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.

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Table of Contents

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].

[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 associations.

2. Terminology

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this 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 follows,

 "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,

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":"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
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"

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

     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.peterson-acme-authority-token].

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

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.

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 function.

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.

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 represent.

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.

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.

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

[I-D.ietf-acme-acme] Barnes, R., Hoffman-Andrews, J., McCarney, D. and J. Kasten, "Automatic Certificate Management Environment (ACME)", Internet-Draft draft-ietf-acme-acme-12, April 2018.
[I-D.peterson-acme-authority-token] Peterson, J., Barnes, M., Hancock, D. and C. Wendt, "ACME Challenges Using an Authority Token", Internet-Draft draft-peterson-acme-authority-token-01, 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.
[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-1000074] ATIS/SIP Forum NNI Task Group, "Signature-based Handling of Asserted information using toKENs (SHAKEN)", January 2017.
[ATIS-1000080] 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 Comcast One Comcast Center Philadelphia, PA 19103, USA EMail:
David Hancock Comcast EMail:
Mary Barnes iconectiv EMail:
Jon Peterson Neustar Inc. 1800 Sutter St Suite 570 Concord, CA 94520, US EMail: