TLS M. Thomson
Internet-Draft Mozilla
Intended status: Standards Track March 8, 2014
Expires: September 9, 2014
Client Authentication Request Extension for (D)TLS
draft-thomson-tls-care-00
Abstract
This document describes an extension to Transport Layer Security
(TLS) and Datagram TLS (DTLS) that allows a client to indicate that
it wants to provide a client certificate.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on September 9, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 2
2. Client Authentication and TLS Renegotiation . . . . . . . . . 2
3. Client Authentication Request Extension . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
7. Normative References . . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4
1. Introduction
In Transport Layer Security (TLS) [RFC5246] and Datagram TLS (DTLS)
[RFC5764] the server decides whether or not to request a certificate
from clients.
In TLS versions 1.2 and earlier, the Certificate message from a
client is not encrypted and is therefore not confidential. TLS
renegotiation is frequently used to provide confidentiality for
client credentials, since renegotiation handshakes are encrypted with
the TLS session keys.
A client that is aware of a need to authenticate can initial
renegotiation, but is unable to induce a CertificateRequest from the
server. The decision to request client authentication is one that
can only be made by a server.
This document defines a client authentication request extension that
can be used by a client to request that the server send a
CertificateRequest in its handshake.
1.1. 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. Client Authentication and TLS Renegotiation
Renegotiation has the potential to create confusion at higher layers
about the security properties that apply to the byte stream. This is
especially difficult when there are protocol constructs that span the
ChangeCipherSpec messages that represent a switch between states.
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For that reason, a client can initiate a new connection when it
detects a need to authenticate, initiating renegotiation to establish
authentication credentials immediately after the initial handshake.
Since the server only conditionally requests client authentication
and it has no context with which to decide that authentication is
needed, the client needs to provide some indication that it might
need to be authentication. The second, renegotiation handshake can
contain the client authentication request extension (Section 3) to
provide this indication. As long as no application data is sent on
the connection prior to completing renegotiation and sending the
corresponding ChangeCipherSpec, there is no possibility for confusion
over the security properties of application content.
This behavior could need to be triggered by a higher level protocol.
This document does not define how that happens.
3. Client Authentication Request Extension
A new extension type ("client_authentication_request(TBD)") is
defined. If a client includes this extension in its ClientHello to
indicate that it wishes the server to issue a CertificateRequest.
enum {
client_authentication_request(TBD), (65535)
} ExtensionType;
The "extension_data" field of this extension MUST be empty.
A server that supports client authentication based on certificates
can use the presence of this extension to decide to include a
CertificateRequest. The server MAY choose to ignore this extension.
A server MUST NOT send this extension to a client.
4. Security Considerations
This document is entirely about security.
5. IANA Considerations
IANA has allocated a TLS extension code point of (TBD) for this
extension.
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6. Acknowledgements
Eric Rescorla helped identify the problem and formulate this
mechanism.
7. 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.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
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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