TLS compression: Transport Layer Security Protocol Compression Methods
Gives TLS a standard compression method (DEFLATE) and a numbering scheme for adding more.
Hollenbeck, S., "Transport Layer Security Protocol Compression Methods", RFC 3749, DOI 10.17487/RFC3749, May 2004, <https://www.rfc-editor.org/info/rfc3749>.
@misc{rfc3749,
series = {Request for Comments},
number = 3749,
howpublished = {RFC 3749},
publisher = {RFC Editor},
doi = {10.17487/RFC3749},
url = {https://www.rfc-editor.org/info/rfc3749},
author = {S. Hollenbeck},
title = {{Transport Layer Security Protocol Compression Methods}},
pagetotal = 8,
year = 2004,
month = may,
} RFC text
Contents
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
Full Copyright Statement
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AI ELI5
On this page
§1Why Compress Inside TLS
The Transport Layer Security (TLS) protocol, as defined in RFC 2246, can negotiate a lossless data compression method in its Handshake Protocol and then apply it in its Record Protocol. But TLS only defined one standard method, CompressionMethod.null, which means no compression.
That kept implementations interoperable but left implementers without a shared way to compress. The RFC points out that TLS is used for long-lived connections that can move thousands or millions of octets, and that XML tends to be verbose. Compression is one way to cut bandwidth and latency while keeping TLS security services.
The document adds one compression method. Standardizing the compressed data formats and algorithms behind it is out of scope.
Secure connections could already squish their messages in theory, but nobody agreed on how. This RFC picks one squishing method everybody can use.
§2Compression Method Identifiers
TLS defines enum { null(0), (255) } CompressionMethod;, which leaves room for up to 256 compression methods. This RFC splits the values into three zones:
- 0 through 63 (0x3F): reserved for IETF Standards Track protocols.
- 64 (0x40) through 223 (0xDF): reserved for assignment to non-Standards Track methods.
- 224 (0xE0) through 255 (0xFF): reserved for private use.
It also assigns an identifier to DEFLATE, so the definition becomes enum { null(0), DEFLATE(1), (255) } CompressionMethod;.
Think of numbered coat hooks. The low hooks are for official methods, the middle hooks are for other methods, and the top hooks are for anyone's private use.
| Values (decimal) | Reserved for | IANA policy |
|---|---|---|
| 0 to 63 | IETF Standards Track protocols | Standards Action |
| 64 to 223 | Non-Standards Track methods | Specification Required |
| 224 to 255 | Private use | Private Use |
§2.1DEFLATE and Stateful Compression
TLS is a stateful protocol. A compression method can be stateful (the compressor keeps its state across all compressed records) or stateless (each record is compressed on its own). The RFC notes there seems to be little known benefit in a stateless method within TLS. The DEFLATE method here is stateful, and the RFC RECOMMENDS that future methods be stateful too.
DEFLATE is described in RFC 1951. It lets the sender pick among options that trade compression ratio, speed and memory, and the receiving decompressor MUST adjust automatically to what the sender chose. All data submitted for compression MUST appear in the compressed output, with nothing held back for a later payload. Flushing ensures every compressed packet payload can be decompressed completely.
Compression can sometimes expand data. A method that exceeds the expansion limits in section 6.2.2 of RFC 2246 MUST NOT be used with TLS.
The squisher remembers what it already squished, so it can do a better job as the conversation goes on. The listener has to follow whatever squishing settings the sender picked.
§3History and Session Resumption
Keeping compression history across packets gives a higher ratio than compressing each packet alone, but one packet may then hold data needed to decompress another. That requires a reliable link and sequenced delivery. Because TLS and the layers below it provide both, compression history MAY be kept if the method supports it.
TLS also lets several connections share one session through resumption. That has operational effects: for example, load balancers would need extra state if compression state were not cleared on resumption. So two rules MUST be followed when resuming a session:
- The compression algorithm MUST be retained.
- The compression state and history MUST be cleared.
When you come back to a conversation you paused, you keep using the same squishing method, but you forget everything the squisher remembered and start fresh.
§5IANA Registry Rules
The RFC describes a registry of compression method identifiers maintained by the IANA, including the DEFLATE assignment. Values 0 to 63 are assigned by Standards Action, values 64 to 223 by Specification Required, and values 224 to 255 are reserved for Private Use, using the terms from RFC 2434.
The RFC also notes that the identifiers are machine-readable numbers, so human internationalization and localization issues are not introduced.
A librarian keeps the official list of numbers. Official methods need a formal process, other methods need a written description, and the top numbers are free for anyone.
§6Security Considerations
The RFC says it does not alter the threat model addressed by TLS, and the security considerations of RFC 2246 still apply. But combining compression with encryption can reveal information. Data that is the same length before compression may differ in length afterwards, so an observer who sees the compressed length may learn something about the original data.
Ciphersuites differ in how much they hide length. Stream cipher encryption without padding hides it not at all, while Cipher Block Chaining (CBC) with padding hides some, depending on how padding is chosen. Use of TLS compression SHOULD take into account that compressed length may leak more than the original length.
Compression algorithms are complex and prone to implementation errors, and a buffer overrun is a risk where the language or operating system gives no protection. Rogue data can also be built to expand to an enormous size. RFC 2246 limits this: compression has to be lossless, the allowed content length increase is limited to 1,024 bytes, and total content length is limited to 2^14 bytes.
If someone can see how big your squished package is, they may guess what is inside. Also, a trick package could unsquish into something huge, so there are size limits.