Network Working Group A. Langley Internet-Draft Google Inc Expires: July 5, 2014 Jan 2014 A TLS padding extension draft-agl-tls-padding-03 Abstract This memo describes the 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 working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on July 5, 2014. Copyright Notice Copyright (c) 2014 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 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 Expires July 5, 2014 [Page 1] Internet-Draft TLS Padding Extension Jan 2014 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 4 3. Padding Extension . . . . . . . . . . . . . . . . . . . . . . 5 4. Example usage . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 8. Normative References . . . . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11 Langley Expires July 5, 2014 [Page 2] Internet-Draft TLS Padding Extension Jan 2014 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. Langley Expires July 5, 2014 [Page 3] Internet-Draft TLS Padding Extension Jan 2014 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]. Langley Expires July 5, 2014 [Page 4] Internet-Draft TLS Padding Extension Jan 2014 3. Padding Extension A new extension type ("padding(TBD)") is defined and MAY be included by the client in its ClientHello message. enum { padding(TBD), (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 TBD TBD 00 00. (TBR values to be filled in once an ExtensionType has been assigned.) A ten byte extension would include 6 bytes of "extension_data" and would be encoded as: xx xx 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. Langley Expires July 5, 2014 [Page 5] Internet-Draft TLS Padding Extension Jan 2014 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 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 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 past 512 bytes in these cases. Langley Expires July 5, 2014 [Page 6] Internet-Draft TLS Padding Extension Jan 2014 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. Servers MAY verify that the extension is either empty or contains only zero bytes, in order to enforce this. Langley Expires July 5, 2014 [Page 7] Internet-Draft TLS Padding Extension Jan 2014 6. IANA Considerations IANA is requested to assign an extension value for the padding extension from its ExtensionType registry. Langley Expires July 5, 2014 [Page 8] Internet-Draft TLS Padding Extension Jan 2014 7. Acknowledgements The author gratefully acknowledges the contributions of Wan-Teh Chang. Also the suggestions of Eric Rescorla. Langley Expires July 5, 2014 [Page 9] Internet-Draft TLS Padding Extension Jan 2014 8. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. Langley Expires July 5, 2014 [Page 10] Internet-Draft TLS Padding Extension Jan 2014 Author's Address Adam Langley Google Inc Email: agl@google.com Langley Expires July 5, 2014 [Page 11]