A TLS ClientHello padding extension
draft-ietf-tls-padding-01

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Document Type Active Internet-Draft (tls WG)
Last updated 2015-08-24 (latest revision 2015-02-17)
Replaces draft-agl-tls-padding
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Network Working Group                                         A. Langley
Internet-Draft                                                Google Inc
Expires: August 21, 2015                               February 17, 2015

                  A TLS ClientHello padding extension
                       draft-ietf-tls-padding-01

Abstract

   This memo describes a TLS extension that can be used to pad
   ClientHello messages to a desired size.

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
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 21, 2015.

Copyright Notice

   Copyright (c) 2015 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
   (http://trustee.ietf.org/license-info) in effect on the date of
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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   2
   3.  Padding Extension . . . . . . . . . . . . . . . . . . . . . .   2
   4.  Example usage . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   4
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   Successive TLS [RFC5246] versions have added support for more cipher
   suites and, over time, more TLS extensions have been defined.  This
   has caused the size of the TLS ClientHello to grow and the additional
   size has caused some implementation bugs to come to light.  At least
   one of these implementation bugs can be ameliorated by making the
   ClientHello even larger.

   This memo describes a TLS extension that can be used to pad a
   ClientHello to a desired size in order to avoid implementation bugs
   caused by certain ClientHello sizes.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Padding Extension

   A new extension type ("padding(21)") is defined and MAY be included
   by the client in its ClientHello message.

   enum {
           padding(21), (65535)
   } ExtensionType;

   The "extension_data" for the extension consists of an arbitary number
   of zero bytes.  For example, the smallest "padding" extension is four
   bytes long and is encoded as 0x00 0x15 0x00 0x00.  A ten byte
   extension would include 6 bytes of "extension_data" and would be
   encoded as:

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   00 15 00 06 00 00 00 00 00 00
   |---| |---| |---------------|
     |     |           |
     |     |           \- extension_data: 6 zero bytes
     |     |
     |     \------------- 16-bit, extension_data length
     |
     \------------------- extension_type for padding extension

   The client MUST fill the padding extension completely with zero
   bytes, although the padding extension may be empty.

   The server MUST NOT echo the extension.

4.  Example usage

   As an example, consider a client that wishes to avoid sending a
   ClientHello with a record size between 256 and 511 bytes (inclusive).
   This case is considered because at least one TLS implementation is
   known to hang the connection when such a ClientHello record is
   received.

   After building a ClientHello as normal, the client can add four to
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