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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 5246 (Obsoleted by RFC 8446) Summary: 1 error (**), 0 flaws (~~), 1 warning (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group A. Langley 3 Internet-Draft Google Inc 4 Updates: 5246 (if approved) Sept 2015 5 Intended status: Standards Track 6 Expires: March 4, 2016 8 A TLS ClientHello padding extension 9 draft-ietf-tls-padding-03 11 Abstract 13 This memo describes a TLS extension that can be used to pad 14 ClientHello messages to a desired size. 16 Status of This Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at http://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on March 4, 2016. 33 Copyright Notice 35 Copyright (c) 2015 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (http://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 51 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 2 52 3. Padding Extension . . . . . . . . . . . . . . . . . . . . . . 2 53 4. Example usage . . . . . . . . . . . . . . . . . . . . . . . . 3 54 5. Security Considerations . . . . . . . . . . . . . . . . . . . 3 55 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 56 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4 57 8. Normative References . . . . . . . . . . . . . . . . . . . . 4 58 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4 60 1. Introduction 62 Successive TLS [RFC5246] versions have added support for more cipher 63 suites and, over time, more TLS extensions have been defined. This 64 has caused the size of the TLS ClientHello to grow and the additional 65 size has caused some implementation bugs to come to light. At least 66 one of these implementation bugs can be ameliorated by making the 67 ClientHello even larger. 69 This memo describes a TLS extension that can be used to pad a 70 ClientHello to a desired size in order to avoid implementation bugs 71 caused by certain ClientHello sizes. 73 2. Requirements Notation 75 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 76 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 77 document are to be interpreted as described in RFC 2119 [RFC2119]. 79 3. Padding Extension 81 A new extension type ("padding(21)") is defined and MAY be included 82 by the client in its ClientHello message. 84 enum { 85 padding(21), (65535) 86 } ExtensionType; 88 The "extension_data" for the extension consists of an arbitrary 89 number of zero bytes. For example, the smallest "padding" extension 90 is four bytes long and is encoded as 0x00 0x15 0x00 0x00. A ten byte 91 extension would include 6 bytes of "extension_data" and would be 92 encoded as: 94 00 15 00 06 00 00 00 00 00 00 95 |---| |---| |---------------| 96 | | | 97 | | \- extension_data: 6 zero bytes 98 | | 99 | \------------- 16-bit, extension_data length 100 | 101 \------------------- extension_type for padding extension 103 The client MUST fill the padding extension completely with zero 104 bytes, although the padding extension_data field may be empty. 106 The server MUST NOT echo the extension. 108 4. Example usage 110 As an example, consider a client that wishes to avoid sending a 111 ClientHello with a record size between 256 and 511 bytes (inclusive). 112 This case is considered because at least one TLS implementation is 113 known to hang the connection when such a ClientHello record is 114 received. 116 After building a ClientHello as normal, the client can add four to 117 the length (to account for the "msg_type" and "length" fields of the 118 handshake protocol) and test whether the resulting length falls into 119 that range. If it does, a padding extension can be added in order to 120 push the length to (at least) 512 bytes. 122 Note that if the original ClientHello size was between 505 and 507 123 bytes then, with the handshake protocol overhead, the record would be 124 between 509 and 511 bytes long. Since it's not possible for an 125 extension to take less than four bytes of space, the additional 126 padding would have to expand the ClientHello record past 512 bytes in 127 these cases. 129 5. Security Considerations 131 The contents of the padding extension could be used as a covert 132 channel. In order to prevent this, the contents are required to be 133 all zeros, although the length of the extension can still be used as 134 a much smaller covert channel. Servers MAY verify that the extension 135 is either empty or contains only zero bytes, in order to enforce 136 this. 138 6. IANA Considerations 140 IANA is requested to permanently assign the early code point for the 141 padding extension in its ExtensionType registry. 143 7. Acknowledgements 145 The author gratefully acknowledges the contributions of Wan-Teh Chang 146 and the suggestions of Eric Rescorla. 148 8. Normative References 150 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 151 Requirement Levels", BCP 14, RFC 2119, 152 DOI 10.17487/RFC2119, March 1997, 153 . 155 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 156 (TLS) Protocol Version 1.2", RFC 5246, 157 DOI 10.17487/RFC5246, August 2008, 158 . 160 Author's Address 162 Adam Langley 163 Google Inc 165 Email: agl@google.com