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Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Informational                                  C. Wendt
Expires: May 6, 2021                                             Comcast
                                                        November 2, 2020

                      Messaging Use Cases 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
   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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   This Internet-Draft will expire on May 6, 2021.

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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
   4.  PASSporTs and Messaging . . . . . . . . . . . . . . . . . . .   4
     4.1.  PASSporTs Conveyance with Messaging . . . . . . . . . . .   5
   5.  Certificates and Messaging  . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  JSON Web Token Claims Registration  . . . . . . . . . . .   6
     7.2.  PASSporT Type Registration  . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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.  Increasingly, 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 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

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   network resources with STIR could be repurposed to help secure the
   messaging ecosystem.

   This specification therefore explores how the PASSporT mechanism
   defined for STIR could be applied to providing protection for textual
   and multimedia messaging, but only for 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 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.

   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.  The
   same would apply to sessions that negotiate text over RTP via
   [RFC4103] or similar mechanisms.  In these cases, STIR for messaging
   should not require any further protocol enhancements.

   [TBD: Also consider the applicability of "mkey" to these schemes, and
   RFC8862?  Also, any interest in MLS interaction?]

   In the second case, SIP also has a method for sending messages a the
   body of a SIP request: the MESSAGE [RFC3428] method.  The interaction

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   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
   bodies of SIP requests are MIME encoded, S/MIME [RFC8591] has been
   proposed as a means of providing integrating for MESSAGE, and
   potentially for securing MSRP as well.  The interaction of [RFC8226]
   STIR certificates with S/MIME for messaging applications would
   require some further explication; and potentially, PASSporT could
   provide its own integrity check for message contents.

   Moreover, the MESSAGE method is not commonly used today to carry
   messages for consumer devices.  A variety of non-SIP protocols, both
   those integrated in to 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.  This
   specification proposes that the STIR credentials assigned to service
   providers could be leveraged to sign for PASSporTs for messages that
   originate from telephone numbers.  In order to apply PASSporT to
   textual or multimedia messaging, a new claim is here defined to
   provide a hash over message contents.

4.  PASSporTs and Messaging

   In order to differentiate a PASSporT for a message from a PASSporT
   used to secure a telephone call, this document defines a new "msg"
   PASSporT Type.  This prevents 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 the contents of a message, which may be
   a text message, or a more complex 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.

   [TBD: Do we need algorithmic agility here?]

   In order to generate the message digest, the following steps are

   [TBD: Canonicalization procedures.  Maybe we need separate procedures
   for plain text (like, SMPP), rich text, and then more complex
   multimedia messages?  Definitely a danger of scope creep.  Anything
   we could easily steal here?]

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   At the end result of the process, the digest becomes the value of the
   JWT "msgi" claim, as per this example:

   "msgi" :

4.1.  PASSporTs 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 4.

   In 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 [I-D.ietf-stir-oob] conveyance would be the only way to
   deliver the PASSporT.

   [TBD: I mean, if you can deliver a PASSporT OOB, you can deliver a
   message OTT - there may be limits to how useful a mechanism like this
   would be.  In any event, the precise way to do OOB for messaging
   would need to be sketched out here.]

5.  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?]

6.  Acknowledgments

   We would like to thank YOU for your contributions to this

7.  IANA Considerations

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7.1.  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]

7.2.  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 4.

8.  Security Considerations


9.  Informative References

              Rescorla, E. and J. Peterson, "STIR Out-of-Band
              Architecture and Use Cases", draft-ietf-stir-oob-07 (work
              in progress), March 2020.

              Peterson, J., "PASSporT Extension for Diverted Calls",
              draft-ietf-stir-passport-divert-09 (work in progress),
              July 2020.

   [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,

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   [RFC3311]  Rosenberg, J., "The Session Initiation Protocol (SIP)
              UPDATE Method", RFC 3311, DOI 10.17487/RFC3311, October
              2002, <https://www.rfc-editor.org/info/rfc3311>.

   [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,

   [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,

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, DOI 10.17487/RFC4916, June
              2007, <https://www.rfc-editor.org/info/rfc4916>.

   [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,

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/info/rfc7159>.

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

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

   [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>.

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   [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,

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

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

Authors' Addresses

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

   Email: jon.peterson@team.neustar

   Chris Wendt
   One Comcast Center
   Philadelphia, PA  19103

   Email: chris-ietf@chriswendt.net

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