Internet Engineering Task Force (IETF) A. Ghedini
Request for Comments: 8879 Cloudflare, Inc.
Category: Standards Track V. Vasiliev
ISSN: 2070-1721 Google
August 2020
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
https://www.rfc-editor.org/info/rfc8879.
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
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described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Notational Conventions
3. Negotiating Certificate Compression
4. Compressed Certificate Message
5. Security Considerations
6. Middlebox Compatibility
7. IANA Considerations
7.1. TLS ExtensionType Values
7.2. TLS HandshakeType
7.3. Compression Algorithms
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses
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
[RFC8878].
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:
+==================+===============================+===========+
| Algorithm Number | Description | Reference |
+==================+===============================+===========+
| 0 | Reserved | RFC 8879 |
+------------------+-------------------------------+-----------+
| 1 | zlib | RFC 8879 |
+------------------+-------------------------------+-----------+
| 2 | brotli | RFC 8879 |
+------------------+-------------------------------+-----------+
| 3 | zstd | RFC 8879 |
+------------------+-------------------------------+-----------+
| 16384 to 65535 | Reserved for Experimental Use | |
+------------------+-------------------------------+-----------+
Table 1: TLS Certificate Compression Algorithm IDs
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,
<https://www.rfc-editor.org/info/rfc1950>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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, <https://www.rfc-editor.org/info/rfc7250>.
[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", RFC 7924,
DOI 10.17487/RFC7924, July 2016,
<https://www.rfc-editor.org/info/rfc7924>.
[RFC7932] Alakuijala, J. and Z. Szabadka, "Brotli Compressed Data
Format", RFC 7932, DOI 10.17487/RFC7932, July 2016,
<https://www.rfc-editor.org/info/rfc7932>.
[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,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/info/rfc8447>.
[RFC8878] Collet, Y. and M. Kucherawy, Ed., "Zstandard Compression
and the 'application/zstd' Media Type", RFC 8878,
DOI 10.17487/RFC8878, August 2020,
<https://www.rfc-editor.org/rfc/rfc8878>.
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,
<https://www.rfc-editor.org/info/rfc5246>.
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