Pre-Shared Key (PSK) Ciphersuites with NULL Encryption for Transport Layer Security (TLS)

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

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].

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].

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

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