Network Working Group

Internet Engineering Task Force (IETF)                       J. Peterson
Internet-Draft
Request for Comments: 9475                                       Neustar
Intended status:
Category: Standards Track                                       C. Wendt
Expires: 8 January 2024
ISSN: 2070-1721                                                    Somos
                                                             7 July
                                                            October 2023

    Messaging Use Cases and Extensions for STIR
                      draft-ietf-stir-messaging-08 Secure Telephone Identity
                            Revisited (STIR)

Abstract

   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 and multimedia
   messaging space.  This document explores the applicability of STIR's
   Personal Assertion Token (PASSporT) and certificate issuance
   framework to text and multimedia messaging use cases, including
   support both for both messages carried as a payload in SIP requests and
   for
   messages sent in sessions negotiated by SIP.

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 an Internet Standards Track document.

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   Internet-Drafts are draft documents valid the IETF community.  It has
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   Internet Standards is available in Section 2 of six months RFC 7841.

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   This Internet-Draft will expire on 8 January 2024.
   https://www.rfc-editor.org/info/rfc9475.

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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 Individual Messages . . . . . . . . . . . .   4
       3.2.1.  PASSporT Conveyance with Messaging  . . . . . . . . .   6
   4.  Certificates and Messaging  . . . . . . . . . . . . . . . . .   7
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     6.1.
     5.1.  JSON Web Token Claims Registration  . . . . . . . . . . .   8
     6.2.
     5.2.  PASSporT Type Registration  . . . . . . . . . . . . . . .   8
   7.
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
   8.
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Acknowledgments
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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 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

   However, the problem of bulk, unsolicited commercial communications
   is not,
   however, not limited to telephone calls.  Spammers and fraudsters are
   increasingly turning to messaging applications to deliver undesired
   content to consumers.  In some respects, mitigating these unwanted
   messages resembles the email spam problem: problem; for example, textual
   analysis of the message contents can be used to fingerprint content
   that is generated by spammers, for example. spammers.  However, encrypted messaging is
   becoming more common, and analysis of message contents may no longer
   be a reliable way to mitigate messaging spam in the future.  And as  As STIR
   sees further deployment in the telephone network, the governance
   structures put in place for securing telephone network 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 attacks and
   ultimately helping help to identify the source of emergency communications in
   general (including swatting attacks, see [RFC7340]).

   This

   Therefore, this specification therefore explores how the PASSporT mechanism
   defined for STIR could be applied to in providing protection for textual
   and multimedia messaging, but it focuses particularly on those
   messages that use telephone numbers as the identity of the sender.  It
   moreover
   Moreover, it considers the reuse of existing STIR certificates, which
   are beginning to see widespread deployment, deployment for signing PASSporTs that
   protect messages.  For that purpose purpose, it defines a new PASSporT type
   and an element that protects message integrity.  It contains a
   mixture of normative and informative guidance that specifies new
   fields
   claims for use in PASSporTs as well as an overview of how STIR might
   be applied to messaging in various environemnts. environments.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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 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,

   1.  a PASSporT could be used to securely negotiate a session over
       which messages will be exchanged; exchanged (see Section 3.1), and second,

   2.  in sessionless scenarios, a PASSporT could be generated on a per-message per-
       message basis with its own built-in message security. security (see
       Section 3.2).

3.1.  Message Sessions

   For

   In the first case, where SIP negotiates a session where in which the media will
   be text messages or MIME content, as, for example, with the Message
   Session Relay Protocol (MSRP) [RFC4975], the [RFC4975].  This 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
   Rich Communication Suite (RCS) [RCC.07].  Note that 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. framework
   described in [RFC8862].  Similar practices would apply to sessions
   that negotiate real-time text over RTP ([RFC4103], [RFC5194]); any
   that can operate over DTLS/SRTP (Secure Real-time Transport Protocol)
   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 multiparty conversations.  Any solution in this
   space would likely require the implementation of STIR connected STIR-connected
   identity
   [I-D.peterson-stir-rfc4916-update], [CONNECT-ID-STIR], but the specification of PASSporT-signed
   session conferencing is outside the scope of this document.

   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
   However, for the most part, STIR does not intend to protect against machine-
   in-the-middle
   machine-in-the-middle attacks so much as spoofed origination, however, origination; so the
   protection offered may be sufficient to mitigate nuisance messaging.

3.2.  PASSporTs and Individual Messages

   In the second case, case described in Section 3, SIP also has a method for
   sending messages in the body of a SIP request: the MESSAGE [RFC3428] method. method
   [RFC3428].  For example, 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
   content, and content; it would potentially be
   reusable for cut-and-paste attacks where the Identity header field
   from a legitimate request for one user is reused in a request for a
   different user.  As the bodies of SIP requests are MIME encoded, S/MIME S/
   MIME [RFC8591] has been proposed as a means of providing integrity
   for MESSAGE (and some MSRP cases as well).  The use of CPIM Common
   Presence and Instant Messaging (CPIM) [RFC3862] as a MIME body allows
   the integrity of messages to withstand interworking with non-SIP protocols. protocols
   that are not SIP.  The interaction of [RFC8226] STIR certificates with S/MIME
   (see [RFC8226]) for messaging applications would require further
   specification; 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. type. "msg" PASSporTs may carry
   a new optional JWT JSON Web Token (JWT) [RFC7519] claim "msgi" "msgi", which
   provides a digest over a MIME body that contains a text or multimedia
   message.  Authentication services MUST NOT include "msgi" elements in
   PASSporT types other than "msg", but "msgi" is OPTIONAL in "msg"
   PASSporTs, as integrity for messages may be provided by some other
   service (e.g.  [RFC8591]).  Verification services MUST ignore the
   presence of "msgi" in non-"msg" PASSporT types.

   The claim value of the "msgi" claim key is a string that defines the
   crypto algorithm used to generate the digest concatenated by a hyphen
   with a digest string.  Implementations MUST support the hash
   algorithms SHA-256, SHA-384, and SHA-512.  These hash algorithms are
   identified by "sha256", "sha384", and "sha512", respectively.  SHA-
   256, SHA-384, and SHA-512 are part of the SHA-2 set of cryptographic
   hash functions [RFC6234] defined by the US National Institute of
   Standards and Technology (NIST).  [SHA2] Implementations implementations MAY support
   additional recommended hash algorithms in [IANA-COSE-ALG]
   (https://www.iana.org/assignments/cose/cose.xhtml#algorithms); the "COSE Algorithms"
   registry (https://www.iana.org/assignments/cose/
   cose.xhtml#algorithms); that is, the hash algorithm has "Yes" in the
   "Recommended" column of the IANA registry.  Hash algorithm
   identifiers MUST use only lowercase letters, and they MUST NOT
   contain hyphen characters.  The character following the algorithm
   string MUST be a hyphen character, "-", character ("-" or ASCII 45. character 45).

   The subsequent characters in the claim value are the base64 encoded base64-encoded
   [RFC4648] digest of a canonicalized and concatenated string or binary
   data based
   binary-data-based MIME body of the message.  A  An "msgi" message digest
   is computed over the entirety of the MIME body (be it carried via SIP
   or
   no), which not); per [RFC3428] [RFC3428], this may be any sort of MIME body, including
   a multipart body in some cases, especially when multimedia content is
   involved.  Those MIME bodies may or may not contain encrypted content
   or not as the sender desires.  The digest becomes the value of the JWT
   "msgi" claim, as per this example:

   "msgi" :
   "sha256-H8BRh8j48O9oYatfu5AZzq6A9RINQZngK7T62em8MUt1FLm52t+eX6xO"

   Per baseline [RFC8224], this specifications specification leaves it to local policy to
   determine how messages are handled after verification succeeds or
   fails.  Broadly, if a SIP-based verification service wants to
   communicate back to the sender that the "msgi" hash does not
   correspond to the received message, using a SIP 438 response code
   would be most appropriate.

   Note that that, in some CPIM environments, intermediaries may add or
   consume CPIM headers used for metadata in messages.  MIME-layer
   integrity protection of "msgi" would be broken by a modification
   along these lines.  Any such environment would require a profile of
   this specification that reduces the scope of protection only to the
   CPIM payload, as discussed in [RFC8591] Section 9.1. 9.1 of [RFC8591].

   Finally, note that messages may be subject to store-and-forward
   treatment that differs from traditional delivery expectations of SIP
   transactions.  In such cases, the expiry freshness window recommended
   by [RFC8224] may be too strict, as routine behavior might dictate the
   delivery of a MESSAGE minutes or hours after it was sent.  The
   potential for replay attacks can, however, be largely mitigated by
   the timestamp in PASSporTs; duplicate messages are easily detected,
   and the timestamp can be used to order messages displayed to in the user
   inbox in a way that precludes showing stale messages as fresh.
   Relaxing the expiry timer would require support for such features on
   the receiving side of the message.

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 in that request.
   The authentication and verification service procedures for populating
   that PASSporT would follow the guidance in [RFC8224], with the
   addition of the "msgi" claim defined in Section 3.2.

   In text messaging today, multimedia message system Multimedia Messaging Service (MMS) messages
   are often conveyed with SMTP.  There are thus  Thus, there is a suite of additional
   email security tools available in this environment for sender
   authentication, such as DMARC Domain-based Message Authentication,
   Reporting, and Conformance (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.

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

   A MESSAGE request can be sent to multiple destinations in order to
   support multiparty messaging.  In those cases, the "dest" field claim 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  However, if 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 [RFC8946] to sign
   for its target set.

4.  Certificates and Messaging

   The [RFC8226] STIR certificate profiles

   "Secure Telephone Identity Credentials: Certificates"[RFC8226]
   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.

   Note that the certificate referenced by the "x5u" of a PASSporT can
   change over time, time due to certificate expiry/rollover; in particular particular,
   the use of short-lived certificates can entail rollover on a daily
   basis,
   basis or even more frequently.  Thus  Thus, any store-and-forward messaging
   system relying on PASSporTs must take into account the possibility
   that the certificate that signed the PASSporT, though valid at the
   time the PASSporT was generated, could expire before a user reads the
   message.  This might require require:

   *  storing some indicator of the validity of the signature and
      certificate at the time the message was received, or

   *  securely storing the certificate along with the
   PASSporT, PASSporT

   so that the "iat" field claim can be compared with the expiry freshness
   window of the certificate prior to validation.

   As the "orig" and "dest" field claims 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 appropriate
   certification authorities who are appropriate
   to sign for non-telephone number designing "orig" values. values that do not
   contain telephone numbers.

5.  Acknowledgments

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

6.  IANA Considerations

6.1.

5.1.  JSON Web Token Claims Registration

   This specification requests that the

   IANA add has added one new claim to the
   JSON "JSON Web Token Claims Claims" registry as
   that was defined in [RFC7519].

   Claim Name: "msgi"  msgi

   Claim Description:  Message Integrity Information

   Change Controller: IESG  IETF

   Specification Document(s): [RFCThis]

6.2.  RFC 9475

5.2.  PASSporT Type Registration

   This specification defines one new PASSporT type for the PASSport
   Extensions Registry "Personal
   Assertion Token (PASSporT) Extensions" registry defined in [RFC8225], which resides at
   https://www.iana.org/assignments/passport/passport.xhtml#passport-
   extensions. [RFC8225].

   ppt value: "msg"  msg

   Reference: [RFCThis]  Section 3.2

7. of RFC 9475

6.  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 STIR are discussed in [RFC8224],
   and those considerations would apply equally here for anonymous
   messaging.  Creating a an "msg" PASSporT does not add any additional
   privacy protections to the original message content.

8.

7.  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 as well.
   The guidance about store-and-forward messaging and replay protection
   in Section 3.2 should also be recognized by implementers.

   Note that a variety of non-SIP protocols, protocols that are not SIP, 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.

8.  References

9.1.

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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc3261>.

   [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,
              <https://www.rfc-editor.org/info/rfc3428>.

   [RFC3862]  Klyne, G. and D. Atkins, "Common Presence and Instant
              Messaging (CPIM): Message Format", RFC 3862,
              DOI 10.17487/RFC3862, August 2004,
              <https://www.rfc-editor.org/info/rfc3862>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc8224>.

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
              <https://www.rfc-editor.org/info/rfc8225>.

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,
              <https://www.rfc-editor.org/info/rfc8226>.

9.2.

8.2.  Informative References

   [I-D.peterson-stir-rfc4916-update]

   [CONNECT-ID-STIR]
              Peterson, J. and C. Wendt, "Connected Identity for STIR",
              Work in Progress, Internet-Draft, draft-peterson-stir-
              rfc4916-update-04, 12 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-peterson-
              stir-rfc4916-update-04>. draft-ietf-stir-rfc4916-
              update-04, 23 October 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-stir-
              rfc4916-update-04>.

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

   [RCC.15]   GSMA PRD-RCC.15 v5.0 | 16 Version
              9.0, May 2018, <https://www.gsma.com/futurenetworks/wp-
              content/uploads/2019/09/RCC.07-v9.0.pdf>.

   [RCC.15]   GSMA, "IMS Device Configuration and Supporting Services", 2018.
              Version 7.0, October 2019, <https://www.gsma.com/newsroom/
              wp-content/uploads//RCC.15-v7.0.pdf>.

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
              <https://www.rfc-editor.org/info/rfc4103>.

   [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,
              <https://www.rfc-editor.org/info/rfc4975>.

   [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,
              <https://www.rfc-editor.org/info/rfc5194>.

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,
              <https://www.rfc-editor.org/info/rfc7340>.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
              <https://www.rfc-editor.org/info/rfc7489>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC8591]  Campbell, B. and R. Housley, "SIP-Based Messaging with S/
              MIME", RFC 8591, DOI 10.17487/RFC8591, April 2019,
              <https://www.rfc-editor.org/info/rfc8591>.

   [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,
              <https://www.rfc-editor.org/info/rfc8816>.

   [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,
              <https://www.rfc-editor.org/info/rfc8862>.

   [RFC8876]  Rosen, B., Schulzrinne, H., Tschofenig, H., and R.
              Gellens, "Non-interactive Emergency Calls", RFC 8876,
              DOI 10.17487/RFC8876, September 2020,
              <https://www.rfc-editor.org/info/rfc8876>.

   [RFC8946]  Peterson, J., "Personal Assertion Token (PASSporT)
              Extension for Diverted Calls", RFC 8946,
              DOI 10.17487/RFC8946, February 2021,
              <https://www.rfc-editor.org/info/rfc8946>.

   [SHA2]     National Institute of Standards and Technology FIPS PUB
              180-3. http://csrc.nist.gov/publications/fips/fips180-3/
              fips180-3_final.pdf, (NIST),
              "Secure Hash Standard (SHS)", 2018. FIPS PUB 180-3, 2008,
              <http://csrc.nist.gov/publications/fips/fips180-3/
              fips180-3_final.pdf>.

   [SMPP]     SMS Forum v5.0 | 19 February 2003, Forum, "Short Message Peer to
              Peer Peer-to-Peer Protocol
              Specification", 2003. Version 5.0, February 2003,
              <https://smpp.org/SMPP_v5.pdf>.

Acknowledgments

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

Authors' Addresses

   Jon Peterson
   Neustar, Inc.
   Email: jon.peterson@team.neustar

   Chris Wendt
   Somos
   Email: chris-ietf@chriswendt.net