TLS Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks
draft-bmoeller-tls-downgrade-scsv-00
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Active Internet-Draft (individual)
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2013-09-25
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draft-ietf-tls-downgrade-scsv, rfc7507
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Network Working Group B. Moeller
Internet-Draft Google
Updates: 2246,4346,5246 September 25, 2013
(if approved)
Intended status: Standards Track
Expires: March 29, 2014
TLS Signaling Cipher Suite Value (SCSV) for Preventing Protocol
Downgrade Attacks
draft-bmoeller-tls-downgrade-scsv-00
Abstract
This document defines a Signaling Cipher Suite Value (SCSV) that can
be used to prevent protocol downgrades for Transport Layer Security
(TLS).
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 March 29, 2014.
Copyright Notice
Copyright (c) 2013 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
Moeller Expires March 29, 2014 [Page 1]
Internet-Draft TLS Downgrade SCSV September 2013
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The TLS_DOWNGRADE_SCSV Signaling Cipher Suite Value . . . . . 4
3. Server behavior . . . . . . . . . . . . . . . . . . . . . . . 5
4. Client behavior . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . . 8
6.2. Informal References . . . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
Moeller Expires March 29, 2014 [Page 2]
Internet-Draft TLS Downgrade SCSV September 2013
1. Introduction
To work around interoperability problems with legacy servers, many
TLS client implementations do not rely on the TLS protocol version
negotiation mechanism alone, but will intentionally reconnect using a
downgraded protocol if initial handshake attempts fail. Such clients
may fall back to connections in which they announce a version as low
as TLS 1.0 (or even its predecessor, SSL 3.0) as the highest
supported version, and make no attempt of using TLS extensions.
While such protocol downgrades can be a useful last resort for
connections to actual legacy servers, there's a risk that active
attackers could exploit the downgrade strategy to weaken the
cryptographic security of connections. (For example, if a server
requires the client's Supported Elliptic Curves Extension [RFC4492]
to choose a forward-secure key exchange algorithm, the attacker could
interfere with connections using this extension to get the client and
server to instead use a key exchange algorithm that does not
providing forward secrecy.) Also, handshake errors due to network
glitches could similary be misinterpreted as interaction with a
legacy server and result in a protocol downgrade.
All unnecessary protocol downgrades are undesirable (e.g., from TLS
1.2 to TLS 1.1 if both the client and the server actually do support
TLS 1.1); they can be particularly critical if they mean losing the
TLS extension feature (when downgrading to TLS 1.0 without
extensions, or to SSL 3.0). This document defines a Signaling Cipher
Suite Value (SCSV) that can be employed to prevent such protocol
downgrades between clients and servers that comply to this document.
This specification applies to implementations of TLS 1.0 [RFC2246],
TLS 1.1 [RFC4346], and TLS 1.2 [RFC5246]. (It is particularly
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