Network Working Group A. Langley
Internet-Draft Google Inc
Updates: 5246 (if approved) Sept 2015
Intended status: Standards Track
Expires: March 4, 2016

A TLS ClientHello padding extension


This memo describes a TLS extension that can be used to pad ClientHello messages to a desired size.

Status of This Memo

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Table of Contents

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.

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;

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 6 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 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.

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.

6. IANA Considerations

IANA is requested to permanently assign the early code point for the padding extension in its ExtensionType registry.

7. Acknowledgements

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

8. 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.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.

Author's Address

Adam Langley Google Inc EMail: