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Versions: 00 01 02 03                                                   
Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Informational                                  C. Wendt
Expires: January 14, 2021                                        Comcast
                                                           July 13, 2020

                      Connected Identity for STIR


   The SIP Identity header conveys cryptographic identity information
   about the originators of SIP requests.  The Secure Telephone Identity
   Revisited (STIR) framework however provides no means for determining
   the identity of the called party in a traditional telephone calling
   scenario.  This document updates prior guidance on the "connected
   identity" problem to reflect the changes to SIP Identity that
   accompanied STIR, and considers a revised problem space for connected
   identity as a means of detecting calls that have been retargeted to a
   party impersonating the intended destination.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 14, 2021.

Copyright Notice

   Copyright (c) 2020 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
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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
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Connected Identity Problem Statement for STIR . . . . . . . .   3
   4.  Authorization Policy for Callers  . . . . . . . . . . . . . .   4
   5.  Pre-Association with Destinations . . . . . . . . . . . . . .   5
   6.  Updates to RFC4916  . . . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   10. Informative References  . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The Session Initiation Protocol (SIP) [RFC3261] initiates sessions,
   and as a step in establishing sessions, it exchanges information
   about the parties at both ends of a session.  Users review
   information about the calling party, for example, to determine
   whether to accept communications initiated by a SIP, in the same way
   that users of the telephone network assess "Caller ID" information
   before picking up calls.  This information may sometimes be consumed
   by automata to make authorization decisions.

   STIR [RFC8224] provides a cryptographic assurance of the identity of
   calling parties in order to prevent impersonation, which is a key
   enabler of unwanted robocalls, swatting, vishing, voicemail hacking,
   and similar attacks (see [RFC7340]).  There also exists a related
   problem: the identity of the party who answers a call can differ from
   that of the initial called party for various innocuous reasons such
   as call forwarding, but in certain network environments it is
   possible for attackers to hijack the route of a called number and
   direct it to a resource controlled by the attacker.  It can
   potentially be difficult to determine why a call reached a target
   other than the one originally intended, and whether the party
   ultimately reached by the call is one that the caller should trust.
   The property of providing identity in the backwards direction of a
   call is here called "connected identity."

   Previous work on connected identity focused on fixing the core
   semantics of SIP.  [RFC4916] allowed a mid-dialog request, such as an

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   UPDATE [RFC3311], to convey identity in either direction within the
   context of an existing INVITE-initiated dialog.  In an update to the
   original [RFC3261] behavior, [RFC4916] allowed that UPDATE to alter
   the From header field value for requests in the backwards direction:
   previously [RFC3261] required that the From header field values sent
   in requests in the backwards direction reflect the To header field
   value of the dialog-forming request, for various backwards-
   compatibility reasons.  In other words, if Alice sent a dialog-
   forming request to Bob, then under the original [RFC3261] rules, even
   if Bob's SIP service forwarded that dialog-forming request to Carol,
   Carol would still be required to put Bob's identity in the From
   header field value in any mid-dialog requests in the backwards

   One of the motivating use cases for [RFC4916] was the use of
   connected identity with the SIP Identity Identity [RFC4474] header
   field.  While a mid-dialog request in the backwards direction (e.g.
   UPDATE) can be signed with Identity like any other SIP request,
   forwarded requests would not be signable without the ability to
   change the mid-dialog From header field value: Carol, say, would not
   be able to furnish a key to sign for Bob's identity, if Carol wanted
   to sign requests in the backwards direction.  Carol would however be
   able to sign for her own identity in the From header field value, if
   mid-dialog requests in the backwards direction were permitted to vary
   from the original To header field value.

   With the obsolence of [RFC4474] by [RFC8224], this specification
   updates [RFC4916] to reflect the changes to the SIP Identity header
   and the revised problem space of STIR.  It also explores some new
   features that would be enabled by connected identity for STIR,
   including the use of connected identity to prevent route hijacking
   and to notify callers when an expected called party has successfully
   been reached.

2.  Terminology

   In this document, the key words "MAY", "MUST, "MUST NOT", "SHOULD",
   and "SHOULD NOT", are to be interpreted as described in [RFC2119].

3.  Connected Identity Problem Statement for STIR

   The STIR problem statement [RFC7340] enumerates robocalling,
   voicemail hacking, vishing, and swatting as problems with the modern
   telephone number that are enabled, or abetted, by impersonation: by
   the ability of a calling party to arbitrarily set the identity that
   will be rendered to end users to identify the caller.

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   Today, sophisticated adversaries can redirect calls on the PSTN to
   destinations other than the intended called party.  For some call
   centers, like those associated with financial institutions,
   healthcare, and emergency services, an attacker could hope to gain
   valuable information about people or to prevent some classes of
   important services.

   Moreover, on the Internet, the lack of any centralized or even
   federated routing system for telephone numbers has resulted in
   deployments where the routing of calls is arbitrary: calls to
   telephone numbers might be unceremoniously dumped on a PSTN gateway,
   they might be sent to a default intermediary that makes forwarding
   decisions based on a local flat file, various mechanisms like private
   ENUM might be consulted, or routing might be determined in some
   other, domain specific way.  In short, there are numerous attack
   surfaces that an adversary could explore to attempt to redirect calls
   to a particular number to someplace other than the intended

   [RFC4916] rightly observed that once a SIP call has been answered,
   the called party can be replaced by a different party with a
   different identity due to call transfer, call park and retrieval, and
   so on.  In some cases, due to the presence of a back-to-back user
   agent, it can be effectively impossible for the calling party to know
   that this has happened.  The problem statement considered for STIR
   focuses solely on call setup, and whether or not media from the
   connected party should be rendered to the caller when a dialog has
   been established.  This specification does not consider further any
   threats that arise from a substitution of the called party.

4.  Authorization Policy for Callers

   In a traditional telephone call, the called party receives an
   alerting signal and can make a decision about whether or not to pick
   up a phone.  They may have access to displayed information, like
   "Caller ID", to help them arrive at an authorization decision.  The
   situation is more complicated for callers, however: callers typically
   expect to be connected to the proper destination and are often
   holding telephones in a position that would not enable them to see
   displayed information, if any were available for them to review--and
   moreover, their most direct response to a security breach would be to
   hang up the call they were in the middle of placing.

   While this specification will not prescribe any user experience
   associated with placing a call, it assumes that callers have some
   authorization posture that will result in the right thing happening
   when the connected identity is not expected.  This is analogous to a
   situation where SRTP negotiation fails because the keys exchanges at

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   the media layer do not match fingerprints exchanged at the signaling
   layer: when a user requests confidentiality services, and they are
   unavailable, media should not be exchanged.  Thus we assume that
   users have a way in their interface to require this criticality, on a
   per-call basis, or perhaps on a per-destination basis.  Similarly,
   users will not always place calls where the connected identity is
   crucial--but when they do, they should have a way to tell their
   devices that the call should not be completed if it arrives at an
   unexpected party.

   Ultimately, authorization policy for called parties is difficult to
   set, as calls can end up at unexpected places for legitimate reasons.
   Some work has been done to make sure that secure diversion works with
   STIR, in for example [I-D.ietf-stir-passport-divert].  Those
   indications can be consumed by on the terminating side by
   verification services to determine that a call has reached its
   eventual destination for the right reasons.  The only way those
   diversion PASSporTs will be seen by the calling party is if
   redirection is used (SIP 3XX responses) instead of retargeting; while
   some network policies may want to conceal service logic from the
   originating party, sending redirections in the backwards direction is
   the only current defined way for secure indications of redirection to
   be revealed to the calling party.  That in turn would allow the
   calling user agent to have a strong assurance that legitimate
   entities in the call path caused the request to reach a party that
   the caller did not anticipate.

5.  Pre-Association with Destinations

   Any connected identity mechanism will work best if the user knows
   before initiating a call that connected identity is supported by the
   destination side.  Not every institution that a user wants to connect
   to securely will support STIR and connected identity out of the gate.

   The user interface of modern smartphones support an address book from
   which users select telephone numbers to dial.  Even when dialing a
   number manually, the interface frequently checks the address book and
   will display to users any provisioned name for the target of the call
   if one exists.  Similarly, when clicking on a telephone number viewed
   on a web page, or similar service, smartphone often prompt users
   approve the access to the outbound dialer.  These sorts of decision
   points, when the user is still interacting with the user interface,
   provide an opportunity to form a pre-association with the
   destination, and potentially even to exchange STIR PASSporTs in order
   to validate whether or not the expected destination can be reached
   securely.  Again, this is probably most meaningful for contacting
   financial, government, or emergency services, for cases where
   reaching an unintended destination may have serious consequences.

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   Future versions of this specification will explore how the security
   features of destinations can be discovered before calls are set up so
   that calling parties can make more informed authorization decisions.
   This may rely on the establishment of a provisional, media-less SIP
   dialog which can then negotiate media when the user approves of the
   destination.  In some environments, that may require the use of
   mechanisms defined by [I-D.ietf-stir-oob].

6.  Updates to RFC4916

   [TBD - ways that UPDATEs in the backwards direction can carry
   additional information in support of the above]

   In general, the guidance of RFC4916 remains valid for RFC8224.

   The deprecation of the Identity-Info header has a number of
   implications for RFC4916; all of the protocol examples need to be
   updated to reflect that.

7.  Acknowledgments

   We would like to thank YOU for your contributions to this

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations


10.  Informative References

              Peterson, J. and T. McGarry, "Modern Problem Statement,
              Use Cases, and Framework", draft-ietf-modern-problem-
              framework-04 (work in progress), March 2018.

              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.

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              Peterson, J., "An Architecture and Information Model for
              Telephone-Related Information (TeRI)", draft-peterson-
              modern-teri-04 (work in progress), March 2018.

   [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,

   [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>.

   [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>.

   [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,

   [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,

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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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