A Transport Layer Security (TLS) ClientHello Padding Extension
RFC 7685

Internet Engineering Task Force (IETF)                        A. Langley
Request for Comments: 7685                                    Google Inc
Updates: 5246                                               October 2015
Category: Standards Track
ISSN: 2070-1721

     A Transport Layer Security (TLS) ClientHello Padding Extension

Abstract

   This memo describes a Transport Layer Security (TLS) extension that
   can be used to pad ClientHello messages to a desired size.

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 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7685.

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
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   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.

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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.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   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 is desirable given that
   fully comprehensive patching of affected implementations is difficult
   to achieve.

   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;

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RFC 7685            TLS ClientHello Padding Extension       October 2015

   The "extension_data" for the extension consists of an arbitrary
   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 six bytes of "extension_data" and would be
   encoded as:

   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_data field 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 TLSCiphertext.length 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 bytes
   to the length (to account for the "msg_type" and "length" fields of
   the handshake protocol) and test whether the resulting length falls
   into that range.  If it does, a padding extension can be added in
   order to push the length to (at least) 512 bytes.

   Note that if the original ClientHello size was between 505 and 507
   bytes, then, with the handshake protocol overhead, the record payload
   would be between 509 and 511 bytes long.  Since it's not possible for
   an extension to take less than four bytes of space, the additional
   padding would have to expand the ClientHello record payload beyond
   512 bytes in these cases.

5.  Security Considerations

   The contents of the padding extension could be used as a covert
   channel.  In order to prevent this, the contents are required to be
   all zeros, although the length of the extension can still be used as
   a much smaller covert channel.

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6.  IANA Considerations

   IANA has permanently registered value 21 (padding) in the
   "ExtensionType Values" registry.

7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

Acknowledgements

   The author gratefully acknowledges the contributions of Wan-Teh Chang
   and the suggestions of Eric Rescorla.

Author's Address

   Adam Langley
   Google Inc

   Email: agl@google.com

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