draft-ietf-tls-padding-03.txt   draft-ietf-tls-padding-04.txt 
Network Working Group A. Langley Network Working Group A. Langley
Internet-Draft Google Inc Internet-Draft Google Inc
Updates: 5246 (if approved) Sept 2015 Updates: 5246 (if approved) Sept 2015
Intended status: Standards Track Intended status: Standards Track
Expires: March 4, 2016 Expires: March 11, 2016
A TLS ClientHello padding extension A TLS ClientHello padding extension
draft-ietf-tls-padding-03 draft-ietf-tls-padding-04
Abstract Abstract
This memo describes a TLS extension that can be used to pad This memo describes a TLS extension that can be used to pad
ClientHello messages to a desired size. ClientHello messages to a desired size.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 4, 2016. This Internet-Draft will expire on March 11, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 2 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 2
3. Padding Extension . . . . . . . . . . . . . . . . . . . . . . 2 3. Padding Extension . . . . . . . . . . . . . . . . . . . . . . 2
4. Example usage . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Example usage . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . 3 5. Security Considerations . . . . . . . . . . . . . . . . . . . 3
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
8. Normative References . . . . . . . . . . . . . . . . . . . . 4 8. Normative References . . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4
1. Introduction 1. Introduction
Successive TLS [RFC5246] versions have added support for more cipher Successive TLS [RFC5246] versions have added support for more cipher
suites and, over time, more TLS extensions have been defined. This suites and, over time, more TLS extensions have been defined. This
has caused the size of the TLS ClientHello to grow and the additional 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 size has caused some implementation bugs to come to light. At least
one of these implementation bugs can be ameliorated by making the one of these implementation bugs can be ameliorated by making the
ClientHello even larger. 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 This memo describes a TLS extension that can be used to pad a
ClientHello to a desired size in order to avoid implementation bugs ClientHello to a desired size in order to avoid implementation bugs
caused by certain ClientHello sizes. caused by certain ClientHello sizes.
2. Requirements Notation 2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
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between 509 and 511 bytes long. Since it's not possible for an between 509 and 511 bytes long. Since it's not possible for an
extension to take less than four bytes of space, the additional extension to take less than four bytes of space, the additional
padding would have to expand the ClientHello record past 512 bytes in padding would have to expand the ClientHello record past 512 bytes in
these cases. these cases.
5. Security Considerations 5. Security Considerations
The contents of the padding extension could be used as a covert The contents of the padding extension could be used as a covert
channel. In order to prevent this, the contents are required to be 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 all zeros, although the length of the extension can still be used as
a much smaller covert channel. Servers MAY verify that the extension a much smaller covert channel.
is either empty or contains only zero bytes, in order to enforce
this.
6. IANA Considerations 6. IANA Considerations
IANA is requested to permanently assign the early code point for the RFC EDITOR: Please delete this section prior to publication.
padding extension in its ExtensionType registry.
An early code-point assignment (21) was made for this draft in the
registry of TLS ExtensionType values. IANA is requested to
permanently assign this code-point.
7. Acknowledgements 7. Acknowledgements
The author gratefully acknowledges the contributions of Wan-Teh Chang The author gratefully acknowledges the contributions of Wan-Teh Chang
and the suggestions of Eric Rescorla. and the suggestions of Eric Rescorla.
8. Normative References 8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
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