A Transport Layer Security (TLS) ClientHello Padding Extension
RFC 7685
Document | Type |
RFC - Proposed Standard
(October 2015; No errata)
Updates RFC 5246
Was draft-ietf-tls-padding (tls WG)
|
|
---|---|---|---|
Author | Adam Langley | ||
Last updated | 2015-10-21 | ||
Replaces | draft-agl-tls-padding | ||
Stream | IETF | ||
Formats | plain text html pdf htmlized bibtex | ||
Reviews | |||
Stream | WG state | Submitted to IESG for Publication | |
Document shepherd | Joseph Salowey | ||
Shepherd write-up | Show (last changed 2015-08-05) | ||
IESG | IESG state | RFC 7685 (Proposed Standard) | |
Consensus Boilerplate | Yes | ||
Telechat date | |||
Responsible AD | Stephen Farrell | ||
Send notices to | (None) | ||
IANA | IANA review state | Version Changed - Review Needed | |
IANA action state | RFC-Ed-Ack |
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 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 2015 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; 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 bytesShow full document text