Network Working Group                                         A. Ghedini
Internet-Draft                                          Cloudflare, Inc.
Intended status: Standards Track                             V. Vasiliev
Expires: September 2, 2017                                        Google
                                                          March 01, 2017

         Transport Layer Security (TLS) Certificate Compression


   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

   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 September 2, 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
   ( 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

Ghedini & Vasiliev      Expires September 2, 2017               [Page 1]

Internet-Draft         TLS Certificate Compression            March 2017

   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

   By sending the compress_server_certificates message, the client
   indicates to the server the certificate compression algorithms it

Ghedini & Vasiliev      Expires September 2, 2017               [Page 2]

Internet-Draft         TLS Certificate Compression            March 2017

   supports.  The "extension_data" field of this extension in the
   ClientHello SHALL contain a CertificateCompressionAlgorithms value:

       enum {
       } CertificateCompressionAlgorithm;

       struct {
           CertificateCompressionAlgorithm algorithms<1..2^8>;
       } 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

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

       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].

Ghedini & Vasiliev      Expires September 2, 2017               [Page 3]

Internet-Draft         TLS Certificate Compression            March 2017

   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

   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.  Implementations MAY
   impose a lower limit on the chain size in addition to the 16777216
   byte limit imposed by TLS framing, in which case they MUST apply the
   same limit to the uncompressed chain before starting to decompress

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]).

Ghedini & Vasiliev      Expires September 2, 2017               [Page 4]

Internet-Draft         TLS Certificate Compression            March 2017

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,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [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,

Ghedini & Vasiliev      Expires September 2, 2017               [Page 5]

Internet-Draft         TLS Certificate Compression            March 2017

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [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,

Authors' Addresses

   Alessandro Ghedini
   Cloudflare, Inc.


   Victor Vasiliev


Ghedini & Vasiliev      Expires September 2, 2017               [Page 6]