Network Working Group S. Josefsson
Internet-Draft SJD
Intended status: Standards Track August 12, 2008
Expires: February 13, 2009
Channel Bindings for TLS based on PRF
draft-josefsson-sasl-tls-cb-01
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Abstract
This document specify how to compute data, "channel bindings", that
is cryptographically bound to a specific Transport Layer Security
(TLS) session. The intention is to use this data as a name of the
secure channel for the purpose of a channel binding. The channel
bindings can be used by authentication protocols to avoid tunneling
attacks and security layer re-use. The data is derived using the TLS
Pseudo-Random Function (PRF).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in this Document . . . . . . . . . . . . . . . 3
3. Channel Bindings Syntax . . . . . . . . . . . . . . . . . . . . 3
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 4
7.1. Normative References . . . . . . . . . . . . . . . . . . . 4
7.2. Informative References . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5
Intellectual Property and Copyright Statements . . . . . . . . . . 6
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1. Introduction
Binding authentication to a specific encrypted session can protect
from certain attacks [mitm]. It can also help to improve performance
by having peers agree to re-use a secure channel rather than to set
up a new.
This document describe how to generate data that can be used by
application protocols to bind authentication to a specific TLS
[RFC4346] session.
2. 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].
3. Channel Bindings Syntax
The channel bindings is computed using the TLS Pseudo-Random Function
(PRF). The PRF takes three inputs, a secret, a fixed label, and a
seed. Here the label will be "channel binding". The key will be the
master secret in a TLS session. The seed will be the hash of the
handshake messages, computed the same way as for the TLS Finished
message. We will use the first 32 octets computed by the PRF.
Using the terminology and pseudo-language in TLS [RFC4346], the
channel bindings is computed as follows:
TLS_channel_bindings =
PRF(SecurityParameters.master_secret,
"channel binding",
MD5(handshake_messages) + SHA-1(handshake_messages)) [0..31]
The derived data MUST NOT be used for any other purpose than channel
bindings as desribed in [RFC5056].
4. IANA Considerations
The IANA is requested to register this channel binding using the
following templates and the process described in [RFC5056].
Subject: Registration of channel binding TLS
Channel binding unique prefix (name): tls-unique-prf
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Channel binding type: unique
Channel type: TLS
Published specification (recommended, optional): This document
Channel binding is secret (requires confidentiality protection): no
Description (optional if a specification is given; required if no
Published specification is specified): See earlier in this document.
Intended usage: COMMON
Person and email address to contact for further information:
simon@josefsson.org
Owner/Change controller name and email address: simon@josefsson.org
Expert reviewer name and contact information:
5. Security Considerations
For the intended use and other important considerations, see
[RFC5056].
We claim that by appropriately using a channel binding an application
can protect itself from the attacks in [mitm]. To guarantee this
property, the derived data is only to be used for the intended
purpose.
The security considerations in TLS should be considered. In
particular, the TLS master secret must be protected.
6. Acknowledgements
Thanks to Eric Rescorla and Sam Hartman who pointed out a problem
with the construct used in earlier versions of this document when TLS
server authentication is not used or checked.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
7.2. Informative References
[mitm] Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
in Tunneled Authentication",
WWW http://www.saunalahti.fi/~asokan/research/mitm.html.
Author's Address
Simon Josefsson
SJD
Email: simon@josefsson.org
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