TNAuthList profile of ACME Authority Token

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Table of Contents

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].¶

2. Terminology

The keywords "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 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 as a base64url encoding, as per [RFC8555] base64url encoding is described in Section 5 of [RFC4648] according to the profile specified in JSON Web Signature in Section 2 of [RFC7515], of the TN Authorization List certificate extension ASN.1 object. 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
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.¶

{
  "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"
}

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

 {
   "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"
 }

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

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

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 with four key values.¶
• Verify the "x5u" parameter is a HTTPS URL with a reference to the public key of 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" parameter.¶
• Verify that "atc" claim contains an "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 new-order for either CA certificate or end-entity certificate¶

If all steps in the token validation process pass, then the CA 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. Usage Considerations

8. 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 a 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 [I-D.ietf-acme-authority-token]. 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 keys if, due to concerns about algorithmic agility, a more robust algorithm were required at a future time. Future specifications can define new keys for the fingerprint object as needed.¶

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 |
                         +------------+-----------+
                         | TNAuthList |  RFCThis  |
                         +------------+-----------+

10. Acknowledgements

We would like to thank Richard Barnes and Russ Housley for valuable contributions to this document.¶

Authors' Addresses