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ART STIR This document defines a canonical string object or 'token' including a digital signature for verifying the author of the token, their authority to author the token and the information asserted in the token, minimally, the originating identity or 'persona' corresponding specifically to the originator of 'personal communications', or signalled communications between a set of parties with identities. The PASSporT token is cryptographically signed to protect the integrity of the identify the originator of a personal communications session (e.g. the telephone number or URI) and verify the assertion of the identity information at the destination. The cryptographic signature is defined with the intention that it can confidently verify the originating persona even when the signature is sent to the destination party over an insecure channel. PASSporT is particularly useful for many personal communications applications over IP networks and other multi-hop interconnection scenarios where the originating and destination parties may not have a direct trusted relationship.

In today's IP-enabled telecommunications world, there is a growing concern about the ability to trust incoming invitations for communications sessions, including video, voice and messaging. As an example, modern telephone networks provide the ability to spoof the calling party telephone number for many legitimate purposes including providing network features and services on the behalf of a legitimate telephone number. However, as we have seen, bad actors have taken advantage of this ability for illegitimate and fraudulent purposes meant to trick telephone users to believe they are someone they are not. This problem can be extended to many emerging forms of personal communications. This document defines a method for creating and validating a token that cryptographically verifies an originating identity, or more generally a URI or telephone number representing the originator of personal communications. Through extensions defined in this document, in Section 5.2, other information relevant to the personal communications can also be added to the token. The goal of PASSporT is to provide a common framework for signing originating identity related information in an extensible way. Additionally, this functionality is independent of any specific personal communications signaling call logic, so that the assertion of originating identity related information can be implemented in a flexible way and can be used in applications including end-to-end applications that require different signaling protocols or gateways between different communications systems. It is anticipated that signaling protocol specific guidance will be provided in other related documents and specifications to specify how to use and transport PASSporT tokens, however this is intentionally out of scope for this document. provides details of the use of PASSporT within the SIP signaling protocol for the signing and verification of telephone numbers.

JSON Web Token (JWT) and JSON Web Signature (JWS) and related specifications define a standard token format that can be used as a way of encapsulating claimed or asserted information with an associated digital signature using X.509 based certificates. JWT provides a set of claims in JSON format that can conveniently accommodate asserted originating identity information and is easily extensible for extension mechanisms defined below. Additionally, JWS provides a path for updating methods and cryptographic algorithms used for the associated digital signatures. JWS defines the use of JSON data structures in a specified canonical format for signing data corresponding to JOSE header, JWS Payload, and JWS Signature. JWT defines a set of claims that are represented by specified key value pairs which can be extended with custom keys for specific applications. The next sections define the header and claims that MUST be minimally used with JWT and JWS for PASSporT.

The JWS token header is a JOSE header Section 4, that defines the type and encryption algorithm used in the token. PASSporT header should include, at a minimum, the following header parameters defined the the next three subsections.

The "typ" (Type) Header Parameter is defined in JWS Section 4.1.9. to declare the media type of the complete JWS. For PASSporT Token the "typ" header MUST be the string "passport". This represents that the encoded token is a JWT of type passport.

The "alg" (Algorithm) Header Parameter is defined in JWS Section 4.1.1. This definition includes the ability to specify the use of a cryptographic algorithm for the signature part of the JWS. It also refers to a list of defined "alg" values as part of a registry established by JSON Web Algorithms (JWA) and defined in Section 3.1. For the creation and verification of PASSporT tokens and their digital signatures ES256 MUST be implemented as defined in JWA Section 3.4 Note that JWA defines other algorithms that may be utilized or updated in the future depending on cryptographic strength requirements guided by current security best practice.

As defined in JWS Section 4.1.5., the "x5u" header parameter defines a URI referring to the resource for the X.509 public key certificate or certificate chain corresponding to the key used to digitally sign the JWS. Generally, as defined in JWS section 4.1.5, this would correspond to an HTTPS or DNSSEC resource using integrity protection.

An example of the header, would be the following, including the specified passport type, ES256 algorithm, and a URI referencing the network location of the certificate needed to validate the PASSporT signature.

The token claims consist of the information which needs to be verified at the destination party. These claims follow the definition of a JWT claim Section 4 and be encoded as defined by the JWS Payload Section 3. PASSporT defines the use of a standard JWT defined claim as well as custom claims corresponding to the two parties associated with personal communications, the originator and destination as detailed below. Any claim key values outside the US-ASCII range should be encoded using percent encoding as described in section 2.1 of , case normalized as described in 6.2.2.1 of .

The JSON claim MUST include the "iat" Section 4.1.6 defined claim Issued At. As defined this should be set to the date and time of the origination of the personal communications. The time value should be of the format defined in Section 2 NumericDate. This is included for securing the token against replay and cut and paste attacks, as explained further in the security considerations in section 6.

PASSporT defines claims that convey the identity of the origination and destination of personal communications. Origination in the context of PASSporT and for a given application's use of PASSporT is the point in the network that has the authority to assert the callers identity. This authority is represented in PASSporT by the certificate holder and is signed at the applications choice of authoritative point(s) in the network, for example, at a device that has authenticated with a user, or at a network entity with an authenticated trust relationship with that device and its user. Destination represents the intended destination of the personal communications, i.e. the identity(s) being called by the caller, The destination point(s) determined by the application must have the capability to verify the PASSporT token and the digital signature. The PASSporT associated certificate is used to validate the authority of the originating signer, generally via a certificate chain to the trust anchor for that application. The origination and destination identities are represented by two claims that are required for PASSporT, the "orig" and "dest" claims. Both "orig" and "dest" MUST have claims where the key represents an identity type and the value is the identity string, both defined in subsequent subsections. Currently, these identities can be represented as either telephone numbers or Uniform Resource Indicators (URIs). The "orig" JSON object MUST only have one key value pair representing the asserted identity of any type (currently either "tn" or "uri") of the originator of the personal communications signaling. The "dest" JSON object MUST have at least have one key value pair, but could have multiple identity types (i.e. "tn" and/or "uri") but only one of each. If both "tn" and "uri" are included, the JSON object should list the "tn" array first and the "uri" array second. Within the "tn" and "uri" arrays, the identity strings should be put in lexicographical order including the scheme-specific portion of the URI characters. Additionally, in the case of "dest" only, the identity type key value MUST be an array signaled by standard JSON brackets, even when there is a single identity value in the identity type key value.

If the originating or destination identity is a telephone number, the key representing the identity MUST be "tn". Telephone Number strings for "tn" MUST be canonicalized according to the procedures specified in Section 8.3.

If any of the originating or destination identities is of the form URI, as defined in , the key representing the identity MUST be "uri" URI form of the identity.

We recognize that in the future there may be other standard mechanisms for representing identities. The "orig" and "dest" claims currently support "tn" and "uri" but could be extended in the future to allow for other identity types with new IANA registered unique types to represent these forms.

Single Originator, with telephone number identity +12155551212, to Single Destination, with URI identity 'sip:alice@example.com', example:

Single Originator, with telephone number identity +12155551212, to Multiple Destination Identities, with telephone number identity +12125551212 and two URI identities, sip:alice@example.com and sip:bob@example.com, example:

Some protocols that use PASSporT may also want to protect media security keys delivered within their signaling in order to bind those keys to the identities established in the signaling layers. The "mky" is an optional PASSporT claim defining the assertion of media key fingerprints carried in SDP via the "a=fingerprint" attribute Section 5. This claim can support either a single or multiple fingerprints appearing in a single SDP body corresponding to one or more media streams offered. The "mky" claim MUST be formated in a JSON form including the "alg" and "dig" keys with the corresponding algorithm and hexadecimal values. If there are more that one fingerprint values associated with different media streams in SDP, the fingerprint values MUST be constructed as a JSON array denoted by bracket characters. For the "dig" key value, the hash value MUST be the hexadecimal value without any colons. The "mky" array MUST order the JSON objects containing both "alg" and "dig" key values in lexicographic order of the "alg" string first followed by the corresponding lexicographic order of the "dig" string values. Within each of those objects the JSON keys MUST have "alg" first and "dig" second. An example claim with "mky" claim is as follows: For an SDP offer that includes the following fingerprint values,

the PASSporT Payload object would be:

The signature of the PASSporT is created as specified by JWS Section 5.1 Steps 1 through 6. PASSporT MUST use the JWS Protected Header. For the JWS Payload and the JWS Protected Header, the lexicographic ordering and white space rules described above, and JSON serialization rules in Section 6 of this document MUST be followed. Appendix A of this document has a detailed example of how to follow the steps to create the JWS Signature. JWS Section 5.1 Step 7 JWS JSON serialization is not supported for PASSporT. JWS Section 5.1 Step 8 describes the method to create the final JWS Compact Serialization form of the PASSporT Token.

PASSporT includes the bare minimum set of claims needed to securely assert the originating identity and support the secure properties discussed in various parts of this document. JWT supports a straight forward way to add additional claims by simply adding new claim key pairs. PASSporT can be extended beyond the defined base set of claims to represent other information requiring assertion or validation beyond the originating identity itself as needed.

For extension of the base set of claims defined in this document, a new JWS header parameter "ppt" MUST be used with a unique string. Any PASSporT extension should be defined in a specification describing the PASSporT extension and the string used in the "ppt" header string that defines any new claims that would extend the base set of claims of PASSporT. An example header with a PASSporT extension type of "foo" is as follows:

Specifications that define extensions to the PASSporT mechanism MUST explicitly specify what claims they include beyond the base set of claims from this document, the order in which they will appear, and any further information necessary to implement the extension. All extensions MUST include the baseline JWT elements specified in Section 3; claims may only be appended to the claims object specified; they can never be removed or re-ordered. Specifying new claims follows the baseline JWT procedures ( Section 10.1). Understanding an extension or new claims defined by the extension on the destination verification of the PASSporT token is optional. The creator of a PASSporT object cannot assume that destination systems will understand any given extension. Verification of PASSporT tokens by destination systems that do support an extension may then trigger appropriate application-level behavior in the presence of an extension; authors of extensions should provide appropriate extension-specific guidance to application developers on this point. An example set of extended claims, extending the first example in Section 4.1.2.4. using "bar" as the newly defined claim would be as follows:

JSON, as a canonical format, can include spaces, line breaks and key value pairs can occur in any order and therefore makes it, from a string format, non-deterministic. In order to make the digital signature verification work deterministically, the JSON representation of the PASSporT Header and Claims, particularly if PASSporT is used across multiple signaling environments, specifically the JWS Protected Header object and JWS Payload object MUST be computed as follows. The JSON object MUST follow the rules for the construction of the thumbprint of a JSON Web Key (JWK) as defined in Section 3 Step 1 only. Step 2 MUST NOT be performed; as noted in JWK this is still a legal JWK object. The PASSporT header and claim direct members MUST follow the lexicographical ordering rules. Any top level JSON members that contain JSON objects or arrays, such as "dest" or "mky" MUST follow their own lexicographical ordering and whitespace and line break rules for the sub-elements. This includes any header or claims defined in future specifications using PASSporT.

This section demonstrate the deterministic JSON serialization for the example PASSporT Payload shown in Section 4.2.2. The initial JSON object is shown here:

The parent members of the JSON object are as follows: "dest" "orig" "iat" "mky" Their lexicographic order is: "dest" "iat" "mky" "orig" The final constructed deterministic JSON serialization representation, with whitespace and line breaks removed, (with line breaks used for display purposes only) is:

There are a number of security considerations for use of the token for avoidance of replay and cut and paste attacks. PASSporT tokens should be sent with other application level protocol information (e.g. for SIP an INVITE as defined in ). In order to make the token signature unique to a specific origination of personal communications there should be a link between various information provided in the token and information provided by the application level protocol information. This uniqueness specified using the following two claims: 'iat' claim should correspond to a date/time the message was originated. It should also be within a relative time that is reasonable for clock drift and transmission time characteristics associated with the application using the PASSporT token. Therefore, validation of the token should consider date and time correlation, which could be influenced by signaling protocol specific use and network time differences. 'dest' claim is included to prevent the valid re-use of a previously originated message to send to another destination party.

The use of PASSporT tokens based on the validation of the digital signature and the associated certificate requires consideration of the authentication and authority or reputation of the signer to attest to the identity being asserted. It should be recognized that the use of this token should not, in it's own right, be considered a full solution for absolute non-repudiation of the identity being asserted. It can and often is the case that the end user that the identity represents and signer are not one in the same. However, applications that use PASSporT should ensure the signer is in an authoritative position to represent the user and authenticate the user onto the communications network and should be the responsible party for protecting the destination party from potential identity spoofing in addition to other abuse of the communications network outside of PASSporT.

Because PASSporT explicitly includes claims of identifiers of parties involved in communications, date and times, and potentially other call detail, care should be taken outside of traditional protected or private telephony communications paths where there may be concerns about exposing information to either unintended or illegitimate actors. These identifiers are often exposed through many communications signaling protocols as of today, but appropriate precautions should be taken.

This section registers the "application/passport" media type in the "Media Types" registry in the manner described in , which can be used to indicate that the content is a PASSporT defined JWT and JWS. Type name: application Subtype name: passport Required parameters: n/a Optional parameters: n/a Encoding considerations: 8bit; application/passport values outside the US-ASCII range are encoded using percent encoding as described in section 2.1 of RFC 3986 (some values may be the empty string), each separated from the next by a single period ('.') character. Security considerations: See the Security Considerations section of RFC 7515. Interoperability considerations: n/a Published specification: draft-ietf-stir-passport-05 Applications that use this media type: STIR and other applications that require identity related assertion Fragment identifier considerations: n/a Additional information: Magic number(s): n/a File extension(s): n/a Macintosh file type code(s): n/a Person and email address to contact for further information: Chris Wendt, chris-ietf@chriswendt.net Intended usage: COMMON Restrictions on usage: none Author: Chris Wendt, chris-ietf@chriswendt.net Change Controller: IESG Provisional registration? No

Claim Name: "orig" Claim Description: Originating Identity String Change Controller: IESG Specification Document(s): Section 3.2 of draft-ietf-stir-passport-05 Claim Name: "dest" Claim Description: Destination Identity String Change Controller: IESG Specification Document(s): Section 3.2 of draft-ietf-stir-passport-05 Claim Name: "mky" Claim Description: Media Key Fingerprint String Change Controller: IESG Specification Document(s): Section 3.2 of draft-ietf-stir-passport-05

Particular thanks to members of the ATIS and SIP Forum NNI Task Group including Jim McEchern, Martin Dolly, Richard Shockey, John Barnhill, Christer Holmberg, Victor Pascual Avila, Mary Barnes, Eric Burger for their review, ideas, and contributions also thanks to Henning Schulzrinne, Russ Housley, Alan Johnston, Richard Barnes, Mark Miller, Ted Hardie, Dave Crocker, Robert Sparks for valuable feedback on the technical and security aspects of the document. Additional thanks to Harsha Bellur for assistance in coding the example tokens.

For PASSporT, there will always be a JWS with the following members: "protected", with the value BASE64URL(UTF8(JWS Protected Header)) "payload", with the value BASE64URL (JWS Payload) "signature", with the value BASE64URL(JWS Signature) This example will follow the steps in JWS Section 5.1, steps 1-6 and 8 and incorporates the additional serialization steps required for PASSporT. Step 1 for JWS references the JWS Payload, an example PASSporT Payload is as follows:

This would be serialized to the form (with line break used for display purposes only):

Step 2 Computes the BASE64URL(JWS Payload) producing this value (with line break used for display purposes only):

For Step 3, an example PASSporT Protected Header comprising the JOSE Header is as follows:

This would be serialized to the form (with line break used for display purposes only):

Step 4 Performs the BASE64URL(UTF8(JWS Protected Header)) operation and encoding produces this value (with line break used for display purposes only):

Step 5 and Step 6 performs the computation of the digital signature of the PASSporT Signing Input ASCII(BASE64URL(UTF8(JWS Protected Header)) || '.' || BASE64URL(JWS Payload)) using ES256 as the algorithm and the BASE64URL(JWS Signature).

Step 8 describes how to create the final PASSporT token, concatenating the values in the order Header.Payload.Signature with period ('.') characters. For the above example values this would produce the following (with line breaks between period used for readability purposes only):