TNAuthList profile of ACME Authority Token

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. 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 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 , which can be carried in using protocols such as SIP . Currently, the only defined credentials for this purpose are the certificates specified in . describes certificate extensions suitable for associating telephone numbers and service provider codes with certificates. Specifically, the TN Authorization List defined in 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 . 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 .

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 .

In , 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 encoding of the TN Authorization List certificate extension ASN.1 object. The TN Authorization List certificate extension ASN.1 syntax is defined in section 9. An example of an ACME order object “identifiers” field containing a TNAuthList certificate would look as follows,

where the “value” object string represents the arbitrary length base64 encoded string. A full new-order request would look as follows,

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.

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.

;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 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 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.

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

The Telephone Number Authority List Authority Token (TNAuthList Authority Token) is an extension of the ACME Authority Token defined in . 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.

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.

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

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

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 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 , 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,

Following Section 5, the authority token should be acquired using a RESTful HTTP POST transaction as follows

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 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:

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:

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 HTTP error condition conventions.

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.

Because this specification specifically involves the TNAuthList defined in 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.

Upon receiving a response to the challenge, the ACME server MUST perform the following steps to determine the validity of the response. Verify that the token contained in the Payload “atc” field is an TNAuthList Authority Token. Verify the TNAuthList Authority Token signature using the public key of the certificate referenced by the token’s “x5u” parameter. Verify that “atc” claim contains an identifier type of “TNAuthList”. Verify that the “atc” claim contains the equivalent base64 encoded TNAuthList certificate extension string value as the Identifier specified in the original challenge. Verify that the remaining claims are valid (e.g., verify that token has not expired) Verify that the “atc” claim “fingerprint” is valid 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”.

There are many scenarios and reasons to have various combinations of SPCs, TNs, and TN Ranges. 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.

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 .

This document requests the addition of a new identifier object type to the “ACME Identifier Types” registry defined in Section 9.7.7 of .

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