TLS Certificate Compression
RFC 8879

Document Type RFC - Proposed Standard (December 2020; No errata)
Authors Alessandro Ghedini  , Victor Vasiliev 
Last updated 2020-12-01
Replaces draft-ghedini-tls-certificate-compression
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Internet Engineering Task Force (IETF)                        A. Ghedini
Request for Comments: 8879                              Cloudflare, Inc.
Category: Standards Track                                    V. Vasiliev
ISSN: 2070-1721                                                   Google
                                                           December 2020

                      TLS Certificate Compression

Abstract

   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

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8879.

Copyright Notice

   Copyright (c) 2020 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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Notational Conventions
   3.  Negotiating Certificate Compression
   4.  Compressed Certificate Message
   5.  Security Considerations
   6.  Middlebox Compatibility
   7.  IANA Considerations
     7.1.  TLS ExtensionType Values
     7.2.  TLS HandshakeType
     7.3.  Compression Algorithms
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgements
   Authors' Addresses

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 all handshakes.

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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 [RFC8446] and newer; if
   TLS 1.2 [RFC5246] 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
   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 support for compressed
   server certificates.  Whenever it is sent by the server as a
   CertificateRequest extension ([RFC8446], Section 4.3.2), it indicates
   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(1),
           brotli(2),
           zstd(3),
           (65535)
       } CertificateCompressionAlgorithm;

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

   The compress_certificate extension is a unidirectional indication; no
   corresponding response extension is needed.

4.  Compressed Certificate Message

   If the peer has indicated that it supports compression, server and
   client MAY compress their corresponding Certificate messages
   (Section 4.4.2 of [RFC8446]) and send them in the form of the
   CompressedCertificate message (replacing the Certificate message).

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