STIR C. Wendt
Internet-Draft Comcast
Intended status: Standards Track J. Peterson
Expires: February 23, 2017 Neustar Inc.
August 22, 2016

Persona Assertion Token


This document defines a canonical string object or 'token' for verifying with non-repudiation the author of the token, their authority to author the token and, minimally, the asserted originating identity or persona contained within the token corresponding specifically to the originator of 'personal communications', or any signalled communications between a set of parties with identities. A cryptographic signature is defined to protect the integrity of the information used to 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. The Persona Assertion Token (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.

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Table of Contents

1. Introduction

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. [RFC7340] 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, 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.

Note: As of the authoring of this document, [I-D.ietf-stir-rfc4474bis] provides details of how to use PASSporT within SIP signaling for the signing and verification of telephone numbers.

2. PASSporT Token Overview

JSON Web Token (JWT) [RFC7519] and JSON Web Signature (JWS) [RFC7515] 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 accomidate 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.

3. PASSporT Components

The PASSporT is constructed based on JWT [RFC7519] and JWS [RFC7515] specifications. 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 specific set of claims that are represented by specified key value pairs which can be extended with custom keys for specific applications.

3.1. PASSporT Header

The JWS token header is a JOSE header [RFC7515] 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.

3.1.1. "typ" (Type) Header Parameter

JWS defines the "typ" (Type) Header Parameter to declare the media type of the JWS.

For PASSporT Token the "typ" header MUST be the string "passport". This represents that the encoded token is a JWT of type passport.

3.1.2. "alg" (Algorithm) Header Parameter

For PASSporT, the "alg" should be defined as follows, for the creation and verification of PASSporT tokens and their digital signatures ES256 MUST be implemented.

Note that JWA [RFC7518] defines other algorithms that may be utilized or updated in the future depending on cryptographic strength requirements guided by current security best practice.

3.1.3. "x5u" (X.509 URL) Header Parameter

As defined in JWS, the "x5u" header parameter is used to provide a URI [RFC3986] referring to the resource for the X.509 public key certificate or certificate chain [RFC5280] corresponding to the key used to digitally sign the JWS. Generally this would correspond to an HTTPS or DNSSEC resource with the guidance that it MUST be TLS protected, per JWS spec.


An example of the header, would be the following,

3.2. PASSporT Payload

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 [RFC7519] and be encoded as defined by the JWS Payload [RFC7515].

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.

Key values outside the US-ASCII range should be encoded using percent encoding as described in section 2.1 of [RFC3986], case normalized as described in of [RFC3986]. Matching of these values should use string exact match.

3.2.1. JWT defined claims "iat" - Issued At claim

The JSON claim MUST include the "iat" [RFC7519] 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 [RFC7519] 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.

3.2.2. PASSporT specific claims Originating and Destination Identity Claims

PASSporT defines claims that convey the identity of the origination and destination of personal communications. There are 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 subsecquent 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. Additionaly, 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. "tn" - Telephone Number identity

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 [I-D.ietf-stir-rfc4474bis] Section 7.2. "uri" - URI identity

If any of the originating or destination identities is of the form URI, as defined in [RFC3986], the key representing the identity MUST be "uri" URI form of the identity. Future identity forms

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. Examples


Single Originator, with telephone number identity +12155551212, to Single Destination, with URI identity '', example:


Single Originator, with telephone number identity +12155551212, to Multiple Destination Identities, with telephone number identity +12125551212 and two URI identities, and, example: "mky" - Media Key claim

Some protocols that use PASSporT convey hashes for media security keys within their signaling in order to bind those keys to the identities established in the signaling layers. One example would be the DTLS-SRTP key fingerprints carried in SDP [RFC4566] via the "a=fingerprint" attribute; multiple instances of that fingerprint may appear in a single SDP body corresponding to 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 is multiple fingerprint values, for example 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.

An example claim with "mky" claim is as follows:

    a=fingerprint:sha-256 02:1A:CC:54:27:AB:EB:9C:53:3F:3E:4B:65:
    a=fingerprint:sha-256 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:

For an SDP offer that includes the following fingerprint values,


the PASSporT Payload object would be:

3.3. PASSporT Signature

The signature of the PASSporT is created as specified by JWS using the private key corresponding to the X.509 public key certificate referenced by the "x5u" header parameter.

4. Extending PASSporT

PASSporT includes the bare minimum set of claims needed to securely assert the originating identity and support the secure propoerties 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.

4.1. "ppt" (PASSporT) header parameter

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" hedaer 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:

4.2. Extended PASSporT Claims

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 ([RFC7519] 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.

5. Deterministic JSON Serialization

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 digitial signature verification work deterministically, the JSON representation of the PASSporT Header and Claims, particularly if PASSporT is used across multiple signaling environments, specifically the JSON header object and JSON Claim 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 [RFC7638] Section 3. Each JSON object MUST contain no whitespace or line breaks before or after any syntactic elements and with the required members ordered lexicographically by the Unicode [UNICODE] code points of the member names.

In addition, the JSON header and claim members MUST follow the lexicographical ordering and character and string rules defined in [RFC7638] Section 3.3.

5.1. Example PASSport deterministic JSON form

    {"dest":{"uri":[""],"iat": 1443208345,"mky"

For the example PASSporT Payload shown in Section, the following is the deterministic JSON object form.

6. Security Considerations

6.1. Avoidance of replay and cut and paste attacks

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 [RFC3261]). 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:

6.2. Solution Considerations

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 persona being asserted. This only provides non-repudiation of the signer of PASSporT. If the signer and the persona are not one in the same, which can and often will be the case in telecommunications networks today, protecting the destination party from being spoofed may take some interpretation or additional verification of the link between the PASSporT signature and the persona being asserted.

In addition, the telecommunications systems and specifications that use PASSporT should in practice provide mechanisms for:

6.3. Privacy Considerations

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.

7. IANA Considerations

7.1. Media Type Registration

7.1.1. Media Type Registry Contents Additions Requested

This section registers the "application/passport" media type [RFC2046] in the "Media Types" registry in the manner described in [RFC6838], which can be used to indicate that the content is a PASSporT defined JWT and JWS.

7.2. JSON Web Token Claims Registration

7.2.1. Registry Contents Additions Requested

8. Acknowledgements

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, and Ted Hardie for valuable feedback on the technical and security aspects of the document. Additional thanks to Harsha Bellur for assistance in coding the example tokens.

9. References

9.1. Normative References

, "
[I-D.ietf-stir-rfc4474bis] Peterson, J., Jennings, C., Rescorla, E. and C. Wendt, Authenticated Identity Management in the Session Initiation Protocol (SIP)", Internet-Draft draft-ietf-stir-rfc4474bis-10, July 2016.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, DOI 10.17487/RFC2046, November 1996.
[RFC3986] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005.
[RFC6838] Freed, N., Klensin, J. and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013.
[RFC7515] Jones, M., Bradley, J. and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 2015.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, DOI 10.17487/RFC7518, May 2015.
[RFC7519] Jones, M., Bradley, J. and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015.
[RFC7638] Jones, M. and N. Sakimura, "JSON Web Key (JWK) Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September 2015.
[UNICODE]The Unicode Consortium, "The Unicode Standard""

9.2. Informative References

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008.
[RFC7340] Peterson, J., Schulzrinne, H. and H. Tschofenig, "Secure Telephone Identity Problem Statement and Requirements", RFC 7340, DOI 10.17487/RFC7340, September 2014.

Appendix A. Example PASSporT JWS Serialization and Signature

For PASSporT, there will always be a JWS with the following members:

Note: there will never be a JWS Unprotected Header for PASSporT.


First, an example PASSporT Protected Header is as follows:


This would be serialized to the form:


Encoding this with UTF8 and BASE64 encoding produces this value:


Second, an example PASSporT Payload is as follows:


This would be serialized to the form:


Encoding this with the UTF8 and BASE64 encoding produces this value:


Computing the digital signature of the PASSporT Signing Input ASCII(BASE64URL(UTF8(JWS Protected Header)) || '.' || BASE64URL(JWS Payload))


The final PASSporT token is produced by concatenating the values in the order Header.Payload.Signature with period (',') characters. For the above example values this would produce the following:

A.1. X.509 Private Key Certificate for Example

    -----BEGIN EC PRIVATE KEY-----
    -----END EC PRIVATE KEY-----

A.2. X.509 Public Key Certificate for Example

    -----BEGIN PUBLIC KEY-----
    -----END PUBLIC KEY-----

Authors' Addresses

Chris Wendt Comcast One Comcast Center Philadelphia, PA 19103 USA EMail:
Jon Peterson Neustar Inc. 1800 Sutter St Suite 570 Concord, CA 94520 US EMail: