TLS A. Ghedini
Internet-Draft Cloudflare, Inc.
Intended status: Standards Track V. Vasiliev
Expires: December 10, 2017 Google
June 08, 2017
Transport Layer Security (TLS) Certificate Compression
draft-ietf-tls-certificate-compression-00
Abstract
In Transport Layer Security (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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 10, 2017.
Copyright Notice
Copyright (c) 2017 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
(http://trustee.ietf.org/license-info) in effect on the date of
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 2
3. Negotiating Certificate Compression . . . . . . . . . . . . . 2
4. Server Certificate Message . . . . . . . . . . . . . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6.1. Update of the TLS ExtensionType Registry . . . . . . . . 4
6.2. Registry for Compression Algorithms . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
In order to reduce latency and improve performance it can be useful
to reduce the amount of data exchanged during a Transport Layer
Security (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 server
certificates to be compressed during full handshakes.
2. Notational Conventions
The words "MUST", "MUST NOT", "SHALL", "SHOULD", and "MAY" are used
in this document. It's not shouting; when they are capitalized, they
have the special meaning defined in [RFC2119].
3. Negotiating Certificate Compression
This document defines a new extension type
(compress_server_certificates(TBD)), which is used by the client and
the server to negotiate the use of compression for the server
certificate chain, as well as the choice of the compression
algorithm.
By sending the compress_server_certificates message, the client
indicates to the server the certificate compression algorithms it
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supports. The "extension_data" field of this extension in the
ClientHello SHALL contain a CertificateCompressionAlgorithms value:
enum {
zlib(0),
brotli(1),
(255)
} CertificateCompressionAlgorithm;
struct {
CertificateCompressionAlgorithm algorithms<1..2^8-1>;
} CertificateCompressionAlgorithms;
If the server supports any of the algorithms offered in the
ClientHello, it MAY respond with an extension indicating which
compression algorithm it chose. In that case, the extension_data
SHALL be a CertificateCompressionAlgorithm value corresponding to the
chosen algorithm. If the server has chosen to not use any
compression, it MUST NOT send the compress_server_certificates
extension.
4. Server Certificate Message
If the server picks a compression algorithm and sends it in the
ServerHello, the format of the Certificate message is altered as
follows:
struct {
uint24 uncompressed_length;
opaque compressed_certificate_message<1..2^24-1>;
} Certificate;
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 client MUST abort the connection
with the "bad_certificate" alert.
compressed_certificate_message The compressed body of the
Certificate message, in the same format as the server would
normally express it. The compression algorithm defines how the
bytes in the compressed_certificate_message 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].
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If the client cannot decompress the received Certificate message from
the server, it MUST tear down the connection with the
"bad_certificate" alert.
The extension only affects the Certificate message from the server.
It does not change the format of the Certificate message sent by the
client.
If the format of the message is altered using the
server_certificate_type extension [RFC7250], the resulting altered
message is compressed instead.
If the server chooses to use the cached_info extension [RFC7924] to
replace the Certificate message with a hash, it MUST NOT send the
compress_server_certificates extension.
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.
Since certificate chains are typically presented on a per-server name
basis, the attacker 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.
The implementations SHOULD bound the memory usage when decompressing
the Certificate message.
The implementations MUST limit the size of the resulting decompressed
chain to the specified uncompressed length, and they MUST abort the
connection if the size exceeds that limit. TLS framing imposes
16777216 byte limit on the certificate message size, and the
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.
6. IANA Considerations
6.1. Update of the TLS ExtensionType Registry
Create an entry, compress_server_certificates(TBD), in the existing
registry for ExtensionType (defined in [RFC5246]).
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6.2. Registry for Compression Algorithms
This document establishes a registry of compression algorithms
supported for compressing the Certificate message, titled
"Certificate Compression Algorithm IDs", under the existing
"Transport Layer Security (TLS) Extensions" heading.
The entries in the registry are:
+------------------+--------------------------+
| Algorithm Number | Description |
+------------------+--------------------------+
| 0 | zlib |
| | |
| 1 | brotli |
| | |
| 224 to 255 | Reserved for Private Use |
+------------------+--------------------------+
The values in this registry shall be allocated under "IETF Review"
policy for values strictly smaller than 64, and under "Specification
Required" policy otherwise (see [RFC5226] for the definition of
relevant policies).
7. 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,
<http://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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,
<http://www.rfc-editor.org/info/rfc4366>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
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[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[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, <http://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,
<http://www.rfc-editor.org/info/rfc7924>.
[RFC7932] Alakuijala, J. and Z. Szabadka, "Brotli Compressed Data
Format", RFC 7932, DOI 10.17487/RFC7932, July 2016,
<http://www.rfc-editor.org/info/rfc7932>.
Authors' Addresses
Alessandro Ghedini
Cloudflare, Inc.
Email: alessandro@cloudflare.com
Victor Vasiliev
Google
Email: vasilvv@google.com
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