Internet Engineering Task Force (IETF)                          C. Wendt
Request for Comments: 9448                                    D. Hancock
Category: Standards Track                                     Somos Inc.
ISSN: 2070-1721                                                M. Barnes
                                                            J. Peterson
                                                           Neustar Inc.
                                                         September 2023


  TNAuthList Profile of Automated Certificate Management Environment
                        (ACME) Authority Token

Abstract

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

Status of This Memo

  This is an Internet Standards Track document.

  This document is a product of the Internet Engineering Task Force
  (IETF).  It represents the consensus of the IETF community.  It has
  received public review and has been approved for publication by the
  Internet Engineering Steering Group (IESG).  Further information on
  Internet Standards is available in Section 2 of RFC 7841.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  https://www.rfc-editor.org/info/rfc9448.

Copyright Notice

  Copyright (c) 2023 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
  (https://trustee.ietf.org/license-info) in effect on the date of
  publication of this document.  Please review these documents
  carefully, as they describe your rights and restrictions with respect
  to this document.  Code Components extracted from this document must
  include 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.  Requirements Language
  3.  ACME New-Order Identifiers for TNAuthList
  4.  TNAuthList Identifier Authorization
  5.  TNAuthList Authority Token
    5.1.  "iss" Claim
    5.2.  "exp" Claim
    5.3.  "jti" Claim
    5.4.  "atc" Claim
    5.5.  Acquiring the Token from the Token Authority
    5.6.  Token Authority Responsibilities
    5.7.  Scope of the TNAuthList
  6.  Validating the TNAuthList Authority Token
  7.  Using ACME-Issued Certificates with JSON Web Signature
  8.  Usage Considerations
    8.1.  Large Number of Noncontiguous TNAuthList Values
  9.  IANA Considerations
  10. Security Considerations
  11. References
    11.1.  Normative References
    11.2.  Informative References
  Acknowledgements
  Authors' Addresses

1.  Introduction

  [RFC8555] describes a mechanism for automating certificate management
  on the Internet.  It enables administrative entities to prove
  effective control over resources like domain names, and it automates
  the process of generating and issuing certificates.  [RFC9447]
  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 [RFC9447].  It is a profile that specifically
  addresses the Secure Telephone Identity Revisited (STIR) problem
  statement described in [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
  Personal Assertion Tokens (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 prove the
  authoritative use of the contents of the TNAuthList, including a
  Service Provider Code (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.  Requirements Language

  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 TNAuthList

  Section 7 of [RFC8555] 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 document defines a new type of identifier
  object called TNAuthList.  A TNAuthList identifier contains the
  identity information to be populated in the TNAuthList 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 string that represents the TNAuthList MUST be constructed using
  base64url encoding, as described in Section 5 of [RFC4648] and as
  defined in Section 2 of JSON Web Signature [RFC7515].  The base64url
  encoding MUST NOT include any padding characters, and the TNAuthList
  ASN.1 object MUST be encoded using DER encoding rules.

  An example of an ACME order object "identifiers" field containing a
  TNAuthList certificate is as follows:

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

  where the "value" object string represents the arbitrary length of
  the 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 [RFC9447] 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 Signature-based
  Handling of Asserted information using toKENs (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 TNAuthList Authority Token is a profile instance of the ACME
  Authority Token defined in [RFC9447].

  The TNAuthList Authority Token protected header MUST comply with
  "Request Authentication" (Section 6.2 of [RFC8555]).

  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 [RFC9447].  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 [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 TNAuthList 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 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 [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 identifier 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
  valid authorization credentials, as defined in [RFC9110],
  Section 11.6.2.

  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"
   }

  If successful, the response to the POST request 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 of a 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 identifier provided does
  not exist, the response code MUST be 403 (Forbidden).  Other 4xx and
  5xx responses MUST follow standard HTTP error condition conventions
  [RFC9110].

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, verified, and 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 that 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, telephone number ranges,
  and individual telephone numbers, 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 CSP is
  represented by a particular SPC (e.g., in North America, an operating
  company number (OCN) or service provider identifier (SPID)).  Based
  on number allocations, that provider is also generally associated
  with a particular set of different telephone number ranges and/or
  telephone numbers.  The TNAuthList can be constructed to define a
  limited scope of the TelephoneNumberRanges or TelephoneNumbers
  ([RFC8226], Section 9) either associated with an SPC or with the
  scope of telephone number ranges or telephone numbers the client has
  authority over.

  As recommended in the Security Considerations section in [RFC9447],
  an Authority Token can either have a scope that attests all of the
  resources that 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.

  1.  Verify that the value of the "atc" claim is a well-formed JSON
      object containing the mandatory key values.

  2.  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 for the ecosystem.

  3.  If there is an "x5c" parameter, verify the certificate array
      contains a certificate representing the trusted issuer of
      Authority Tokens for the ecosystem.

  4.  Verify the TNAuthList Authority Token signature using the public
      key of the certificate referenced by the token's "x5u" or "x5c"
      parameter.

  5.  Verify that "atc" claim contains a "tktype" identifier with the
      value "TNAuthList".

  6.  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.

  7.  Verify that the remaining claims are valid (e.g., verify that
      token has not expired).

  8.  Verify that the "atc" claim "fingerprint" is valid and matches
      the account key of the client making the request.

  9.  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 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 certification 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 life cycle.

  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 Noncontiguous 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
  noncontiguous set of telephone numbers are being managed by a CSP.
  Best practice may be simply to split a set of noncontiguous numbers
  under management into multiple STI certificates to represent the
  various contiguous parts of the greater noncontiguous set of TNs,
  particularly if the length of the set of values in an identifier
  object grows to be too large.

9.  IANA Considerations

  Per this document, IANA has added a new identifier object type to the
  "ACME Identifier Types" registry defined in Section 9.7.7 of
  [RFC8555].

                       +============+===========+
                       | Label      | Reference |
                       +============+===========+
                       | TNAuthList | RFC 9448  |
                       +------------+-----------+

                                Table 1

10.  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.

11.  References

11.1.  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/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>.

  [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>.

  [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
             Ed., "HTTP Semantics", STD 97, RFC 9110,
             DOI 10.17487/RFC9110, June 2022,
             <https://www.rfc-editor.org/info/rfc9110>.

  [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/info/rfc9447>.

11.2.  Informative References

  [ATIS-1000080]
             ATIS, "Signature-based Handling of Asserted information
             using toKENs (SHAKEN): Governance Model and Certificate
             Management", ATIS-1000080.v005, December 2022,
             <https://access.atis.org/apps/group_public/
             download.php/69428/ATIS-1000080.v005.pdf>.

  [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>.

Acknowledgements

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

Authors' Addresses

  Chris Wendt
  Somos Inc.
  United States of America
  Email: [email protected]


  David Hancock
  Somos Inc.
  United States of America
  Email: [email protected]


  Mary Barnes
  Neustar Inc.
  United States of America
  Email: [email protected]


  Jon Peterson
  Neustar Inc.
  Suite 570
  1800 Sutter St
  Concord, CA 94520
  United States of America
  Email: [email protected]

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