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PROPOSED STANDARD
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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