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Versions: (draft-wendt-acme-authority-token-tnauthlist) 00

Network Working Group                                           C. Wendt
Internet-Draft                                                D. Hancock
Intended status: Informational                                   Comcast
Expires: January 2, 2019                                       M. Barnes
                                                             J. Peterson
                                                            Neustar Inc.
                                                           July 01, 2018

               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

   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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 2, 2019.

Copyright Notice

   Copyright (c) 2018 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
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   (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

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   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 . . . . . . . . . . . . .   4
   5.  TNAuthList Authority Token  . . . . . . . . . . . . . . . . .   6
     5.1.  "iss" claim . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  "exp" claim . . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  "jti" claim . . . . . . . . . . . . . . . . . . . . . . .   7
     5.4.  "atc" claim . . . . . . . . . . . . . . . . . . . . . . .   7
     5.5.  Acquiring the token from the Authority  . . . . . . . . .   8
     5.6.  Authority Responsibilities  . . . . . . . . . . . . . . .   8
   6.  Validating the TNAuthList Authority Token . . . . . . . . . .   8
   7.  Usage Considerations  . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Large number of Non-contiguous TNAuthList values  . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   [I-D.ietf-acme-acme] 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.peterson-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 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
   only defined credentials for this purpose are the certificates
   specified in [RFC8226].

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   [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 (SPC), Telephone Numbers
   (TNs), or Telephone Number ranges (TN ranges).  Typically, these
   identifiers have been associated 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

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 [I-D.ietf-acme-acme], Section 7.4 defines the procedure that an
   ACME client uses to order a new certificate from a Certificate
   Authority.  The new-order request contains an identifier object that
   specifies the identifiers the order corresponds to.  For the
   TNAuthList identifier, the new-order request MUST include a type set
   to the string "TNAuthList".  The value of the 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 request for a TNAuthList certificate would look as

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

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

   A full new-order request would look as follows,

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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/1",
    "nonce": "5XJ1L3lEkMG7tR6pA00clA",
    "url": "https://example.com/acme/new-order"
  "payload": base64url({
    "identifiers": [{"type:"TNAuthList","value":"F83n2a...avn27DN3=="}],
    "notBefore": "2018-01-01T00:00:00Z",
    "notAfter": "2018-01-08T00:00:00Z"
  "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"

4.  TNAuthList Identifier Authorization

   On receiving a valid new-order request, the CA creates an
   authorization challenge and can be queried by the following example
   request and response.

   GET /acme/authz/1234 HTTP/1.1
   Host: example.com

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   HTTP/1.1 200 OK
   Content-Type: application/json
   Link: <https://example.com/acme/some-directory>;rel="index"

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

     "identifier": {

     "challenges": [
         "type": "tkauth-01",
         "tkauth-type": "ATC",
         "token-authority": "https://authority.example.org/authz",
         "url": "https://boulder.example.com/authz/asdf/0"
         "token": "IlirfxKKXAsHtmzK29Pj8A"

   This follows [I-D.peterson-acme-authority-token] with a challenge
   with the specific identifier of type "TNAuthList" corresponding to
   new-order defined previously in this document.

   When processing a certificate order containing an identifier of type
   "TNAuthList", a CA MUST use the Authority Token challenge mechanism
   defined in [I-D.peterson-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 a CA to determine
   the 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 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
   Authority based on locally configured information or local policies.

   A client responds to this challenge by providing an TNAuthList
   Authority Token to the CA.  The ACME client MUST respond to the
   challenge by posting the TNAuthList Authority Token to the URL

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   identified in the ACME challenge with a request, an example of which

        POST /acme/authz/asdf/0 HTTP/1.1
        Host: sti-ca.com
        Content-Type: application/jose+json

         "protected": base64url({
         "alg": "ES256",
         "kid": "https://sti-ca.com/acme/reg/asdf",
         "nonce": "Q_s3MWoqT05TrdkM2MTDcw",
         "url": "https://sti-ca.com/acme/authz/asdf/0"
         "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.peterson-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 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 Authority that issued the TNAuthList Authority Token
   beyond the "x5u" Header claim that identifies the location of the
   certificate of the Authority used to validate the Authority Token.

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5.2.  "exp" claim

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

5.3.  "jti" claim

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

5.4.  "atc" claim

   The "atc" claim is the only claim specifically defined in this
   document.  It contains an array of three elements; a string set to
   "TNAuthList", the base64 encoded TNAuthList certificate extension
   string, and a fingerprint.

   The "fingerprint" value is a certificate fingerprint of the ACME
   credentials, defined in [RFC4949].  The fingerprint is of the
   certificate the SP used to create an account with the ACME server.  A
   certificate fingerprint is a secure one-way hash of the Distinguished
   Encoding Rules (DER) form of the certificate.  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

   An example of the TNAuthList Authority Token is as follows,

   { "typ":"JWT",

      "SHA256 56:3E:CF:AE:83:CA:4D:15:B0:29:FF:1B:71:D3:BA:B9:19:81:F8:50:

   Similar to how the TNAuthList identifier value is defined, the
   identifier value in the "atc" should also include the same base64
   encoded TNAuthList certificate extension string.

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5.5.  Acquiring the token from the Authority

   The specifics of how the token is acquired from the authority can
   vary and is out of the scope of this document.

5.6.  Authority Responsibilities

   When the Authority creates the Authority Token, it is the
   responsibility of the 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

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)

   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

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   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.

8.  Security Considerations


9.  Acknowledgements

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

10.  References

10.1.  Normative References

              Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", draft-ietf-acme-acme-12 (work in progress), April

              Peterson, J., Barnes, M., Hancock, D., and C. Wendt, "ACME
              Challenges Using an Authority Token", draft-peterson-acme-
              authority-token-01 (work in progress), March 2018.

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

   [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,

   [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,

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   [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,

10.2.  Informative References

              ATIS/SIP Forum NNI Task Group, "Signature-based Handling
              of Asserted information using toKENs (SHAKEN)", January

              ATIS/SIP Forum NNI Task Group, "Signature-based Handling
              of Asserted information using toKENs (SHAKEN) Governance
              Model and Certificate Management", 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,

Authors' Addresses

   Chris Wendt
   One Comcast Center
   Philadelphia, PA  19103

   Email: chris-ietf@chriswendt.net

   David Hancock

   Email: davidhancock.ietf@gmail.com

   Mary Barnes

   Email: mary.ietf.barnes@gmail.com

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

   Email: jon.peterson@neustar.biz

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