TLS                                                           A. Ghedini
Internet-Draft                                          Cloudflare, Inc.
Intended status: Standards Track                             V. Vasiliev
Expires: June 12, 2018                                            Google
                                                       December 09, 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
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   This Internet-Draft will expire on June 12, 2018.

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   Copyright (c) 2017 IETF Trust and the persons identified as the
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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
   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 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_certificates(TBD)), which is used by the client and the
   server to negotiate the use of compression for their certificate
   chains, as well as the choice of the compression algorithm.

   By sending the compress_certificates extension, the client indicates
   to the server the certificate compression algorithms it supports.

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

       enum {
       } 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_certificates extension.

4.  Compressed Certificate Message

   If a compression algorithm has been negotiated, 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 {
            uint24 uncompressed_length;
            opaque compressed_certificate_message<1..2^24-1>;
       } CompressedCertificate;

   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"

   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.

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   A peer is not required to compress their own Certificate messages
   even if the compress_certficates extension has been negotiated, but
   MUST be able to decompress a received CompressedCertificate message.

   If the negotiated compression algorithm is zlib, then the Certificate
   message MUST be compressed with the ZLIB compression algorithm, as
   defined in [RFC1950].  If the negotiated 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 tore 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.

   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_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
   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

   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.

   However this is not a problem when using TLS version 1.3 [draft-ietf-
   tls-tls13] and higher, due to the fact that the Certificate (and thus
   the CompressedCertificate) message is encrypted, preventing
   middleboxes from intercepting it.

7.  IANA Considerations

7.1.  Update of the TLS ExtensionType Registry

   Create an entry, compress_certificates(TBD), in the existing registry
   for ExtensionType (defined in [RFC5246]).

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 [RFC5226] for the definition of
   relevant policies).

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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, <https://www.rfc-

   [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-

   [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", RFC 5226,
              DOI 10.17487/RFC5226, May 2008, <https://www.rfc-

   [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-

   [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, <https://www.rfc-

   [RFC7932]  Alakuijala, J. and Z. Szabadka, "Brotli Compressed Data
              Format", RFC 7932, DOI 10.17487/RFC7932, July 2016,

Authors' Addresses

   Alessandro Ghedini
   Cloudflare, Inc.


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   Victor Vasiliev


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