TLS compression: Transport Layer Security Protocol Compression Methods

Network Working Group                                      S. Hollenbeck
Request for Comments: 3749                                VeriSign, Inc.
Category: Standards Track                                       May 2004



         Transport Layer Security Protocol Compression Methods

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 Internet Society (2004).  All Rights Reserved.

Abstract

   The Transport Layer Security (TLS) protocol (RFC 2246) includes
   features to negotiate selection of a lossless data compression method
   as part of the TLS Handshake Protocol and to then apply the algorithm
   associated with the selected method as part of the TLS Record
   Protocol.  TLS defines one standard compression method which
   specifies that data exchanged via the record protocol will not be
   compressed.  This document describes an additional compression method
   associated with a lossless data compression algorithm for use with
   TLS, and it describes a method for the specification of additional
   TLS compression methods.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Compression Methods  . . . . . . . . . . . . . . . . . . . . .  2
       2.1.  DEFLATE Compression. . . . . . . . . . . . . . . . . . .  3
   3.  Compression History and Packet Processing  . . . . . . . . . .  4
   4.  Internationalization Considerations  . . . . . . . . . . . . .  4
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  4
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  6
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
       8.1.  Normative References . . . . . . . . . . . . . . . . . .  6
       8.2.  Informative References . . . . . . . . . . . . . . . . .  6
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  7
       Full Copyright Statement . . . . . . . . . . . . . . . . . . .  8

1.  Introduction

   The Transport Layer Security (TLS) protocol (RFC 2246, [2]) includes
   features to negotiate selection of a lossless data compression method
   as part of the TLS Handshake Protocol and to then apply the algorithm
   associated with the selected method as part of the TLS Record
   Protocol.  TLS defines one standard compression method,
   CompressionMethod.null, which specifies that data exchanged via the
   record protocol will not be compressed.  While this single
   compression method helps ensure that TLS implementations are
   interoperable, the lack of additional standard compression methods
   has limited the ability of implementers to develop interoperable
   implementations that include data compression.

   TLS is used extensively to secure client-server connections on the
   World Wide Web.  While these connections can often be characterized
   as short-lived and exchanging relatively small amounts of data, TLS
   is also being used in environments where connections can be long-
   lived and the amount of data exchanged can extend into thousands or
   millions of octets.  XML [4], for example, is increasingly being used
   as a data representation method on the Internet, and XML tends to be
   verbose.  Compression within TLS is one way to help reduce the
   bandwidth and latency requirements associated with exchanging large
   amounts of data while preserving the security services provided by
   TLS.

   This document describes an additional compression method associated
   with a lossless data compression algorithm for use with TLS.
   Standardization of the compressed data formats and compression
   algorithms associated with this compression method is beyond the
   scope of 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 RFC 2119 [1].

2.  Compression Methods

   TLS [2] includes the following compression method structure in
   sections 6.1 and 7.4.1.2 and Appendix sections A.4.1 and A.6:

   enum { null(0), (255) } CompressionMethod;

   which allows for later specification of up to 256 different
   compression methods.  This definition is updated to segregate the
   range of allowable values into three zones:

   1. Values from 0 (zero) through 63 decimal (0x3F) inclusive are
      reserved for IETF Standards Track protocols.

   2. Values from 64 decimal (0x40) through 223 decimal (0xDF) inclusive
      are reserved for assignment for non-Standards Track methods.

   3. Values from 224 decimal (0xE0) through 255 decimal (0xFF)
      inclusive are reserved for private use.

   Additional information describing the role of the IANA in the
   allocation of compression method identifiers is described in Section
   5.

   In addition, this definition is updated to include assignment of an
   identifier for the DEFLATE compression method:

   enum { null(0), DEFLATE(1), (255) } CompressionMethod;

   As described in section 6 of RFC 2246 [2], TLS is a stateful
   protocol.  Compression methods used with TLS can be either stateful
   (the compressor maintains its state through all compressed records)
   or stateless (the compressor compresses each record independently),
   but there seems to be little known benefit in using a stateless
   compression method within TLS.

   The DEFLATE compression method described in this document is
   stateful.  It is RECOMMENDED that other compression methods that
   might be standardized in the future be stateful as well.

   Compression algorithms can occasionally expand, rather than compress,
   input data.  A compression method that exceeds the expansion limits
   described in section 6.2.2 of RFC 2246 [2] MUST NOT be used with TLS.

2.1.  DEFLATE Compression

   The DEFLATE compression method and encoding format is described in
   RFC 1951 [5].  Examples of DEFLATE use in IETF protocols can be found
   in RFC 1979 [6], RFC 2394 [7], and RFC 3274 [8].

   DEFLATE allows the sending compressor to select from among several
   options to provide varying compression ratios, processing speeds, and
   memory requirements.  The receiving decompressor MUST automatically
   adjust to the parameters selected by the sender.  All data that was
   submitted for compression MUST be included in the compressed output,
   with no data retained to be included in a later output payload.
   Flushing ensures that each compressed packet payload can be
   decompressed completely.

3.  Compression History and Packet Processing

   Some compression methods have the ability to maintain state/history
   information when compressing and decompressing packet payloads.  The
   compression history allows a higher compression ratio to be achieved
   on a stream as compared to per-packet compression, but maintaining a
   history across packets implies that a packet might contain data
   needed to completely decompress data contained in a different packet.
   History maintenance thus requires both a reliable link and sequenced
   packet delivery.  Since TLS and lower-layer protocols provide
   reliable, sequenced packet delivery, compression history information
   MAY be maintained and exploited if supported by the compression
   method.

   As described in section 7 of RFC 2246 [2], TLS allows multiple
   connections to be instantiated using the same session through the
   resumption feature of the TLS Handshake Protocol.  Session resumption
   has operational implications when multiple compression methods are
   available for use within the session.  For example, load balancers
   will need to maintain additional state information if the compression
   state is not cleared when a session is resumed.  As a result, the
   following restrictions MUST be observed when resuming a session:

   1.  The compression algorithm MUST be retained when resuming a
       session.

   2.  The compression state/history MUST be cleared when resuming a
       session.

4.  Internationalization Considerations

   The compression method identifiers specified in this document are
   machine-readable numbers.  As such, issues of human
   internationalization and localization are not introduced.

5.  IANA Considerations

   Section 2 of this document describes a registry of compression method
   identifiers to be maintained by the IANA, including assignment of an
   identifier for the DEFLATE compression method.  Identifier values
   from the range 0-63 (decimal) inclusive are assigned via RFC 2434
   Standards Action [3].  Values from the range 64-223 (decimal)
   inclusive are assigned via RFC 2434 Specification Required [3].
   Identifier values from 224-255 (decimal) inclusive are reserved for
   RFC 2434 Private Use [3].

6.  Security Considerations

   This document does not introduce any topics that alter the threat
   model addressed by TLS.  The security considerations described
   throughout RFC 2246 [2] apply here as well.

   However, combining compression with encryption can sometimes reveal
   information that would not have been revealed without compression:
   data that is the same length before compression might be a different
   length after compression, so adversaries that observe the length of
   the compressed data might be able to derive information about the
   corresponding uncompressed data.  Some symmetric encryption
   ciphersuites do not hide the length of symmetrically encrypted data
   at all.  Others hide it to some extent, but still do not hide it
   fully.  For example, ciphersuites that use stream cipher encryption
   without padding do not hide length at all; ciphersuites that use
   Cipher Block Chaining (CBC) encryption with padding provide some
   length hiding, depending on how the amount of padding is chosen.  Use
   of TLS compression SHOULD take into account that the length of
   compressed data may leak more information than the length of the
   original uncompressed data.

   Compression algorithms tend to be mathematically complex and prone to
   implementation errors.  An implementation error that can produce a
   buffer overrun introduces a potential security risk for programming
   languages and operating systems that do not provide buffer overrun
   protections.  Careful consideration should thus be given to
   protections against implementation errors that introduce security
   risks.

   As described in Section 2, compression algorithms can occasionally
   expand, rather than compress, input data.  This feature introduces
   the ability to construct rogue data that expands to some enormous
   size when compressed or decompressed.  RFC 2246 describes several
   methods to ameliorate this kind of attack.  First, compression has to
   be lossless.  Second, a limit (1,024 bytes) is placed on the amount
   of allowable compression content length increase.  Finally, a limit
   (2^14 bytes) is placed on the total content length.  See section
   6.2.2 of RFC 2246 [2] for complete details.

7.  Acknowledgements

   The concepts described in this document were originally discussed on
   the IETF TLS working group mailing list in December, 2000.  The
   author acknowledges the contributions to that discussion provided by
   Jeffrey Altman, Eric Rescorla, and Marc Van Heyningen.  Later
   suggestions that have been incorporated into this document were
   provided by Tim Dierks, Pasi Eronen, Peter Gutmann, Elgin Lee, Nikos
   Mavroyanopoulos, Alexey Melnikov, Bodo Moeller, Win Treese, and the
   IESG.

8.  References

8.1.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
        2246, January 1999.

   [3]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

8.2.  Informative References

   [4]  Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler,
        "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C REC-xml,
        October 2000, <http://www.w3.org/TR/REC-xml>.

   [5]  Deutsch, P., "DEFLATE Compressed Data Format Specification
        version 1.3", RFC 1951, May 1996.

   [6]  Woods, J., "PPP Deflate Protocol", RFC 1979, August 1996.

   [7]  Pereira, R., "IP Payload Compression Using DEFLATE", RFC 2394,
        December 1998.

   [8]  Gutmann, P., "Compressed Data Content Type for Cryptographic
        Message Syntax (CMS)", RFC 3274, June 2002.

Author's Address

   Scott Hollenbeck
   VeriSign, Inc.
   21345 Ridgetop Circle
   Dulles, VA  20166-6503
   US

   EMail: shollenbeck@verisign.com

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