TLS                                                           A. Ghedini
Internet-Draft                                          Cloudflare, Inc.
Intended status: Standards Track                             V. Vasiliev
Expires: October 7, 2019                                          Google
                                                          April 05, 2019

                      TLS Certificate Compression


   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

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on October 7, 2019.

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   Copyright (c) 2019 IETF Trust and the persons identified as the
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   ( in effect on the date 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 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 key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 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(27)), which can be used to signal the supported

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   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 the support for compressed
   server certificates.  Whenever it is sent by the server as a
   CertificateRequest extension ([RFC8446], 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 {
       } CertificateCompressionAlgorithm;

       struct {
           CertificateCompressionAlgorithm algorithms<2..2^8-2>;
       } CertificateCompressionAlgorithms;

   There is no ServerHello extension that the server is required to echo

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.  The presence of this field allows the receiver to pre-
      allocate the buffer for the uncompressed Certificate message and
      to enforce limits on the message size before performing

   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 specified
   compression algorithm is zstd, the Certificate message MUST be
   compressed with the Zstandard compression algorithm as defined in

   It is possible to define a certificate compression algorithm that
   uses a pre-shared dictionary to achieve higher compression ratio.
   This document does not define any such algorithms.

   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 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 be deterministic and it 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, 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

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   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.

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.  Update of the TLS ExtensionType Registry

   Create an entry, compress_certificate(27), in the existing registry
   for ExtensionType (defined in [RFC8446]), with "TLS 1.3" column
   values being set to "CH, CR", and "Recommended" column being set to

7.2.  Update of the TLS HandshakeType Registry

   Create an entry, compressed_certificate(25), in the existing registry
   for HandshakeType (defined in [RFC8446]).

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:

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           | Algorithm Number | Description                   |
           | 0                | Reserved                      |
           |                  |                               |
           | 1                | zlib                          |
           |                  |                               |
           | 2                | brotli                        |
           |                  |                               |
           | 3                | zstd                          |
           |                  |                               |
           | 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 [RFC8447].

8.  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,

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   [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,

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.


   Victor Vasiliev


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