Network Working Group U. Blumenthal
Request for Comments: 4785 P. Goel
Category: Standards Track Intel Corporation
January 2007
Pre-Shared Key (PSK) Ciphersuites with NULL Encryption for
Transport Layer Security (TLS)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document specifies authentication-only ciphersuites (with no
encryption) for the Pre-Shared Key (PSK) based Transport Layer
Security (TLS) protocol. These ciphersuites are useful when
authentication and integrity protection is desired, but
confidentiality is not needed or not permitted.
Table of Contents
1. Introduction ....................................................2
1.1. Applicability Statement ....................................2
2. Conventions Used in This Document ...............................2
3. Cipher Usage ....................................................3
4. Security Considerations .........................................3
5. IANA Considerations .............................................3
6. Acknowledgments .................................................3
7. References ......................................................4
7.1. Normative References .......................................4
7.2. Informative References .....................................4
Blumenthal & Goel Standards Track [Page 1]RFC 4785 PSK NULL Encryption Ciphersuites for TLS January 2007
1. Introduction
The RFC for Pre-Shared Key (PSK) based Transport Layer Security (TLS)
[TLS-PSK] specifies ciphersuites for supporting TLS using pre-shared
symmetric keys. However, all the ciphersuites defined in [TLS-PSK]
require encryption. However there are cases when only authentication
and integrity protection is required, and confidentiality is not
needed. There are also cases when confidentiality is not permitted -
e.g., for implementations that must meet import restrictions in some
countries. Even though no encryption is used, these ciphersuites
support authentication of the client and server to each other, and
message integrity. This document augments [TLS-PSK] by adding three
more ciphersuites (PSK, DHE_PSK, RSA_PSK) with authentication and
integrity only - no encryption. The reader is expected to become
familiar with [TLS-PSK] standards prior to studying this document.
1.1. Applicability Statement
The ciphersuites defined in this document are intended for a rather
limited set of applications, usually involving only a very small
number of clients and servers. Even in such environments, other
alternatives may be more appropriate.
If the main goal is to avoid Public-key Infrastructures (PKIs),
another possibility worth considering is using self-signed
certificates with public key fingerprints. Instead of manually
configuring a shared secret in, for instance, some configuration
file, a fingerprint (hash) of the other party's public key (or
certificate) could be placed there instead.
It is also possible to use the Secure Remote Password (SRP)
ciphersuites for shared secret authentication [SRP]. SRP was
designed to be used with passwords, and it incorporates protection
against dictionary attacks. However, it is computationally more
expensive than the PSK ciphersuites in [TLS-PSK].
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].
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RFC 4785 PSK NULL Encryption Ciphersuites for TLS January 2007
3. Cipher Usage
The three new ciphersuites proposed here match the three cipher
suites defined in [TLS-PSK], except that we define suites with null
encryption.
The ciphersuites defined here use the following options for key
exchange and hash part of the protocol:
CipherSuite Key Exchange Cipher Hash
TLS_PSK_WITH_NULL_SHA PSK NULL SHA
TLS_DHE_PSK_WITH_NULL_SHA DHE_PSK NULL SHA
TLS_RSA_PSK_WITH_NULL_SHA RSA_PSK NULL SHA
For the meaning of the terms PSK, please refer to section 1 in [TLS-
PSK]. For the meaning of the terms DHE, RSA, and SHA, please refer
to appendixes A.5 and B in [TLS].
4. Security Considerations
As with all schemes involving shared keys, special care should be
taken to protect the shared values and to limit their exposure over
time. As this document augments [TLS-PSK], everything stated in its
Security Consideration section applies here. In addition, as cipher
suites defined here do not support confidentiality, care should be
taken not to send sensitive information (such as passwords) over
connections protected with one of the ciphersuites defined in this
document.
5. IANA Considerations
This document defines three new ciphersuites whose values are in the
TLS Cipher Suite registry defined in [TLS].
CipherSuite TLS_PSK_WITH_NULL_SHA = { 0x00, 0x2C };
CipherSuite TLS_DHE_PSK_WITH_NULL_SHA = { 0x00, 0x2D };
CipherSuite TLS_RSA_PSK_WITH_NULL_SHA = { 0x00, 0x2E };
6. Acknowledgments
The ciphersuites defined in this document are an augmentation to and
based on [TLS-PSK].
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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.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[TLS-PSK] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279, December
2005.
7.2. Informative References
[SRP] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
"Using SRP for TLS Authentication", Work in Progress,
December 2006.
Authors' Addresses
Uri Blumenthal
Intel Corporation
1515 State Route 10,
PY2-1 10-4
Parsippany, NJ 07054
USA
EMail: urimobile@optonline.net
Purushottam Goel
Intel Corporation
2111 N.E. 25 Ave.
JF3-414
Hillsboro, OR 97124
USA
EMail: Purushottam.Goel@intel.com
Blumenthal & Goel Standards Track [Page 4]RFC 4785 PSK NULL Encryption Ciphersuites for TLS January 2007
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