TNAuthList profile of ACME Authority Token

Introduction 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 (CA). This document is a profile document using the Authority Token mechanism defined in . It is a profile that specifically 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 using the TNAuthList. This document defines the use of the TNAuthList Authority Token in the ACME challenge to proof the authoritative use of the contents of the TNAuthList, including a Service Provider Token (SPC), a Telephone Number, or a set of telephone numbers or telephone number blocks. This document also describes the ability for a telephone authority to authorize the creation of CA types of certificates for delegation as defined in .

Terminology The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

ACME new-order identifiers for TNAuthList In , Section 7 defines the procedure that an ACME client uses to order a new certificate from a CA. 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 includes 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 using base64url encoding, as per base64url encoding described in Section 5 of according to the profile specified in JSON Web Signature in Section 2 of . The base64url encoding MUST NOT include any padding characters and the TNAuthList ASN.1 object MUST encoded using DER encoding rules. 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 base64url encoded string. A full new-order request would look as follows,

On receiving a valid new-order request, the ACME server creates an authorization object, Section 7.1.4, 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.

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.

;rel="index" { "status": "pending", "expires": "2022-01-08T00:00:00Z", "identifier": { "type:"TNAuthList", "value":"F83n2a...avn27DN3" }, "challenges": [ { "type": "tkauth-01", "tkauth-type": "atc", "token-authority": "https://authority.example.org", "url": "https://example.com/acme/chall/prV_B7yEyA4", "token": "IlirfxKKXAsHtmzK29Pj8A" } ] } ]]> When processing a certificate order containing an identifier of type "TNAuthList", a CA uses 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 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 responds 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 "tkauth" field is defined as a new field in the challenge object specific to the tkauth-01 challenge type that should contain the TNAuthList Authority Token defined in the next section.

TNAuthList Authority Token The Telephone Number Authority List Authority Token (TNAuthList Authority Token) is a profile instance 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.

"iss" claim The "iss" claim is an optional claim defined in Section 4.1.1. It can be used as a URL identifying the Token Authority that issued the TNAuthList Authority Token beyond the "x5u" or other Header claims that identify the location of the certificate or certificate chain of the Token Authority used to validate the TNAuthList Authority Token.

"exp" claim The "exp" claim, defined in Section 4.1.4, MUST be included and contains the DateTime value of the ending date and time that the TNAuthList Authority Token expires.

"jti" claim The "jti" claim, defined in Section 4.1.7, MUST be included and contains a unique identifier for this TNAuthList Authority Token transaction.

"atc" claim The "atc" claim MUST be included and is defined in . It contains a JSON object with the following elements: a "tktype" key with a string value equal to "TNAuthList" to represent a TNAuthList profile of the authority token defined by this document. "tktype" is a required key and MUST be included. a "tkvalue" key with a string value equal to the base64url encoding of the TN Authorization List certificate extension ASN.1 object using DER encoding rules. "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 is not intended to be a CA cert, 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 is constructed as defined in Section 8.1 corresponding to the computation of the "Thumbprint" step using the ACME account key credentials. "fingerprint" is a required key and MUST be included. An example of the TNAuthList Authority Token is as follows:

Acquiring the token from the Token Authority 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 communications service provider (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 a JSON object with key value pairs corresponding to values that are requested as the content of the claims in the issued token. As an example, the body SHOULD contain a JSON object as follows:

The response to the POST request if successful returns 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 base64url 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. 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 or if the Account ID provided does not exist, the response code MUST be 403 - Forbidden. Other 4xx and 5xx responses MUST follow standard HTTP error condition conventions.

Token Authority Responsibilities When creating the TNAuthList Authority Token, the Token Authority MUST validate that the information contained in the ASN.1 TNAuthList accurately represents the service provider code (SPC) or telephone number (TN) resources the requesting party is authorized to represent based on their pre-established and verified secure relationship between the Token Authority and the requesting party. Note that the fingerprint in the token request is not meant to be verified by the Token Authority, but rather is meant to be signed as part of the token so that the party that requests the token can, as part of the challenge response, allow the ACME server to validate the token requested and used came from the same party that controls the ACME client.

Scope of the TNAuthList 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 as 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 operating company number (OCN) or service provider identifier (SPID)). That provider is also generally associated, based on number allocations, with a particular set of different TN Blocks and/or TNs. 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. As recommended in security considerations, an Authority Token can either have a scope that attests all of the resources which a client is eligible to receive certificates for, or potentially a more limited scope that is intended to capture only those resources for which a client will receive a certificate from a particular certification authority. Any certification authority that sees an Authority Token can learn information about the resources a client can claim. In cases where this incurs a privacy risk, Authority Token scopes should be limited to only the resources that will be attested by the requested ACME certificate.

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. Verify that the value of the "atc" claim is a well-formed JSON object containing the mandatory key values. Verify the "x5u" parameter is a HTTPS URL with a reference to a certificate representing the trusted issuer of authority tokens for the eco-system. 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 a "tktype" identifier with the value "TNAuthList". Verify that the "atc" claim "tkvalue" identifier contains the equivalent base64url 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 and matches the account key of the client making the request Verify that the "atc" claim "ca" identifier boolean corresponds to the CA boolean in the Basic Constraints extension in the CSR 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".

Using ACME-issued Certificates with JSON Web Signature JSON Web Signature (JWS, ) objects can include an "x5u" header parameter to refer to a certificate that is used to validate the JWS signature. For example, the STIR PASSporT framework uses "x5u" to indicate the STIR certificate used to validate the PASSporT JWS object. The URLs used in "x5u" are expected to provide the required certificate in response to a GET request, not a POST-as-GET as required for the "certificate" URL in the ACME order object. Thus the current mechanism generally requires the ACME client to download the certificate and host it on a public URL to make it accessible to relying parties. This section defines an optional mechanism for the Certificate Authority (CA) to host the certificate directly and provide a URL that the ACME client owner can directly reference in the "x5u" of their signed PASSporTs. As described in Section 7.4 of when the certificate is ready for making a finalize request, the server will return a 200 (OK) with the updated order object. In this response, an ACME Server can add a newly defined field called "x5u" that can pass this URL to the ACME client for usage in generated PASSporTs as a publically available URL for PASSporT validation.

x5u (optional, string):: A URL that can be used to reference the certificate in the "x5u" parameter of a JWS object

The publishing of the certificates at the new "x5u" URL should follow the GET request requirement as mentioned above and should be consistent with the timely publication according to the durations of the certificate lifecycle. The following is an example of the use of "x5u" in the response when the certificate status is "valid".

;rel="index" Location: https://example.com/acme/order/TOlocE8rfgo { "status": "valid", "expires": "2016-01-20T14:09:07.99Z", "notBefore": "2016-01-01T00:00:00Z", "notAfter": "2016-01-08T00:00:00Z", "identifiers": [ "type:"TNAuthList", "value":"F83n2a...avn27DN3" ], "authorizations": ["https://sti-ca.com/acme/authz/1234"], "finalize": "https://example.com/acme/order/TOlocE8rfgo/finalize", "certificate": "https://example.com/acme/cert/mAt3xBGaobw", "x5u": "https://example.com/cert-repo/giJI53km23.pem" } ]]>

Usage Considerations

Large number of Non-contiguous TNAuthList values 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.

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 . Implementations should follow the best practices identified in . This document only specifies SHA256 for the fingerprint hash. However, the syntax of the fingerprint object would permit other algorithms if, due to concerns about algorithmic agility, a more robust algorithm were required at a future time. Future specifications can define new algorithms for the fingerprint object as needed.

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 .

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