Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Standards Track                                C. Wendt
Expires: May 13, 2022                                              Somos
                                                        November 9, 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 text and
   multimedia messaging use cases, both for instant messages carried or
   negotiated by SIP.

Status of This Memo

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   carefully, as they describe your rights and restrictions with respect
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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  . . . . . . . . . . . . . . .   7
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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, 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

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   message contents may no longer be a reliable way to mitigate
   messaging spam in the future.  And as STIR sees further deployment in
   the telephone network, the governance structures put in place for
   securing telephone network resources with STIR could be repurposed to
   help secure the messaging ecosystem.

   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 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.  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.  For the most basic use cases, STIR for
   messaging should not require any further protocol enhancements.

   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 likely require the
   implementation of STIR connected identity
   [I-D.peterson-stir-rfc4916-update], but the specification of
   PASSporT-signed session conferencing is outside the scope of this

   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.  MESSAGE is used
   for example 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

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   content, and potentially be reusable for cut-and-paste attacks.  As
   the bodies of SIP requests are MIME encoded, S/MIME [RFC8591] has
   been proposed as a means of providing integrity for MESSAGE (and some
   MSRP cases as well).  The use of CPIM [RFC3862] as a MIME body allows
   the integrity of messages to withstand interworking with non-SIP
   protocols.  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 through a new claim 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. "msg" PASSporTs may carry
   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.  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.

   A MESSAGE request can be sent to multiple destinations in order to
   support multiparty messaging.  In those cases, the "dest" field of
   the PASSporT can accommodate the multiple targets of a MESSAGE
   without the need to generate a PASSporT for each target of the
   message.  If however the request is forked to multiple targets by an
   intermediary later in the call flow, and the list of targets is not
   available to the authentication service, then that forking
   intermediary would need to use diversion [RFC8946] PASSporTs to sign
   for its target set.

4.  Certificates and Messaging

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

   As the "orig" and "dest" field of PASSporTs may contain URIs
   containing SIP URIs without telephone numbers, the STIR for messaging
   mechanism contained in this specification is not inherently
   restricted to the use of telephone numbers.  This specification
   offers no guidance on certification authorities who are appropriate
   to sign for non-telephone number "orig" values.

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"

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   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.

7.  Privacy Considerations

   Signing messages or message sessions with STIR has little direct
   bearing on the privacy of messaging for SIP as described in [RFC3428]
   or [RFC4975].  An authentication service signing a MESSAGE method may
   compute the "msgi" hash over the message contents; if the message is
   in cleartext, that will reveal its contents to the authentication
   service, which might not otherwise be in the call path.

   The implications for anonymity of SITR are discussed in [RFC8224],
   and those considerations would apply equally here for anonymous

8.  Security Considerations

   This specification inherits the security considerations of [RFC8224].
   The carriage of messages within SIP per Section 3.2 has a number of
   security and privacy implications as documented in [RFC3428], which
   are expanded in [RFC8591]; these considerations apply here well.

   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.

9.  References

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9.1.  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,

   [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,

   [RFC3862]  Klyne, G. and D. Atkins, "Common Presence and Instant
              Messaging (CPIM): Message Format", RFC 3862,
              DOI 10.17487/RFC3862, August 2004,

   [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,

9.2.  Informative References

              Peterson, J. and C. Wendt, "Connected Identity for STIR",
              draft-peterson-stir-rfc4916-update-04 (work in progress),
              July 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, <>.

   [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,

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

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   [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.


   Chris Wendt


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