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Versions: (draft-ghedini-tls-certificate-compression)
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TLS A. Ghedini
Internet-Draft Cloudflare, Inc.
Intended status: Standards Track V. Vasiliev
Expires: July 30, 2018 Google
January 26, 2018
Transport Layer Security (TLS) Certificate Compression
draft-ietf-tls-certificate-compression-02
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 30, 2018.
Copyright Notice
Copyright (c) 2018 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
(https://trustee.ietf.org/license-info) 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
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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. Compressed Certificate Message . . . . . . . . . . . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. Middlebox Compatibility . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7.1. Update of the TLS ExtensionType Registry . . . . . . . . 5
7.2. Update of the TLS HandshakeType Registry . . . . . . . . 5
7.3. Registry for Compression Algorithms . . . . . . . . . . . 5
8. Normative References . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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 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 extension is only supported with TLS 1.3 and newer; if TLS 1.2
or earlier is negotiated, the peers MUST ignore this extension.
This document defines a new extension type
(compress_certificate(TBD)), which can be used to signal the
supported compression formats for the Certificate message to the
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peer. Whenever it is sent by the client as a ClientHello message
extension ([I-D.ietf-tls-tls13], Section 4.1.2), it indicates the
support for compressed server certificates. Whenever it is sent by
the server as a CertificateRequest extension ([I-D.ietf-tls-tls13],
Section 4.3.2), it indicates the 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(0),
brotli(1),
(255)
} CertificateCompressionAlgorithm;
struct {
CertificateCompressionAlgorithm algorithms<1..2^8-1>;
} CertificateCompressionAlgorithms;
There is no ServerHello extension that the server is required to echo
back.
4. Compressed Certificate Message
If the peer has indicated that it supports compression, server and
client MAY compress their corresponding Certificate messages 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
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message MUST abort the connection with the "bad_certificate"
alert.
compressed_certificate_message The compressed body of the
Certificate message, in the same format as it would normally be
expressed in. 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 received CompressedCertificate message cannot be decompressed,
the connection MUST be torn down with the "bad_certificate" alert.
If the format of the Certificate message is altered using the
server_certificate_type extension [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.
Since certificate chains are typically presented on a per-server name
or per-user 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 CompressedCertificate 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.
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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 [I-D.ietf-tls-tls13] and
higher.
7. IANA Considerations
7.1. Update of the TLS ExtensionType Registry
Create an entry, compress_certificate(TBD), in the existing registry
for ExtensionType (defined in [I-D.ietf-tls-tls13]), with "TLS 1.3"
column values being set to "CH, CR".
7.2. Update of the TLS HandshakeType Registry
Create an entry, compressed_certificate(TBD), in the existing
registry for HandshakeType (defined in [RFC5246]).
7.3. 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 [RFC8126] for the definition of
relevant policies).
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8. Normative References
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-23 (work in progress),
January 2018.
[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>.
[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,
<https://www.rfc-editor.org/info/rfc4366>.
[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>.
[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>.
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Appendix A. 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, 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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