notBefore (optional, string):  The requested value of the notBefore
      field in the certificate, in the date format defined in [RFC3339].

   notAfter (optional, string):  The requested value of the notAfter
      field in the certificate, in the date format defined in [RFC3339].






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RFC 8555                          ACME                        March 2019


   error (optional, object):  The error that occurred while processing
      the order, if any.  This field is structured as a problem document
      [RFC7807].

   authorizations (required, array of string):  For pending orders, the
      authorizations that the client needs to complete before the
      requested certificate can be issued (see Section 7.5), including
      unexpired authorizations that the client has completed in the past
      for identifiers specified in the order.  The authorizations
      required are dictated by server policy; there may not be a 1:1
      relationship between the order identifiers and the authorizations
      required.  For final orders (in the "valid" or "invalid" state),
      the authorizations that were completed.  Each entry is a URL from
      which an authorization can be fetched with a POST-as-GET request.

   finalize (required, string):  A URL that a CSR must be POSTed to once
      all of the order's authorizations are satisfied to finalize the
      order.  The result of a successful finalization will be the
      population of the certificate URL for the order.

   certificate (optional, string):  A URL for the certificate that has
      been issued in response to this order.

   {
     "status": "valid",
     "expires": "2016-01-20T14:09:07.99Z",

     "identifiers": [
       { "type": "dns", "value": "www.example.org" },
       { "type": "dns", "value": "example.org" }
     ],

     "notBefore": "2016-01-01T00:00:00Z",
     "notAfter": "2016-01-08T00:00:00Z",

     "authorizations": [
       "https://example.com/acme/authz/PAniVnsZcis",
       "https://example.com/acme/authz/r4HqLzrSrpI"
     ],

     "finalize": "https://example.com/acme/order/TOlocE8rfgo/finalize",

     "certificate": "https://example.com/acme/cert/mAt3xBGaobw"
   }

   Any identifier of type "dns" in a newOrder request MAY have a
   wildcard domain name as its value.  A wildcard domain name consists
   of a single asterisk character followed by a single full stop



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   character ("*.") followed by a domain name as defined for use in the
   Subject Alternate Name Extension by [RFC5280].  An authorization
   returned by the server for a wildcard domain name identifier MUST NOT
   include the asterisk and full stop ("*.") prefix in the authorization
   identifier value.  The returned authorization MUST include the
   optional "wildcard" field, with a value of true.

   The elements of the "authorizations" and "identifiers" arrays are
   immutable once set.  The server MUST NOT change the contents of
   either array after they are created.  If a client observes a change
   in the contents of either array, then it SHOULD consider the order
   invalid.

   The "authorizations" array of the order SHOULD reflect all
   authorizations that the CA takes into account in deciding to issue,
   even if some authorizations were fulfilled in earlier orders or in
   pre-authorization transactions.  For example, if a CA allows multiple
   orders to be fulfilled based on a single authorization transaction,
   then it SHOULD reflect that authorization in all of the orders.

   Note that just because an authorization URL is listed in the
   "authorizations" array of an order object doesn't mean that the
   client is required to take action.  There are several reasons that
   the referenced authorizations may already be valid:

   o  The client completed the authorization as part of a previous order

   o  The client previously pre-authorized the identifier (see
      Section 7.4.1)

   o  The server granted the client authorization based on an external
      account

   Clients SHOULD check the "status" field of an order to determine
   whether they need to take any action.

7.1.4.  Authorization Objects

   An ACME authorization object represents a server's authorization for
   an account to represent an identifier.  In addition to the
   identifier, an authorization includes several metadata fields, such
   as the status of the authorization (e.g., "pending", "valid", or
   "revoked") and which challenges were used to validate possession of
   the identifier.







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RFC 8555                          ACME                        March 2019


   The structure of an ACME authorization resource is as follows:

   identifier (required, object):  The identifier that the account is
      authorized to represent.

      type (required, string):  The type of identifier (see below and
         Section 9.7.7).

      value (required, string):  The identifier itself.

   status (required, string):  The status of this authorization.
      Possible values are "pending", "valid", "invalid", "deactivated",
      "expired", and "revoked".  See Section 7.1.6.

   expires (optional, string):  The timestamp after which the server
      will consider this authorization invalid, encoded in the format
      specified in [RFC3339].  This field is REQUIRED for objects with
      "valid" in the "status" field.

   challenges (required, array of objects):  For pending authorizations,
      the challenges that the client can fulfill in order to prove
      possession of the identifier.  For valid authorizations, the
      challenge that was validated.  For invalid authorizations, the
      challenge that was attempted and failed.  Each array entry is an
      object with parameters required to validate the challenge.  A
      client should attempt to fulfill one of these challenges, and a
      server should consider any one of the challenges sufficient to
      make the authorization valid.

   wildcard (optional, boolean):  This field MUST be present and true
      for authorizations created as a result of a newOrder request
      containing a DNS identifier with a value that was a wildcard
      domain name.  For other authorizations, it MUST be absent.
      Wildcard domain names are described in Section 7.1.3.

   The only type of identifier defined by this specification is a fully
   qualified domain name (type: "dns").  The domain name MUST be encoded
   in the form in which it would appear in a certificate.  That is, it
   MUST be encoded according to the rules in Section 7 of [RFC5280].
   Servers MUST verify any identifier values that begin with the ASCII-
   Compatible Encoding prefix "xn--" as defined in [RFC5890] are
   properly encoded.  Wildcard domain names (with "*" as the first
   label) MUST NOT be included in authorization objects.  If an
   authorization object conveys authorization for the base domain of a
   newOrder DNS identifier containing a wildcard domain name, then the
   optional authorizations "wildcard" field MUST be present with a value
   of true.




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   Section 8 describes a set of challenges for domain name validation.

   {
     "status": "valid",
     "expires": "2015-03-01T14:09:07.99Z",

     "identifier": {
       "type": "dns",
       "value": "www.example.org"
     },

     "challenges": [
       {
         "url": "https://example.com/acme/chall/prV_B7yEyA4",
         "type": "http-01",
         "status": "valid",
         "token": "DGyRejmCefe7v4NfDGDKfA",
         "validated": "2014-12-01T12:05:58.16Z"
       }
     ],

     "wildcard": false
   }

7.1.5.  Challenge Objects

   An ACME challenge object represents a server's offer to validate a
   client's possession of an identifier in a specific way.  Unlike the
   other objects listed above, there is not a single standard structure
   for a challenge object.  The contents of a challenge object depend on
   the validation method being used.  The general structure of challenge
   objects and an initial set of validation methods are described in
   Section 8.

7.1.6.  Status Changes

   Each ACME object type goes through a simple state machine over its
   lifetime.  The "status" field of the object indicates which state the
   object is currently in.

   Challenge objects are created in the "pending" state.  They
   transition to the "processing" state when the client responds to the
   challenge (see Section 7.5.1) and the server begins attempting to
   validate that the client has completed the challenge.  Note that
   within the "processing" state, the server may attempt to validate the
   challenge multiple times (see Section 8.2).  Likewise, client





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   requests for retries do not cause a state change.  If validation is
   successful, the challenge moves to the "valid" state; if there is an
   error, the challenge moves to the "invalid" state.

            pending
               |
               | Receive
               | response
               V
           processing <-+
               |   |    | Server retry or
               |   |    | client retry request
               |   +----+
               |
               |
   Successful  |   Failed
   validation  |   validation
     +---------+---------+
     |                   |
     V                   V
   valid              invalid

                  State Transitions for Challenge Objects

   Authorization objects are created in the "pending" state.  If one of
   the challenges listed in the authorization transitions to the "valid"
   state, then the authorization also changes to the "valid" state.  If
   the client attempts to fulfill a challenge and fails, or if there is
   an error while the authorization is still pending, then the
   authorization transitions to the "invalid" state.  Once the
   authorization is in the "valid" state, it can expire ("expired"), be
   deactivated by the client ("deactivated", see Section 7.5.2), or
   revoked by the server ("revoked").


















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                      pending --------------------+
                         |                        |
       Challenge failure |                        |
              or         |                        |
             Error       |  Challenge valid       |
               +---------+---------+              |
               |                   |              |
               V                   V              |
            invalid              valid            |
                                   |              |
                                   |              |
                                   |              |
                    +--------------+--------------+
                    |              |              |
                    |              |              |
             Server |       Client |   Time after |
             revoke |   deactivate |    "expires" |
                    V              V              V
                 revoked      deactivated      expired

                State Transitions for Authorization Objects

   Order objects are created in the "pending" state.  Once all of the
   authorizations listed in the order object are in the "valid" state,
   the order transitions to the "ready" state.  The order moves to the
   "processing" state after the client submits a request to the order's
   "finalize" URL and the CA begins the issuance process for the
   certificate.  Once the certificate is issued, the order enters the
   "valid" state.  If an error occurs at any of these stages, the order
   moves to the "invalid" state.  The order also moves to the "invalid"
   state if it expires or one of its authorizations enters a final state
   other than "valid" ("expired", "revoked", or "deactivated").



















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    pending --------------+
       |                  |
       | All authz        |
       | "valid"          |
       V                  |
     ready ---------------+
       |                  |
       | Receive          |
       | finalize         |
       | request          |
       V                  |
   processing ------------+
       |                  |
       | Certificate      | Error or
       | issued           | Authorization failure
       V                  V
     valid             invalid

                    State Transitions for Order Objects

   Account objects are created in the "valid" state, since no further
   action is required to create an account after a successful newAccount
   request.  If the account is deactivated by the client or revoked by
   the server, it moves to the corresponding state.

                     valid
                       |
                       |
           +-----------+-----------+
    Client |                Server |
   deactiv.|                revoke |
           V                       V
      deactivated               revoked

                   State Transitions for Account Objects

   Note that some of these states may not ever appear in a "status"
   field, depending on server behavior.  For example, a server that
   issues synchronously will never show an order in the "processing"
   state.  A server that deletes expired authorizations immediately will
   never show an authorization in the "expired" state.










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7.2.  Getting a Nonce

   Before sending a POST request to the server, an ACME client needs to
   have a fresh anti-replay nonce to put in the "nonce" header of the
   JWS.  In most cases, the client will have gotten a nonce from a
   previous request.  However, the client might sometimes need to get a
   new nonce, e.g., on its first request to the server or if an existing
   nonce is no longer valid.

   To get a fresh nonce, the client sends a HEAD request to the newNonce
   resource on the server.  The server's response MUST include a Replay-
   Nonce header field containing a fresh nonce and SHOULD have status
   code 200 (OK).  The server MUST also respond to GET requests for this
   resource, returning an empty body (while still providing a Replay-
   Nonce header) with a status code of 204 (No Content).

   HEAD /acme/new-nonce HTTP/1.1
   Host: example.com

   HTTP/1.1 200 OK
   Replay-Nonce: oFvnlFP1wIhRlYS2jTaXbA
   Cache-Control: no-store
   Link: <https://example.com/acme/directory>;rel="index"

   Proxy caching of responses from the newNonce resource can cause
   clients to receive the same nonce repeatedly, leading to "badNonce"
   errors.  The server MUST include a Cache-Control header field with
   the "no-store" directive in responses for the newNonce resource, in
   order to prevent caching of this resource.

7.3.  Account Management

   In this section, we describe how an ACME client can create an account
   on an ACME server and perform some modifications to the account after
   it has been created.

   A client creates a new account with the server by sending a POST
   request to the server's newAccount URL.  The body of the request is a
   stub account object containing some subset of the following fields:

   contact (optional, array of string):  Same meaning as the
      corresponding server field defined in Section 7.1.2.

   termsOfServiceAgreed (optional, boolean):  Same meaning as the
      corresponding server field defined in Section 7.1.2.






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   onlyReturnExisting (optional, boolean):  If this field is present
      with the value "true", then the server MUST NOT create a new
      account if one does not already exist.  This allows a client to
      look up an account URL based on an account key (see
      Section 7.3.1).

   externalAccountBinding (optional, object):  Same meaning as the
      corresponding server field defined in Section 7.1.2

   POST /acme/new-account HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "jwk": {...},
       "nonce": "6S8IqOGY7eL2lsGoTZYifg",
       "url": "https://example.com/acme/new-account"
     }),
     "payload": base64url({
       "termsOfServiceAgreed": true,
       "contact": [
         "mailto:cert-admin@example.org",
         "mailto:admin@example.org"
       ]
     }),
     "signature": "RZPOnYoPs1PhjszF...-nh6X1qtOFPB519I"
   }

   The server MUST ignore any values provided in the "orders" fields in
   account objects sent by the client, as well as any other fields that
   it does not recognize.  If new fields are specified in the future,
   the specification of those fields MUST describe whether they can be
   provided by the client.  The server MUST NOT reflect the
   "onlyReturnExisting" field or any unrecognized fields in the
   resulting account object.  This allows clients to detect when servers
   do not support an extension field.

   The server SHOULD validate that the contact URLs in the "contact"
   field are valid and supported by the server.  If the server validates
   contact URLs, it MUST support the "mailto" scheme.  Clients MUST NOT
   provide a "mailto" URL in the "contact" field that contains "hfields"
   [RFC6068] or more than one "addr-spec" in the "to" component.  If a
   server encounters a "mailto" contact URL that does not meet these
   criteria, then it SHOULD reject it as invalid.





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   If the server rejects a contact URL for using an unsupported scheme,
   it MUST return an error of type "unsupportedContact", with a
   description of the error and what types of contact URLs the server
   considers acceptable.  If the server rejects a contact URL for using
   a supported scheme but an invalid value, then the server MUST return
   an error of type "invalidContact".

   If the server wishes to require the client to agree to terms under
   which the ACME service is to be used, it MUST indicate the URL where
   such terms can be accessed in the "termsOfService" subfield of the
   "meta" field in the directory object, and the server MUST reject
   newAccount requests that do not have the "termsOfServiceAgreed" field
   set to "true".  Clients SHOULD NOT automatically agree to terms by
   default.  Rather, they SHOULD require some user interaction for
   agreement to terms.

   The server creates an account and stores the public key used to
   verify the JWS (i.e., the "jwk" element of the JWS header) to
   authenticate future requests from the account.  The server returns
   this account object in a 201 (Created) response, with the account URL
   in a Location header field.  The account URL is used as the "kid"
   value in the JWS authenticating subsequent requests by this account
   (see Section 6.2).  The account URL is also used for requests for
   management actions on this account, as described below.

   HTTP/1.1 201 Created
   Content-Type: application/json
   Replay-Nonce: D8s4D2mLs8Vn-goWuPQeKA
   Link: <https://example.com/acme/directory>;rel="index"
   Location: https://example.com/acme/acct/evOfKhNU60wg

   {
     "status": "valid",

     "contact": [
       "mailto:cert-admin@example.org",
       "mailto:admin@example.org"
     ],

     "orders": "https://example.com/acme/acct/evOfKhNU60wg/orders"
   }

7.3.1.  Finding an Account URL Given a Key

   If the server receives a newAccount request signed with a key for
   which it already has an account registered with the provided account
   key, then it MUST return a response with status code 200 (OK) and
   provide the URL of that account in the Location header field.  The



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   body of this response represents the account object as it existed on
   the server before this request; any fields in the request object MUST
   be ignored.  This allows a client that has an account key but not the
   corresponding account URL to recover the account URL.

   If a client wishes to find the URL for an existing account and does
   not want an account to be created if one does not already exist, then
   it SHOULD do so by sending a POST request to the newAccount URL with
   a JWS whose payload has an "onlyReturnExisting" field set to "true"
   ({"onlyReturnExisting": true}).  If a client sends such a request and
   an account does not exist, then the server MUST return an error
   response with status code 400 (Bad Request) and type
   "urn:ietf:params:acme:error:accountDoesNotExist".

7.3.2.  Account Update

   If the client wishes to update this information in the future, it
   sends a POST request with updated information to the account URL.
   The server MUST ignore any updates to the "orders" field,
   "termsOfServiceAgreed" field (see Section 7.3.3), the "status" field
   (except as allowed by Section 7.3.6), or any other fields it does not
   recognize.  If the server accepts the update, it MUST return a
   response with a 200 (OK) status code and the resulting account
   object.

   For example, to update the contact information in the above account,
   the client could send the following request:

   POST /acme/acct/evOfKhNU60wg HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "ax5RnthDqp_Yf4_HZnFLmA",
       "url": "https://example.com/acme/acct/evOfKhNU60wg"
     }),
     "payload": base64url({
       "contact": [
         "mailto:certificates@example.org",
         "mailto:admin@example.org"
       ]
     }),
     "signature": "hDXzvcj8T6fbFbmn...rDzXzzvzpRy64N0o"
   }




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7.3.3.  Changes of Terms of Service

   As described above, a client can indicate its agreement with the CA's
   terms of service by setting the "termsOfServiceAgreed" field in its
   account object to "true".

   If the server has changed its terms of service since a client
   initially agreed, and the server is unwilling to process a request
   without explicit agreement to the new terms, then it MUST return an
   error response with status code 403 (Forbidden) and type
   "urn:ietf:params:acme:error:userActionRequired".  This response MUST
   include a Link header field with link relation "terms-of-service" and
   the latest terms-of-service URL.

   The problem document returned with the error MUST also include an
   "instance" field, indicating a URL that the client should direct a
   human user to visit in order for instructions on how to agree to the
   terms.

 HTTP/1.1 403 Forbidden
 Replay-Nonce: T81bdZroZ2ITWSondpTmAw
 Link: <https://example.com/acme/directory>;rel="index"
 Link: <https://example.com/acme/terms/2017-6-02>;rel="terms-of-service"
 Content-Type: application/problem+json
 Content-Language: en

 {
   "type": "urn:ietf:params:acme:error:userActionRequired",
   "detail": "Terms of service have changed",
   "instance": "https://example.com/acme/agreement/?token=W8Ih3PswD-8"
 }

7.3.4.  External Account Binding

   The server MAY require a value for the "externalAccountBinding" field
   to be present in "newAccount" requests.  This can be used to
   associate an ACME account with an existing account in a non-ACME
   system, such as a CA customer database.

   To enable ACME account binding, the CA operating the ACME server
   needs to provide the ACME client with a MAC key and a key identifier,
   using some mechanism outside of ACME.  The key identifier MUST be an
   ASCII string.  The MAC key SHOULD be provided in base64url-encoded
   form, to maximize compatibility between non-ACME provisioning systems
   and ACME clients.






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   The ACME client then computes a binding JWS to indicate the external
   account holder's approval of the ACME account key.  The payload of
   this JWS is the ACME account key being registered, in JWK form.  The
   protected header of the JWS MUST meet the following criteria:

   o  The "alg" field MUST indicate a MAC-based algorithm

   o  The "kid" field MUST contain the key identifier provided by the CA

   o  The "nonce" field MUST NOT be present

   o  The "url" field MUST be set to the same value as the outer JWS

   The "signature" field of the JWS will contain the MAC value computed
   with the MAC key provided by the CA.

   POST /acme/new-account HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "jwk": /* account key */,
       "nonce": "K60BWPrMQG9SDxBDS_xtSw",
       "url": "https://example.com/acme/new-account"
     }),
     "payload": base64url({
       "contact": [
         "mailto:cert-admin@example.org",
         "mailto:admin@example.org"
       ],
       "termsOfServiceAgreed": true,

       "externalAccountBinding": {
         "protected": base64url({
           "alg": "HS256",
           "kid": /* key identifier from CA */,
           "url": "https://example.com/acme/new-account"
         }),
         "payload": base64url(/* same as in "jwk" above */),
         "signature": /* MAC using MAC key from CA */
       }
     }),
     "signature": "5TWiqIYQfIDfALQv...x9C2mg8JGPxl5bI4"
   }





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   If such a CA requires that newAccount requests contain an
   "externalAccountBinding" field, then it MUST provide the value "true"
   in the "externalAccountRequired" subfield of the "meta" field in the
   directory object.  If the CA receives a newAccount request without an
   "externalAccountBinding" field, then it SHOULD reply with an error of
   type "externalAccountRequired".

   When a CA receives a newAccount request containing an
   "externalAccountBinding" field, it decides whether or not to verify
   the binding.  If the CA does not verify the binding, then it MUST NOT
   reflect the "externalAccountBinding" field in the resulting account
   object (if any).  To verify the account binding, the CA MUST take the
   following steps:

   1.  Verify that the value of the field is a well-formed JWS

   2.  Verify that the JWS protected field meets the above criteria

   3.  Retrieve the MAC key corresponding to the key identifier in the
       "kid" field

   4.  Verify that the MAC on the JWS verifies using that MAC key

   5.  Verify that the payload of the JWS represents the same key as was
       used to verify the outer JWS (i.e., the "jwk" field of the outer
       JWS)

   If all of these checks pass and the CA creates a new account, then
   the CA may consider the new account associated with the external
   account corresponding to the MAC key.  The account object the CA
   returns MUST include an "externalAccountBinding" field with the same
   value as the field in the request.  If any of these checks fail, then
   the CA MUST reject the newAccount request.

7.3.5.  Account Key Rollover

   A client may wish to change the public key that is associated with an
   account in order to recover from a key compromise or proactively
   mitigate the impact of an unnoticed key compromise.

   To change the key associated with an account, the client sends a
   request to the server containing signatures by both the old and new
   keys.  The signature by the new key covers the account URL and the
   old key, signifying a request by the new key holder to take over the
   account from the old key holder.  The signature by the old key covers
   this request and its signature, and indicates the old key holder's
   assent to the rollover request.




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   To create this request object, the client first constructs a
   keyChange object describing the account to be updated and its account
   key:

   account (required, string):  The URL for the account being modified.
      The content of this field MUST be the exact string provided in the
      Location header field in response to the newAccount request that
      created the account.

   oldKey (required, JWK):  The JWK representation of the old key.

   The client then encapsulates the keyChange object in an "inner" JWS,
   signed with the requested new account key.  This "inner" JWS becomes
   the payload for the "outer" JWS that is the body of the ACME request.

   The outer JWS MUST meet the normal requirements for an ACME JWS
   request body (see Section 6.2).  The inner JWS MUST meet the normal
   requirements, with the following differences:

   o  The inner JWS MUST have a "jwk" header parameter, containing the
      public key of the new key pair.

   o  The inner JWS MUST have the same "url" header parameter as the
      outer JWS.

   o  The inner JWS MUST omit the "nonce" header parameter.

   This transaction has signatures from both the old and new keys so
   that the server can verify that the holders of the two keys both
   agree to the change.  The signatures are nested to preserve the
   property that all signatures on POST messages are signed by exactly
   one key.  The "inner" JWS effectively represents a request by the
   holder of the new key to take over the account form the holder of the
   old key.  The "outer" JWS represents the current account holder's
   assent to this request.
















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   POST /acme/key-change HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "S9XaOcxP5McpnTcWPIhYuB",
       "url": "https://example.com/acme/key-change"
     }),
     "payload": base64url({
       "protected": base64url({
         "alg": "ES256",
         "jwk": /* new key */,
         "url": "https://example.com/acme/key-change"
       }),
       "payload": base64url({
         "account": "https://example.com/acme/acct/evOfKhNU60wg",
         "oldKey": /* old key */
       }),
       "signature": "Xe8B94RD30Azj2ea...8BmZIRtcSKPSd8gU"
     }),
     "signature": "5TWiqIYQfIDfALQv...x9C2mg8JGPxl5bI4"
   }

   On receiving a keyChange request, the server MUST perform the
   following steps in addition to the typical JWS validation:

   1.  Validate the POST request belongs to a currently active account,
       as described in Section 6.

   2.  Check that the payload of the JWS is a well-formed JWS object
       (the "inner JWS").

   3.  Check that the JWS protected header of the inner JWS has a "jwk"
       field.

   4.  Check that the inner JWS verifies using the key in its "jwk"
       field.

   5.  Check that the payload of the inner JWS is a well-formed
       keyChange object (as described above).

   6.  Check that the "url" parameters of the inner and outer JWSs are
       the same.





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   7.  Check that the "account" field of the keyChange object contains
       the URL for the account matching the old key (i.e., the "kid"
       field in the outer JWS).

   8.  Check that the "oldKey" field of the keyChange object is the same
       as the account key for the account in question.

   9.  Check that no account exists whose account key is the same as the
       key in the "jwk" header parameter of the inner JWS.

   If all of these checks pass, then the server updates the
   corresponding account by replacing the old account key with the new
   public key and returns status code 200 (OK).  Otherwise, the server
   responds with an error status code and a problem document describing
   the error.  If there is an existing account with the new key
   provided, then the server SHOULD use status code 409 (Conflict) and
   provide the URL of that account in the Location header field.

   Note that changing the account key for an account SHOULD NOT have any
   other impact on the account.  For example, the server MUST NOT
   invalidate pending orders or authorization transactions based on a
   change of account key.

7.3.6.  Account Deactivation

   A client can deactivate an account by posting a signed update to the
   account URL with a status field of "deactivated".  Clients may wish
   to do this when the account key is compromised or decommissioned.  A
   deactivated account can no longer request certificate issuance or
   access resources related to the account, such as orders or
   authorizations.  If a server receives a POST or POST-as-GET from a
   deactivated account, it MUST return an error response with status
   code 401 (Unauthorized) and type
   "urn:ietf:params:acme:error:unauthorized".

















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   POST /acme/acct/evOfKhNU60wg HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "ntuJWWSic4WVNSqeUmshgg",
       "url": "https://example.com/acme/acct/evOfKhNU60wg"
     }),
     "payload": base64url({
       "status": "deactivated"
     }),
     "signature": "earzVLd3m5M4xJzR...bVTqn7R08AKOVf3Y"
   }

   The server MUST verify that the request is signed by the account key.
   If the server accepts the deactivation request, it replies with a 200
   (OK) status code and the current contents of the account object.

   Once an account is deactivated, the server MUST NOT accept further
   requests authorized by that account's key.  The server SHOULD cancel
   any pending operations authorized by the account's key, such as
   certificate orders.  A server may take a variety of actions in
   response to an account deactivation, e.g., deleting data related to
   that account or sending mail to the account's contacts.  Servers
   SHOULD NOT revoke certificates issued by the deactivated account,
   since this could cause operational disruption for servers using these
   certificates.  ACME does not provide a way to reactivate a
   deactivated account.

7.4.  Applying for Certificate Issuance

   The client begins the certificate issuance process by sending a POST
   request to the server's newOrder resource.  The body of the POST is a
   JWS object whose JSON payload is a subset of the order object defined
   in Section 7.1.3, containing the fields that describe the certificate
   to be issued:

   identifiers (required, array of object):  An array of identifier
      objects that the client wishes to submit an order for.

      type (required, string):  The type of identifier.

      value (required, string):  The identifier itself.





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   notBefore (optional, string):  The requested value of the notBefore
      field in the certificate, in the date format defined in [RFC3339].

   notAfter (optional, string):  The requested value of the notAfter
      field in the certificate, in the date format defined in [RFC3339].

   POST /acme/new-order HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "5XJ1L3lEkMG7tR6pA00clA",
       "url": "https://example.com/acme/new-order"
     }),
     "payload": base64url({
       "identifiers": [
         { "type": "dns", "value": "www.example.org" },
         { "type": "dns", "value": "example.org" }
       ],
       "notBefore": "2016-01-01T00:04:00+04:00",
       "notAfter": "2016-01-08T00:04:00+04:00"
     }),
     "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
   }

   The server MUST return an error if it cannot fulfill the request as
   specified, and it MUST NOT issue a certificate with contents other
   than those requested.  If the server requires the request to be
   modified in a certain way, it should indicate the required changes
   using an appropriate error type and description.

   If the server is willing to issue the requested certificate, it
   responds with a 201 (Created) response.  The body of this response is
   an order object reflecting the client's request and any
   authorizations the client must complete before the certificate will
   be issued.












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   HTTP/1.1 201 Created
   Replay-Nonce: MYAuvOpaoIiywTezizk5vw
   Link: <https://example.com/acme/directory>;rel="index"
   Location: https://example.com/acme/order/TOlocE8rfgo

   {
     "status": "pending",
     "expires": "2016-01-05T14:09:07.99Z",

     "notBefore": "2016-01-01T00:00:00Z",
     "notAfter": "2016-01-08T00:00:00Z",

     "identifiers": [
       { "type": "dns", "value": "www.example.org" },
       { "type": "dns", "value": "example.org" }
     ],

     "authorizations": [
       "https://example.com/acme/authz/PAniVnsZcis",
       "https://example.com/acme/authz/r4HqLzrSrpI"
     ],

     "finalize": "https://example.com/acme/order/TOlocE8rfgo/finalize"
   }

   The order object returned by the server represents a promise that if
   the client fulfills the server's requirements before the "expires"
   time, then the server will be willing to finalize the order upon
   request and issue the requested certificate.  In the order object,
   any authorization referenced in the "authorizations" array whose
   status is "pending" represents an authorization transaction that the
   client must complete before the server will issue the certificate
   (see Section 7.5).  If the client fails to complete the required
   actions before the "expires" time, then the server SHOULD change the
   status of the order to "invalid" and MAY delete the order resource.
   Clients MUST NOT make any assumptions about the sort order of
   "identifiers" or "authorizations" elements in the returned order
   object.













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   Once the client believes it has fulfilled the server's requirements,
   it should send a POST request to the order resource's finalize URL.
   The POST body MUST include a CSR:

   csr (required, string):  A CSR encoding the parameters for the
      certificate being requested [RFC2986].  The CSR is sent in the
      base64url-encoded version of the DER format.  (Note: Because this
      field uses base64url, and does not include headers, it is
      different from PEM.)

   POST /acme/order/TOlocE8rfgo/finalize HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "MSF2j2nawWHPxxkE3ZJtKQ",
       "url": "https://example.com/acme/order/TOlocE8rfgo/finalize"
     }),
     "payload": base64url({
       "csr": "MIIBPTCBxAIBADBFMQ...FS6aKdZeGsysoCo4H9P",
     }),
     "signature": "uOrUfIIk5RyQ...nw62Ay1cl6AB"
   }

   The CSR encodes the client's requests with regard to the content of
   the certificate to be issued.  The CSR MUST indicate the exact same
   set of requested identifiers as the initial newOrder request.
   Identifiers of type "dns" MUST appear either in the commonName
   portion of the requested subject name or in an extensionRequest
   attribute [RFC2985] requesting a subjectAltName extension, or both.
   (These identifiers may appear in any sort order.)  Specifications
   that define new identifier types must specify where in the
   certificate signing request these identifiers can appear.

   A request to finalize an order will result in an error if the CA is
   unwilling to issue a certificate corresponding to the submitted CSR.
   For example:

   o  If the CSR and order identifiers differ

   o  If the account is not authorized for the identifiers indicated in
      the CSR

   o  If the CSR requests extensions that the CA is not willing to
      include



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   In such cases, the problem document returned by the server SHOULD use
   error code "badCSR" and describe specific reasons the CSR was
   rejected in its "detail" field.  After returning such an error, the
   server SHOULD leave the order in the "ready" state, to allow the
   client to submit a new finalize request with an amended CSR.

   A request to finalize an order will result in error if the order is
   not in the "ready" state.  In such cases, the server MUST return a
   403 (Forbidden) error with a problem document of type
   "orderNotReady".  The client should then send a POST-as-GET request
   to the order resource to obtain its current state.  The status of the
   order will indicate what action the client should take (see below).

   If a request to finalize an order is successful, the server will
   return a 200 (OK) with an updated order object.  The status of the
   order will indicate what action the client should take:

   o  "invalid": The certificate will not be issued.  Consider this
      order process abandoned.

   o  "pending": The server does not believe that the client has
      fulfilled the requirements.  Check the "authorizations" array for
      entries that are still pending.

   o  "ready": The server agrees that the requirements have been
      fulfilled, and is awaiting finalization.  Submit a finalization
      request.

   o  "processing": The certificate is being issued.  Send a POST-as-GET
      request after the time given in the Retry-After header field of
      the response, if any.

   o  "valid": The server has issued the certificate and provisioned its
      URL to the "certificate" field of the order.  Download the
      certificate.
















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   HTTP/1.1 200 OK
   Replay-Nonce: CGf81JWBsq8QyIgPCi9Q9X
   Link: <https://example.com/acme/directory>;rel="index"
   Location: https://example.com/acme/order/TOlocE8rfgo

   {
     "status": "valid",
     "expires": "2016-01-20T14:09:07.99Z",

     "notBefore": "2016-01-01T00:00:00Z",
     "notAfter": "2016-01-08T00:00:00Z",

     "identifiers": [
       { "type": "dns", "value": "www.example.org" },
       { "type": "dns", "value": "example.org" }
     ],

     "authorizations": [
       "https://example.com/acme/authz/PAniVnsZcis",
       "https://example.com/acme/authz/r4HqLzrSrpI"
     ],

     "finalize": "https://example.com/acme/order/TOlocE8rfgo/finalize",

     "certificate": "https://example.com/acme/cert/mAt3xBGaobw"
   }

7.4.1.  Pre-authorization

   The order process described above presumes that authorization objects
   are created reactively, in response to a certificate order.  Some
   servers may also wish to enable clients to obtain authorization for
   an identifier proactively, outside of the context of a specific
   issuance.  For example, a client hosting virtual servers for a
   collection of names might wish to obtain authorization before any
   virtual servers are created and only create a certificate when a
   virtual server starts up.

   In some cases, a CA running an ACME server might have a completely
   external, non-ACME process for authorizing a client to issue
   certificates for an identifier.  In these cases, the CA should
   provision its ACME server with authorization objects corresponding to
   these authorizations and reflect them as already valid in any orders
   submitted by the client.

   If a CA wishes to allow pre-authorization within ACME, it can offer a
   "new authorization" resource in its directory by adding the field
   "newAuthz" with a URL for the newAuthz resource.



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   To request authorization for an identifier, the client sends a POST
   request to the newAuthz resource specifying the identifier for which
   authorization is being requested.

   identifier (required, object):  The identifier to appear in the
      resulting authorization object (see Section 7.1.4).

      type (required, string):  The type of identifier.

      value (required, string):  The identifier itself.

   POST /acme/new-authz HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "uQpSjlRb4vQVCjVYAyyUWg",
       "url": "https://example.com/acme/new-authz"
     }),
     "payload": base64url({
       "identifier": {
         "type": "dns",
         "value": "example.org"
       }
     }),
     "signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps"
   }

   Note that because the identifier in a pre-authorization request is
   the exact identifier to be included in the authorization object, pre-
   authorization cannot be used to authorize issuance of certificates
   containing wildcard domain names.

   Before processing the authorization request, the server SHOULD
   determine whether it is willing to issue certificates for the
   identifier.  For example, the server should check that the identifier
   is of a supported type.  Servers might also check names against a
   blacklist of known high-value identifiers.  If the server is
   unwilling to issue for the identifier, it SHOULD return an error with
   status code 403 (Forbidden), with a problem document describing the
   reason for the rejection.







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   If the server is willing to proceed, it builds a pending
   authorization object from the inputs submitted by the client:

   o  "identifier" the identifier submitted by the client

   o  "status" MUST be "pending" unless the server has out-of-band
      information about the client's authorization status

   o  "challenges" as selected by the server's policy for this
      identifier

   The server allocates a new URL for this authorization and returns a
   201 (Created) response with the authorization URL in the Location
   header field and the JSON authorization object in the body.  The
   client then follows the process described in Section 7.5 to complete
   the authorization process.

7.4.2.  Downloading the Certificate

   To download the issued certificate, the client simply sends a POST-
   as-GET request to the certificate URL.

   The default format of the certificate is application/pem-certificate-
   chain (see Section 9).

   The server MAY provide one or more link relation header fields
   [RFC8288] with relation "alternate".  Each such field SHOULD express
   an alternative certificate chain starting with the same end-entity
   certificate.  This can be used to express paths to various trust
   anchors.  Clients can fetch these alternates and use their own
   heuristics to decide which is optimal.




















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   POST /acme/cert/mAt3xBGaobw HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json
   Accept: application/pem-certificate-chain

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "uQpSjlRb4vQVCjVYAyyUWg",
       "url": "https://example.com/acme/cert/mAt3xBGaobw"
     }),
     "payload": "",
     "signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps"
   }

   HTTP/1.1 200 OK
   Content-Type: application/pem-certificate-chain
   Link: <https://example.com/acme/directory>;rel="index"

   -----BEGIN CERTIFICATE-----
   [End-entity certificate contents]
   -----END CERTIFICATE-----
   -----BEGIN CERTIFICATE-----
   [Issuer certificate contents]
   -----END CERTIFICATE-----
   -----BEGIN CERTIFICATE-----
   [Other certificate contents]
   -----END CERTIFICATE-----

   A certificate resource represents a single, immutable certificate.
   If the client wishes to obtain a renewed certificate, the client
   initiates a new order process to request one.

   Because certificate resources are immutable once issuance is
   complete, the server MAY enable the caching of the resource by adding
   Expires and Cache-Control header fields specifying a point in time in
   the distant future.  These header fields have no relation to the
   certificate's period of validity.

   The ACME client MAY request other formats by including an Accept
   header field [RFC7231] in its request.  For example, the client could
   use the media type "application/pkix-cert" [RFC2585] or "application/
   pkcs7-mime" [RFC5751] to request the end-entity certificate in DER
   format.  Server support for alternate formats is OPTIONAL.  For
   formats that can only express a single certificate, the server SHOULD





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   provide one or more "Link: rel="up"" header fields pointing to an
   issuer or issuers so that ACME clients can build a certificate chain
   as defined in TLS (see Section 4.4.2 of [RFC8446]).

7.5.  Identifier Authorization

   The identifier authorization process establishes the authorization of
   an account to manage certificates for a given identifier.  This
   process assures the server of two things:

   1.  That the client controls the private key of the account key pair,
       and

   2.  That the client controls the identifier in question.

   This process may be repeated to associate multiple identifiers with
   an account (e.g., to request certificates with multiple identifiers)
   or to associate multiple accounts with an identifier (e.g., to allow
   multiple entities to manage certificates).

   Authorization resources are created by the server in response to
   newOrder or newAuthz requests submitted by an account key holder;
   their URLs are provided to the client in the responses to these
   requests.  The authorization object is implicitly tied to the account
   key used to sign the request.

   When a client receives an order from the server in reply to a
   newOrder request, it downloads the authorization resources by sending
   POST-as-GET requests to the indicated URLs.  If the client initiates
   authorization using a request to the newAuthz resource, it will have
   already received the pending authorization object in the response to
   that request.



















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   POST /acme/authz/PAniVnsZcis HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "uQpSjlRb4vQVCjVYAyyUWg",
       "url": "https://example.com/acme/authz/PAniVnsZcis"
     }),
     "payload": "",
     "signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps"
   }

   HTTP/1.1 200 OK
   Content-Type: application/json
   Link: <https://example.com/acme/directory>;rel="index"

   {
     "status": "pending",
     "expires": "2016-01-02T14:09:30Z",

     "identifier": {
       "type": "dns",
       "value": "www.example.org"
     },

     "challenges": [
       {
         "type": "http-01",
         "url": "https://example.com/acme/chall/prV_B7yEyA4",
         "token": "DGyRejmCefe7v4NfDGDKfA"
       },
       {
         "type": "dns-01",
         "url": "https://example.com/acme/chall/Rg5dV14Gh1Q",
         "token": "DGyRejmCefe7v4NfDGDKfA"
       }
     ]
   }

7.5.1.  Responding to Challenges

   To prove control of the identifier and receive authorization, the
   client needs to provision the required challenge response based on
   the challenge type and indicate to the server that it is ready for
   the challenge validation to be attempted.



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   The client indicates to the server that it is ready for the challenge
   validation by sending an empty JSON body ("{}") carried in a POST
   request to the challenge URL (not the authorization URL).

   For example, if the client were to respond to the "http-01" challenge
   in the above authorization, it would send the following request:

   POST /acme/chall/prV_B7yEyA4 HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "Q_s3MWoqT05TrdkM2MTDcw",
       "url": "https://example.com/acme/chall/prV_B7yEyA4"
     }),
     "payload": base64url({}),
     "signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ"
   }

   The server updates the authorization document by updating its
   representation of the challenge with the response object provided by
   the client.  The server MUST ignore any fields in the response object
   that are not specified as response fields for this type of challenge.
   Note that the challenges in this document do not define any response
   fields, but future specifications might define them.  The server
   provides a 200 (OK) response with the updated challenge object as its
   body.

   If the client's response is invalid for any reason or does not
   provide the server with appropriate information to validate the
   challenge, then the server MUST return an HTTP error.  On receiving
   such an error, the client SHOULD undo any actions that have been
   taken to fulfill the challenge, e.g., removing files that have been
   provisioned to a web server.

   The server is said to "finalize" the authorization when it has
   completed one of the validations.  This is done by assigning the
   authorization a status of "valid" or "invalid", corresponding to
   whether it considers the account authorized for the identifier.  If
   the final state is "valid", then the server MUST include an "expires"
   field.  When finalizing an authorization, the server MAY remove
   challenges other than the one that was completed, and it may modify
   the "expires" field.  The server SHOULD NOT remove challenges with
   status "invalid".




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   Usually, the validation process will take some time, so the client
   will need to poll the authorization resource to see when it is
   finalized.  For challenges where the client can tell when the server
   has validated the challenge (e.g., by seeing an HTTP or DNS request
   from the server), the client SHOULD NOT begin polling until it has
   seen the validation request from the server.

   To check on the status of an authorization, the client sends a POST-
   as-GET request to the authorization URL, and the server responds with
   the current authorization object.  In responding to poll requests
   while the validation is still in progress, the server MUST return a
   200 (OK) response and MAY include a Retry-After header field to
   suggest a polling interval to the client.






































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   POST /acme/authz/PAniVnsZcis HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "uQpSjlRb4vQVCjVYAyyUWg",
       "url": "https://example.com/acme/authz/PAniVnsZcis"
     }),
     "payload": "",
     "signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps"
   }

   HTTP/1.1 200 OK
   Content-Type: application/json
   Link: <https://example.com/acme/directory>;rel="index"

   {
     "status": "valid",
     "expires": "2018-09-09T14:09:01.13Z",

     "identifier": {
       "type": "dns",
       "value": "www.example.org"
     },

     "challenges": [
       {
         "type": "http-01",
         "url": "https://example.com/acme/chall/prV_B7yEyA4",
         "status": "valid",
         "validated": "2014-12-01T12:05:13.72Z",
         "token": "IlirfxKKXAsHtmzK29Pj8A"
       }
     ]
   }

7.5.2.  Deactivating an Authorization

   If a client wishes to relinquish its authorization to issue
   certificates for an identifier, then it may request that the server
   deactivate each authorization associated with it by sending POST
   requests with the static object {"status": "deactivated"} to each
   authorization URL.





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   POST /acme/authz/PAniVnsZcis HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "xWCM9lGbIyCgue8di6ueWQ",
       "url": "https://example.com/acme/authz/PAniVnsZcis"
     }),
     "payload": base64url({
       "status": "deactivated"
     }),
     "signature": "srX9Ji7Le9bjszhu...WTFdtujObzMtZcx4"
   }

   The server MUST verify that the request is signed by the account key
   corresponding to the account that owns the authorization.  If the
   server accepts the deactivation, it should reply with a 200 (OK)
   status code and the updated contents of the authorization object.

   The server MUST NOT treat deactivated authorization objects as
   sufficient for issuing certificates.

7.6.  Certificate Revocation

   To request that a certificate be revoked, the client sends a POST
   request to the ACME server's revokeCert URL.  The body of the POST is
   a JWS object whose JSON payload contains the certificate to be
   revoked:

   certificate (required, string):  The certificate to be revoked, in
      the base64url-encoded version of the DER format.  (Note: Because
      this field uses base64url, and does not include headers, it is
      different from PEM.)

   reason (optional, int):  One of the revocation reasonCodes defined in
      Section 5.3.1 of [RFC5280] to be used when generating OCSP
      responses and CRLs.  If this field is not set, the server SHOULD
      omit the reasonCode CRL entry extension when generating OCSP
      responses and CRLs.  The server MAY disallow a subset of
      reasonCodes from being used by the user.  If a request contains a
      disallowed reasonCode, then the server MUST reject it with the
      error type "urn:ietf:params:acme:error:badRevocationReason".  The
      problem document detail SHOULD indicate which reasonCodes are
      allowed.




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   Revocation requests are different from other ACME requests in that
   they can be signed with either an account key pair or the key pair in
   the certificate.

   Example using an account key pair for the signature:

   POST /acme/revoke-cert HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "JHb54aT_KTXBWQOzGYkt9A",
       "url": "https://example.com/acme/revoke-cert"
     }),
     "payload": base64url({
       "certificate": "MIIEDTCCAvegAwIBAgIRAP8...",
       "reason": 4
     }),
     "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
   }

   Example using the certificate key pair for the signature:

   POST /acme/revoke-cert HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "RS256",
       "jwk": /* certificate's public key */,
       "nonce": "JHb54aT_KTXBWQOzGYkt9A",
       "url": "https://example.com/acme/revoke-cert"
     }),
     "payload": base64url({
       "certificate": "MIIEDTCCAvegAwIBAgIRAP8...",
       "reason": 1
     }),
     "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
   }








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   Before revoking a certificate, the server MUST verify that the key
   used to sign the request is authorized to revoke the certificate.
   The server MUST consider at least the following accounts authorized
   for a given certificate:

   o  the account that issued the certificate.

   o  an account that holds authorizations for all of the identifiers in
      the certificate.

   The server MUST also consider a revocation request valid if it is
   signed with the private key corresponding to the public key in the
   certificate.

   If the revocation succeeds, the server responds with status code 200
   (OK).  If the revocation fails, the server returns an error.  For
   example, if the certificate has already been revoked, the server
   returns an error response with status code 400 (Bad Request) and type
   "urn:ietf:params:acme:error:alreadyRevoked".

   HTTP/1.1 200 OK
   Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw
   Content-Length: 0
   Link: <https://example.com/acme/directory>;rel="index"

   --- or ---

   HTTP/1.1 403 Forbidden
   Replay-Nonce: lXfyFzi6238tfPQRwgfmPU
   Content-Type: application/problem+json
   Content-Language: en
   Link: <https://example.com/acme/directory>;rel="index"

   {
     "type": "urn:ietf:params:acme:error:unauthorized",
     "detail": "No authorization provided for name example.org"
   }

8.  Identifier Validation Challenges

   There are few types of identifiers in the world for which there is a
   standardized mechanism to prove possession of a given identifier.  In
   all practical cases, CAs rely on a variety of means to test whether
   an entity applying for a certificate with a given identifier actually
   controls that identifier.






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   Challenges provide the server with assurance that an account holder
   is also the entity that controls an identifier.  For each type of
   challenge, it must be the case that, in order for an entity to
   successfully complete the challenge, the entity must both:

   o  Hold the private key of the account key pair used to respond to
      the challenge, and

   o  Control the identifier in question.

   Section 10 documents how the challenges defined in this document meet
   these requirements.  New challenges will need to document how they
   do.

   ACME uses an extensible challenge/response framework for identifier
   validation.  The server presents a set of challenges in the
   authorization object it sends to a client (as objects in the
   "challenges" array), and the client responds by sending a response
   object in a POST request to a challenge URL.

   This section describes an initial set of challenge types.  The
   definition of a challenge type includes:

   1.  Content of challenge objects

   2.  Content of response objects

   3.  How the server uses the challenge and response to verify control
       of an identifier

   Challenge objects all contain the following basic fields:

   type (required, string):  The type of challenge encoded in the
      object.

   url (required, string):  The URL to which a response can be posted.

   status (required, string):  The status of this challenge.  Possible
      values are "pending", "processing", "valid", and "invalid" (see
      Section 7.1.6).

   validated (optional, string):  The time at which the server validated
      this challenge, encoded in the format specified in [RFC3339].
      This field is REQUIRED if the "status" field is "valid".







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   error (optional, object):  Error that occurred while the server was
      validating the challenge, if any, structured as a problem document
      [RFC7807].  Multiple errors can be indicated by using subproblems
      Section 6.7.1.  A challenge object with an error MUST have status
      equal to "invalid".

   All additional fields are specified by the challenge type.  If the
   server sets a challenge's "status" to "invalid", it SHOULD also
   include the "error" field to help the client diagnose why the
   challenge failed.

   Different challenges allow the server to obtain proof of different
   aspects of control over an identifier.  In some challenges, like HTTP
   and DNS, the client directly proves its ability to do certain things
   related to the identifier.  The choice of which challenges to offer
   to a client under which circumstances is a matter of server policy.

   The identifier validation challenges described in this section all
   relate to validation of domain names.  If ACME is extended in the
   future to support other types of identifiers, there will need to be
   new challenge types, and they will need to specify which types of
   identifier they apply to.

8.1.  Key Authorizations

   All challenges defined in this document make use of a key
   authorization string.  A key authorization is a string that
   concatenates the token for the challenge with a key fingerprint,
   separated by a "." character:

   keyAuthorization = token || '.' || base64url(Thumbprint(accountKey))

   The "Thumbprint" step indicates the computation specified in
   [RFC7638], using the SHA-256 digest [FIPS180-4].  As noted in
   [RFC7518] any prepended zero octets in the fields of a JWK object
   MUST be stripped before doing the computation.

   As specified in the individual challenges below, the token for a
   challenge is a string comprised entirely of characters in the URL-
   safe base64 alphabet.  The "||" operator indicates concatenation of
   strings.










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8.2.  Retrying Challenges

   ACME challenges typically require the client to set up some network-
   accessible resource that the server can query in order to validate
   that the client controls an identifier.  In practice, it is not
   uncommon for the server's queries to fail while a resource is being
   set up, e.g., due to information propagating across a cluster or
   firewall rules not being in place.

   Clients SHOULD NOT respond to challenges until they believe that the
   server's queries will succeed.  If a server's initial validation
   query fails, the server SHOULD retry the query after some time, in
   order to account for delay in setting up responses such as DNS
   records or HTTP resources.  The precise retry schedule is up to the
   server, but server operators should keep in mind the operational
   scenarios that the schedule is trying to accommodate.  Given that
   retries are intended to address things like propagation delays in
   HTTP or DNS provisioning, there should not usually be any reason to
   retry more often than every 5 or 10 seconds.  While the server is
   still trying, the status of the challenge remains "processing"; it is
   only marked "invalid" once the server has given up.

   The server MUST provide information about its retry state to the
   client via the "error" field in the challenge and the Retry-After
   HTTP header field in response to requests to the challenge resource.
   The server MUST add an entry to the "error" field in the challenge
   after each failed validation query.  The server SHOULD set the Retry-
   After header field to a time after the server's next validation
   query, since the status of the challenge will not change until that
   time.

   Clients can explicitly request a retry by re-sending their response
   to a challenge in a new POST request (with a new nonce, etc.).  This
   allows clients to request a retry when the state has changed (e.g.,
   after firewall rules have been updated).  Servers SHOULD retry a
   request immediately on receiving such a POST request.  In order to
   avoid denial-of-service attacks via client-initiated retries, servers
   SHOULD rate-limit such requests.

8.3.  HTTP Challenge

   With HTTP validation, the client in an ACME transaction proves its
   control over a domain name by proving that it can provision HTTP
   resources on a server accessible under that domain name.  The ACME
   server challenges the client to provision a file at a specific path,
   with a specific string as its content.





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   As a domain may resolve to multiple IPv4 and IPv6 addresses, the
   server will connect to at least one of the hosts found in the DNS A
   and AAAA records, at its discretion.  Because many web servers
   allocate a default HTTPS virtual host to a particular low-privilege
   tenant user in a subtle and non-intuitive manner, the challenge must
   be completed over HTTP, not HTTPS.

   type (required, string):  The string "http-01".

   token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy.
      It MUST NOT contain any characters outside the base64url alphabet
      and MUST NOT include base64 padding characters ("=").  See
      [RFC4086] for additional information on randomness requirements.

   {
     "type": "http-01",
     "url": "https://example.com/acme/chall/prV_B7yEyA4",
     "status": "pending",
     "token": "LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowyjxAjEuX0"
   }

   A client fulfills this challenge by constructing a key authorization
   from the "token" value provided in the challenge and the client's
   account key.  The client then provisions the key authorization as a
   resource on the HTTP server for the domain in question.

   The path at which the resource is provisioned is comprised of the
   fixed prefix "/.well-known/acme-challenge/", followed by the "token"
   value in the challenge.  The value of the resource MUST be the ASCII
   representation of the key authorization.

   GET /.well-known/acme-challenge/LoqXcYV8...jxAjEuX0
   Host: example.org

   HTTP/1.1 200 OK
   Content-Type: application/octet-stream

   LoqXcYV8...jxAjEuX0.9jg46WB3...fm21mqTI

   (In the above, "..." indicates that the token and the JWK thumbprint
   in the key authorization have been truncated to fit on the page.)

   A client responds with an empty object ({}) to acknowledge that the
   challenge can be validated by the server.






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   POST /acme/chall/prV_B7yEyA4
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "UQI1PoRi5OuXzxuX7V7wL0",
       "url": "https://example.com/acme/chall/prV_B7yEyA4"
     }),
     "payload": base64url({}),
     "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
   }

   On receiving a response, the server constructs and stores the key
   authorization from the challenge "token" value and the current client
   account key.

   Given a challenge/response pair, the server verifies the client's
   control of the domain by verifying that the resource was provisioned
   as expected.

   1.  Construct a URL by populating the URL template [RFC6570]
       "http://{domain}/.well-known/acme-challenge/{token}", where:

       *  the domain field is set to the domain name being verified; and

       *  the token field is set to the token in the challenge.

   2.  Verify that the resulting URL is well-formed.

   3.  Dereference the URL using an HTTP GET request.  This request MUST
       be sent to TCP port 80 on the HTTP server.

   4.  Verify that the body of the response is a well-formed key
       authorization.  The server SHOULD ignore whitespace characters at
       the end of the body.

   5.  Verify that key authorization provided by the HTTP server matches
       the key authorization stored by the server.

   The server SHOULD follow redirects when dereferencing the URL.
   Clients might use redirects, for example, so that the response can be
   provided by a centralized certificate management server.  See
   Section 10.2 for security considerations related to redirects.





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   If all of the above verifications succeed, then the validation is
   successful.  If the request fails, or the body does not pass these
   checks, then it has failed.

   The client SHOULD de-provision the resource provisioned for this
   challenge once the challenge is complete, i.e., once the "status"
   field of the challenge has the value "valid" or "invalid".

   Note that because the token appears both in the request sent by the
   ACME server and in the key authorization in the response, it is
   possible to build clients that copy the token from request to
   response.  Clients should avoid this behavior because it can lead to
   cross-site scripting vulnerabilities; instead, clients should be
   explicitly configured on a per-challenge basis.  A client that does
   copy tokens from requests to responses MUST validate that the token
   in the request matches the token syntax above (e.g., that it includes
   only characters from the base64url alphabet).

8.4.  DNS Challenge

   When the identifier being validated is a domain name, the client can
   prove control of that domain by provisioning a TXT resource record
   containing a designated value for a specific validation domain name.

   type (required, string):  The string "dns-01".

   token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy.
      It MUST NOT contain any characters outside the base64url alphabet,
      including padding characters ("=").  See [RFC4086] for additional
      information on randomness requirements.

   {
     "type": "dns-01",
     "url": "https://example.com/acme/chall/Rg5dV14Gh1Q",
     "status": "pending",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
   }

   A client fulfills this challenge by constructing a key authorization
   from the "token" value provided in the challenge and the client's
   account key.  The client then computes the SHA-256 digest [FIPS180-4]
   of the key authorization.

   The record provisioned to the DNS contains the base64url encoding of
   this digest.  The client constructs the validation domain name by
   prepending the label "_acme-challenge" to the domain name being
   validated, then provisions a TXT record with the digest value under



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   that name.  For example, if the domain name being validated is
   "www.example.org", then the client would provision the following DNS
   record:

   _acme-challenge.www.example.org. 300 IN TXT "gfj9Xq...Rg85nM"

   A client responds with an empty object ({}) to acknowledge that the
   challenge can be validated by the server.

   POST /acme/chall/Rg5dV14Gh1Q
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/evOfKhNU60wg",
       "nonce": "SS2sSl1PtspvFZ08kNtzKd",
       "url": "https://example.com/acme/chall/Rg5dV14Gh1Q"
     }),
     "payload": base64url({}),
     "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
   }

   On receiving a response, the server constructs and stores the key
   authorization from the challenge "token" value and the current client
   account key.

   To validate a DNS challenge, the server performs the following steps:

   1.  Compute the SHA-256 digest [FIPS180-4] of the stored key
       authorization

   2.  Query for TXT records for the validation domain name

   3.  Verify that the contents of one of the TXT records match the
       digest value

   If all of the above verifications succeed, then the validation is
   successful.  If no DNS record is found, or DNS record and response
   payload do not pass these checks, then the validation fails.

   The client SHOULD de-provision the resource record(s) provisioned for
   this challenge once the challenge is complete, i.e., once the
   "status" field of the challenge has the value "valid" or "invalid".






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9.  IANA Considerations

9.1.  Media Type: application/pem-certificate-chain

   A file of this type contains one or more certificates encoded with
   the PEM textual encoding, according to [RFC7468].  The textual
   encoding of certificates in this file MUST use the strict encoding
   and MUST NOT include explanatory text.  The ABNF for this format is
   as follows, where "stricttextualmsg" and "eol" are as defined in
   Section 3 of RFC 7468:

   certchain = stricttextualmsg *(eol stricttextualmsg)

   In order to provide easy interoperation with TLS, the first
   certificate MUST be an end-entity certificate.  Each following
   certificate SHOULD directly certify the one preceding it.  Because
   certificate validation requires that trust anchors be distributed
   independently, a certificate that represents a trust anchor MAY be
   omitted from the chain, provided that supported peers are known to
   possess any omitted certificates.

   The following has been registered in the "Media Types" registry:
   Type name: application

   Subtype name: pem-certificate-chain

   Required parameters: None

   Optional parameters: None

   Encoding considerations: 7bit

   Security considerations: Carries a cryptographic certificate and its
   associated certificate chain.  This media type carries no active
   content.

   Interoperability considerations: None

   Published specification: RFC 8555

   Applications that use this media type: ACME clients and servers, HTTP
   servers, other applications that need to be configured with a
   certificate chain

   Additional information:

      Deprecated alias names for this type: n/a
      Magic number(s): n/a



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      File extension(s): .pem
      Macintosh file type code(s): n/a

   Person & email address to contact for further information: See
   Authors' Addresses section.

   Intended usage: COMMON

   Restrictions on usage: n/a

   Author: See Authors' Addresses section.

   Change controller: IETF <iesg@ietf.org>

9.2.  Well-Known URI for the HTTP Challenge

   The following value has been registered in the "Well-Known URIs"
   registry (using the template from [RFC5785]):

   URI suffix: acme-challenge

   Change controller: IETF

   Specification document(s): RFC 8555, Section 8.3

   Related information: N/A

9.3.  Replay-Nonce HTTP Header

   The following value has been registered in the "Message Headers"
   registry:

   +-------------------+----------+----------+-------------------------+
   | Header Field Name | Protocol | Status   | Reference               |
   +-------------------+----------+----------+-------------------------+
   | Replay-Nonce      | http     | standard | RFC 8555, Section 6.5.1 |
   +-------------------+----------+----------+-------------------------+














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9.4.  "url" JWS Header Parameter

   The following value has been registered in the "JSON Web Signature
   and Encryption Header Parameters" registry:

   o  Header Parameter Name: "url"

   o  Header Parameter Description: URL

   o  Header Parameter Usage Location(s): JWE, JWS

   o  Change Controller: IESG

   o  Specification Document(s): RFC 8555, Section 6.4.1

9.5.  "nonce" JWS Header Parameter

   The following value has been registered in the "JSON Web Signature
   and Encryption Header Parameters" registry:

   o  Header Parameter Name: "nonce"

   o  Header Parameter Description: Nonce

   o  Header Parameter Usage Location(s): JWE, JWS

   o  Change Controller: IESG

   o  Specification Document(s): RFC 8555, Section 6.5.2

9.6.  URN Sub-namespace for ACME (urn:ietf:params:acme)

   The following value has been registered in the "IETF URN Sub-
   namespace for Registered Protocol Parameter Identifiers" registry,
   following the template in [RFC3553]:

   Registry name:  acme

   Specification:  RFC 8555

   Repository:  http://www.iana.org/assignments/acme

   Index value:  No transformation needed.








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9.7.  New Registries

   IANA has created the following registries:

   1.  ACME Account Object Fields (Section 9.7.1)

   2.  ACME Order Object Fields (Section 9.7.2)

   3.  ACME Authorization Object Fields (Section 9.7.3)

   4.  ACME Error Types (Section 9.7.4)

   5.  ACME Resource Types (Section 9.7.5)

   6.  ACME Directory Metadata Fields (Section 9.7.6)

   7.  ACME Identifier Types (Section 9.7.7)

   8.  ACME Validation Methods (Section 9.7.8)

   All of these registries are under a heading of "Automated Certificate
   Management Environment (ACME) Protocol" and are administered under a
   Specification Required policy [RFC8126].

9.7.1.  Fields in Account Objects

   The "ACME Account Object Fields" registry lists field names that are
   defined for use in ACME account objects.  Fields marked as
   "configurable" may be included in a newAccount request.

   Template:

   o  Field name: The string to be used as a field name in the JSON
      object

   o  Field type: The type of value to be provided, e.g., string,
      boolean, array of string

   o  Requests: Either the value "none" or a list of types of requests
      where the field is allowed in a request object, taken from the
      following values:

      *  "new" - Requests to the "newAccount" URL

      *  "account" - Requests to an account URL

   o  Reference: Where this field is defined




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   Initial contents: The fields and descriptions defined in
   Section 7.1.2.

   +------------------------+---------------+--------------+-----------+
   | Field Name             | Field Type    | Requests     | Reference |
   +------------------------+---------------+--------------+-----------+
   | status                 | string        | new, account | RFC 8555  |
   |                        |               |              |           |
   | contact                | array of      | new, account | RFC 8555  |
   |                        | string        |              |           |
   |                        |               |              |           |
   | externalAccountBinding | object        | new          | RFC 8555  |
   |                        |               |              |           |
   | termsOfServiceAgreed   | boolean       | new          | RFC 8555  |
   |                        |               |              |           |
   | onlyReturnExisting     | boolean       | new          | RFC 8555  |
   |                        |               |              |           |
   | orders                 | string        | none         | RFC 8555  |
   +------------------------+---------------+--------------+-----------+

9.7.2.  Fields in Order Objects

   The "ACME Order Object Fields" registry lists field names that are
   defined for use in ACME order objects.  Fields marked as
   "configurable" may be included in a newOrder request.

   Template:

   o  Field name: The string to be used as a field name in the JSON
      object

   o  Field type: The type of value to be provided, e.g., string,
      boolean, array of string

   o  Configurable: Boolean indicating whether the server should accept
      values provided by the client

   o  Reference: Where this field is defined













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   Initial contents: The fields and descriptions defined in
   Section 7.1.3.

      +----------------+-----------------+--------------+-----------+
      | Field Name     | Field Type      | Configurable | Reference |
      +----------------+-----------------+--------------+-----------+
      | status         | string          | false        | RFC 8555  |
      |                |                 |              |           |
      | expires        | string          | false        | RFC 8555  |
      |                |                 |              |           |
      | identifiers    | array of object | true         | RFC 8555  |
      |                |                 |              |           |
      | notBefore      | string          | true         | RFC 8555  |
      |                |                 |              |           |
      | notAfter       | string          | true         | RFC 8555  |
      |                |                 |              |           |
      | error          | string          | false        | RFC 8555  |
      |                |                 |              |           |
      | authorizations | array of string | false        | RFC 8555  |
      |                |                 |              |           |
      | finalize       | string          | false        | RFC 8555  |
      |                |                 |              |           |
      | certificate    | string          | false        | RFC 8555  |
      +----------------+-----------------+--------------+-----------+

9.7.3.  Fields in Authorization Objects

   The "ACME Authorization Object Fields" registry lists field names
   that are defined for use in ACME authorization objects.  Fields
   marked as "configurable" may be included in a newAuthz request.

   Template:

   o  Field name: The string to be used as a field name in the JSON
      object

   o  Field type: The type of value to be provided, e.g., string,
      boolean, array of string

   o  Configurable: Boolean indicating whether the server should accept
      values provided by the client

   o  Reference: Where this field is defined








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   Initial contents: The fields and descriptions defined in
   Section 7.1.4.

        +------------+-----------------+--------------+-----------+
        | Field Name | Field Type      | Configurable | Reference |
        +------------+-----------------+--------------+-----------+
        | identifier | object          | true         | RFC 8555  |
        |            |                 |              |           |
        | status     | string          | false        | RFC 8555  |
        |            |                 |              |           |
        | expires    | string          | false        | RFC 8555  |
        |            |                 |              |           |
        | challenges | array of object | false        | RFC 8555  |
        |            |                 |              |           |
        | wildcard   | boolean         | false        | RFC 8555  |
        +------------+-----------------+--------------+-----------+

9.7.4.  Error Types

   The "ACME Error Types" registry lists values that are used within URN
   values that are provided in the "type" field of problem documents in
   ACME.

   Template:

   o  Type: The label to be included in the URN for this error,
      following "urn:ietf:params:acme:error:"

   o  Description: A human-readable description of the error

   o  Reference: Where the error is defined

   Initial contents: The types and descriptions in the table in
   Section 6.7 above, with the Reference field set to point to this
   specification.

9.7.5.  Resource Types

   The "ACME Resource Types" registry lists the types of resources that
   ACME servers may list in their directory objects.

   Template:

   o  Field name: The value to be used as a field name in the directory
      object

   o  Resource type: The type of resource labeled by the field




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   o  Reference: Where the resource type is defined

   Initial contents:

              +------------+--------------------+-----------+
              | Field Name | Resource Type      | Reference |
              +------------+--------------------+-----------+
              | newNonce   | New nonce          | RFC 8555  |
              |            |                    |           |
              | newAccount | New account        | RFC 8555  |
              |            |                    |           |
              | newOrder   | New order          | RFC 8555  |
              |            |                    |           |
              | newAuthz   | New authorization  | RFC 8555  |
              |            |                    |           |
              | revokeCert | Revoke certificate | RFC 8555  |
              |            |                    |           |
              | keyChange  | Key change         | RFC 8555  |
              |            |                    |           |
              | meta       | Metadata object    | RFC 8555  |
              +------------+--------------------+-----------+

9.7.6.  Fields in the "meta" Object within a Directory Object

   The "ACME Directory Metadata Fields" registry lists field names that
   are defined for use in the JSON object included in the "meta" field
   of an ACME directory object.

   Template:

   o  Field name: The string to be used as a field name in the JSON
      object

   o  Field type: The type of value to be provided, e.g., string,
      boolean, array of string

   o  Reference: Where this field is defined














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   Initial contents: The fields and descriptions defined in
   Section 7.1.1.

         +-------------------------+-----------------+-----------+
         | Field Name              | Field Type      | Reference |
         +-------------------------+-----------------+-----------+
         | termsOfService          | string          | RFC 8555  |
         |                         |                 |           |
         | website                 | string          | RFC 8555  |
         |                         |                 |           |
         | caaIdentities           | array of string | RFC 8555  |
         |                         |                 |           |
         | externalAccountRequired | boolean         | RFC 8555  |
         +-------------------------+-----------------+-----------+

9.7.7.  Identifier Types

   The "ACME Identifier Types" registry lists the types of identifiers
   that can be present in ACME authorization objects.

   Template:

   o  Label: The value to be put in the "type" field of the identifier
      object

   o  Reference: Where the identifier type is defined

   Initial contents:

                           +-------+-----------+
                           | Label | Reference |
                           +-------+-----------+
                           | dns   | RFC 8555  |
                           +-------+-----------+

9.7.8.  Validation Methods

   The "ACME Validation Methods" registry lists identifiers for the ways
   that CAs can validate control of identifiers.  Each method's entry
   must specify whether it corresponds to an ACME challenge type.  The
   "Identifier Type" field must be contained in the Label column of the
   "ACME Identifier Types" registry.









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   Template:

   o  Label: The identifier for this validation method

   o  Identifier Type: The type of identifier that this method applies
      to

   o  ACME: "Y" if the validation method corresponds to an ACME
      challenge type; "N" otherwise

   o  Reference: Where the validation method is defined

   This registry may also contain reserved entries (e.g., to avoid
   collisions).  Such entries should have the "ACME" field set to "N"
   and the "Identifier Type" set to "RESERVED".

   Initial Contents

            +------------+-----------------+------+-----------+
            | Label      | Identifier Type | ACME | Reference |
            +------------+-----------------+------+-----------+
            | http-01    | dns             | Y    | RFC 8555  |
            |            |                 |      |           |
            | dns-01     | dns             | Y    | RFC 8555  |
            |            |                 |      |           |
            | tls-sni-01 | RESERVED        | N    | RFC 8555  |
            |            |                 |      |           |
            | tls-sni-02 | RESERVED        | N    | RFC 8555  |
            +------------+-----------------+------+-----------+

   When evaluating a request for an assignment in this registry, the
   designated expert should ensure that the method being registered has
   a clear, interoperable definition and does not overlap with existing
   validation methods.  That is, it should not be possible for a client
   and server to follow the same set of actions to fulfill two different
   validation methods.

   The values "tls-sni-01" and "tls-sni-02" are reserved because they
   were used in pre-RFC versions of this specification to denote
   validation methods that were removed because they were found not to
   be secure in some cases.

   Validation methods do not have to be compatible with ACME in order to
   be registered.  For example, a CA might wish to register a validation
   method to support its use with the ACME extensions to CAA [ACME-CAA].






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10.  Security Considerations

   ACME is a protocol for managing certificates that attest to
   identifier/key bindings.  Thus, the foremost security goal of ACME is
   to ensure the integrity of this process, i.e., to ensure that the
   bindings attested by certificates are correct and that only
   authorized entities can manage certificates.  ACME identifies clients
   by their account keys, so this overall goal breaks down into two more
   precise goals:

   1.  Only an entity that controls an identifier can get an
       authorization for that identifier

   2.  Once authorized, an account key's authorizations cannot be
       improperly used by another account

   In this section, we discuss the threat model that underlies ACME and
   the ways that ACME achieves these security goals within that threat
   model.  We also discuss the denial-of-service risks that ACME servers
   face, and a few other miscellaneous considerations.

10.1.  Threat Model

   As a service on the Internet, ACME broadly exists within the Internet
   threat model [RFC3552].  In analyzing ACME, it is useful to think of
   an ACME server interacting with other Internet hosts along two
   "channels":

   o  An ACME channel, over which the ACME HTTPS requests are exchanged

   o  A validation channel, over which the ACME server performs
      additional requests to validate a client's control of an
      identifier


















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   +------------+
   |    ACME    |     ACME Channel
   |   Client   |--------------------+
   +------------+                    |
                                     V
                               +------------+
                               |    ACME    |
                               |   Server   |
                               +------------+
   +------------+                    |
   | Validation |<-------------------+
   |   Server   |  Validation Channel
   +------------+

                   Communications Channels Used by ACME

   In practice, the risks to these channels are not entirely separate,
   but they are different in most cases.  Each channel, for example,
   uses a different communications pattern: the ACME channel will
   comprise inbound HTTPS connections to the ACME server and the
   validation channel outbound HTTP or DNS requests.

   Broadly speaking, ACME aims to be secure against active and passive
   attackers on any individual channel.  Some vulnerabilities arise
   (noted below) when an attacker can exploit both the ACME channel and
   one of the others.

   On the ACME channel, in addition to network-layer attackers, we also
   need to account for man-in-the-middle (MitM) attacks at the
   application layer and for abusive use of the protocol itself.
   Protection against application-layer MitM addresses potential
   attackers such as Content Distribution Networks (CDNs) and
   middleboxes with a TLS MitM function.  Preventing abusive use of ACME
   means ensuring that an attacker with access to the validation channel
   can't obtain illegitimate authorization by acting as an ACME client
   (legitimately, in terms of the protocol).

   ACME does not protect against other types of abuse by a MitM on the
   ACME channel.  For example, such an attacker could send a bogus
   "badSignatureAlgorithm" error response to downgrade a client to the
   lowest-quality signature algorithm that the server supports.  A MitM
   that is present on all connections (such as a CDN) can cause denial-
   of-service conditions in a variety of ways.








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10.2.  Integrity of Authorizations

   ACME allows anyone to request challenges for an identifier by
   registering an account key and sending a newOrder request using that
   account key.  The integrity of the authorization process thus depends
   on the identifier validation challenges to ensure that the challenge
   can only be completed by someone who both (1) holds the private key
   of the account key pair and (2) controls the identifier in question.

   Validation responses need to be bound to an account key pair in order
   to avoid situations where a MitM on ACME HTTPS requests can switch
   out a legitimate domain holder's account key for one of his choosing.
   Such MitMs can arise, for example, if a CA uses a CDN or third-party
   reverse proxy in front of its ACME interface.  An attack by such an
   MitM could have the following form:

   1.  Legitimate domain holder registers account key pair A

   2.  MitM registers account key pair B

   3.  Legitimate domain holder sends a newOrder request signed using
       account key A

   4.  MitM suppresses the legitimate request but sends the same request
       signed using account key B

   5.  ACME server issues challenges and MitM forwards them to the
       legitimate domain holder

   6.  Legitimate domain holder provisions the validation response

   7.  ACME server performs validation query and sees the response
       provisioned by the legitimate domain holder

   8.  Because the challenges were issued in response to a message
       signed with account key B, the ACME server grants authorization
       to account key B (the MitM) instead of account key A (the
       legitimate domain holder)













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   Domain                                         ACME
   Holder                  MitM                  Server
     |                      |                      |
     | newAccount(A)        |                      |
     |--------------------->|--------------------->|
     |                      |                      |
     |                      | newAccount(B)        |
     |                      |--------------------->|
     | newOrder(domain, A)  |                      |
     |--------------------->|                      |
     |                      | newOrder(domain, B)  |
     |                      |--------------------->|
     |                      |                      |
     |   authz, challenges  |   authz, challenges  |
     |<---------------------|<---------------------|
     |                      |                      |
     | response(chall, A)   | response(chall, B)   |
     |--------------------->|--------------------->|
     |                      |                      |
     |  validation request  |                      |
     |<--------------------------------------------|
     |                      |                      |
     | validation response  |                      |
     |-------------------------------------------->|
     |                      |                      |
     |                      |                      | Considers challenge
     |                      |                      | fulfilled by B
     |                      |                      |

             Man-in-the-Middle Attack Exploiting a Validation
                    Method without Account Key Binding

   All of the challenges defined in this document have a binding between
   the account private key and the validation query made by the server,
   via the key authorization.  The key authorization reflects the
   account public key and is provided to the server in the validation
   response over the validation channel.

   The association of challenges to identifiers is typically done by
   requiring the client to perform some action that only someone who
   effectively controls the identifier can perform.  For the challenges
   in this document, the actions are as follows:

   o  HTTP: Provision files under .well-known on a web server for the
      domain

   o  DNS: Provision DNS resource records for the domain




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   There are several ways that these assumptions can be violated, both
   by misconfiguration and by attacks.  For example, on a web server
   that allows non-administrative users to write to .well-known, any
   user can claim to own the web server's hostname by responding to an
   HTTP challenge.  Similarly, if a server that can be used for ACME
   validation is compromised by a malicious actor, then that malicious
   actor can use that access to obtain certificates via ACME.

   The use of hosting providers is a particular risk for ACME
   validation.  If the owner of the domain has outsourced operation of
   DNS or web services to a hosting provider, there is nothing that can
   be done against tampering by the hosting provider.  As far as the
   outside world is concerned, the zone or website provided by the
   hosting provider is the real thing.

   More limited forms of delegation can also lead to an unintended party
   gaining the ability to successfully complete a validation
   transaction.  For example, suppose an ACME server follows HTTP
   redirects in HTTP validation and a website operator provisions a
   catch-all redirect rule that redirects requests for unknown resources
   to a different domain.  Then the target of the redirect could use
   that to get a certificate through HTTP validation since the
   validation path will not be known to the primary server.

   The DNS is a common point of vulnerability for all of these
   challenges.  An entity that can provision false DNS records for a
   domain can attack the DNS challenge directly and can provision false
   A/AAAA records to direct the ACME server to send its HTTP validation
   query to a remote server of the attacker's choosing.  There are a few
   different mitigations that ACME servers can apply:

   o  Always querying the DNS using a DNSSEC-validating resolver
      (enhancing security for zones that are DNSSEC-enabled)

   o  Querying the DNS from multiple vantage points to address local
      attackers

   o  Applying mitigations against DNS off-path attackers, e.g., adding
      entropy to requests [DNS0x20] or only using TCP

   Given these considerations, the ACME validation process makes it
   impossible for any attacker on the ACME channel or a passive attacker
   on the validation channel to hijack the authorization process to
   authorize a key of the attacker's choice.

   An attacker that can only see the ACME channel would need to convince
   the validation server to provide a response that would authorize the
   attacker's account key, but this is prevented by binding the



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   validation response to the account key used to request challenges.  A
   passive attacker on the validation channel can observe the correct
   validation response and even replay it, but that response can only be
   used with the account key for which it was generated.

   An active attacker on the validation channel can subvert the ACME
   process, by performing normal ACME transactions and providing a
   validation response for his own account key.  The risks due to
   hosting providers noted above are a particular case.

   Attackers can also exploit vulnerabilities in Internet routing
   protocols to gain access to the validation channel (see, e.g.,
   [RFC7132]).  In order to make such attacks more difficult, it is
   RECOMMENDED that the server perform DNS queries and make HTTP
   connections from multiple points in the network.  Since routing
   attacks are often localized or dependent on the position of the
   attacker, forcing the attacker to attack multiple points (the
   server's validation vantage points) or a specific point (the DNS /
   HTTP server) makes it more difficult to subvert ACME validation using
   attacks on routing.

10.3.  Denial-of-Service Considerations

   As a protocol run over HTTPS, standard considerations for TCP-based
   and HTTP-based DoS mitigation also apply to ACME.

   At the application layer, ACME requires the server to perform a few
   potentially expensive operations.  Identifier validation transactions
   require the ACME server to make outbound connections to potentially
   attacker-controlled servers, and certificate issuance can require
   interactions with cryptographic hardware.

   In addition, an attacker can also cause the ACME server to send
   validation requests to a domain of its choosing by submitting
   authorization requests for the victim domain.

   All of these attacks can be mitigated by the application of
   appropriate rate limits.  Issues closer to the front end, like POST
   body validation, can be addressed using HTTP request limiting.  For
   validation and certificate requests, there are other identifiers on
   which rate limits can be keyed.  For example, the server might limit
   the rate at which any individual account key can issue certificates
   or the rate at which validation can be requested within a given
   subtree of the DNS.  And in order to prevent attackers from
   circumventing these limits simply by minting new accounts, servers
   would need to limit the rate at which accounts can be registered.





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10.4.  Server-Side Request Forgery

   Server-Side Request Forgery (SSRF) attacks can arise when an attacker
   can cause a server to perform HTTP requests to an attacker-chosen
   URL.  In the ACME HTTP challenge validation process, the ACME server
   performs an HTTP GET request to a URL in which the attacker can
   choose the domain.  This request is made before the server has
   verified that the client controls the domain, so any client can cause
   a query to any domain.

   Some ACME server implementations include information from the
   validation server's response (in order to facilitate debugging).
   Such implementations enable an attacker to extract this information
   from any web server that is accessible to the ACME server, even if it
   is not accessible to the ACME client.  For example, the ACME server
   might be able to access servers behind a firewall that would prevent
   access by the ACME client.

   It might seem that the risk of SSRF through this channel is limited
   by the fact that the attacker can only control the domain of the URL,
   not the path.  However, if the attacker first sets the domain to one
   they control, then they can send the server an HTTP redirect (e.g., a
   302 response) which will cause the server to query an arbitrary URL.

   In order to further limit the SSRF risk, ACME server operators should
   ensure that validation queries can only be sent to servers on the
   public Internet, and not, say, web services within the server
   operator's internal network.  Since the attacker could make requests
   to these public servers himself, he can't gain anything extra through
   an SSRF attack on ACME aside from a layer of anonymization.

10.5.  CA Policy Considerations

   The controls on issuance enabled by ACME are focused on validating
   that a certificate applicant controls the identifier he claims.
   Before issuing a certificate, however, there are many other checks
   that a CA might need to perform, for example:

   o  Has the client agreed to a subscriber agreement?

   o  Is the claimed identifier syntactically valid?

   o  For domain names:

      *  If the leftmost label is a '*', then have the appropriate
         checks been applied?

      *  Is the name on the Public Suffix List?



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      *  Is the name a high-value name?

      *  Is the name a known phishing domain?

   o  Is the key in the CSR sufficiently strong?

   o  Is the CSR signed with an acceptable algorithm?

   o  Has issuance been authorized or forbidden by a Certification
      Authority Authorization (CAA) record ([RFC6844])?

   CAs that use ACME to automate issuance will need to ensure that their
   servers perform all necessary checks before issuing.

   CAs using ACME to allow clients to agree to terms of service should
   keep in mind that ACME clients can automate this agreement, possibly
   not involving a human user.

   ACME does not specify how the server constructs the URLs that it uses
   to address resources.  If the server operator uses URLs that are
   predictable to third parties, this can leak information about what
   URLs exist on the server, since an attacker can probe for whether a
   POST-as-GET request to the URL returns 404 (Not Found) or 401
   (Unauthorized).

   For example, suppose that the CA uses highly structured URLs with
   guessable fields:

   o  Accounts: https://example.com/:accountID

   o  Orders: https://example.com/:accountID/:domainName

   o  Authorizations: https://example.com/:accountID/:domainName

   o  Certificates: https://example.com/:accountID/:domainName

   Under that scheme, an attacker could probe for which domain names are
   associated with which accounts, which may allow correlation of
   ownership between domain names, if the CA does not otherwise permit
   it.











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   To avoid leaking these correlations, CAs SHOULD assign URLs with an
   unpredictable component.  For example, a CA might assign URLs for
   each resource type from an independent namespace, using unpredictable
   IDs for each resource:

   o  Accounts: https://example.com/acct/:accountID

   o  Orders: https://example.com/order/:orderID

   o  Authorizations: https://example.com/authz/:authorizationID

   o  Certificates: https://example.com/cert/:certID

   Such a scheme would leak only the type of resource, hiding the
   additional correlations revealed in the example above.

11.  Operational Considerations

   There are certain factors that arise in operational reality that
   operators of ACME-based CAs will need to keep in mind when
   configuring their services.  See the subsections below for examples.

11.1.  Key Selection

   ACME relies on two different classes of key pair:

   o  Account key pairs, which are used to authenticate account holders

   o  Certificate key pairs, which are used to sign and verify CSRs (and
      whose public keys are included in certificates)

   Compromise of the private key of an account key pair has more serious
   consequences than compromise of a private key corresponding to a
   certificate.  While the compromise of a certificate key pair allows
   the attacker to impersonate the entities named in the certificate for
   the lifetime of the certificate, the compromise of an account key
   pair allows the attacker to take full control of the victim's ACME
   account and take any action that the legitimate account holder could
   take within the scope of ACME:

   1.  Issuing certificates using existing authorizations

   2.  Revoking existing certificates

   3.  Accessing and changing account information (e.g., contacts)

   4.  Changing the account key pair for the account, locking out the
       legitimate account holder



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   For this reason, it is RECOMMENDED that each account key pair be used
   only for authentication of a single ACME account.  For example, the
   public key of an account key pair MUST NOT be included in a
   certificate.  If an ACME client receives a request from a user for
   account creation or key rollover using an account key that the client
   knows to be used elsewhere, then the client MUST return an error.
   Clients MUST generate a fresh account key for every account creation
   or rollover operation.  Note that given the requirements of
   Section 7.3.1, servers will not create accounts with reused keys
   anyway.

   ACME clients and servers MUST verify that a CSR submitted in a
   finalize request does not contain a public key for any known account
   key pair.  In particular, when a server receives a finalize request,
   it MUST verify that the public key in a CSR is not the same as the
   public key of the account key pair used to authenticate that request.
   This assures that vulnerabilities in the protocols with which the
   certificate is used (e.g., signing oracles in TLS [JSS15]) do not
   result in compromise of the ACME account.  Because ACME accounts are
   uniquely identified by their account key pair (see Section 7.3.1),
   the server MUST not allow account key pair reuse across multiple
   accounts.

11.2.  DNS Security

   As noted above, DNS forgery attacks against the ACME server can
   result in the server making incorrect decisions about domain control
   and thus mis-issuing certificates.  Servers SHOULD perform DNS
   queries over TCP, which provides better resistance to some forgery
   attacks than DNS over UDP.

   An ACME-based CA will often need to make DNS queries, e.g., to
   validate control of DNS names.  Because the security of such
   validations ultimately depends on the authenticity of DNS data, every
   possible precaution should be taken to secure DNS queries done by the
   CA.  Therefore, it is RECOMMENDED that ACME-based CAs make all DNS
   queries via DNSSEC-validating stub or recursive resolvers.  This
   provides additional protection to domains that choose to make use of
   DNSSEC.

   An ACME-based CA must only use a resolver if it trusts the resolver
   and every component of the network route by which it is accessed.
   Therefore, it is RECOMMENDED that ACME-based CAs operate their own
   DNSSEC-validating resolvers within their trusted network and use
   these resolvers both for CAA record lookups and all record lookups in
   furtherance of a challenge scheme (A, AAAA, TXT, etc.).





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11.3.  Token Entropy

   The http-01 and dns-01 validation methods mandate the use of a random
   token value to uniquely identify the challenge.  The value of the
   token is required to contain at least 128 bits of entropy for the
   following security properties.  First, the ACME client should not be
   able to influence the ACME server's choice of token as this may allow
   an attacker to reuse a domain owner's previous challenge responses
   for a new validation request.  Second, the entropy requirement makes
   it more difficult for ACME clients to implement a "naive" validation
   server that automatically replies to challenges without being
   configured per challenge.

11.4.  Malformed Certificate Chains

   ACME provides certificate chains in the widely used format known
   colloquially as PEM (though it may diverge from the actual Privacy
   Enhanced Mail specification [RFC1421], as noted in [RFC7468]).  Some
   current software will allow the configuration of a private key and a
   certificate in one PEM file by concatenating the textual encodings of
   the two objects.  In the context of ACME, such software might be
   vulnerable to key replacement attacks.  A malicious ACME server could
   cause a client to use a private key of its choosing by including the
   key in the PEM file returned in response to a query for a certificate
   URL.

   When processing a file of type "application/pem-certificate-chain", a
   client SHOULD verify that the file contains only encoded
   certificates.  If anything other than a certificate is found (i.e.,
   if the string "-----BEGIN" is ever followed by anything other than
   "CERTIFICATE"), then the client MUST reject the file as invalid.

12.  References

12.1.  Normative References

   [FIPS180-4]
              National Institute of Standards and Technology (NIST),
              "Secure Hash Standard (SHS)", FIPS PUB 180-4,
              DOI 10.6028/NIST.FIPS.180-4, August 2015,
              <http://csrc.nist.gov/publications/fips/fips180-4/
              fips-180-4.pdf>.









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   [JSS15]    Somorovsky, J., Schwenk, J., and J. Somorovsky, "On the
              Security of TLS 1.3 and QUIC Against Weaknesses in PKCS#1
              v1.5 Encryption", CSS '15 Proceedings of the 22nd ACM
              SIGSAC Conference on Computer and Communications
              Security Pages 1185-1196, DOI 10.1145/2810103.2813657,
              <https://dl.acm.org/citation.cfm?id=2813657>.

   [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>.

   [RFC2585]  Housley, R. and P. Hoffman, "Internet X.509 Public Key
              Infrastructure Operational Protocols: FTP and HTTP",
              RFC 2585, DOI 10.17487/RFC2585, May 1999,
              <https://www.rfc-editor.org/info/rfc2585>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.

   [RFC2985]  Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
              Classes and Attribute Types Version 2.0", RFC 2985,
              DOI 10.17487/RFC2985, November 2000,
              <https://www.rfc-editor.org/info/rfc2985>.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <https://www.rfc-editor.org/info/rfc2986>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC3986]  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>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.



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   [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>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, DOI 10.17487/RFC5751, January
              2010, <https://www.rfc-editor.org/info/rfc5751>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.

   [RFC6068]  Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
              URI Scheme", RFC 6068, DOI 10.17487/RFC6068, October 2010,
              <https://www.rfc-editor.org/info/rfc6068>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <https://www.rfc-editor.org/info/rfc6570>.

   [RFC6844]  Hallam-Baker, P. and R. Stradling, "DNS Certification
              Authority Authorization (CAA) Resource Record", RFC 6844,
              DOI 10.17487/RFC6844, January 2013,
              <https://www.rfc-editor.org/info/rfc6844>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [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>.




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   [RFC7515]  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>.

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

   [RFC7638]  Jones, M. and N. Sakimura, "JSON Web Key (JWK)
              Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
              2015, <https://www.rfc-editor.org/info/rfc7638>.

   [RFC7797]  Jones, M., "JSON Web Signature (JWS) Unencoded Payload
              Option", RFC 7797, DOI 10.17487/RFC7797, February 2016,
              <https://www.rfc-editor.org/info/rfc7797>.

   [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>.

   [RFC8037]  Liusvaara, I., "CFRG Elliptic Curve Diffie-Hellman (ECDH)
              and Signatures in JSON Object Signing and Encryption
              (JOSE)", RFC 8037, DOI 10.17487/RFC8037, January 2017,
              <https://www.rfc-editor.org/info/rfc8037>.

   [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>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

   [RFC8446]  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>.





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12.2.  Informative References

   [ACME-CAA]
              Landau, H., "CAA Record Extensions for Account URI and
              ACME Method Binding", Work in Progress, draft-ietf-acme-
              caa-06, January 2019.

   [ACME-IP]  Shoemaker, R., "ACME IP Identifier Validation Extension",
              Work in Progress, draft-ietf-acme-ip-05, February 2019.

   [ACME-TELEPHONE]
              Peterson, J. and R. Barnes, "ACME Identifiers and
              Challenges for Telephone Numbers", Work in Progress,
              draft-ietf-acme-telephone-01, October 2017.

   [CABFBR]   CA/Browser Forum, "CA/Browser Forum Baseline
              Requirements", September 2018,
              <https://cabforum.org/baseline-requirements-documents/>.

   [DNS0x20]  Vixie, P. and D. Dagon, "Use of Bit 0x20 in DNS Labels to
              Improve Transaction Identity", Work in Progress,
              draft-vixie-dnsext-dns0x20-00, March 2008.

   [RFC1421]  Linn, J., "Privacy Enhancement for Internet Electronic
              Mail: Part I: Message Encryption and Authentication
              Procedures", RFC 1421, DOI 10.17487/RFC1421, February
              1993, <https://www.rfc-editor.org/info/rfc1421>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [RFC3553]  Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
              IETF URN Sub-namespace for Registered Protocol
              Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
              2003, <https://www.rfc-editor.org/info/rfc3553>.
   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,
              <https://www.rfc-editor.org/info/rfc5785>.

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,
              <https://www.rfc-editor.org/info/rfc6960>.




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   [RFC7132]  Kent, S. and A. Chi, "Threat Model for BGP Path Security",
              RFC 7132, DOI 10.17487/RFC7132, February 2014,
              <https://www.rfc-editor.org/info/rfc7132>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [W3C.REC-cors-20140116]
              Kesteren, A., Ed., "Cross-Origin Resource Sharing", W3C
              Recommendation REC-cors-20140116, January 2014,
              <http://www.w3.org/TR/2014/REC-cors-20140116>.





































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Acknowledgements

   In addition to the editors listed on the front page, this document
   has benefited from contributions from a broad set of contributors,
   all the way back to its inception.

   o  Andrew Ayer, SSLMate

   o  Karthik Bhargavan, INRIA

   o  Peter Eckersley, EFF

   o  Alex Halderman, University of Michigan

   o  Sophie Herold, Hemio

   o  Tim Hollebeek, DigiCert

   o  Eric Rescorla, Mozilla

   o  Seth Schoen, EFF

   o  Roland Shoemaker, Let's Encrypt

   o  Rob Stradling, Sectigo

   o  Martin Thomson, Mozilla

   o  Jakub Warmuz, University of Oxford

   This document draws on many concepts established by Eric Rescorla's
   "Automated Certificate Issuance Protocol" draft.  Martin Thomson
   provided helpful guidance in the use of HTTP.


















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Authors' Addresses

   Richard Barnes
   Cisco

   Email: rlb@ipv.sx


   Jacob Hoffman-Andrews
   EFF

   Email: jsha@eff.org


   Daniel McCarney
   Let's Encrypt

   Email: cpu@letsencrypt.org


   James Kasten
   University of Michigan

   Email: jdkasten@umich.edu



























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