Network Working Group J. Peterson Internet-Draft Neustar Intended status: Informational C. Wendt Expires: September 6, 2018 Comcast March 5, 2018 Connected Identity for STIR draft-peterson-stir-rfc4916-update-00.txt Abstract 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 6, 2018. Copyright Notice Copyright (c) 2018 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 Peterson & Wendt Expires September 6, 2018 [Page 1] Internet-Draft RFC4916 Update March 2018 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 . . . . . . . . . . . . . . . . . . . . . 5 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 9. Security Considerations . . . . . . . . . . . . . . . . . . . 6 10. Informative References . . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 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 reasons such as call forwarding, call distribution and call pick-up. It can potentially be difficult to determine why a call reaches a target other than the one originally intended, and whether the party ultimately reached by the call is one that the caller should trust [RFC4916] allowed a mid-dialog request, such as an UPDATE [RFC3311], to convey what is commonly called "connected identity" information-- that is, the identity of the connected user--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 Peterson & Wendt Expires September 6, 2018 [Page 2] Internet-Draft RFC4916 Update March 2018 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 that dialog-forming request reached 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 direction. [RFC4916] furthermore created the "from-change" option tag to negotiate this capability during dialog establishment. [RFC4916] was created to work with the original SIP Identity [RFC4474] mechanism, as that mechanism only allowed requests to be signed, but not responses. Since a mid-dialog request in the backwards direction can be signed with Identity like any other SIP request, this created a practical problem: 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. This specification updates [RFC4916] to reflect the changes to the SIP Identity header as defined in [RFC4474] made by [RFC8224], and the revised problem space of STIR. 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. 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 a telephone numbers might be unceremoniously dumped on a PSTN gateway, they might be sent to a default intermediary that makes forwarding Peterson & Wendt Expires September 6, 2018 [Page 3] Internet-Draft RFC4916 Update March 2018 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. While the MODERN framework hopes to foster a more credible story about how to establish authority for telephone numbers on the Internet, in the interim, 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 destination. [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 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 the media layer do not match fingerprints exchanged at the signaling layer: when a user requests confidentiality services, and they are available, 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. Peterson & Wendt Expires September 6, 2018 [Page 4] Internet-Draft RFC4916 Update March 2018 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. There is currently no way to expose similar information to the calling party however: only if redirection is used (SIP 3XX responses) instead of retargeting will the originating side participate in setting a new destination for calls. Future versions of this specification will explore ways that the results of mechanisms like [I-D.ietf-stir-passport-divert] could be communicated back to the originating authentication service. 5. Pre-Association with Destinations Any connected identity mechanism will work best if the user knows before initiating a call that security services are 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. 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 reuse 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 specification. Peterson & Wendt Expires September 6, 2018 [Page 5] Internet-Draft RFC4916 Update March 2018 8. IANA Considerations This memo includes no request to IANA. 9. Security Considerations TBD. 10. Informative References [I-D.ietf-modern-problem-framework] Peterson, J. and T. McGarry, "Modern Problem Statement, Use Cases, and Framework", draft-ietf-modern-problem- framework-03 (work in progress), July 2017. [I-D.ietf-stir-oob] Rescorla, E. and J. Peterson, "STIR Out-of-Band Architecture and Use Cases", draft-ietf-stir-oob-01 (work in progress), October 2017. [I-D.ietf-stir-passport-divert] Peterson, J., "PASSporT Extension for Diverted Calls", draft-ietf-stir-passport-divert-01 (work in progress), October 2017. [I-D.peterson-modern-teri] Peterson, J., "An Architecture and Information Model for Telephone-Related Information (TeRI)", draft-peterson- modern-teri-03 (work in progress), July 2017. [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, . Peterson & Wendt Expires September 6, 2018 [Page 6] Internet-Draft RFC4916 Update March 2018 [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, . [RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014, . [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, . Authors' Addresses Jon Peterson Neustar, Inc. 1800 Sutter St Suite 570 Concord, CA 94520 US Email: jon.peterson@neustar.biz Chris Wendt Comcast One Comcast Center Philadelphia, PA 19103 USA Email: chris-ietf@chriswendt.net Peterson & Wendt Expires September 6, 2018 [Page 7]