Internet-Draft ACME TNAuthList Auth Token February 2021
Wendt, et al. Expires 26 August 2021 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-ietf-acme-authority-token-tnauthlist-07
Published:
Intended Status:
Standards Track
Expires:
Authors:
C. Wendt
Comcast
D. Hancock
Comcast
M. Barnes
Independent
J. Peterson
Neustar Inc.

TNAuthList profile of 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 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 working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 26 August 2021.

▲

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.

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 (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", "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 [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=="}]

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
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":"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.

HTTP/1.1 201 Created
Replay-Nonce: MYAuvOpaoIiywTezizk5vw
Location: https://example.com/acme/order/1234

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

  "notBefore": "2021-01-01T00:00:00Z",
  "notAfter": "2021-01-08T00:00:00Z",
  "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/1",
        "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": "2018-03-03T14:09:00Z",

  "identifier": {
    "type:"TNAuthList",
    "value":"F83n2a...avn27DN3=="
  },

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

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.

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

{
  "protected": base64url({
  "alg": "ES256",
  "kid": "https://example.com/acme/acct/1",
  "nonce": "Q_s3MWoqT05TrdkM2MTDcw",
  "url": "https://boulder.example.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.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.

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.

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.

  • 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.
  • 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.
  • 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.
  • 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,

{
  "protected": base64url({
    "typ":"JWT",
    "alg":"ES256",
    "x5u":https://authority.example.org/cert
  }),
  "payload": base64url({
    "iss":"https://authority.example.org/authz",
    "exp":1300819380,
    "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 [I-D.ietf-acme-authority-token] 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.com
  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:

 {
   "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"}
 }

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.

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

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].

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.

11. References

11.1. Normative References

[I-D.ietf-acme-authority-token]
Peterson, J., Barnes, M., Hancock, D., and C. Wendt, "ACME Challenges Using an Authority Token", Work in Progress, Internet-Draft, draft-ietf-acme-authority-token-05, , <http://www.ietf.org/internet-drafts/draft-ietf-acme-authority-token-05.txt>.
[I-D.ietf-stir-cert-delegation]
Peterson, J., "STIR Certificate Delegation", Work in Progress, Internet-Draft, draft-ietf-stir-cert-delegation-03, , <http://www.ietf.org/internet-drafts/draft-ietf-stir-cert-delegation-03.txt>.
[RFC4648]
Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, , <https://www.rfc-editor.org/info/rfc4648>.
[RFC7231]
Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, , <https://www.rfc-editor.org/info/rfc7231>.
[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, , <https://www.rfc-editor.org/info/rfc8224>.
[RFC8225]
Wendt, C. and J. Peterson, "PASSporT: Personal Assertion Token", RFC 8225, DOI 10.17487/RFC8225, , <https://www.rfc-editor.org/info/rfc8225>.
[RFC8226]
Peterson, J. and S. Turner, "Secure Telephone Identity Credentials: Certificates", RFC 8226, DOI 10.17487/RFC8226, , <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, , <https://www.rfc-editor.org/info/rfc8555>.

11.2. Informative References

[ATIS-1000080]
ATIS/SIP Forum NNI Task Group, "Signature-based Handling of Asserted information using toKENs (SHAKEN) Governance Model and Certificate Management <https://access.atis.org/apps/group_public/download.php/32237/ATIS-1000080.pdf>", .
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC4949]
Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, , <https://www.rfc-editor.org/info/rfc4949>.
[RFC7340]
Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure Telephone Identity Problem Statement and Requirements", RFC 7340, DOI 10.17487/RFC7340, , <https://www.rfc-editor.org/info/rfc7340>.

Authors' Addresses

Chris Wendt
Comcast
One Comcast Center
Philadelphia, PA 19103,
United States of America
David Hancock
Comcast
Mary Barnes
Independent
Jon Peterson
Neustar Inc.
1800 Sutter St Suite 570
Concord, CA 94520,
United States of America