Internet-Draft ACME JWTClaimConstraints Auth Token March 2025
Wendt & Hancock Expires 4 September 2025 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-wendt-acme-authority-token-jwtclaimcon-00
Published:
Intended Status:
Standards Track
Expires:
Authors:
C. Wendt
Somos Inc.
D. Hancock
Somos Inc.

JWTClaimConstraints profile of ACME Authority Token

Abstract

This document defines an authority token profile for handling the validation of JWTClaimConstraints and Enhanced JWTClaimConstraints. This profile follows the model established in Authority Token for the validation of TNAuthList but is specifically tailored for the JWTClaimConstraints certificate extensions. The profile enables validation and challenge processes necessary to support certificates containing both TNAuthList and JWTClaimConstraints, particularly in the context of Secure Telephone Identity (STI).

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 4 September 2025.

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Table of Contents

1. Introduction

The validation of JWTClaimConstraints as part of an STI certificate defined in [RFC8226] is critical for ensuring the integrity and scope of claims used in PASSporTs. This document specifies an authority token profile for validating JWTClaimConstraints, modeled after the authority token framework established in [RFC9447] and the TNAuthList validation defined in [RFC9448]. This profile facilitates proper delegation and authorization for entities requesting certificates under ACME [RFC8555] and similar frameworks.

This Authority Token profile specifically addresses the inclusions of the STI certificate extensions JWTClaimConstraints, as defined in [RFC8226], and Enhanced JWTClaimConstraints, as defined in [RFC9118]. The STI certificate credentials are used to sign PASSporTs [RFC8225], which can be carried in using protocols such as SIP [RFC8224]. This document defines the use of the JWTClaimConstraints Authority Token in the ACME challenge to proof an authoritative or trusted use of the contents of the JWTClaimsConstraint based on the issuer of the token. The Certification Authority (CA) issuing the STI Certificate can be informed of the proper use and contents of the JWTClaimConstraints based on the STI eco-system policies, best practices, or authoritative information which is out of scope of this document and will be defined by the STI eco-system.

This document also discusses the ability for a telephone 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). The new-order request contains an identifier field that specifies the identifier objects the order corresponds to. 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 Enhanced JWTClaimConstraints of the new certificate. 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 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 encoded using DER encoding rules.

An example of an ACME order object “identifiers” field containing a JWTClaimConstraints certificate would look as follows,

 "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 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 as shown in the following example request and response.

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"
    }

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://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" 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 and is only used in cases where the VoIP telephone network requires the CA to identify the Token Authority. If a "token-authority" parameter is present, then the ACME client MAY use the "token-authority" 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, then 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, an example of which follows.

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

{
  "protected": base64url({
  "alg": "ES256",
  "kid": "https://example.com/acme/acct/evOfKhNU60wg",
  "nonce": "Q_s3MWoqT05TrdkM2MTDcw",
  "url": "https://boulder.example.com/acme/authz/asdf/0"
  }),
  "payload": base64url({
  "tkauth": "DGyRejmCefe7v4N...vb29HhjjLPSggwiE"
  }),
  "signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ"
}

The "tkauth" field is defined as a new field in the challenge object specific to the tkauth-01 challenge type that should contain the JWTClaimConstraints Authority Token defined in the next section.

5. JWTClaimConstraints Authority Token

The Telephone Number Authority List Authority Token (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].

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 detailed in the next subsections.

5.1. "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.

5.2. "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.

5.3. "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.

5.4. "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 TN Authorization List 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 either true when the requested certificate is allowed to be a CA cert for delegation uses or false when the requested certificate is not intended to be a CA cert, only an end-entity certificate. "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 "Thumbprint" 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:

{
  "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"
}

5.5. 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". This string will be managed as an identifier specific to the Token Authority's relationship with the entity that is creating and signing a PASSporT [RFC8225] and making the Certificate Signing Request via ACME. There is assumed to also be a corresponding authentication procedure that can be verified for the success of this transaction. For example, an HTTP authorization header containing a 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:

 {
   "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"}

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.

5.6. 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. Note that the fingerprint in the token request is not meant to be verified by the Token Authority, but rather 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 the token requested and used came from the same party that controls the ACME client.

5.7. Scope of the JWTClaimConstraints

Because this specification specifically involves the JWTClaimConstraints and Enhanced JWTClaimConstraints defined in [RFC8226] and [RFC9118] which involves the required or disallowed different claim types or claim values, the client may also request an Authority Token with some subset of its own authority as the JWTClaimConstraints provided in the "tkvalue" element in the "atc" JSON object. JWTClaimConstraints can be constructed to define a limited scope of claims and claim values the client has authority over.

As recommended in [RFC9447] security considerations, an Authority Token can either have a scope that attests all of the resources which a client is eligible to receive certificates for, or potentially a more limited scope that is intended to capture only those resources for which a client will receive a certificate from a particular certification authority. Any certification authority that sees an Authority Token can learn information about the resources a client can claim. In cases where this incurs a privacy risk, Authority Token scopes should be limited to only the resources that will be attested by the requested ACME certificate.

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.

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, [RFC7515]) objects can include an "x5u" header parameter to refer to a certificate that is used to validate the JWS signature. For example, the STIR PASSporT framework [RFC8225] uses "x5u" to indicate the STIR certificate used to validate the PASSporT JWS object. The URLs used in "x5u" are expected to provide the required certificate in response to a GET request, not a POST-as-GET as required for the "certificate" URL in the ACME order object. Thus the current mechanism generally requires the ACME client to download the certificate and host it on a public URL to make it accessible to relying parties. This section defines an optional mechanism for the Certificate Authority (CA) to host the certificate directly and provide a URL that the ACME client owner can directly reference in the "x5u" of their signed PASSporTs.

As described in Section 7.4 of [RFC8555] when the certificate is ready for making a finalize request, the server will return a 200 (OK) with the updated order object. In this response, an ACME Server can add a newly defined field called "x5u" that can pass this URL to the ACME client for usage in generated PASSporTs as a publicly available URL for PASSporT validation.

x5u (optional, string):

A URL that can be used to reference the certificate in the "x5u" parameter of a JWS object [RFC7515]

The publishing of the certificates at the new "x5u" URL should follow the GET request requirement as mentioned above and should be consistent with the timely publication according to the durations of the certificate lifecycle.

The following is an example of the use of "x5u" in the response when the certificate status is "valid".

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":"TNAuthList",
    "value":"F83n2a...avn27DN3"
  ],

  "authorizations": ["https://sti-ca.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. Usage Considerations

8.1. Large number of Non-contiguous TNAuthList values

There are many scenarios and reasons to have various combinations of SPCs, TNs, and TN Ranges. [RFC8226] has provided a somewhat unbounded set of combinations. It's possible that a complex non-contiguous set of telephone numbers are being managed by a CSP. Best practice may be simply to split a set of non-contiguous numbers under management into multiple STI certificates to represent the various contiguous parts of the greater non-contiguous set of TNs, particularly if length of the set of values in identifier object grows to be too large.

9. Security Considerations

The token represented by this document has the credentials to represent the scope of a telephone number, a block of telephone numbers, or an entire set of telephone numbers represented by an SPC. 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 telephone numbers.

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

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.

10. 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  |
+---------------------+-----------+

11. Acknowledgements

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

12. References

12.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC4648]
Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, , <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, , <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, , <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, , <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, , <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8226]
Peterson, J. and S. Turner, "Secure Telephone Identity Credentials: Certificates", RFC 8226, DOI 10.17487/RFC8226, , <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, , <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, , <https://www.rfc-editor.org/rfc/rfc8725>.
[RFC9060]
Peterson, J., "Secure Telephone Identity Revisited (STIR) Certificate Delegation", RFC 9060, DOI 10.17487/RFC9060, , <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, , <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, , <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, , <https://www.rfc-editor.org/rfc/rfc9448>.

12.2. Informative References

[RFC8224]
Peterson, J., Jennings, C., Rescorla, E., and C. Wendt, "Authenticated Identity Management in the Session Initiation Protocol (SIP)", RFC 8224, DOI 10.17487/RFC8224, , <https://www.rfc-editor.org/rfc/rfc8224>.
[RFC8225]
Wendt, C. and J. Peterson, "PASSporT: Personal Assertion Token", RFC 8225, DOI 10.17487/RFC8225, , <https://www.rfc-editor.org/rfc/rfc8225>.

Authors' Addresses

Chris Wendt
Somos Inc.
United States of America
David Hancock
Somos Inc.
United States of America