TNAuthList profile of ACME Authority Token

Internet-Draft	ACME TNAuthList Auth Token	February 2023
Wendt, et al.	Expires 11 August 2023	[Page]

Abstract

This document defines a profile of the Automated Certificate Management Environment (ACME) Authority Token for the automated and authorized creation of certificates for VoIP Telephone Providers to support Secure Telephony Identity (STI) using the TNAuthList defined by STI certificates.¶

1. Introduction

[RFC8555] is a mechanism for automating certificate management on the Internet. It enables administrative entities to prove effective control over resources like domain names, and automates the process of generating and issuing certificates. [I-D.ietf-acme-authority-token] extends ACME to provide a general method of extending the authority and authorization of entities to control a resource via a third party Token Authority beyond the Certification Authority (CA).¶

This document is a profile document using the Authority Token mechanism defined in [I-D.ietf-acme-authority-token]. It is a profile that specifically addresses the STIR problem statement [RFC7340] which identifies the need for Internet credentials that can attest authority for the originator of VoIP calls in order to detect impersonation, which is currently an enabler for common attacks associated with illegal robocalling, voicemail hacking, and swatting. These credentials are used to sign PASSporTs [RFC8225], which can be carried in using protocols such as SIP [RFC8224]. Currently, the only defined credentials for this purpose are the certificates specified in [RFC8226] using the TNAuthList. This document defines the use of the TNAuthList Authority Token in the ACME challenge to proof the authoritative use of the contents of the TNAuthList, including a Service Provider Token (SPC), a Telephone Number, or a set of telephone numbers or telephone number blocks.¶

This document also describes the ability for a telephone authority to authorize the creation of CA types of certificates for delegation as defined in [RFC9060].¶

3. ACME new-order identifiers for TNAuthList

In [RFC8555], Section 7 defines the procedure that an ACME client uses to order a new certificate from a 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 TNAuthList. A TNAuthList identifier contains the identity information to be populated in the TN Authorization List of the new certificate. For the TNAuthList identifier, the new-order request includes a type set to the string "TNAuthList". The value of the TNAuthList identifier MUST be set to the details of the TNAuthList requested.¶

The format of the string that represents the TNAuthList 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 TNAuthList ASN.1 object MUST encoded using DER encoding rules.¶

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

 "identifiers": [{"type":"TNAuthList","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":"TNAuthList","value":"F83n...n27DN3"}],
    "notBefore": "2021-01-01T00:00:00Z",
    "notAfter": "2021-01-08T00:00:00Z"
  }),
  "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 TNAuthList 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": "2022-01-08T00:00:00Z",

  "notBefore": "2022-01-01T00:00:00Z",
  "notAfter": "2022-01-08T00:00:00Z",
  "identifiers":[{"type":"TNAuthList",
                 "value":"F83n2a...avn27DN3"}],

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

4. TNAuthList Identifier 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": "2022-01-08T00:00:00Z",

  "identifier": {
    "type":"TNAuthList",
    "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 "TNAuthList", a CA uses the Authority Token challenge type of "tkauth-01" with a "tkauth-type" of "atc" in [I-D.ietf-acme-authority-token] to verify that the requesting ACME client has authenticated and authorized control over the requested resources represented by the "TNAuthList" value.¶

The challenge "token-authority" parameter is only used in cases where the VoIP telephone network requires the CA to identify the Token Authority. This is currently not the case for the SHAKEN [ATIS-1000080] certificate framework governance, but may be used by other frameworks. 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 TNAuthList 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 TNAuthList 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 TNAuthList Authority Token defined in the next section.¶

5. TNAuthList Authority Token

The Telephone Number Authority List Authority Token (TNAuthList Authority Token) is a profile instance of the ACME Authority Token defined in [I-D.ietf-acme-authority-token].¶

The TNAuthList Authority Token Protected header MUST comply with the Authority Token Protected header as defined in [I-D.ietf-acme-authority-token].¶

The TNAuthList 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 TNAuthList 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 TNAuthList 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 TNAuthList 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 TNAuthList Authority Token transaction.¶

5.4. "atc" claim

The "atc" claim MUST be included and is defined in [I-D.ietf-acme-authority-token]. It contains a JSON object with the following elements:¶

a "tktype" key with a string value equal to "TNAuthList" to represent a TNAuthList profile of the authority token [I-D.ietf-acme-authority-token] 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 TNAuthList 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":"TNAuthList",
      "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 [I-D.ietf-acme-authority-token] 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 a communications service provider (CSP). 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":"TNAuthList",
   "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 TNAuthList 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 TNAuthList Authority Token, the Token Authority MUST validate that the information contained in the ASN.1 TNAuthList accurately represents the service provider code (SPC) or telephone number (TN) 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 TNAuthList

Because this specification specifically involves the TNAuthList defined in [RFC8226] which involves SPC, TNBlock, and individual TNs, the client may also request an Authority Token with some subset of its own authority as the TNAuthList provided in the "tkvalue" element in the "atc" JSON object. Generally, the scope of authority representing a communications service provider is represented by a particular SPC (e.g. in North America, an operating company number (OCN) or service provider identifier (SPID)). That provider is also generally associated, based on number allocations, with a particular set of different TN Blocks and/or TNs. TNAuthList can be constructed to define a limited scope of the TNBlocks or TNs either associated with an SPC or with the scope of TN Blocks or TNs the client has authority over.¶

As recommended in [I-D.ietf-acme-authority-token] 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 TNAuthList Authority Token

Upon receiving a response to the challenge, the ACME server MUST perform the following steps to determine the validity of the response.¶

Verify that the value of the "atc" claim is a well-formed JSON object containing the mandatory key values.¶
If there is an "x5u" parameter verify the "x5u" parameter is a HTTPS URL with a reference to a certificate representing the trusted issuer of authority tokens for the eco-system.¶
If there is an "x5c" parameter verify the certificate array contains a certificate representing the trusted issuer of authority tokens for the eco-system.¶
Verify the TNAuthList Authority Token signature using the public key of the certificate referenced by the token's "x5u" or "x5c" parameter.¶
Verify that "atc" claim contains a "tktype" identifier with the value "TNAuthList".¶
Verify that the "atc" claim "tkvalue" identifier contains the equivalent base64url encoded TNAuthList certificate extension string value as the Identifier specified in the original challenge.¶
Verify that the remaining claims are valid (e.g., verify that token has not expired)¶
Verify that the "atc" claim "fingerprint" is valid and matches the account key of the client making the request¶
Verify that the "atc" claim "ca" identifier boolean corresponds to the CA boolean in the Basic Constraints extension in the CSR for either CA certificate or end-entity certificate¶

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 publically 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"
}

12. References

12.1. Normative References

[I-D.ietf-acme-authority-token]: Peterson, J., Barnes, M., Hancock, D., and C. Wendt, "ACME Challenges Using an Authority Token", Work in Progress, Internet-Draft, draft-ietf-acme-authority-token-09, 24 October 2022, <https://www.ietf.org/archive/id/draft-ietf-acme-authority-token-09.txt>.
[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>.
[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>.
[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>.
[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>.
[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/info/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/info/rfc8174>.
[RFC8226]: Peterson, J. and S. Turner, "Secure Telephone Identity Credentials: Certificates", RFC 8226, DOI 10.17487/RFC8226, February 2018, <https://www.rfc-editor.org/info/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/info/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/info/rfc8725>.
[RFC9060]: Peterson, J., "Secure Telephone Identity Revisited (STIR) Certificate Delegation", RFC 9060, DOI 10.17487/RFC9060, September 2021, <https://www.rfc-editor.org/info/rfc9060>.

12.2. Informative References

[ATIS-1000080]: ATIS/SIP Forum NNI Task Group, "Signature-based Handling of Asserted information using toKENs (SHAKEN) Governance Model and Certificate Management <https://access.atis.org/apps/group_public/download.php/32237/ATIS-1000080.pdf>", July 2017.
[RFC7340]: Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure Telephone Identity Problem Statement and Requirements", RFC 7340, DOI 10.17487/RFC7340, September 2014, <https://www.rfc-editor.org/info/rfc7340>.
[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, February 2018, <https://www.rfc-editor.org/info/rfc8224>.
[RFC8225]: Wendt, C. and J. Peterson, "PASSporT: Personal Assertion Token", RFC 8225, DOI 10.17487/RFC8225, February 2018, <https://www.rfc-editor.org/info/rfc8225>.

TNAuthList profile of ACME Authority Token

Abstract

Status of This Memo

Copyright Notice

Table of Contents

1. Introduction

2. Terminology