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Messaging Use Cases and Extensions for STIR

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Authors Jon Peterson , Chris Wendt
Last updated 2021-07-12
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Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Standards Track                                C. Wendt
Expires: January 13, 2022                                        Comcast
                                                           July 12, 2021

              Messaging Use Cases and Extensions for STIR


   Secure Telephone Identity Revisited (STIR) provides a means of
   attesting the identity of a telephone caller via a signed token in
   order to prevent impersonation of a calling party number, which is a
   key enabler for illegal robocalling.  Similar impersonation is
   sometimes leveraged by bad actors in the text messaging space.  This
   document considers the applicability of STIR's Persona Assertion
   Token (PASSporT) and certificate issuance framework to instant text
   and multimedia messaging use cases, both for messages carried or
   negotiated by SIP, and for non-SIP messaging.

Status of This Memo

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   Copyright (c) 2021 IETF Trust and the persons identified as the
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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Applicability to Messaging Systems  . . . . . . . . . . . . .   3
     3.1.  Message Sessions  . . . . . . . . . . . . . . . . . . . .   4
     3.2.  PASSporTs and Messaging . . . . . . . . . . . . . . . . .   4
       3.2.1.  PASSporT Conveyance with Messaging  . . . . . . . . .   5
   4.  Certificates and Messaging  . . . . . . . . . . . . . . . . .   6
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  JSON Web Token Claims Registration  . . . . . . . . . . .   6
     6.2.  PASSporT Type Registration  . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The STIR problem statement [RFC7340] describes widespread problems
   enabled by impersonation in the telephone network, including illegal
   robocalling, voicemail hacking, and swatting.  As telephone services
   are increasingly migrating onto the Internet and using Voice over IP
   (VoIP) protocols such as SIP [RFC3261], it is necessary for these
   protocols to support stronger identity mechanisms to prevent
   impersonation.  [RFC8224] defines a SIP Identity header field capable
   of carrying PASSporT [RFC8225] objects in SIP as a means to
   cryptographically attest that the originator of a telephone call is
   authorized to use the calling party number (or, for native SIP cases,
   SIP URI) associated with the originator of the call.

   The problem of bulk, unsolicited commercial communications is not
   however limited to telephone calls.  Although the problem is not
   currently widespread, in some environments spammers and fraudsters
   are turning to messaging applications to deliver undesired content to
   consumers.  In some respects, mitigating these unwanted messages
   resembles the email spam problem: textual analysis of the message
   contents can be used to fingerprint content that is generated by
   spammers, for example.  However, encrypted messaging is becoming more
   common, and analysis of message contents may no longer be a reliably

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   way to mitigate messaging spam in the future.  And as STIR sees
   further deployment in the telephone network, it seems likely that the
   governance structures put in place for securing telephone network
   resources with STIR could be repurposed to help secure the messaging

   One of the more sensitive applications for message security is
   emergency services.  As next-generation emergency services
   increasingly incorporate messaging as a mode of communication with
   public safety personnel (see [RFC8876]), providing an identity
   assurance could help to mitigate denial-of-service attacks, as well
   as ultimately helping to identify the source of emergency
   communications in general (including the swatting attacks, see

   This specification therefore explores how the PASSporT mechanism
   defined for STIR could be applied to providing protection for textual
   and multimedia messaging, but focuses particularly on those messages
   that use telephone numbers as the identity of the sender.  It
   moreover considers the reuse of existing STIR certificates, which are
   beginning to see widespread deployment, for signing PASSporTs that
   protect messages.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

3.  Applicability to Messaging Systems

   At a high level, baseline PASSporT [RFC8225] claims provide similar
   value to number-based messaging as they do to traditional telephone
   calls.  A signature over the calling and called party numbers, along
   with a timestamp, could already help to prevent impersonation in the
   mobile messaging ecosystem.  When it comes to protecting message
   contents, broadly, there are a few ways that the PASSporT mechanism
   of STIR could apply to messaging: first, a PASSporT could be used to
   securely negotiate a session over which messages will be exchanged;
   and second, in sessionless scenarios, a PASSporT could be generated
   on a per-message basis with its own built-in message security.

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3.1.  Message Sessions

   For the first case, where SIP negotiates a session where the media
   will be text messages, as for example with the Message Session Relay
   Protocol (MSRP) [RFC4975], the usage of STIR would deviate little
   from [RFC8224].  An INVITE request sent with an Identity header
   containing a PASSporT with the proper calling and called party
   numbers would then negotiate an MSRP session the same way that an
   INVITE for a telephone call would negotiate an audio session.  This
   could be applicable to MSRP sessions negotiated for RCS [RCC.07].
   Note that if TLS is used to secure MSRP (per RCS [RCC.15]),
   fingerprints of those TLS keys could be secured via the "mky" claim
   of PASSporT using the [RFC8862] framework.  Similar practices would
   apply to sessions that negotiate text over RTP via [RFC4103] or
   similar mechanisms.  For the most basic use cases, STIR for messaging
   should not require any further protocol enhancements.

   Messages can also be sent over a variety of other transports
   negotiated by SIP (including for example Real-Time Text [RFC5194];
   any that can operate over DTLS/SRTP should work with the "mky"
   PASSporT claim.

   Current usage of baseline [RFC8224] Identity is largely confined to
   INVITE requests that initiate telephone calls.  RCS-style
   applications would require PASSporTs for all conversation
   participants, which could become complex in multi-party
   conversations.  Any solution in this space would in turn require the
   implementation of STIR connected identity

   Also note that the assurance offered by [RFC8862] is "end-to-end" in
   the sense that it offers assurance between an authentication service
   and verification service.  If those are not implemented by the
   endpoints themselves, there are still potential opportunities for
   tampering before messages are signed and after they are verified.
   For the most part, STIR does not intend to protect against man-in-
   the-middle attacks so much as spoofed origination, however, so the
   protection offered may be sufficient to mitigate nuisance messaging.

3.2.  PASSporTs and Messaging

   In the second case, SIP also has a method for sending messages in the
   body of a SIP request: the MESSAGE [RFC3428] method, which is used in
   some North American emergency services use cases.  The interaction of
   STIR with MESSAGE is not as straightforward as the potential use case
   with MSRP.  An Identity header could be added to any SIP MESSAGE
   request, but without some extension to the PASSporT claims, the
   PASSporT would offer no protection to the message content.  As the

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   bodies of SIP requests are MIME encoded, S/MIME [RFC8591] has been
   proposed as a means of providing integrating for MESSAGE (and some
   MSRP cases as well).  The interaction of [RFC8226] STIR certificates
   with S/MIME for messaging applications requires some further
   explication; and additionally, PASSporT can provide its own integrity
   check for message contents as part of its assertions through, a new
   claim is here defined to provide a hash over message contents.

   In order to differentiate a PASSporT for an individual message from a
   PASSporT used to secure a telephone call or message stream, this
   document defines a new "msg" PASSporT Type.  This helps to prevent
   the replay of a PASSporT for a message to putatively secure a call,
   or vice versa.

   This specification defines a new optional JWT [RFC7519] claim "msgi"
   which provides a digest over a MIME body that contains a text or
   multimedia message. "msgi" MUST NOT appear in PASSporTs with a type
   other than "msg", but they are OPTIONAL in "msg" PASSporTs, as
   integrity for messages may be provided by some other service (e.g.
   [RFC8591]).  Implementations of "msgi" MUST support the following
   hash algorithms: "SHA256", "SHA384", or "SHA512", which are defined
   as part of the SHA-2 set of cryptographic hash functions by the NIST.

   A "msgi" message digest is computed over the entire MIME body of a
   SIP message, which per [RFC3428] may any sort of MIME body, including
   a multipart body in some cases, especially when multimedia content is
   involved.  At the end result of the process, the digest becomes the
   value of the JWT "msgi" claim, as per this example:

   "msgi" :

3.2.1.  PASSporT Conveyance with Messaging

   If the message is being conveyed in SIP, via the MESSAGE method, then
   the PASSporT could be conveyed in an Identity header field in that
   request.  The authentication and verification service procedures for
   populating that PASSporT would follow [RFC8224], with the addition of
   the "msgi" claim defined in Section 3.2.

   In text messaging today, multimedia message system (MMS) messages are
   often conveyed with SMTP.  There are thus a suite of additional email
   security tools available in this environment for sender
   authentication, such as DMARC [RFC7489].  The interaction of these
   mechanisms with STIR certificates and/or PASSporTs would require
   further study and is outside the scope of this document.

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   For other cases where messages are conveyed by some protocol other
   than SIP, that protocol might itself have some way of conveying
   PASSporTs.  But there will surely be cases where legacy transmission
   of messages will not permit an accompanying PASSporT, in which case
   something like out-of-band [RFC8816] conveyance would be the only way
   to deliver the PASSporT.  This may be necessary to support cases
   where legacy SMPP systems cannot be upgraded, for example.

4.  Certificates and Messaging

   The [RFC8226] STIR certificate profiles defines a way to issue
   certificates that sign PASSporTs, which attest through their
   TNAuthList either a Service Provider Code (SPC), or a set of one or
   more telephone numbers.  This specification proposes that the
   semantics of this certificates should suffice for signing for
   messages from a telephone number without further modification.

   [TBD: Or should there be?  Should for example certificates have to
   have some special authority to sign for messages instead of calls?]

5.  Acknowledgments

   We would like to thank Christer Holmberg, Brian Rosen, Ben Campbell,
   and Alex Bobotek for their contributions to this specification.

6.  IANA Considerations

6.1.  JSON Web Token Claims Registration

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

   Claim Name: "msgi"

   Claim Description: Message Integrity Information

   Change Controller: IESG

   Specification Document(s): [RFCThis]

6.2.  PASSporT Type Registration

   This specification defines one new PASSporT type for the PASSport
   Extensions Registry defined in [RFC8225], which resides at
   extensions.  It is:

   "msg" as defined in [RFCThis] Section 3.2.

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7.  Security Considerations

   Note that a variety of non-SIP protocols, both those integrated into
   the traditional telephone network and those based on over-the-top
   applications, are responsible for most of the messaging that is sent
   to and from telephone numbers today.  Introducing this capability for
   SIP-based messaging will help to mitigate spoofing and nuisance
   messaging for SIP-based platforms only.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474,
              DOI 10.17487/RFC4474, August 2006,

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [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,

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,

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   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,

8.2.  Informative References

              Peterson, J. and C. Wendt, "Connected Identity for STIR",
              draft-peterson-stir-rfc4916-update-03 (work in progress),
              February 2021.

   [RCC.07]   GSMA RCC.07 v9.0 | 16 May 2018, "Rich Communication Suite
              8.0 Advanced Communications Services and Client
              Specification", 2018.

   [RCC.15]   GSMA PRD-RCC.15 v5.0 | 16 May 2018, "IMS Device
              Configuration and Supporting Services", 2018.

   [RFC3311]  Rosenberg, J., "The Session Initiation Protocol (SIP)
              UPDATE Method", RFC 3311, DOI 10.17487/RFC3311, October
              2002, <>.

   [RFC3428]  Campbell, B., Ed., Rosenberg, J., Schulzrinne, H.,
              Huitema, C., and D. Gurle, "Session Initiation Protocol
              (SIP) Extension for Instant Messaging", RFC 3428,
              DOI 10.17487/RFC3428, December 2002,

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, DOI 10.17487/RFC4916, June
              2007, <>.

   [RFC4975]  Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
              "The Message Session Relay Protocol (MSRP)", RFC 4975,
              DOI 10.17487/RFC4975, September 2007,

   [RFC5194]  van Wijk, A., Ed. and G. Gybels, Ed., "Framework for Real-
              Time Text over IP Using the Session Initiation Protocol
              (SIP)", RFC 5194, DOI 10.17487/RFC5194, June 2008,

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   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,

   [RFC8591]  Campbell, B. and R. Housley, "SIP-Based Messaging with
              S/MIME", RFC 8591, DOI 10.17487/RFC8591, April 2019,

   [RFC8816]  Rescorla, E. and J. Peterson, "Secure Telephone Identity
              Revisited (STIR) Out-of-Band Architecture and Use Cases",
              RFC 8816, DOI 10.17487/RFC8816, February 2021,

   [RFC8862]  Peterson, J., Barnes, R., and R. Housley, "Best Practices
              for Securing RTP Media Signaled with SIP", BCP 228,
              RFC 8862, DOI 10.17487/RFC8862, January 2021,

   [RFC8876]  Rosen, B., Schulzrinne, H., Tschofenig, H., and R.
              Gellens, "Non-interactive Emergency Calls", RFC 8876,
              DOI 10.17487/RFC8876, September 2020,

   [RFC8946]  Peterson, J., "Personal Assertion Token (PASSporT)
              Extension for Diverted Calls", RFC 8946,
              DOI 10.17487/RFC8946, February 2021,

Authors' Addresses

   Jon Peterson
   Neustar, Inc.
   1800 Sutter St Suite 570
   Concord, CA  94520


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   Chris Wendt
   One Comcast Center
   Philadelphia, PA  19103


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