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Internet Engineering Task Force (IETF)                        A. LangleyRequest for Comments: 7685                                    Google IncUpdates:5246                                               October 2015Category: Standards TrackISSN: 2070-1721A Transport Layer Security (TLS) ClientHello Padding ExtensionAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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   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.Langley                      Standards Track                    [Page 1]

RFC 7685            TLS ClientHello Padding Extension       October 2015Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Requirements Notation . . . . . . . . . . . . . . . . . . . .23.  Padding Extension . . . . . . . . . . . . . . . . . . . . . .24.  Example Usage . . . . . . . . . . . . . . . . . . . . . . . .35.  Security Considerations . . . . . . . . . . . . . . . . . . .36.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .47.  Normative References  . . . . . . . . . . . . . . . . . . . .4   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .4   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .41.  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 inRFC 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;Langley                      Standards Track                    [Page 2]

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.Langley                      Standards Track                    [Page 3]

RFC 7685            TLS ClientHello Padding Extension       October 20156.  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.comLangley                      Standards Track                    [Page 4]