RFC 8879 - TLS Certificate Compression

RFC 8879 - TLS Certificate Compression
RFC 8879
TLS Certificate Compression
December 2020
Ghedini & Vasiliev
Standards Track
[Page]
Stream:
Internet Engineering Task Force (IETF)
RFC:
8879
Category:
Standards Track
Published:
December 2020
ISSN:
2070-1721
Authors:
A. Ghedini
Cloudflare, Inc.
V. Vasiliev
Google
RFC 8879
TLS Certificate Compression
Abstract
In TLS handshakes, certificate chains often take up
the majority of the bytes transmitted.
This document describes how certificate chains can be compressed to reduce the
amount of data transmitted and avoid some round trips.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by
the Internet Engineering Steering Group (IESG). Further
information on Internet Standards is available in Section 2 of
RFC 7841.
Information about the current status of this document, any
errata, and how to provide feedback on it may be obtained at
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect to this document. Code Components extracted from this
document must include Simplified BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Simplified BSD License.
Table of Contents
1.
Introduction
In order to reduce latency and improve performance, it can be useful to reduce
the amount of data exchanged during a TLS handshake.
RFC7924
describes a mechanism that allows a client and a server to avoid
transmitting certificates already shared in an earlier handshake, but it
doesn't help when the client connects to a server for the first time and
doesn't already have knowledge of the server's certificate chain.
This document describes a mechanism that would allow certificates to be
compressed during all handshakes.
2.
Notational Conventions
The key words "
MUST
", "
MUST NOT
", "
REQUIRED
", "
SHALL
", "
SHALL NOT
", "
SHOULD
", "
SHOULD NOT
", "
RECOMMENDED
", "
NOT RECOMMENDED
",
MAY
", and "
OPTIONAL
" in this document are to be interpreted as
described in BCP 14
RFC2119
RFC8174
when, and only when, they appear in all capitals, as shown here.
3.
Negotiating Certificate Compression
This extension is only supported with TLS 1.3
RFC8446
and newer; if TLS 1.2
RFC5246
or earlier is negotiated, the peers
MUST
ignore this extension.
This document defines a new extension type (compress_certificate(27)), which
can be used to signal the supported compression formats for the Certificate
message to the peer. Whenever it is sent by the client as a ClientHello message
extension (
RFC8446
],
Section 4.1.2
), it indicates support for
compressed server certificates. Whenever it is sent by the server as a
CertificateRequest extension (
RFC8446
],
Section 4.3.2
), it indicates support for compressed client certificates.
By sending a compress_certificate extension, the sender indicates to the peer
the certificate-compression algorithms it is willing to use for decompression.
The "extension_data" field of this extension
SHALL
contain a
CertificateCompressionAlgorithms value:
enum {
zlib(1),
brotli(2),
zstd(3),
(65535)
} CertificateCompressionAlgorithm;

struct {
CertificateCompressionAlgorithm algorithms<2..2^8-2>;
} CertificateCompressionAlgorithms;
The compress_certificate extension is a unidirectional indication; no
corresponding response extension is needed.
4.
Compressed Certificate Message
If the peer has indicated that it supports compression, server and
client
MAY
compress their corresponding Certificate messages (
Section 4.4.2
of [
RFC8446
and send them in the form of the CompressedCertificate message (replacing the
Certificate message).
The CompressedCertificate message is formed as follows:
struct {
CertificateCompressionAlgorithm algorithm;
uint24 uncompressed_length;
opaque compressed_certificate_message<1..2^24-1>;
} CompressedCertificate;
algorithm:
The algorithm used to compress the certificate. The algorithm
MUST
be one of
the algorithms listed in the peer's compress_certificate extension.
uncompressed_length:
The length of the Certificate message once it is uncompressed. If, after
decompression, the specified length does not match the actual length, the
party receiving the invalid message
MUST
abort the connection with the
"bad_certificate" alert. The presence of this field allows the receiver to
preallocate the buffer for the uncompressed Certificate message and enforce
limits on the message size before performing decompression.
compressed_certificate_message:
The result of applying the indicated compression algorithm to the encoded
Certificate message that would have been sent if certificate compression was not
in use. The compression algorithm defines how the
bytes in the compressed_certificate_message field are converted into the
Certificate message.
If the specified compression algorithm is zlib, then the Certificate message
MUST
be compressed with the ZLIB compression algorithm, as defined in
RFC1950
If the specified compression algorithm is brotli, the Certificate message
MUST
be compressed with the Brotli compression algorithm, as defined in
RFC7932
. If
the specified compression algorithm is zstd, the Certificate message
MUST
be
compressed with the Zstandard compression algorithm, as defined in
RFC8478
It is possible to define a certificate compression algorithm that uses a
preshared dictionary to achieve a higher compression ratio. This document does
not define any such algorithms, but additional codepoints may be allocated for
such use per the policy in
Section 7.3
If the received CompressedCertificate message cannot be decompressed, the
connection
MUST
be terminated with the "bad_certificate" alert.
If the format of the Certificate message is altered using the
server_certificate_type or client_certificate_type extensions
RFC7250
, the
resulting altered message is compressed instead.
5.
Security Considerations
After decompression, the Certificate message
MUST
be processed as if it were
encoded without being compressed. This way, the parsing and the verification
have the same security properties as they would have in TLS normally.
In order for certificate compression to function correctly, the underlying
compression algorithm
MUST
output the same data
that was provided as input by the peer.
Since certificate chains are typically presented on a per-server-name or
per-user basis, a malicious application does not have control over any individual fragments
in the Certificate message, meaning that they cannot leak information about the
certificate by modifying the plaintext.
Implementations
SHOULD
bound the memory usage when decompressing the
CompressedCertificate message.
Implementations
MUST
limit the size of the resulting decompressed chain to
the specified uncompressed length, and they
MUST
abort the connection if the
size of the output of the decompression function exceeds that limit. TLS framing
imposes a 16777216-byte limit on the certificate message size, and implementations
MAY
impose a limit that is lower than that; in both cases, they
MUST
apply the same
limit as if no compression were used.
While the Certificate message in TLS 1.3 is encrypted, third parties can draw
inferences from the message length observed on the wire. TLS 1.3 provides a padding
mechanism (discussed in Sections
5.4
and
E.3
of
RFC8446
) to counteract such
analysis. Certificate compression alters the length of the Certificate message,
and the change in length is dependent on the actual contents of the certificate.
Any padding scheme covering the Certificate message has to address compression
within its design or disable it altogether.
6.
Middlebox Compatibility
It's been observed that a significant number of middleboxes intercept and try
to validate the Certificate message exchanged during a TLS handshake. This
means that middleboxes that don't understand the CompressedCertificate message
might misbehave and drop connections that adopt certificate compression.
Because of that, the extension is only supported in the versions of TLS where
the certificate message is encrypted in a way that prevents middleboxes from
intercepting it -- that is, TLS version 1.3
RFC8446
and higher.
7.
IANA Considerations
7.1.
TLS ExtensionType Values
IANA has created an entry, compress_certificate(27), in the
"TLS ExtensionType Values" registry (defined in
RFC8446
) with the values in the "TLS 1.3" column
set to "CH, CR" and the "Recommended" column entry set to "Yes".
7.2.
TLS HandshakeType
IANA has created an entry, compressed_certificate(25), in
the "TLS Handshake Type" registry (defined in
RFC8446
), with the "DTLS-OK" column value set to
"Yes".
7.3.
Compression Algorithms
This document establishes a registry of compression algorithms supported for
compressing the Certificate message, titled "TLS Certificate Compression Algorithm
IDs", under the existing "Transport Layer Security (TLS) Extensions" registry.
The entries in the registry are:
Table 1
TLS Certificate Compression Algorithm IDs
Algorithm Number
Description
Reference
Reserved
RFC 8879
zlib
RFC 8879
brotli
RFC 8879
zstd
RFC 8879
16384 to 65535
Reserved for Experimental Use
The values in this registry shall be allocated under "IETF Review" policy for
values strictly smaller than 256, under "Specification Required" policy for
values 256-16383, and under "Experimental Use" otherwise (see
RFC8126
for the
definition of relevant policies). Experimental Use extensions can be used both
on private networks and over the open Internet.
The procedures for requesting values in the Specification Required space are
specified in
Section 17
of [
RFC8447
8.
References
8.1.
Normative References
[RFC1950]
Deutsch, P.
and J-L. Gailly
"ZLIB Compressed Data Format Specification version 3.3"
RFC 1950
DOI 10.17487/RFC1950
May 1996
[RFC2119]
Bradner, S.
"Key words for use in RFCs to Indicate Requirement Levels"
BCP 14
RFC 2119
DOI 10.17487/RFC2119
March 1997
[RFC7250]
Wouters, P., Ed.
, Tschofenig, H., Ed.
, Gilmore, J.
, Weiler, S.
, and T. Kivinen
"Using Raw Public Keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)"
RFC 7250
DOI 10.17487/RFC7250
June 2014
[RFC7924]
Santesson, S.
and H. Tschofenig
"Transport Layer Security (TLS) Cached Information Extension"
RFC 7924
DOI 10.17487/RFC7924
July 2016
[RFC7932]
Alakuijala, J.
and Z. Szabadka
"Brotli Compressed Data Format"
RFC 7932
DOI 10.17487/RFC7932
July 2016
[RFC8126]
Cotton, M.
, Leiba, B.
, and T. Narten
"Guidelines for Writing an IANA Considerations Section in RFCs"
BCP 26
RFC 8126
DOI 10.17487/RFC8126
June 2017
[RFC8174]
Leiba, B.
"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"
BCP 14
RFC 8174
DOI 10.17487/RFC8174
May 2017
[RFC8446]
Rescorla, E.
"The Transport Layer Security (TLS) Protocol Version 1.3"
RFC 8446
DOI 10.17487/RFC8446
August 2018
[RFC8447]
Salowey, J.
and S. Turner
"IANA Registry Updates for TLS and DTLS"
RFC 8447
DOI 10.17487/RFC8447
August 2018
[RFC8478]
Collet, Y.
and M. Kucherawy, Ed.
"Zstandard Compression and the application/zstd Media Type"
RFC 8478
DOI 10.17487/RFC8478
October 2018
8.2.
Informative References
[RFC5246]
Dierks, T.
and E. Rescorla
"The Transport Layer Security (TLS) Protocol Version 1.2"
RFC 5246
DOI 10.17487/RFC5246
August 2008
Acknowledgements
Certificate compression was originally introduced in the QUIC Crypto protocol,
designed by
Adam Langley
and
Wan-Teh Chang
This document has benefited from contributions and suggestions from
David Benjamin
Ryan Hamilton
Christian Huitema
Benjamin Kaduk
Ilari Liusvaara
Piotr Sikora
Ian Swett
Martin Thomson
Sean Turner
, and many others.
Authors' Addresses
Alessandro Ghedini
Cloudflare, Inc.
Email:
alessandro@cloudflare.com
Victor Vasiliev
Google
Email:
vasilvv@google.com
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- Proposed Standard
December 2020
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