PASSporT SHAKEN Extension (SHAKEN)
draft-ietf-stir-passport-shaken-04

Abstract

This document extends PASSporT, which is a token object that conveys cryptographically-signed information about the participants involved in communications, to include information defined as part of the SHAKEN specification from ATIS (Alliance for Telecommunications Industry Solutions) and the SIP Forum IP-NNI Joint Task Force. These extensions provide a level of confidence in the correctness of the originating identity for a telephone network that has communications coming from both STIR participating originating communications as well as communications that does not include STIR information.

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Overview of ‘shaken’ PASSporT extension
• 4. PASSporT ‘attest’ Claim
• 5. PASSporT ‘origid’ Claim
• 6. Example “shaken” PASSporT
• 7. Using ‘shaken’ in SIP
• 8. Order of Claim Keys
• 9. Security Considerations
• 10. IANA Considerations
• 10.1. JSON Web Token claims
• 10.2. PASSporT Types
• 11. Acknowledgements
• 12. References
• 12.1. Normative References
• 12.2. Informative References
• Authors' Addresses

1. Introduction

The SHAKEN [ATIS-1000074] specification defines a framework for using STIR protocols including PASSporT [RFC8225], SIP Authenticated Identity Management [RFC8224] and the STIR certificate framework [RFC8226] for implementing the cryptographic validation of an authorized originator of telephone calls using SIP. Because the current telephone network contains both VoIP and TDM/SS7 originated traffic, there are many scenarios that need to be accounted for where PASSporT signatures may represent either direct or indirect call origination scenarios. The SHAKEN [ATIS-1000074] specification defines levels of attestation of the origination of the call as well as an origination identifier that can help create a unique association with the origination of calls from various parts of the VoIP or TDM telephone network. This document specifies these indicators as a specified PASSporT extension.

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. Overview of ‘shaken’ PASSporT extension

The SHAKEN framework is designed to use PASSporT [RFC8225] as a method of asserting the telephone number calling identity. In addition to the PASSporT base claims, there are two additional claims that have been defined for the needs of a service provider to signal information beyond just the telephone identity. First, in order to help bridge the transition of the state of the current telephone network which has calls with no authentication and non-SIP [RFC3261] signaling not compatible with the use of PASSporT and Secure Telephone Identity (STI) in general, there is an attestation claim. This provides three levels of attestation, including a full attestation when the service provider can fully attest to the calling identity, a partial attestation, when the service provider originated a telephone call but can not fully attest to the calling identity, and a gateway attestation which is the lowest level of attestation and represents the service provider receiving a call from a non PASSporT or STI supporting telephone gateway.

The second claim is a unique origination identifier that should be used by the service provider to identify different sources of telephone calls to support a traceback mechanism that can be used for enforcement and identification of a source of illegitimate calls.

The use of the compact form of PASSporT is not specified in this document and is not specified for use in SHAKEN [ATIS-1000074].

The next two sections define these new claims.

4. PASSporT ‘attest’ Claim

This indicator allows for both identifying the service provider that is vouching for the call as well as clearly indicating what information the service provider is attesting to. The ‘attest’ claim can be one of the following three values, ‘A’, ‘B’, or ‘C’ as defined in [ATIS-1000074].

‘A’ represents ‘Full Attestation’ where the signing provider MUST satisfy all of the following conditions:

• Is responsible for the origination of the call onto the IP based service provider voice network.
• Has a direct authenticated relationship with the initiator of the call and can identify the customer associated with the initiator.
• Has established a verified association with the calling party telephone number used for the call.

‘B’ represents ‘Partial Attestation’ where the signing provider MUST satisfy all of the following conditions:

• Is responsible for the origination of the call onto its IP-based voice network.
• Has a direct authenticated relationship with the initiator of the call and can identify the customer associated with the initiator.
• Has NOT established a verified association with the calling party telephone number being used for the call.

‘C’ represents ‘Gateway Attestation’ where the signing provider MUST satisfy all of the following conditions:

• Is the entry point of the call into its VoIP network.
• Has no relationship with the initiator of the call (e.g., international gateways)

5. PASSporT ‘origid’ Claim

The purpose of the unique origination identifier is to assign an opaque identifier corresponding to the service provider-initiated calls themselves, customers, classes of devices, or other groupings that a service provider might want to use for determining things like reputation or trace back identification of customers or gateways. The value of ‘origid’ claim is a UUID as defined in [RFC4122]. SHAKEN isn’t prescriptive in the exact usage of origid other than the UUID format as a globally unique identifier representing the originator of the call to whatever granularity the PASSporT signer determines is sufficient for the ability to trace the original origination point of the call. There will likely be best practices documents that more precisely guide it’s usage in real deployments.

6. Example “shaken” PASSporT

Protected Header
{
   "alg":"ES256",
   "typ":"passport",
   “ppt”:”shaken”,
   "x5u":"https://cert.example.org/passport.cer"
}
Payload
{
   “attest”:”A”
   "dest":{“uri”:["sip:alice@example.com"]}
   "iat":"1443208345",
   "orig":{“tn”:"12155551212"},
   “origid”:”123e4567-e89b-12d3-a456-426655440000”
}

7. Using ‘shaken’ in SIP

The use of the ‘shaken’ PASSporT type and the claims ‘attest’ and ‘origid’ are formally defined in [ATIS-1000074] for usage in SIP [RFC3261] aligned with the use of the identity header defined in [RFC8224].

8. Order of Claim Keys

The order of the claim keys MUST follow the rules of [RFC8225] Section 9; the claim keys MUST appear in lexicographic order. Therefore, the claim keys discussed in this document appear in the PASSporT Payload in the following order,

• attest
• dest
• iat
• orig
• origid

9. Security Considerations

This document defines a new PASSporT [RFC8225] extension. The considerations related to the security of the PASSporT object itself are the same as those described in [RFC8225].

[RFC8224] defines how to compare the values of the "dest", "orig" and "iat" claims against fields in a SIP containing a PASSporT as part of validating that request. The values of the new "attest" and "origid" claims added by this extension are not used in such a validation step. They are not compared to fields in the SIP message. Instead, they simply carry additional information from the signer to the consumer of the PASSport. This new information shares the same integrity protection and non-repudiation properties as the base claims in the PASSporT.

10. IANA Considerations

10.1. JSON Web Token claims

This specification requests that the IANA add two new claims to the JSON Web Token Claims registry as defined in [RFC7519].

Claim Name: “attest”

Claim Description: Attestation level as defined in SHAKEN framework

Change Controller: IESG

Specification Document(s): [RFCThis]

Claim Name: “origid”

Claim Description: Originating Identifier as defined in SHAKEN framework

Change Controller: IESG

Specification Document(s): [RFCThis]

10.2. PASSporT Types

This specification requests that the IANA add a new entry to the Personal Assertion Token (PASSporT) Extensions registry for the type “shaken” which is specified in [RFCThis].

11. Acknowledgements

The authors would like to thank those that helped review and contribute to this document including specific contributions from Jon Peterson, Russ Housley, and Andrew Jurczak. The authors would like acknowledge the work of the ATIS/SIP Forum IP-NNI Task Force to develop the concepts behind this document.

12. References

12.1. Normative References

12.2. Informative References

Authors' Addresses