TLS M. Thomson
Internet-Draft Mozilla
Intended status: Standards Track March 6, 2014
Expires: September 7, 2014
Authenticated Content Promise Extension for (D)TLS
draft-thomson-tls-acp-00
Abstract
This document describes an extension to Transport Layer Security
(TLS) and Datagram TLS (DTLS) that enables the negotiation of a
promise to protect session content from modification and
eavesdropping by third parties.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Authenticated Content Promise . . . . . . . . . . . . . . 3
1.2. Conventions and Terminology . . . . . . . . . . . . . . . 3
2. Authenticated Content Promise . . . . . . . . . . . . . . . . 3
3. Security Considerations . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1. Normative References . . . . . . . . . . . . . . . . . . 4
6.2. Informative References . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
WebRTC [I-D.ietf-rtcweb-overview] creates a new understanding of the
way that "user-generated content" is used on the world wide web. The
established definition identifies content that is generated by users
and used by sites; after all, the primary mode of interaction on the
web is between users and sites.
WebRTC changes that by enabling users to communicate directly, with
secure channels between established between user agents (or
browsers). These channels might be established with the aid of a web
site, but the content of the communication session can be made
inaccessible to the site [I-D.ietf-rtcweb-security-arch]. With peer
authentication, each user is able to be sure that:
o the content they are generating is only accessible to the
authenticated peer; and
o the content they are receiving can be attributed solely to the
authenticated peer.
On the originating end of a communications session, this guarantee is
easy to provide. A web site is able to provide instructions for
session setup that allow the browser to protect content from the
site, and to restrict where content is delivered based on identity.
On the receiving side, this is more complicated. Since there is a
desire to enable use cases where sites do have access to content that
is received, there is a need for a signal of some form to distinguish
the cases.
It is possible to use the WebRTC signaling channel for this purpose,
but only with restrictions. The signaling channel is considered
untrustworthy, so additional protection would be required to ensure
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that any indicators could not be erased or re-attributed to other
keying material. Furthermore, this would also require protection
against replay. Prohibiting key reuse between confidential and non-
confidential sessions would suffice for this purpose, though this is
undesirable for other reasons.
1.1. Authenticated Content Promise
This document describes a Transport Layer Security (TLS) [RFC5246]
extension, which, if negotiated, establishes a session as being
confidential. Peers that negotiate this extension promise that:
o Any content that is written to or read from the connection MUST be
protected from modification by entities other than the one that is
authenticated (i.e., the user).
o Any content that is written to or read from the connection MUST
NOT be recorded or forwarded to any entity other than the one that
is authenticated.
In addition to establishing an authenticated channel for
communications, this provides a key advantage over signaling-based
methods for ensuring privacy. Key continuity is possible, which
allows clients to operate without identity providers and still have a
stable basis for establishing continuity of identity with peers.
1.2. Conventions and Terminology
At times, this document falls back on shorthands for establishing
interoperability requirements on implementations: the capitalized
words "MUST", "SHOULD" and "MAY". These terms are defined in
[RFC2119].
2. Authenticated Content Promise
A new extension type ("authenticated_content_promise(TBD)") is
defined. If this extension is negotiated, both client and server are
bound by a promise to protect content.
enum {
authenticated_content_promise(TBD), (65535)
} ExtensionType;
The "extension_data" field of this extension MUST be empty.
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3. Security Considerations
Endpoints need to take care to avoid rendering of authenticated
content alongside other content in a way that could cause user
confusion equivalent to the effect of modifying content. For
instance, unauthenticated audio could be played at higher volume
levels than authenticated audio, potentially misleading users about
what sounds can be attributed to each.
This looks a little like digital rights management (DRM), but it
really doesn't promise to protect content to the degree required by
DRM schemes. It relies solely on users and their trust each other
(and their user agents, operating system and hardware). Nothing in
this mechanism stops a compromised end system from modifying or
eavesdropping on communications, from information being overhead or
seen by people nearby, or from any action taken on the part of the
authentiated entities, such as screen recording.
A little care is needed to avoid side channels, some of which are
quite obvious. For example, even with echo cancellation, audio
played over speakers can be picked up by nearby microphones; video
playback might be observable in a mirror.
4. IANA Considerations
IANA has allocated a TLS extension code point of (TBD) for this
extension.
5. Acknowledgements
Eric Rescorla helped identify the problem and formulate this
mechanism.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
6.2. Informative References
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[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for Brower-
based Applications", draft-ietf-rtcweb-overview-09 (work
in progress), February 2014.
[I-D.ietf-rtcweb-security-arch]
Rescorla, E., "WebRTC Security Architecture", draft-ietf-
rtcweb-security-arch-09 (work in progress), February 2014.
Author's Address
Martin Thomson
Mozilla
Suite 300
650 Castro Street
Mountain View, CA 94041
US
Email: martin.thomson@gmail.com
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