Network Working Group                                           C. Wendt
Internet-Draft                                                D. Hancock
Intended status: Standards Track                                 Comcast
Expires: August 26, 2021                                       M. Barnes
                                                             J. Peterson
                                                            Neustar Inc.
                                                       February 22, 2021

               TNAuthList profile of ACME Authority Token


   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.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 26, 2021.

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   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  ACME new-order identifiers for TNAuthList . . . . . . . . . .   3
   4.  TNAuthList Identifier Authorization . . . . . . . . . . . . .   5
   5.  TNAuthList Authority Token  . . . . . . . . . . . . . . . . .   7
     5.1.  "iss" claim . . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  "exp" claim . . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  "jti" claim . . . . . . . . . . . . . . . . . . . . . . .   8
     5.4.  "atc" claim . . . . . . . . . . . . . . . . . . . . . . .   8
     5.5.  Acquiring the token from the Token Authority  . . . . . .   9
     5.6.  Token Authority Responsibilities  . . . . . . . . . . . .  10
     5.7.  Scope of the TNAuthList token authority . . . . . . . . .  10
   6.  Validating the TNAuthList Authority Token . . . . . . . . . .  11
   7.  Usage Considerations  . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Large number of Non-contiguous TNAuthList values  . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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.

   This document 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

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   only defined credentials for this purpose are the certificates
   specified in [RFC8226].

   [RFC8226] describes certificate extensions suitable for associating
   telephone numbers and service provider codes with certificates.
   Specifically, the TN Authorization List defined in [RFC8226]
   Section 9, defines the ability to associate a STI certificate with a
   specific set of Service Provider Codes (SPCs), Telephone Numbers
   (TNs), or Telephone Number ranges (TN ranges).  Typically, these
   identifiers have been assigned to a Communications Service Provider
   (CSP) that is authorized to use a set of telephone numbers or
   telephone number ranges in association with a Service Provider Code
   as defined in [RFC8226].  The SPC is a unique code or string managed
   by a national regulatory body that has the authority over those code-
   to-CSP associations.

   This document also describes the ability for a telephone authority to
   authorize the creation of CA types of certificates for delegation as
   defined in [I-D.ietf-stir-cert-delegation].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  ACME new-order identifiers for TNAuthList

   In [RFC8555], Section 7.4 defines the procedure that an ACME client
   uses to order a new certificate from a Certification Authority.  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 MUST include 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 base64 [RFC4648] encoding of the TN Authorization
   List certificate extension ASN.1 object.  The TN Authorization List
   certificate extension ASN.1 syntax is defined in [RFC8226] section 9.

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

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

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   where the "value" object string represents the arbitrary length
   base64 encoded string.

   A full new-order request would look as follows,

   POST /acme/new-order HTTP/1.1
   Content-Type: application/jose+json

     "protected": base64url({
       "alg": "ES256",
       "kid": "",
       "nonce": "5XJ1L3lEkMG7tR6pA00clA",
       "url": ""
     "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 CA creates an
   authorization object 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.

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   HTTP/1.1 201 Created
   Replay-Nonce: MYAuvOpaoIiywTezizk5vw

     "status": "pending",
     "expires": "2015-03-01T14:09:00Z",

     "notBefore": "2021-01-01T00:00:00Z",
     "notAfter": "2021-01-08T00:00:00Z",

     "authorizations": [
     "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
       Content-Type: application/jose+json

         "protected": base64url({
           "alg": "ES256",
           "kid": "",
           "nonce": "uQpSjlRb4vQVCjVYAyyUWg",
           "url": "",
         "payload": "",
         "signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps"

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   HTTP/1.1 200 OK
   Content-Type: application/json
   Link: <>;rel="index"

     "status": "pending",
     "expires": "2018-03-03T14:09:00Z",

     "identifier": {

     "challenges": [
         "type": "tkauth-01",
         "tkauth-type": "atc",
         "token-authority": "",
         "url": ""
         "token": "IlirfxKKXAsHtmzK29Pj8A"

   When processing a certificate order containing an identifier of type
   "TNAuthList", a CA MUST use 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 optional and 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

   The ACME client MUST respond 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.

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   POST /acme/authz/asdf/0 HTTP/1.1
   Content-Type: application/jose+json

     "protected": base64url({
     "alg": "ES256",
     "kid": "",
     "nonce": "Q_s3MWoqT05TrdkM2MTDcw",
     "url": ""
     "payload": base64url({
     "atc": "DGyRejmCefe7v4N...vb29HhjjLPSggwiE"
     "signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ"

   The specifics of the construction of the TNAuthList specific "atc"
   token is defined in the next section.

5.  TNAuthList Authority Token

   The Telephone Number Authority List Authority Token (TNAuthList
   Authority Token) is an extension 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

   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.  It can be used as a URL
   identifying the Token Authority that issued the TNAuthList Authority
   Token beyond the "x5u" Header claim that identifies the location of
   the certificate of the Token Authority used to validate the
   TNAuthList Authority Token.

5.2.  "exp" claim

   The "exp" claim contains the DateTime value of the ending date and
   time that the TNAuthList Authority Token expires.

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5.3.  "jti" claim

   The "jti" claim contains a unique identifier for this TNAuthList
   Authority Token transaction.

5.4.  "atc" claim

   The "atc" claim is the only claim specifically defined in this
   document.  It contains a JSON object of three elements.

   o  a "tktype" key that is required 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.

   o  a "tkvalue" key with a string value equal to the TNAuthList
      identifier "value" string which MUST contain the base64 encoding
      of the TN Authorization List certificate extension ASN.1 object.
      "tkvalue" is a required key and MUST be included.

   o  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 MUST NOT be a CA cert
      and only an end-entity certificate. "ca" is an optional key, if it
      not included the "ca" value is considered false by default.

   o  a "fingerprint" key with a fingerprint value equal to the
      fingerprint, as defined in [RFC4949], of the ACME account
      credentials.  Specifically, the fingerprint value is a secure one-
      way hash of the Distinguished Encoding Rules (DER) form of the
      public key corresponding to the key pair the SP used to create the
      account with the ACME server.  The fingerprint value consists of
      the name of the hash function, which shall be 'SHA256' for this
      specification, followed by the hash value itself.  The hash value
      is represented as a sequence of uppercase hexadecimal bytes,
      separated by colons.  The number of bytes is defined by the hash
      function. "fingerprint" is a required key and MUST be included.

   An example of the TNAuthList Authority Token is as follows,

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     "protected": base64url({
     "payload": base64url({
         "fingerprint":"SHA256 56:3E:CF:AE:83:CA:4D:15:B0:29:FF:1B:71:
     "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

     POST /at/account/:id/token HTTP/1.1
     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 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 the "atc" JSON object that
   should be embedded in the token that is requested, for example the
   body should contain a JSON object as shown:

        "fingerprint":"SHA256 56:3E:CF:AE:83:CA:4D:15:B0:29:FF:1B:71:D3

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   The response to the POST request if successful MUST return 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 base64
   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, however,
   the "token" key MUST be passed at a minimum.

   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, the response code MUST be 403 - Forbidden.
   If the Account ID provided does not exist or does not match
   credentials in Authorization header, the response MUST be 404 -
   Invalid account ID.  Other 4xx and 5xx responses SHOULD follow
   standard [RFC7231] HTTP error condition conventions.

5.6.  Token Authority Responsibilities

   When the Token Authority creates the TNAuthList Authority Token, it
   is the responsibility of the Token Authority to validate that the
   information contained in the ASN.1 TNAuthList accurately represents
   the SPC or telephone number resources the ACME client is authorized
   to represent.

5.7.  Scope of the TNAuthList token authority

   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 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 OCN or SPID) which is
   associated with a particular set of different TN Blocks and/or TNs,
   although more often the former typically through a set of regulated
   authoritative registries or databases.  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.

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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

   o  Verify that the token contained in the Payload "atc" field is an
      TNAuthList Authority Token.

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

   o  Verify that "atc" claim contains an identifier type of

   o  Verify that the "atc" claim contains the equivalent base64 encoded
      TNAuthList certificate extension string value as the Identifier
      specified in the original challenge.

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

   o  Verify that the "atc" claim "fingerprint" is valid

   o  Verify that the "ca" claim boolean corresponds to the CSR request
      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

7.1.  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.

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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

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

                            |   Label    | Reference |
                            | TNAuthList |  RFCThis  |

10.  Acknowledgements

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

11.  References

11.1.  Normative References

              Peterson, J., Barnes, M., Hancock, D., and C. Wendt, "ACME
              Challenges Using an Authority Token", draft-ietf-acme-
              authority-token-05 (work in progress), March 2020.

              Peterson, J., "STIR Certificate Delegation", draft-ietf-
              stir-cert-delegation-03 (work in progress), July 2020.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,

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   [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,

   [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,

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,

11.2.  Informative References

              ATIS/SIP Forum NNI Task Group, "Signature-based Handling
              of Asserted information using toKENs (SHAKEN) Governance
              Model and Certificate Management
              download.php/32237/ATIS-1000080.pdf>", July 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,

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Authors' Addresses

   Chris Wendt
   One Comcast Center
   Philadelphia, PA  19103


   David Hancock


   Mary Barnes


   Jon Peterson
   Neustar Inc.
   1800 Sutter St Suite 570
   Concord, CA  94520


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