    Internet Engineering Task Force (IETF)                          C. Wendt
    Request for Comments: 8588                                       Comcast
    Category: Standards Track                                      M. Barnes
    ISSN: 2070-1721                                                iconectiv
                                                                    May 2019

       Personal Assertion Token (PaSSporT) Extension for Signature-based
             Handling of Asserted information using toKENs (SHAKEN)

    Abstract

       This document extends the Personal Assertion Token (PASSporT), which
       is a token object that conveys cryptographically signed information
       about the participants involved in communications.  The extension is
       defined based on the "Signature-based Handling of Asserted
       information using toKENs (SHAKEN)" specification by the ATIS/SIP
       Forum IP-NNI Task Group.  It provides both (1) a specific set of
       levels of confidence in the correctness of the originating identity
       of a call originated in a SIP-based telephone network as well as (2)
       an identifier that allows the Service Provider (SP) to uniquely
       identify the origin of the call within its network.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8588.

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    Copyright Notice

       Copyright (c) 2019 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (https://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
       2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
       3.  Overview of   "shaken" PASSporT Extension . . . . . . . . . .   4
       4.  PASSporT "attest" Claim . . . . . . . . . . . . . . . . . . .   4
       5.  PASSporT "origid" Claim . . . . . . . . . . . . . . . . . . .   4
       6.  Example "shaken" PASSporT . . . . . . . . . . . . . . . . . .   5
       7.  Using "shaken" in SIP . . . . . . . . . . . . . . . . . . . .   5
       8.  Order of Claim Keys . . . . . . . . . . . . . . . . . . . . .   5
       9.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
       10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
       11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
         11.1.  JSON Web Token claims  . . . . . . . . . . . . . . . . .   7
         11.2.  PASSporT Types . . . . . . . . . . . . . . . . . . . . .   7
       12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
         12.1.  Normative References . . . . . . . . . . . . . . . . . .   7
         12.2.  Informative References . . . . . . . . . . . . . . . . .   8
       Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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    1.  Introduction

       The Signature-based Handling of Asserted information using toKENs
       (SHAKEN) [ATIS-1000074] specification defines a framework for using
       Secure Telephone Identity Revisited (STIR) protocols including the
       Personal Assertion Token (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 traffic originated from both VoIP
       and TDM/SS7 (Time Division Multiplexing / Signaling System 7), 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 between the
       origin of a particular call to the point in the VoIP or TDM telephone
       network the call came from to identify, for example, either a
       customer or class of service that call represents.  This document
       specifies these values as claims to extend the base set of PASSporT
       claims.

    2.  Terminology

       The key words "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 [RFC2119] [RFC8174] when, and only when, they appear in all
       capitals, as shown here.

       In addition, the following terms are used in this document:

       o  Verified association: Typically defined as an authenticated
          relationship between a customer and a device that initiated a call
          on behalf of that customer, for example, a subscriber account with
          a specific SIM card or set of SIP credentials.

       o  PASSporT: Defined in [RFC8225] is a JSON Web Token [RFC7519]
          defined specifically for securing the identity of an initiator of
          personal communication.  This document defines a specific
          extension to PASSporT.

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    3.  Overview of "shaken" PASSporT Extension

       The SHAKEN framework is designed to use PASSporT [RFC8225] as a
       method of asserting the caller's telephone 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: 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 cannot 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 telephone gateway that
       does not support PASSporT or STI.

       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'.  These
       values correspond to 'Full Attestation', 'Partial Attestation', and
       'Gateway Attestation', respectively.  See [ATIS-1000074] for the
       definitions of these three levels of attestation.

    5.  PASSporT "origid" Claim

       The purpose of the "origid" claim is described in [ATIS-1000074].
       The value of "origid" claim is a Universally Unique Identifier (UUID)
       as defined in [RFC4122].  Please refer to Section 10 for a discussion
       of the privacy considerations around the use of this value.

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    6.  Example "shaken" PASSporT

       Protected Header
       {
          "alg":"ES256",
          "typ":"passport",
          "ppt":"shaken",
          "x5u":"https://cert.example.org/passport.cer"
       }
       Payload
       {
          "attest":"A"
          "dest":{"tn":["12155550131"]}
          "iat":"1443208345",
          "orig":{"tn":"12155550121"},
          "origid":"123e4567-e89b-12d3-a456-426655440000"
       }

    7.  Using "shaken" in SIP

       The use of the "shaken" PASSporT type and the "attest" and "origid"
       claims for SIP is formally defined in [ATIS-1000074] using the SIP
       [RFC3261] Identity header field defined in [RFC8224].

    8.  Order of Claim Keys

       The order of the claim keys MUST follow the rules of Section 9 of
       [RFC8225]; 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:

       o  attest

       o  dest

       o  iat

       o  orig

       o  origid

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    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 message 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.  Privacy Considerations

       As detailed in Section 26 of [RFC3261], SIP messages inherently carry
       identifying information of the caller and callee.  The addition of
       STIR cryptographically attests that the signing party vouches for the
       information given about the callee, as is discussed in the Privacy
       Considerations of [RFC8224].

       SHAKEN [ATIS-1000074] furthermore adds an "origid" value to the STIR
       PASSporT, which is an opaque unique identifier representing an
       element on the path of a given SIP request.  This identifier is
       generated by an originating telephone service provider to identify
       where within their network (e.g. a gateway or particular service
       element) a call was initiated; "origid" can facilitate forensic
       analysis of call origins when identifying and stopping bad actors
       trying to spoof identities or make fraudulent calls.

       The opacity of the "origid" claim value is intended to minimize
       exposure of information about the origination of calls labeled with
       an "origid" value.  It is therefore RECOMMENDED that implementations
       generate a unique "origid" value per call in such a way that only the
       generator of the "origid" can determine when two "origid" values
       represent the same or different elements.  If deployed systems
       instead use a common or related "origid" for service elements in
       their network, the potential for discovering patterns through
       correlation of those calls exists.  This could allow a recipient of
       calls to, for instance, learn that a set of callers are using a
       particular service or coming through a common gateway.  It is
       expected that SHAKEN PASSporTs are shared only within an [RFC3324]
       trust domain and will be stripped before calls exit that trust
       domain, but this information still could be used by analytics on

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       intermediary and terminating systems to reveal information that could
       include geographic location and even device-level information,
       depending on how the "origid" is generated.

    11.  IANA Considerations

    11.1.  JSON Web Token claims

       IANA has added 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): RFC 8588

       Claim Name: origid
       Claim Description: Originating Identifier as defined in SHAKEN
          framework
       Change Controller: IESG
       Specification Document(s): RFC 8588

    11.2.  PASSporT Types

       IANA has added a new entry in the "Personal Assertion Token
       (PASSporT) Extensions" registry for the type "shaken", which is
       specified in this document.

    12.  References

    12.1.  Normative References

       [ATIS-1000074]
                  ATIS/SIP Forum IP-NNI Task Group, "Signature-based
                  Handling of Asserted information using toKENs (SHAKEN)",
                  January 2017, <https://access.atis.org/apps/group_public/
                  download.php/32237/ATIS-1000074.pdf>.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
                  Unique IDentifier (UUID) URN Namespace", RFC 4122,
                  DOI 10.17487/RFC4122, July 2005,
                  <https://www.rfc-editor.org/info/rfc4122>.

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       [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
                  (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
                  <https://www.rfc-editor.org/info/rfc7519>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                  2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                  May 2017, <https://www.rfc-editor.org/info/rfc8174>.

       [RFC8224]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
                  "Authenticated Identity Management in the Session
                  Initiation Protocol (SIP)", RFC 8224,
                  DOI 10.17487/RFC8224, February 2018,
                  <https://www.rfc-editor.org/info/rfc8224>.

       [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
                  Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
                  <https://www.rfc-editor.org/info/rfc8225>.

       [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
                  Credentials: Certificates", RFC 8226,
                  DOI 10.17487/RFC8226, February 2018,
                  <https://www.rfc-editor.org/info/rfc8226>.

    12.2.  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,
                  <https://www.rfc-editor.org/info/rfc3261>.

       [RFC3324]  Watson, M., "Short Term Requirements for Network Asserted
                  Identity", RFC 3324, DOI 10.17487/RFC3324, November 2002,
                  <https://www.rfc-editor.org/info/rfc3324>.

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    Acknowledgements

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

    Authors' Addresses

       Chris Wendt
       Comcast
       One Comcast Center
       Philadelphia, PA  19103
       United States of America

       Email: chris-ietf@chriswendt.net

       Mary Barnes
       iconectiv

       Email: mary.ietf.barnes@gmail.com

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