Internet DRAFT - draft-ietf-sipbrandy-rtpsec
draft-ietf-sipbrandy-rtpsec
Network Working Group J. Peterson
Internet-Draft Neustar
Intended status: Best Current Practice R. Barnes
Expires: October 27, 2019 Cisco
R. Housley
Vigil Security
April 25, 2019
Best Practices for Securing RTP Media Signaled with SIP
draft-ietf-sipbrandy-rtpsec-08
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 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 October 27, 2019.
Copyright Notice
Copyright (c) 2019 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
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(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Security at the SIP and SDP layer . . . . . . . . . . . . . . 3
4. STIR Profile for Endpoint Authentication and Verification
Services . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Credentials . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Anonymous Communications . . . . . . . . . . . . . . . . 6
4.3. Connected Identity Usage . . . . . . . . . . . . . . . . 7
4.4. Authorization Decisions . . . . . . . . . . . . . . . . . 8
5. Media Security Protocols . . . . . . . . . . . . . . . . . . 8
6. Relayed Media and Conferencing . . . . . . . . . . . . . . . 9
7. ICE and Connected Identity . . . . . . . . . . . . . . . . . 9
8. Best Current Practices . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Security Considerations . . . . . . . . . . . . . . . . . . . 11
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Normative References . . . . . . . . . . . . . . . . . . 11
12.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The Session Initiation Protocol (SIP) [RFC3261] includes a suite of
security services, including Digest authentication, for
authenticating entities with a shared secret, TLS for transport
security, and S/MIME (optionally) 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 Secure RTP [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
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[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 [I-D.ietf-sipbrandy-osrtp], which builds on the prior efforts of
[I-D.kaplan-mmusic-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 SIPBRANDY profile of 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
place. Various specifications that user agents 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: Session Initiation
Protocol (SIP) [RFC3261], the Session Description Protocol (SDP)
[RFC4566], and the Real-time Protocol (RTP) [RFC3550], in particular
its secure profile Secure RTP (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.
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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 establishing 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 PASSporT
[RFC8225] object 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.
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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, user agent
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 user
agent, 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 either contains 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
[I-D.ietf-acme-authority-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, either for
greenfield identifiers or for 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. This profile of STIR
employs more relaxed authority requirements of [RFC8224] to allow the
use of self-signed public keys for authentication services that are
composed with user agents, by generating a certificate (per the
guidance in [RFC8226]) with a subject corresponding to the user's
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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 user agents 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 user agents 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 implementation an anonymization service as specified in
[RFC3323].
Even for anonymous sessions, providing media confidentiality and
partial SDP integrity is still desirable. This specification
RECOMMENDS using one-time self-signed certificates for anonymous
communications, with 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.
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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 "mkey"
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.
The problem of providing "Connected Identity" in [RFC4474], which is
obsoleted by STIR, was explored in [RFC4916], which uses a
provisional or mid-dialog UPDATE request in the backwards direction
to convey an Identity header field for the recipient of an INVITE.
The procedures in that specification are largely compatible with the
revision of the Identity header in STIR [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 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.
The text in Section 4.4.1 of [RFC4916] regarding the receipt at a
UAC of error codes 428, 436, 437 and 438 in response to a mid-
dialog request RECOMMENDS treating the dialog 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 the original
[RFC4474], and will not match signatures using STIR [RFC8224].
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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 ICE; 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 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. 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 user agent receives a signed request in
the backwards direction per Section 4.3. If no such requests are
received, the UA may do one or 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
[I-D.ietf-sipbrandy-osrtp], opportunistic approaches considered
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include DTLS-SRTP, security descriptions [RFC4568], and ZRTP
[RFC6189].
Support for DTLS-SRTP is REQUIRED by this specification.
The "mkey" 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 the hop-by-hop media between a user agent and a
centralized server as well as the end-to-end media between user
agents, but is not sufficiently mature at this time to make a
recommendation for a best practice here. 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 those devices to act as media relays while
still permitting end-to-end confidentiality between user agents.
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
trusted entities that can decrypt media and then 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.
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In order to best enable end-to-end connectivity between user agents,
and to avoid media relays as much as possible, implementations of
this specification MUST support ICE [RFC8445]. To speed up call
establishment, it is RECOMMENDED that implementations support trickle
ICE [I-D.ietf-mmusic-trickle-ice-sip].
Note that in the comprehensive protection case, the use of Connected
Identity [RFC4916] with ICE entails 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
provisional acknowledgment (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 STUN usage
for consent freshness defined in [RFC7675].
8. Best Current Practices
The following are the best practices for SIP user agents to provide
media confidentiality for SIP sessions.
Implementations MUST support the STIR endpoint profile given in
Section 4, and signal that in PASSporT with the "msec" header
element.
Implementations MUST follow the authorization decision behavior in
Section 4.4.
Implementations MUST support DTLS-SRTP for key-management, as
described in Section 5.
Implementations MUST support the 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," and the IANA is
requested to add that entry to the registry with a value pointing to
[RFCThis].
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10. Security Considerations
This document describes the security features that provide media
sessions established with SIP with confidentiality, integrity, and
authentication.
11. Acknowledgments
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.
12. References
12.1. Normative References
[I-D.ietf-mmusic-trickle-ice-sip]
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)", draft-ietf-
mmusic-trickle-ice-sip-18 (work in progress), June 2018.
[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>.
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[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>.
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[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>.
12.2. Informative References
[I-D.ietf-acme-authority-token]
Peterson, J., Barnes, M., Hancock, D., and C. Wendt, "ACME
Challenges Using an Authority Token", draft-ietf-acme-
authority-token-03 (work in progress), March 2019.
[I-D.ietf-sipbrandy-osrtp]
Johnston, A., Aboba, B., Hutton, A., Jesske, R., and T.
Stach, "An Opportunistic Approach for Secure Real-time
Transport Protocol (OSRTP)", draft-ietf-sipbrandy-osrtp-08
(work in progress), March 2019.
[I-D.kaplan-mmusic-best-effort-srtp]
Audet, F. and H. Kaplan, "Session Description Protocol
(SDP) Offer/Answer Negotiation For Best-Effort Secure
Real-Time Transport Protocol", draft-kaplan-mmusic-best-
effort-srtp-01 (work in progress), October 2006.
[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>.
Peterson, et al. Expires October 27, 2019 [Page 13]
Internet-Draft RTP Security April 2019
[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>.
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
Peterson, et al. Expires October 27, 2019 [Page 14]
