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From: draft-ietf-sipbrandy-rtpsec-08               Best Current Practice
Internet Engineering Task Force (IETF)                       J. Peterson
Request for Comments: 8862                                       Neustar
BCP: 228                                                       R. Barnes
Category: Best Current Practice                                    Cisco
ISSN: 2070-1721                                               R. Housley
                                                          Vigil Security
                                                            January 2021


        Best Practices for Securing RTP Media Signaled with SIP

Abstract

   Although the Session Initiation Protocol (SIP) includes a suite of
   security services that has been expanded by numerous specifications
   over the years, there is no single place that explains how to use SIP
   to establish confidential media sessions.  Additionally, existing
   mechanisms have some feature gaps that need to be identified and
   resolved in order for them to address the pervasive monitoring threat
   model.  This specification describes best practices for negotiating
   confidential media with SIP, including a comprehensive protection
   solution that binds the media layer to SIP layer identities.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8862.

Copyright Notice

   Copyright (c) 2021 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
   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.  Terminology
   3.  Security at the SIP and SDP Layer
   4.  STIR Profile for Endpoint Authentication and Verification
           Services
     4.1.  Credentials
     4.2.  Anonymous Communications
     4.3.  Connected Identity Usage
     4.4.  Authorization Decisions
   5.  Media Security Protocols
   6.  Relayed Media and Conferencing
   7.  ICE and Connected Identity
   8.  Best Current Practices
   9.  IANA Considerations
   10. Security Considerations
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The Session Initiation Protocol (SIP) [RFC3261] includes a suite of
   security services, including Digest Authentication [RFC7616] for
   authenticating entities with a shared secret, TLS [RFC8446] for
   transport security, and (optionally) S/MIME [RFC8551] for body
   security.  SIP is frequently used to establish media sessions -- in
   particular, audio or audiovisual sessions, which have their own
   security mechanisms available, such as the Secure Real-time Transport
   Protocol (SRTP) [RFC3711].  However, the practices needed to bind
   security at the media layer to security at the SIP layer, to provide
   an assurance that protection is in place all the way up the stack,
   rely on a great many external security mechanisms and practices.
   This document provides documentation to explain their optimal use as
   a best practice.

   Revelations about widespread pervasive monitoring of the Internet
   have led to a greater desire to protect Internet communications
   [RFC7258].  In order to maximize the use of security features,
   especially of media confidentiality, opportunistic measures serve as
   a stopgap when a full suite of services cannot be negotiated all the
   way up the stack.  Opportunistic media security for SIP is described
   in [RFC8643], which builds on the prior efforts of
   [Best-Effort-SRTP].  With opportunistic encryption, there is an
   attempt to negotiate the use of encryption, but if the negotiation
   fails, then cleartext is used.  Opportunistic encryption approaches
   typically have no integrity protection for the keying material.

   This document contains the SIP Best-practice Recommendations Against
   Network Dangers to privacY (SIPBRANDY) profile of Secure Telephone
   Identity Revisited (STIR) [RFC8224] for media confidentiality,
   providing a comprehensive security solution for SIP media that
   includes integrity protection for keying material and offers
   application-layer assurance that media confidentiality is in place.
   Various specifications that User Agents (UAs) must implement to
   support media confidentiality are given in the sections below; a
   summary of the best current practices appears in Section 8.

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.  Security at the SIP and SDP Layer

   There are two approaches to providing confidentiality for media
   sessions set up with SIP: comprehensive protection and opportunistic
   security (as defined in [RFC7435]).  This document only addresses
   comprehensive protection.

   Comprehensive protection for media sessions established by SIP
   requires the interaction of three protocols: the Session Initiation
   Protocol (SIP) [RFC3261], the Session Description Protocol (SDP)
   [RFC4566], and the Real-time Transport Protocol (RTP) [RFC3550] --
   in particular, its secure profile SRTP [RFC3711].  Broadly, it is the
   responsibility of SIP to provide integrity protection for the media
   keying attributes conveyed by SDP, and those attributes will in turn
   identify the keys used by endpoints in the RTP media session(s) that
   SDP negotiates.

   Note that this framework does not apply to keys that also require
   confidentiality protection in the signaling layer, such as the SDP
   "k=" line, which MUST NOT be used in conjunction with this profile.

   In that way, once SIP and SDP have exchanged the necessary
   information to initiate a session, media endpoints will have a strong
   assurance that the keys they exchange have not been tampered with by
   third parties and that end-to-end confidentiality is available.

   To establish the identity of the endpoints of a SIP session, this
   specification uses STIR [RFC8224].  The STIR Identity header has been
   designed to prevent a class of impersonation attacks that are
   commonly used in robocalling, voicemail hacking, and related threats.
   STIR generates a signature over certain features of SIP requests,
   including header field values that contain an identity for the
   originator of the request, such as the From header field or
   P-Asserted-Identity field, and also over the media keys in SDP if
   they are present.  As currently defined, STIR provides a signature
   over the "a=fingerprint" attribute, which is a fingerprint of the key
   used by DTLS-SRTP [RFC5763]; consequently, STIR only offers
   comprehensive protection for SIP sessions in concert with SDP and
   SRTP when DTLS-SRTP is the media security service.  The underlying
   Personal Assertion Token (PASSporT) object [RFC8225] used by STIR is
   extensible, however, and it would be possible to provide signatures
   over other SDP attributes that contain alternate keying material.  A
   profile for using STIR to provide media confidentiality is given in
   Section 4.

4.  STIR Profile for Endpoint Authentication and Verification Services

   STIR [RFC8224] defines the Identity header field for SIP, which
   provides a cryptographic attestation of the source of communications.
   This document includes a profile of STIR, called the SIPBRANDY
   profile, where the STIR verification service will act in concert with
   an SRTP media endpoint to ensure that the key fingerprints, as given
   in SDP, match the keys exchanged to establish DTLS-SRTP.  To satisfy
   this condition, the verification service function would in this case
   be implemented in the SIP User Agent Server (UAS), which would be
   composed with the media endpoint.  If the STIR authentication service
   or verification service functions are implemented at an intermediary
   rather than an endpoint, this introduces the possibility that the
   intermediary could act as a man in the middle, altering key
   fingerprints.  As this attack is not in STIR's core threat model,
   which focuses on impersonation rather than man-in-the-middle attacks,
   STIR offers no specific protections against such interference.

   The SIPBRANDY profile for media confidentiality thus shifts these
   responsibilities to the endpoints rather than the intermediaries.
   While intermediaries MAY provide the verification service function of
   STIR for SIPBRANDY transactions, the verification needs to be
   repeated at the endpoint to obtain end-to-end assurance.
   Intermediaries supporting this specification MUST NOT block or
   otherwise redirect calls if they do not trust the signing credential.
   The SIPBRANDY profile is based on an end-to-end trust model, so it is
   up to the endpoints to determine if they support signing credentials,
   not intermediaries.

   In order to be compliant with best practices for SIP media
   confidentiality with comprehensive protection, UA implementations
   MUST implement both the authentication service and verification
   service roles described in [RFC8224].  STIR authentication services
   MUST signal their compliance with this specification by including the
   "msec" claim defined in this specification to the PASSporT payload.
   Implementations MUST provide key fingerprints in SDP and the
   appropriate signatures over them as specified in [RFC8225].

   When generating either an offer or an answer [RFC3264], compliant
   implementations MUST include an "a=fingerprint" attribute containing
   the fingerprint of an appropriate key (see Section 4.1).

4.1.  Credentials

   In order to implement the authentication service function in the UA,
   SIP endpoints will need to acquire the credentials needed to sign for
   their own identity.  That identity is typically carried in the From
   header field of a SIP request and contains either a greenfield SIP
   URI (e.g., "sip:alice@example.com") or a telephone number (which can
   appear in a variety of ways, e.g.,
   "sip:+17004561212@example.com;user=phone").  Section 8 of [RFC8224]
   contains guidance for separating the two and determining what sort of
   credential is needed to sign for each.

   To date, few commercial certification authorities (CAs) issue
   certificates for SIP URIs or telephone numbers; though work is
   ongoing on systems for this purpose (such as [ACME-Auth-Token]), it
   is not yet mature enough to be recommended as a best practice.  This
   is one reason why STIR permits intermediaries to act as an
   authentication service on behalf of an entire domain, just as in SIP
   a proxy server can provide domain-level SIP service.  While CAs that
   offer proof-of-possession certificates similar to those used for
   email could be offered for SIP -- for either greenfield identifiers
   or telephone numbers -- this specification does not require their
   use.

   For users who do not possess such certificates, DTLS-SRTP [RFC5763]
   permits the use of self-signed public keys.  The profile of STIR in
   this document, called the SIPBRANDY profile, employs the more relaxed
   authority requirements of [RFC8224] to allow the use of self-signed
   public keys for authentication services that are composed with UAs,
   by generating a certificate (per the guidance in [RFC8226]) with a
   subject corresponding to the user's identity.  To obtain
   comprehensive protection with a self-signed certificate, some out-of-
   band verification is needed as well.  Such a credential could be used
   for trust on first use (see [RFC7435]) by relying parties.  Note that
   relying parties SHOULD NOT use certificate revocation mechanisms or
   real-time certificate verification systems for self-signed
   certificates, as they will not increase confidence in the
   certificate.

   Users who wish to remain anonymous can instead generate self-signed
   certificates as described in Section 4.2.

   Generally speaking, without access to out-of-band information about
   which certificates were issued to whom, it will be very difficult for
   relying parties to ascertain whether or not the signer of a SIP
   request is genuinely an "endpoint".  Even the term "endpoint" is a
   problematic one, as SIP UAs can be composed in a variety of
   architectures and may not be devices under direct user control.
   While it is possible that techniques based on certificate
   transparency [RFC6962] or similar practices could help UAs to
   recognize one another's certificates, those operational systems will
   need to ramp up with the CAs that issue credentials to end-user
   devices going forward.

4.2.  Anonymous Communications

   In some cases, the identity of the initiator of a SIP session may be
   withheld due to user or provider policy.  Following the
   recommendations of [RFC3323], this may involve using an identity such
   as "anonymous@anonymous.invalid" in the identity fields of a SIP
   request.  [RFC8224] does not currently permit authentication services
   to sign for requests that supply this identity.  It does, however,
   permit signing for valid domains, such as "anonymous@example.com", as
   a way of implementing an anonymization service as specified in
   [RFC3323].

   Even for anonymous sessions, providing media confidentiality and
   partial SDP integrity is still desirable.  One-time self-signed
   certificates for anonymous communications SHOULD include a
   subjectAltName of "sip:anonymous@anonymous.invalid".  After a session
   is terminated, the certificate SHOULD be discarded, and a new one,
   with fresh keying material, SHOULD be generated before each future
   anonymous call.  As with self-signed certificates, relying parties
   SHOULD NOT use certificate revocation mechanisms or real-time
   certificate verification systems for anonymous certificates, as they
   will not increase confidence in the certificate.

   Note that when using one-time anonymous self-signed certificates, any
   man in the middle could strip the Identity header and replace it with
   one signed by its own one-time certificate, changing the "mky"
   parameters of PASSporT and any "a=fingerprint" attributes in SDP as
   it chooses.  This signature only provides protection against
   non-Identity-aware entities that might modify SDP without altering
   the PASSporT conveyed in the Identity header.

4.3.  Connected Identity Usage

   STIR [RFC8224] provides integrity protection for the fingerprint
   attributes in SIP request bodies but not SIP responses.  When a
   session is established, therefore, any SDP body carried by a
   200-class response in the backwards direction will not be protected
   by an authentication service and cannot be verified.  Thus, sending a
   secured SDP body in the backwards direction will require an extra
   RTT, typically a request sent in the backwards direction.

   [RFC4916] explored the problem of providing "connected identity" to
   implementations of [RFC4474] (which is obsoleted by [RFC8224]);
   [RFC4916] uses a provisional or mid-dialog UPDATE request in the
   backwards (reverse) direction to convey an Identity header field for
   the recipient of an INVITE.  The procedures in [RFC4916] are largely
   compatible with the revision of the Identity header in [RFC8224].
   However, the following need to be considered:

   *  The UPDATE carrying signed SDP with a fingerprint in the backwards
      direction needs to be sent during dialog establishment, following
      the receipt of a Provisional Response Acknowledgement (PRACK)
      after a provisional 1xx response.

   *  For use with this SIPBRANDY profile for media confidentiality, the
      UAS that responds to the INVITE request needs to act as an
      authentication service for the UPDATE sent in the backwards
      direction.

   *  Per the text in Section 4.4.1 of [RFC4916] regarding the receipt
      at a User Agent Client (UAC) of error code 428, 436, 437, or 438
      in response to a mid-dialog request, it is RECOMMENDED that the
      dialog be treated as terminated.  However, Section 6.1.1 of
      [RFC8224] allows the retransmission of requests with repairable
      error conditions.  In particular, an authentication service might
      retry a mid-dialog rather than treating the dialog as terminated,
      although only one such retry is permitted.

   *  Note that the examples in [RFC4916] are based on [RFC4474] and
      will not match signatures using [RFC8224].

   Future work may be done to revise [RFC4916] for STIR; that work
   should take into account any impacts on the SIPBRANDY profile
   described in this document.  The use of [RFC4916] has some further
   interactions with Interactive Connectivity Establishment (ICE)
   [RFC8445]; see Section 7.

4.4.  Authorization Decisions

   [RFC8224] grants STIR verification services a great deal of latitude
   when making authorization decisions based on the presence of the
   Identity header field.  It is largely a matter of local policy
   whether an endpoint rejects a call based on the absence of an
   Identity header field, or even the presence of a header that fails an
   integrity check against the request.

   For this SIPBRANDY profile of STIR, however, a compliant verification
   service that receives a dialog-forming SIP request containing an
   Identity header with a PASSporT type of "msec", after validating the
   request per the steps described in Section 6.2 of [RFC8224], MUST
   reject the request if there is any failure in that validation process
   with the appropriate status code per Section 6.2.2 of [RFC8224].  If
   the request is valid, then if a terminating user accepts the request,
   it MUST then follow the steps in Section 4.3 to act as an
   authentication service and send a signed request with the "msec"
   PASSporT type in its Identity header as well, in order to enable
   end-to-end bidirectional confidentiality.

   For the purposes of this profile, the "msec" PASSporT type can be
   used by authentication services in one of two ways: as a mandatory
   request for media security or as a merely opportunistic request for
   media security.  As any verification service that receives an
   Identity header field in a SIP request with an unrecognized PASSporT
   type will simply ignore that Identity header, an authentication
   service will know whether or not the terminating side supports "msec"
   based on whether or not its UA receives a signed request in the
   backwards direction per Section 4.3.  If no such requests are
   received, the UA may do one of two things: shut down the dialog, if
   the policy of the UA requires that "msec" be supported by the
   terminating side for this dialog; or, if policy permits (e.g., an
   explicit acceptance by the user), allow the dialog to continue
   without media security.

5.  Media Security Protocols

   As there are several ways to negotiate media security with SDP, any
   of which might be used with either opportunistic or comprehensive
   protection, further guidance to implementers is needed.  In
   [RFC8643], opportunistic approaches considered include DTLS-SRTP,
   security descriptions [RFC4568], and ZRTP [RFC6189].

   Support for DTLS-SRTP is REQUIRED by this specification.

   The "mky" claim of PASSporT provides integrity protection for
   "a=fingerprint" attributes in SDP, including cases where multiple
   "a=fingerprint" attributes appear in the same SDP.

6.  Relayed Media and Conferencing

   Providing end-to-end media confidentiality for SIP is complicated by
   the presence of many forms of media relays.  While many media relays
   merely proxy media to a destination, others present themselves as
   media endpoints and terminate security associations before
   re-originating media to its destination.

   Centralized conference bridges are one type of entity that typically
   terminates a media session in order to mux media from multiple
   sources and then to re-originate the muxed media to conference
   participants.  In many such implementations, only hop-by-hop media
   confidentiality is possible.  Work is ongoing to specify a means to
   encrypt both (1) the hop-by-hop media between a UA and a centralized
   server and (2) the end-to-end media between UAs, but it is not
   sufficiently mature at this time to become a best practice.  Those
   protocols are expected to identify their own best-practice
   recommendations as they mature.

   Another class of entities that might relay SIP media are Back-to-Back
   User Agents (B2BUAs).  If a B2BUA follows the guidance in [RFC7879],
   it may be possible for B2BUAs to act as media relays while still
   permitting end-to-end confidentiality between UAs.

   Ultimately, if an endpoint can decrypt media it receives, then that
   endpoint can forward the decrypted media without the knowledge or
   consent of the media's originator.  No media confidentiality
   mechanism can protect against these sorts of relayed disclosures or
   against a legitimate endpoint that can legitimately decrypt media and
   record a copy to be sent elsewhere (see [RFC7245]).

7.  ICE and Connected Identity

   Providing confidentiality for media with comprehensive protection
   requires careful timing of when media streams should be sent and when
   a user interface should signify that confidentiality is in place.

   In order to best enable end-to-end connectivity between UAs and to
   avoid media relays as much as possible, implementations of this
   specification MUST support ICE [RFC8445] [RFC8839].  To speed up call
   establishment, it is RECOMMENDED that implementations support Trickle
   ICE [RFC8838] [RFC8840].

   Note that in the comprehensive protection case, the use of connected
   identity [RFC4916] with ICE implies that the answer containing the
   key fingerprints, and thus the STIR signature, will come in an UPDATE
   sent in the backwards direction, a provisional response, and a PRACK,
   rather than in any earlier SDP body.  Only at such a time as that
   UPDATE is received will the media keys be considered exchanged in
   this case.

   Similarly, in order to prevent, or at least mitigate, the denial-of-
   service attack described in Section 19.5.1 of [RFC8445], this
   specification incorporates best practices for ensuring that
   recipients of media flows have consented to receive such flows.
   Implementations of this specification MUST implement the Session
   Traversal Utilities for NAT (STUN) usage for consent freshness
   defined in [RFC7675].

8.  Best Current Practices

   The following are the best practices for SIP UAs to provide media
   confidentiality for SIP sessions.

   *  Implementations MUST support the SIPBRANDY profile as defined in
      Section 4 and signal such support in PASSporT via the "msec"
      header element.

   *  Implementations MUST follow the authorization decision behavior
      described in Section 4.4.

   *  Implementations MUST support DTLS-SRTP for management of keys, as
      described in Section 5.

   *  Implementations MUST support ICE and the STUN consent freshness
      mechanism, as specified in Section 7.

9.  IANA Considerations

   This specification defines a new value for the "Personal Assertion
   Token (PASSporT) Extensions" registry called "msec".  IANA has added
   the entry to the registry with a value pointing to this document.

10.  Security Considerations

   This document describes the security features that provide media
   sessions established with SIP with confidentiality, integrity, and
   authentication.

11.  References

11.1.  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>.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <https://www.rfc-editor.org/info/rfc3264>.

   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323,
              DOI 10.17487/RFC3323, November 2002,
              <https://www.rfc-editor.org/info/rfc3323>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, DOI 10.17487/RFC4568, July 2006,
              <https://www.rfc-editor.org/info/rfc4568>.

   [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>.

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
              2010, <https://www.rfc-editor.org/info/rfc5763>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [RFC7675]  Perumal, M., Wing, D., Ravindranath, R., Reddy, T., and M.
              Thomson, "Session Traversal Utilities for NAT (STUN) Usage
              for Consent Freshness", RFC 7675, DOI 10.17487/RFC7675,
              October 2015, <https://www.rfc-editor.org/info/rfc7675>.

   [RFC7879]  Ravindranath, R., Reddy, T., Salgueiro, G., Pascual, V.,
              and P. Ravindran, "DTLS-SRTP Handling in SIP Back-to-Back
              User Agents", RFC 7879, DOI 10.17487/RFC7879, May 2016,
              <https://www.rfc-editor.org/info/rfc7879>.

   [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>.

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8838]  Ivov, E., Uberti, J., and P. Saint-Andre, "Trickle ICE:
              Incremental Provisioning of Candidates for the Interactive
              Connectivity Establishment (ICE) Protocol", RFC 8838,
              DOI 10.17487/RFC8838, January 2021,
              <https://www.rfc-editor.org/info/rfc8838>.

   [RFC8839]  Petit-Huguenin, M., Nandakumar, S., Holmberg, C., Keränen,
              A., and R. Shpount, "Session Description Protocol (SDP)
              Offer/Answer Procedures for Interactive Connectivity
              Establishment (ICE)", RFC 8839, DOI 10.17487/RFC8839,
              January 2021, <https://www.rfc-editor.org/info/rfc8839>.

   [RFC8840]  Ivov, E., Stach, T., Marocco, E., and C. Holmberg, "A
              Session Initiation Protocol (SIP) Usage for Incremental
              Provisioning of Candidates for the Interactive
              Connectivity Establishment (Trickle ICE)", RFC 8840,
              DOI 10.17487/RFC8840, January 2021,
              <https://www.rfc-editor.org/info/rfc8840>.

11.2.  Informative References

   [ACME-Auth-Token]
              Peterson, J., Barnes, M., Hancock, D., and C. Wendt, "ACME
              Challenges Using an Authority Token", Work in Progress,
              Internet-Draft, draft-ietf-acme-authority-token-05, 9
              March 2020, <https://tools.ietf.org/html/draft-ietf-acme-
              authority-token-05>.

   [Best-Effort-SRTP]
              Kaplan, H. and F. Audet, "Session Description Protocol
              (SDP) Offer/Answer Negotiation For Best-Effort Secure
              Real-Time Transport Protocol", Work in Progress, Internet-
              Draft, draft-kaplan-mmusic-best-effort-srtp-01, 25 October
              2006, <https://tools.ietf.org/html/draft-kaplan-mmusic-
              best-effort-srtp-01>.

   [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,
              <https://www.rfc-editor.org/info/rfc4474>.

   [RFC6189]  Zimmermann, P., Johnston, A., Ed., and J. Callas, "ZRTP:
              Media Path Key Agreement for Unicast Secure RTP",
              RFC 6189, DOI 10.17487/RFC6189, April 2011,
              <https://www.rfc-editor.org/info/rfc6189>.

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
              <https://www.rfc-editor.org/info/rfc6962>.

   [RFC7245]  Hutton, A., Ed., Portman, L., Ed., Jain, R., and K. Rehor,
              "An Architecture for Media Recording Using the Session
              Initiation Protocol", RFC 7245, DOI 10.17487/RFC7245, May
              2014, <https://www.rfc-editor.org/info/rfc7245>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <https://www.rfc-editor.org/info/rfc7435>.

   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
              Digest Access Authentication", RFC 7616,
              DOI 10.17487/RFC7616, September 2015,
              <https://www.rfc-editor.org/info/rfc7616>.

   [RFC8551]  Schaad, J., Ramsdell, B., and S. Turner, "Secure/
              Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
              Message Specification", RFC 8551, DOI 10.17487/RFC8551,
              April 2019, <https://www.rfc-editor.org/info/rfc8551>.

   [RFC8643]  Johnston, A., Aboba, B., Hutton, A., Jesske, R., and T.
              Stach, "An Opportunistic Approach for Secure Real-time
              Transport Protocol (OSRTP)", RFC 8643,
              DOI 10.17487/RFC8643, August 2019,
              <https://www.rfc-editor.org/info/rfc8643>.

Acknowledgements

   We thank Eric Rescorla, Adam Roach, Andrew Hutton, and Ben Campbell
   for contributions to this problem statement and framework.  We thank
   Liang Xia and Alissa Cooper for their careful review.

Authors' Addresses

   Jon Peterson
   Neustar, Inc.

   Email: jon.peterson@team.neustar


   Richard Barnes
   Cisco

   Email: rlb@ipv.sx


   Russ Housley
   Vigil Security, LLC

   Email: housley@vigilsec.com