Network Working Group N. Williams
Internet-Draft Sun
Intended status: Standards Track April 10, 2009
Expires: October 12, 2009
TLS Extension for Optimizing Application Protocols, Specifically SASL
draft-williams-tls-app-sasl-opt-00.txt
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Abstract
This document specifies an extension to Transport Layer Security
(TLS) for carrying application data which is suitable for delayed
integrity protection and does not require privacy protection. In
particular we describe how to use this extension to reduce the number
of round-trips needed for application-layer authentication,
specifically Simple Authentication (SASL), and through it, Generic
Security Services (GSS-API).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . 3
2. The Extension and Optimization of SASL . . . . . . . . . . . . 4
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Normative References . . . . . . . . . . . . . . . . . . . 9
5.2. Informative References . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
Many applications use TLS [RFC5246] and then Simple Authentication
and Security Layers (SASL) [RFC4422] on top of TLS. This requires at
least two round-trips for TLS, then one round-trip for SASL mechanism
negotiation, then as many round-trips as the negotiated SASL
mechanism requires. One and a half of the TLS round-trips can carry
extensions such that we could piggyback some application data on
those TLS messages to save up to two round-trips. This document
specifies how to take advantage of TLS extensions to reduce the
number of round-trips needed altogether.
First we define a TLS extension for use in Client Hello and Handshake
messages. This extension will carry typed application data. Then we
describe how to reduce the number of round-trips for SASL
applications. And through the new SASL/GSS-API bridge
[I-D.ietf-sasl-gs2] we obtain support for use of GSS-API [RFC2743]
mechanisms as well. [RFC2743] applications. We achieve a one and a
half round-trip reduction for SASL applications.
In the case of SASL applications we use the first TLS round-trip to
optimize the SASL mechanism negotiation. Then we use the client's
handshake message to send the first authentication message of the
selected SASL mechanism. Note that the TLS channel binding [RFC5056]
is available at that time, thus no special considerations apply to
how channel binding is done. Use of channel binding protects against
man-in-the-middle attacks, including downgrade attacks on mechanism
negotiation.
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. The Extension and Optimization of SASL
When a client application wishes to exchange one or more application
messages prior to the conclusion of a TLS exchange it uses the TLS
Client Hello message extension to a) indicate its intention to the
server, and b) optionally send the first application message to the
server. These messages will not have any privacy or integrity
protection applied by TLS unless a ChangeCipherSpec has been done
earlier (i.e., unless the application has already done one TLS
handshake).
When this message is received the server MUST either ignore the
extension or pass it to the application, which then MUST respond to
that application data via a new handshake message (see below). If
the server ignores it then the client will discover that the server
does not support this extension when the client receives the server's
handshake messages. Otherwise there must be a corresponding
application data handshake message in the server's response, and that
indicates that the server TLS and application implementations support
this extension.
The extension contents are defined by the application. In order to
save the application having to encode application data types and
lengths we define two application data extension types and we allow
the Client Hello to carry one of each of these extensions:
o pfapp_data (<TBD>)
o sasl_sml_req (<TBD>)
The "pf" prefix indicates "pre-Finished message exchange". It is the
application's responsibility to define the contents of the pfapp_data
extension.
The sasl_sml_req (SASL server mechanism list request) message
contains an empty payload.
We also define new Handshake messages that may be used after the
Client Hello messages:
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enum {
finished(20), pfapp_data(<TBD>),
sasl_sml(<TBD>), sasl_msg(<TBD>), (255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (HandshakeType) {
case hello_request: HelloRequest;
...
case pfapp_data: PFAppData;
case sasl_sml: SaslSML;
case sasl_msg: SaslMsg;
} body;
} Handshake;
opaque PFAppData<2^16-1>;
opaque SaslSML<2^16-1>;
opaque SaslMsg<2^16-1>;
A generic application protocol using these extensions might look
like:
Client Server
ClientHello w/ sasl_sml_req -------->
ServerHello
SaslSML*
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
SaslMsg*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
SASL auth messages <-------> SASL auth messages
Application Data <-------> Application Data
The TLS channel binding types that are suitable for use with SASL in
this facility are:
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o tls-server-end-point
o tls-unique
See the IANA channel binding type registry for more information about
these channel binding types. The channel binding type to use is to
be selected as described in [I-D.ietf-sasl-channel-bindings] (namely:
if there is a server certificate, then use tls-server-end-point, else
use tls-unique).
Note that the application has to construct its first SASL
authentication message for sending in the same half-round trip as the
client's Finished message, yet the client's Finished message is used
in the tls-unique channel binding type. This means that the Finished
message MUST be constructed before the client's SaslMsg, and the
SaslMsg is not integrity protected by the client's Finished message,
though it will be integrity protected by the server's Finished
message.
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3. IANA Considerations
When this document is approved for the Standards-Track the &lgt;TBD>
values above will be filled in and the IANA TLS ExtensionType and
HandshakeType registries will have to be updated to reflect these
assignments. (These registries require IETF Consensus and Standards
action, respectively.)
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4. Security Considerations
The security considerations of [RFC4422],
[I-D.ietf-sasl-channel-bindings], [RFC5246] and [RFC5056] apply, as
do those of [RFC2743] when used via the SASL/GS2 bridge
[I-D.ietf-sasl-gs2].
As usual with TLS there is no privacy protection of client identity
unless the client first completes a handshake without authenticating
itself, changes the cipher spec, then initiates a new handshake where
it does authenticate itself. In this case, client authentication
being done via SASL, this means not sending a SaslMsg until after the
initial ChangeCipherSpec exchange.
The use of SASL mechanisms that do not provide channel binding
support is NOT RECOMMENDED, but if they are used then the application
MUST ensure that the server identity authenticated by TLS corresponds
to the server identity authenticated by SASL if any, and to the
server name provided by the user.
The initial SASL authentication message is not protected by the TLS
client's Finished message, but it is protected by the server's
Finished message. If channel binding is used, as it should be, the
initial SASL authentication message will be bound to the TLS channel.
Therefore the server can detect modifications to the initial SASL
authentication message to the best of the selected SASL mechanism's
ability, and the client can detect modifications to its initial SASL
authentication message through the server's TLS Finished message.
The SASL mechanism negotiation is protected by the TLS Finished
messages.
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5. References
5.1. Normative References
[I-D.ietf-sasl-channel-bindings]
Williams, N., "SASL And Channel Binding",
draft-ietf-sasl-channel-bindings-00 (work in progress),
April 2009.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
5.2. Informative References
[I-D.ietf-sasl-gs2]
Josefsson, S. and N. Williams, "Using GSS-API Mechanisms
in SASL: The GS2 Mechanism Family", draft-ietf-sasl-gs2-11
(work in progress), March 2009.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
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Author's Address
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
US
Email: Nicolas.Williams@sun.com
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