JWTClaimConstraints profile of ACME Authority Token
draft-ietf-acme-authority-token-jwtclaimcon-01

Network Working Group                                           C. Wendt
Internet-Draft                                                D. Hancock
Intended status: Standards Track                              Somos Inc.
Expires: 25 September 2026                                 24 March 2026

          JWTClaimConstraints profile of ACME Authority Token
             draft-ietf-acme-authority-token-jwtclaimcon-01

Abstract

   This document defines an authority token profile for the validation
   of JWTClaimConstraints and EnhancedJWTClaimConstraints certificate
   extensions within the Automated Certificate Management Environment
   (ACME) protocol.  This profile is based on the Authority Token
   framework and establishes the specific ACME identifier type,
   challenge mechanism, and token format necessary to authorize a client
   to request a certificate containing these constraints.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 25 September 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

Wendt & Hancock         Expires 25 September 2026               [Page 1]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  ACME new-order Identifiers for JWTClaimConstraints  . . . . .   3
   4.  JWTClaimConstraints Authorization . . . . . . . . . . . . . .   5
   5.  JWTClaimConstraints Authority Token . . . . . . . . . . . . .   7
     5.1.  JWTClaimConstraints Authority Token Payload . . . . . . .   7
       5.1.1.  "iss" claim . . . . . . . . . . . . . . . . . . . . .   7
       5.1.2.  "exp" claim . . . . . . . . . . . . . . . . . . . . .   8
       5.1.3.  "jti" claim . . . . . . . . . . . . . . . . . . . . .   8
       5.1.4.  "atc" claim . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Acquiring the token from the Token Authority  . . . . . .   9
     5.3.  Token Authority Responsibilities  . . . . . . . . . . . .  10
     5.4.  Scope of the JWTClaimConstraints  . . . . . . . . . . . .  11
     5.5.  ACME Challenges requiring multiple Authority Tokens . . .  11
       5.5.1.  ACME Procedures when Challenge requires two Authority
               Tokens  . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Validating the JWTClaimConstraints Authority Token  . . . . .  13
   7.  Using ACME-issued Certificates with JSON Web Signature  . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  16
   Appendix A.  JWTClaimConstraints Identifier Value Examples  . . .  17
     A.1.  No Extended Claims Authorized . . . . . . . . . . . . . .  18
     A.2.  Extended Claims Authorized (Uniform Constraints)  . . . .  19
     A.3.  Extended Claims Authorized (Per-TN Subset Constraints)  .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   The validation of certificate extensions that constrain the use of a
   certificate's credentials, such as the JWTClaimConstraints extension
   defined in [RFC8226] and the EnhancedJWTClaimConstraints extension
   defined in [RFC9118], is critical for defining the scope of an issued
   certificate.  This document specifies an authority token profile for
   validating these constraints, modeled after the authority token
   framework established in [RFC9447] and the TNAuthList validation

Wendt & Hancock         Expires 25 September 2026               [Page 2]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   defined in [RFC9448].

   This profile facilitates proper delegation and authorization for
   entities requesting certificates under ACME and similar frameworks.
   It defines the use of the JWTClaimConstraints Authority Token in the
   ACME challenge to prove an authoritative or trusted use of the
   contents of the JWTClaimConstraints and EnhancedJWTClaimConstraints
   extensions based on the issuer of the token.

   This document is intended for use by Secure Telephone Identity (STI)
   Certification Authorities.  In this ecosystem, certificates contain
   telephone-number-related (TNAuthList) extensions and
   JWTClaimConstraints extensions defined in [RFC8226] and [RFC9118],
   and certificate issuance is governed by a set of STI-specific Token
   Authorities.  The TNAuthList Authority Token profile ([RFC9448]) is
   the parallel specification for telephone number authorization in this
   ecosystem and is already widely deployed.  This document follows the
   same pattern to address authorization of the JWTClaimConstraints and
   EnhancedJWTClaimConstraints extensions.  ACME implementers unfamiliar
   with the STIR ecosystem can treat the validation of the
   JWTClaimConstraints token value as a domain-specific authorization
   step analogous to TNAuthList validation: the ACME wire protocol
   mechanics are identical, and the Token Authority is responsible for
   the domain-specific semantic validation described in [RFC8226] and
   [RFC9118].

   This document also discusses the ability for an authority to
   authorize the creation of CA types of certificates for delegation as
   defined in [RFC9060].

2.  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.

3.  ACME new-order Identifiers for JWTClaimConstraints

   In [RFC8555], Section 7 defines the procedure that an ACME client
   uses to order a new certificate from a Certification Authority (CA).
   This draft defines a new type of identifier object called
   JWTClaimConstraints.  A JWTClaimConstraints identifier contains the
   Token Claim Constraints information to be populated in the
   JWTClaimConstraints or EnhancedJWTClaimConstraints of the new
   certificate.

Wendt & Hancock         Expires 25 September 2026               [Page 3]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   For the JWTClaimConstraints identifier, the new-order request
   includes a type set to the string "JWTClaimConstraints".  The value
   of the JWTClaimConstraints identifier MUST be set to the details of
   the JWTClaimConstraints or EnhancedJWTClaimConstraints extension
   requested.

   The format of the string that represents the JWTClaimConstraints MUST
   be constructed using base64url encoding, as per [RFC8555] base64url
   encoding described in Section 5 of [RFC4648] according to the profile
   specified in JSON Web Signature in Section 2 of [RFC7515].  The
   base64url encoding MUST NOT include any padding characters and the
   JWTClaimConstraints ASN.1 object MUST be encoded using DER encoding
   rules.

   An example of an ACME order object "identifiers" field containing a
   JWTClaimConstraints certificate:

    "identifiers": [{"type":"JWTClaimConstraints",
      "value":"F83n2a...avn27DN3"}]

   where the "value" object string represents the arbitrary length
   base64url encoded string.

   A full new-order request would look as follows,

   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":"JWTClaimConstraints",
         "value":"F83n...n27DN3"}],
       "notBefore": "2025-01-01T00:00:00Z",
       "notAfter": "2025-01-08T00:00:00Z"
     }),
     "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
   }

   On receiving a valid new-order request, the ACME server creates an
   authorization object, [RFC8555] Section 7.1.4, containing the
   challenge that the ACME client must satisfy to demonstrate authority

Wendt & Hancock         Expires 25 September 2026               [Page 4]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   for the identifiers specified by the new order (in this case, the
   JWTClaimConstraints identifier).  The CA adds the authorization
   object URL to the "authorizations" field of the order object, and
   returns the order object to the ACME client in the body of a 201
   (Created) response.

   HTTP/1.1 201 Created
   Content-Type: application/json
   Replay-Nonce: MYAuvOpaoIiywTezizk5vw
   Location: https://example.com/acme/order/1234

   {
     "status": "pending",
     "expires": "2025-01-08T00:00:00Z",

     "notBefore": "2025-01-01T00:00:00Z",
     "notAfter": "2025-01-08T00:00:00Z",
     "identifiers": [
       {"type": "JWTClaimConstraints",
        "value": "F83n2a...avn27DN3"}
     ],

     "authorizations": [
      "https://example.com/acme/authz/1234"
     ],
     "finalize": "https://example.com/acme/order/1234/finalize"
   }

4.  JWTClaimConstraints Authorization

   On receiving the new-order response, the ACME client queries the
   referenced authorization object to obtain the challenges for the
   identifier contained in the new-order request.

   POST /acme/authz/1234 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/1234"
     }),
     "payload": "",
     "signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps"
   }

Wendt & Hancock         Expires 25 September 2026               [Page 5]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

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

   {
     "status": "pending",
     "expires": "2025-01-08T00:00:00Z",

     "identifier": {
       "type": "JWTClaimConstraints",
       "value": "F83n2a...avn27DN3"
     },

     "challenges": [
       {
         "type": "tkauth-01",
         "tkauth-type": "atc",
         "token-authority": "https://authority.example.org",
         "url": "https://boulder.example.com/acme/chall/prV_B7yEyA4",
         "token": "IlirfxKKXAsHtmzK29Pj8A"
       }
     ]
   }

   When processing a certificate order containing an identifier of type
   "JWTClaimConstraints", a CA uses the Authority Token challenge type
   of "tkauth-01" with a "tkauth-type" of "atc", defined in [RFC9447],
   to verify that the requesting ACME client has authenticated and
   authorized control over the requested resources represented by the
   "JWTClaimConstraints" value.

   The challenge "token-authority" parameter is OPTIONAL.  If a "token-
   authority" parameter is present, the ACME client MAY use this value
   to identify the URL representing the Token Authority that will
   provide the JWTClaimConstraints Authority Token response to the
   challenge.  If the "token-authority" parameter is not present, the
   ACME client MUST identify the Token Authority based on locally
   configured information or local policies.

   The ACME client responds to the challenge by posting the
   JWTClaimConstraints Authority Token to the challenge URL identified
   in the returned ACME authorization object.

Wendt & Hancock         Expires 25 September 2026               [Page 6]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   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({
       "tkauth": "DGyRejmCefe7v4N...vb29HhjjLPSggwiE"
     }),
     "signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ"
   }

   The "tkauth" field in the challenge response object is specific to
   the tkauth-01 challenge type.  When responding to a challenge for a
   JWTClaimConstraints identifier, this field SHALL contain the
   JWTClaimConstraints Authority Token defined in the next section.

5.  JWTClaimConstraints Authority Token

   The JWTClaimConstraints Authority Token is a profile instance of the
   ACME Authority Token defined in [RFC9447].

   The JWTClaimConstraints Authority Token Protected header MUST comply
   with the Authority Token Protected header as defined in [RFC9447].

5.1.  JWTClaimConstraints Authority Token Payload

   The JWTClaimConstraints Authority Token Payload MUST include the
   mandatory claims "exp", "jti", and "atc", and MAY include the
   optional claims defined for the Authority Token.

5.1.1.  "iss" claim

   The "iss" claim is an optional claim defined in [RFC7519]
   Section 4.1.1.  It can be used as a URL identifying the Token
   Authority that issued the JWTClaimConstraints Authority Token beyond
   the "x5u" or other Header claims that identify the location of the
   certificate or certificate chain of the Token Authority used to
   validate the JWTClaimConstraints Authority Token.

Wendt & Hancock         Expires 25 September 2026               [Page 7]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

5.1.2.  "exp" claim

   The "exp" claim, defined in [RFC7519] Section 4.1.4, MUST be included
   and contains the DateTime value of the ending date and time that the
   JWTClaimConstraints Authority Token expires.

5.1.3.  "jti" claim

   The "jti" claim, defined in [RFC7519] Section 4.1.7, MUST be included
   and contains a unique identifier for this JWTClaimConstraints
   Authority Token transaction.

5.1.4.  "atc" claim

   The "atc" claim MUST be included and is defined in [RFC9447].  It
   contains a JSON object with the following elements:

   *  a "tktype" key with a string value equal to "JWTClaimConstraints"
      to represent a JWTClaimConstraints profile of the authority token
      [RFC9447] defined by this document. "tktype" is a required key and
      MUST be included.

   *  a "tkvalue" key with a string value equal to the base64url
      encoding of the JWTClaimConstraints or EnhancedJWTClaimConstraints
      certificate extension ASN.1 object using DER encoding rules.
      "tkvalue" is a required key and MUST be included.

   *  a "ca" key with a boolean value set to false (since the
      JWTClaimConstraints extension is applicable only to end-entity
      certificates). "ca" is an optional key; if not included, the "ca"
      value is considered false by default.

   *  a "fingerprint" key is constructed as defined in [RFC8555]
      Section 8.1 corresponding to the computation of the fingerprint
      step using the ACME account key credentials. "fingerprint" is a
      required key and MUST be included.

   An example of the JWTClaimConstraints Authority Token is as follows:

Wendt & Hancock         Expires 25 September 2026               [Page 8]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   {
     "protected": base64url({
       "typ": "JWT",
       "alg": "ES256",
       "x5u": "https://authority.example.org/cert"
     }),
     "payload": base64url({
       "iss": "https://authority.example.org",
       "exp": 1640995200,
       "jti": "id6098364921",
       "atc": {
         "tktype": "JWTClaimConstraints",
         "tkvalue": "F83n2a...avn27DN3",
         "ca": false,
         "fingerprint": "SHA256 56:3E:CF:AE:83:CA:4D:15:B0:29:FF:1B:71:
          D3:BA:B9:19:81:F8:50:9B:DF:4A:D4:39:72:E2:B1:F0:B9:38:E3"
       }
     }),
     "signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ"
   }

5.2.  Acquiring the token from the Token Authority

   Following [RFC9447] Section 5, the authority token should be acquired
   using a RESTful HTTP POST transaction as follows:

   POST /at/account/:id/token HTTP/1.1
   Host: authority.example.org
   Content-Type: application/json

   The request will pass the account id as a string in the request
   parameter "id".  Note that this account identifier refers to the ACME
   client's account with the Token Authority, and is distinct from the
   ACME account used with the CA.  There is assumed to be a
   corresponding authentication procedure that can be verified for the
   success of this transaction, for example, an HTTP Authorization
   header field containing valid authorization credentials as defined in
   [RFC7231] Section 14.8.

   The body of the POST request MUST contain a JSON object with key
   value pairs corresponding to values that are requested as the content
   of the claims in the issued token.

   As an example, the body SHOULD contain a JSON object as follows:

Wendt & Hancock         Expires 25 September 2026               [Page 9]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   {
     "atc": {
       "tktype": "JWTClaimConstraints",
       "tkvalue": "F83n2a...avn27DN3",
       "ca": false,
       "fingerprint": "SHA256 56:3E:CF:AE:83:CA:4D:15:B0:29:FF:1B:71:D3
         :BA:B9:19:81:F8:50:9B:DF:4A:D4:39:72:E2:B1:F0:B9:38:E3"
     }
   }

   The response to the POST request if successful returns a 200 OK with
   a JSON body that contains, at a minimum, the JWTClaimConstraints
   Authority Token as a JSON object with a key of "token" and the
   base64url encoded string representing the atc token.  JSON is easily
   extensible, so users of this specification may want to pass other
   pieces of information relevant to a specific application.

   An example successful response would be as follows:

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

   {"token": "DGyRejmCefe7v4N...vb29HhjjLPSggwiE"}

   The ACME client then uses the value of the "token" field as the
   "tkauth" field in the challenge response POST to the ACME challenge
   URL, as described in Section 4.

   If the request is not successful, the response should indicate the
   error condition.  Specifically, for the case that the authorization
   credentials are invalid or if the Account ID provided does not exist,
   the response code MUST be 403 - Forbidden.  Other 4xx and 5xx
   responses MUST follow standard [RFC7231] HTTP error condition
   conventions.

5.3.  Token Authority Responsibilities

   When creating the JWTClaimConstraints Authority Token, the Token
   Authority MUST validate that the information contained in the ASN.1
   JWTClaimConstraints accurately represents the corresponding
   JWTClaimConstraint resources the requesting party is authorized to
   represent based on their pre-established and verified secure
   relationship between the Token Authority and the requesting party.

Wendt & Hancock         Expires 25 September 2026              [Page 10]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   The fingerprint in the token request is not meant to be verified by
   the Token Authority.  Rather, it is meant to be signed as part of the
   token so that the party that requests the token can, as part of the
   challenge response, allow the ACME server to validate that the token
   requested and used came from the same party that controls the ACME
   client.

5.4.  Scope of the JWTClaimConstraints

   Because this specification involves the JWTClaimConstraints and
   EnhancedJWTClaimConstraints extensions, the client MAY request an
   Authority Token with some subset of its own authority as the
   JWTClaimConstraints provided in the "tkvalue" element of the "atc"
   JSON object.  JWTClaimConstraints can be constructed to define a
   limited scope of claims and claim values the client has authority
   over.

5.5.  ACME Challenges requiring multiple Authority Tokens

   The ACME new-order request may include multiple identifiers, each of
   which is authorized separately.  With the introduction of this
   specification, for STIR certificates [RFC8226], a certificate order
   may require two Authority Token identifier types:

   *  The JWTClaimConstraints identifier defined in this document, and

   *  The TNAuthList identifier defined in [RFC9448].

   Other Authority Token types may be introduced in future Authority
   Token profile specifications with similar requirements.

   This section describes scenarios where a new-order request contains
   both of these identifier types.  In such cases, the CA requires the
   ACME client to provide both a JWTClaimConstraints Authority Token and
   a TNAuthList Authority Token as part of the challenge response.

   The TNAuthList Authority Token authorizes the token holder to obtain
   certificates containing a TNAuthList extension whose scope is less
   than or equal to the scope of the TNAuthList identifier in the token.

   The JWTClaimConstraints Authority Token authorizes the token holder
   to obtain a certificate containing a JWTClaimConstraints or
   EnhancedJWTClaimConstraints extension, provided that the extension is
   within the scope of the JWTClaimConstraints identifier in the token.
   Since these two certificate extensions constrain the resources and
   claims available to the certificate, there is an inherent interaction
   between these two types of Authority Tokens.

Wendt & Hancock         Expires 25 September 2026              [Page 11]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   The "value" field of the JWTClaimConstraints identifier, and the
   corresponding "tkvalue" in the "atc" claim, is a base64url-encoded
   DER representation of a JWTClaimConstraints or
   EnhancedJWTClaimConstraints ASN.1 object as defined in [RFC8226] and
   [RFC9118].  From the ACME server's perspective this value is opaque:
   validation step 5 in Section 6 requires only that the value in the
   token matches the value in the order.  The semantic content of the
   extension — which claims are permitted or excluded, and for which
   telephone numbers — is a STIR ecosystem concern governed by [RFC8226]
   and [RFC9118], and is validated by the Token Authority prior to token
   issuance (see Section 5.3).  Informative examples of
   JWTClaimConstraints ASN.1 structures, their DER encodings, and
   corresponding base64url values are provided in Appendix A.

5.5.1.  ACME Procedures when Challenge requires two Authority Tokens

   Sections 3 and 4 describe the ACME procedures for issuing a
   certificate based on a single JWTClaimConstraints identifier.  This
   section describes how these procedures are modified to support the
   case where the new-order request contains both a TNAuthList and
   JWTClaimConstraints identifier.

   First, the "identifiers" field in the new-order request includes both
   identifier types:

        "identifiers": [
        {"type": "TNAuthList",
         "value": "KHn6xf...jw4A1vgh"},
        {"type": "JWTClaimConstraints",
         "value": "F83n2a...avn27DN3"}]

   The CA includes two "authorizations" URLs in the 201 (Created)
   response to the new-order request, one for each identifier:

        "authorizations": [
           "https://example.com/acme/authz/1234",
           "https://example.com/acme/authz/5678"]

   The ACME client then queries each "authorizations" URL as shown in
   Section 4.  The CA returns the Authority Token challenge for each
   identifier.  The ACME client responds to each challenge by providing
   an Authority Token of the appropriate type.

Wendt & Hancock         Expires 25 September 2026              [Page 12]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

6.  Validating the JWTClaimConstraints Authority Token

   Upon receiving a response to the challenge, the ACME server MUST
   perform the following steps to determine the validity of the
   response.  Steps 1 through 3 and steps 6 through 8 are general ACME
   authority token validation steps applicable to any Authority Token
   profile.  Steps 4 and 5 are specific to the JWTClaimConstraints
   profile.  The semantic validation of whether the JWTClaimConstraints
   or EnhancedJWTClaimConstraints content accurately reflects the
   requesting party's authorized resources is the responsibility of the
   Token Authority prior to issuing the token (see Section 5.3), and is
   governed by [RFC8226] and [RFC9118].

   1.  Verify that the value of the "atc" claim is a well-formed JSON
       object containing the mandatory key values ("tktype", "tkvalue",
       "fingerprint").

   2.  Verify Token Issuer: If there is an "x5u" parameter, verify the
       "x5u" parameter is an HTTPS URL with a reference to a certificate
       representing the trusted issuer of authority tokens.  If there is
       an "x5c" parameter, verify the certificate array contains a
       certificate representing the trusted issuer of authority tokens.

   3.  Verify Signature: Verify the JWTClaimConstraints Authority Token
       signature using the public key of the certificate referenced by
       the token's "x5u" or "x5c" parameter.

   4.  Verify Token Type: Verify that the "atc" claim contains a
       "tktype" identifier with the value "JWTClaimConstraints".

   5.  Verify Constraints Match: Verify that the "atc" claim "tkvalue"
       identifier contains the equivalent base64url encoded
       JWTClaimConstraints or EnhancedJWTClaimConstraints certificate
       extension string value as the Identifier specified in the
       original challenge.

   6.  Verify Claims: Verify that the remaining claims are valid (e.g.,
       verify that the token has not expired using the "exp" claim).

   7.  Verify Account Control: Verify that the "atc" claim "fingerprint"
       is valid and matches the account key of the client making the
       request.

   8.  Verify CA Flag: Verify that the "atc" claim "ca" identifier
       boolean corresponds to the CA boolean in the Basic Constraints
       extension in the Certificate Signing Request (CSR) for either a
       CA certificate or an end-entity certificate.

Wendt & Hancock         Expires 25 September 2026              [Page 13]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   If all steps in the token validation process pass, then the ACME
   server MUST set the challenge object "status" to "valid".  If any
   step of the validation process fails, the "status" in the challenge
   object MUST be set to "invalid".

7.  Using ACME-issued Certificates with JSON Web Signature

   JSON Web Signature (JWS) objects can include an "x5u" header
   parameter to refer to a certificate for signature validation.  In
   order to support this usage, the Certificate Authority (CA) MAY host
   the newly issued certificate and provide a URL that the ACME client
   owner can directly reference in the "x5u" header parameter of their
   signed JWS objects.

   To facilitate this, the CA MAY add a newly defined field called "x5u"
   to the 200 (OK) order object response when the certificate is ready
   for the finalize request:

   x5u (optional, string): A URL that can be used to reference the
   certificate in the "x5u" parameter of a JWS object.

   An example of a 200 (OK) response containing the new "x5u" field:

Wendt & Hancock         Expires 25 September 2026              [Page 14]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   HTTP/1.1 200 OK
   Content-Type: application/json
   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": "JWTClaimConstraints",
       "value": "F83n2a...avn27DN3"
     ],

     "authorizations": ["https://example.com/acme/authz/1234"],

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

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

     "x5u": "https://example.com/cert-repo/giJI53km23.pem"
   }

8.  Security Considerations

   The token represented by this document has the credentials to
   represent JWTClaimConstraints and EnhancedJWTClaimConstraints, which
   constrain the resources and claims a certificate holder can assert.
   The creation, transport, and any storage of this token MUST follow
   the strictest of security best practices, beyond the recommendations
   of the use of encrypted transport protocols in this document, to
   protect it from getting in the hands of bad actors with illegitimate
   intent to impersonate or misuse the constrained resources.

   This document inherits the security properties of [RFC9447].
   Implementations SHOULD follow the best practices identified in
   [RFC8725] for cryptographic security.

   This document only specifies SHA256 for the fingerprint hash.
   However, the syntax of the fingerprint object would permit other
   algorithms if, due to concerns about algorithmic agility, a more
   robust algorithm were required at a future time.  Future
   specifications CAN define new algorithms for the fingerprint object
   as needed.

Wendt & Hancock         Expires 25 September 2026              [Page 15]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

9.  IANA Considerations

   This document requests the addition of a new identifier object type
   to the "ACME Identifier Types" registry defined in Section 9.7.7 of
   [RFC8555].

   +---------------------+-----------+
   |        Label        | Reference |
   +---------------------+-----------+
   | JWTClaimConstraints |  RFCThis  |
   +---------------------+-----------+

10.  Acknowledgements

   We would like to thank ACME and STIR working groups for valuable
   contributions to the authority token framework used in this document.

11.  Normative References

   [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/rfc/rfc2119>.

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

   [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/rfc/rfc7231>.

   [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/rfc/rfc7515>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7519>.

   [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/rfc/rfc8174>.

Wendt & Hancock         Expires 25 September 2026              [Page 16]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,
              <https://www.rfc-editor.org/rfc/rfc8226>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/rfc/rfc8555>.

   [RFC8725]  Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
              Current Practices", BCP 225, RFC 8725,
              DOI 10.17487/RFC8725, February 2020,
              <https://www.rfc-editor.org/rfc/rfc8725>.

   [RFC9060]  Peterson, J., "Secure Telephone Identity Revisited (STIR)
              Certificate Delegation", RFC 9060, DOI 10.17487/RFC9060,
              September 2021, <https://www.rfc-editor.org/rfc/rfc9060>.

   [RFC9118]  Housley, R., "Enhanced JSON Web Token (JWT) Claim
              Constraints for Secure Telephone Identity Revisited (STIR)
              Certificates", RFC 9118, DOI 10.17487/RFC9118, August
              2021, <https://www.rfc-editor.org/rfc/rfc9118>.

   [RFC9447]  Peterson, J., Barnes, M., Hancock, D., and C. Wendt,
              "Automated Certificate Management Environment (ACME)
              Challenges Using an Authority Token", RFC 9447,
              DOI 10.17487/RFC9447, September 2023,
              <https://www.rfc-editor.org/rfc/rfc9447>.

   [RFC9448]  Wendt, C., Hancock, D., Barnes, M., and J. Peterson,
              "TNAuthList Profile of Automated Certificate Management
              Environment (ACME) Authority Token", RFC 9448,
              DOI 10.17487/RFC9448, September 2023,
              <https://www.rfc-editor.org/rfc/rfc9448>.

Appendix A.  JWTClaimConstraints Identifier Value Examples

   This appendix provides informative examples of JWTClaimConstraints
   and EnhancedJWTClaimConstraints ASN.1 structures as defined in
   [RFC8226] and [RFC9118], along with their DER encodings and base64url
   values as used in the ACME identifier "value" field and the "tkvalue"
   of the "atc" claim.  These examples are intended for implementers of
   STI Token Authorities and certificate requestors in the STIR/SHAKEN
   ecosystem.  ACME server implementations treat the identifier value as
   opaque and are not required to parse or validate its internal
   structure.

Wendt & Hancock         Expires 25 September 2026              [Page 17]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   Note: single-quote delimiters in the ASN.1 notation below denote
   string values; inner double quotes are part of the JSON-formatted
   claim values as defined in [RFC8226].

A.1.  No Extended Claims Authorized

   In this case, the requestor is authorized to use a set of telephone
   numbers but no optional claim information.  The
   EnhancedJWTClaimConstraints extension uses a mustExclude constraint
   to prohibit all optional claims relevant to the application.

   SEQUENCE {
     mustExclude [2] {
       SEQUENCE {
         IA5String 'attest'
         IA5String 'origid'
         IA5String 'div'
         IA5String 'rph'
         IA5String 'sph'
         IA5String 'rcd'
         IA5String 'rcdi'
         IA5String 'crn'
         }
       }
     }

   DER encoding (51 bytes, hex):

   30 31 a2 2f 16 06 61 74 74 65 73 74 16 06 6f 72
   69 67 69 64 16 03 64 69 76 16 03 72 70 68 16 03
   73 70 68 16 03 72 63 64 16 04 72 63 64 69 16 03
   63 72 6e

   base64url value:

   MDGiLxYGYXR0ZXN0FgZvcmlnaWQWA2RpdhYDcnBoFgNzcGgWA3JjZBYEcmNkaRYDY3Ju

   A simpler alternative for requestors not authorized to include
   optional claims is to submit a new-order request containing only a
   TNAuthList identifier.  In this case, the absence of a
   JWTClaimConstraints identifier MAY trigger local policy in the CA to
   include a restrictive EnhancedJWTClaimConstraints extension in the
   issued certificate.

Wendt & Hancock         Expires 25 September 2026              [Page 18]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

A.2.  Extended Claims Authorized (Uniform Constraints)

   In this case, the requestor is authorized to assert a specific set of
   claim information that applies uniformly across all authorized
   telephone numbers.  The extension uses a permittedValues constraint
   for the authorized claims and a mustExclude constraint for the
   remainder.

   SEQUENCE {
     permittedValues [1] {
       SEQUENCE {
         SEQUENCE {
           IA5String 'rcd'
           SEQUENCE {
             UTF8String '"nam": "James Bond"'
             }
           IA5String 'crn'
           SEQUENCE {
             UTF8String '"For your ears only"'
             }
           }
         }
       }
     mustExclude [2] {
       SEQUENCE {
         IA5String 'attest'
         IA5String 'origid'
         IA5String 'div'
         IA5String 'rph'
         IA5String 'sph'
         IA5String 'rcdi'
         }
       }
     }

   DER encoding (104 bytes, hex):

   30 66 a1 3d 30 3b 30 39 16 03 72 63 64 30 15 0c
   13 22 6e 61 6d 22 3a 20 22 4a 61 6d 65 73 20 42
   6f 6e 64 22 16 03 63 72 6e 30 16 0c 14 22 46 6f
   72 20 79 6f 75 72 20 65 61 72 73 20 6f 6e 6c 79
   22 a2 25 16 06 61 74 74 65 73 74 16 06 6f 72 69
   67 69 64 16 03 64 69 76 16 03 72 70 68 16 03 73
   70 68 16 04 72 63 64 69

   base64url value:

Wendt & Hancock         Expires 25 September 2026              [Page 19]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   MGahPTA7MDkWA3JjZDAVDBMibmFtIjogIkphbWVzIEJvbmQiFgNjcm4wFgwUIkZv
   ciB5b3VyIGVhcnMgb25seSKiJRYGYXR0ZXN0FgZvcmlnaWQWA2RpdhYDcnBoFgNz
   cGgWBHJjZGk

A.3.  Extended Claims Authorized (Per-TN Subset Constraints)

   In this case, the requestor is authorized to assert different sets of
   claims for distinct subsets of telephone numbers.  This is expressed
   by including an "orig" claim in the permittedValues entry to
   associate specific claim values with a particular set of telephone
   numbers.

   SEQUENCE {
     permittedValues [1] {
       SEQUENCE {
         SEQUENCE {
           IA5String 'rcd'
           SEQUENCE {
             UTF8String '"nam": "James Bond"'
             }
           IA5String 'crn'
           SEQUENCE {
             UTF8String '"For your ears only"'
             }
           IA5String 'orig'
           SEQUENCE {
             UTF8String '"12025551000"'
             UTF8String '"12025551001"'
             }
           }
         }
       }
     mustExclude [2] {
       SEQUENCE {
         IA5String 'attest'
         IA5String 'origid'
         IA5String 'div'
         IA5String 'rph'
         IA5String 'sph'
         IA5String 'rcdi'
         }
       }
     }

   DER encoding (143 bytes, hex):

Wendt & Hancock         Expires 25 September 2026              [Page 20]
Internet-Draft     ACME JWTClaimConstraints Auth Token        March 2026

   30 81 8c a1 63 30 61 30 5f 16 03 72 63 64 30 15
   0c 13 22 6e 61 6d 22 3a 20 22 4a 61 6d 65 73 20
   42 6f 6e 64 22 16 03 63 72 6e 30 16 0c 14 22 46
   6f 72 20 79 6f 75 72 20 65 61 72 73 20 6f 6e 6c
   79 22 16 04 6f 72 69 67 30 1e 0c 0d 22 31 32 30
   32 35 35 35 31 30 30 30 22 0c 0d 22 31 32 30 32
   35 35 35 31 30 30 31 22 a2 25 16 06 61 74 74 65
   73 74 16 06 6f 72 69 67 69 64 16 03 64 69 76 16
   03 72 70 68 16 03 73 70 68 16 04 72 63 64 69

   base64url value:

   MIGMoWMwYTBfFgNyY2QwFQwTIm5hbSI6ICJKYW1lcyBCb25kIhYDY3JuMBYMFCJG
   b3IgeW91ciBlYXJzIG9ubHkiFgRvcmlnMB4MDSIxMjAyNTU1MTAwMCIMDSIxMjAy
   NTU1MTAwMSKiJRYGYXR0ZXN0FgZvcmlnaWQWA2RpdhYDcnBoFgNzcGgWBHJjZGk

Authors' Addresses

   Chris Wendt
   Somos Inc.
   United States of America
   Email: chris@appliedbits.com

   David Hancock
   Somos Inc.
   United States of America
   Email: davidhancock.ietf@gmail.com

Wendt & Hancock         Expires 25 September 2026              [Page 21]