Network Working Group N. Mavrogiannopoulos
Internet-Draft B. Preneel
Intended status: Standards Track KU Leuven - IBBT
Expires: May 13, 2013 November 9, 2012
Preventing cross-protocol attacks on the TLS protocol
draft-mavrogiannopoulos-tls-cross-protocol-03
Abstract
This memo proposes a fix in the TLS key exchange signature generation
to prevent cross-protocol attacks.
Status of This Memo
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This Internet-Draft will expire on May 13, 2013.
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1. Introduction
The TLS protocol [RFC5246] suffers from an issue in the
ServerKeyExchange message signature discovered by Wagner and Schneier
in [WS-ATTACK]. They describe a cross-protocol attack on the SSL 3.0
[RFC6101] protocol, that re-uses a signed ServerKeyExchange packet in
another session with a different key exchange algorithm. In effect
the attack uses a server as an oracle to obtain signed
ServerKeyExchange messages that are relayed to another, unrelated,
session. The described attack turned to be impossible to implement
in practice, but the underlying idea is applicable to all TLS
protocol versions, and it provides a tool for new attacks on the
protocol. The [CROSS-PROTOCOL] attack is a prominent example, which
takes advantage of interactions between the Diffie-Hellman and
Elliptic Curve Diffie-Hellman ciphersuites to perform a TLS server
impersonation after obtaining 2^40 signed messages.
In this document we propose a fix for the TLS protocol which makes it
immune to these attacks, but does not require a protocol version
upgrade.
2. Terminology
This document uses the same notation and terminology used in the TLS
Protocol specification [RFC5246].
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 [RFC2119].
3. The new ServerKeyExchange signature
The goal of this memo is to restrict the applicability of the server
provided signed ServerKeyExchange to the current session. A simple
fix may be to include the negotiated ciphersuite into the signature.
However, the TLS protocol is complex and a key exchange method does
not always imply a single format of the ServerKeyExchange signature.
For example, the elliptic curves key exchange method may be used with
an arbitrary elliptic curve [RFC4492] which requires different data
in the ServerKeyExchange than when used with a named curve. Such key
exchange suboptions are negotiated using TLS extensions and such
extensions should be covered by the signature to prevent any attack
that takes advantage of the different signature format.
For that we propose that the signature of the ServerKeyExchange
message to be modified to include in addition to explicit identifiers
of the algorithms, all the previously exchanged messages. The
proposed signature for a ServerKeyExchange message is shown below.
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enum { server (0), client (1) } ConnectionEnd;
enum { dhe_dss (0), dhe_rsa (1),
ec_diffie_hellman (2)
} KeyExchangeAlgorithm;
struct {
KeyExchangeAlgorithm kx_algorithm;
select (KeyExchangeAlgorithm) {
case dhe_dss:
case dhe_rsa:
ServerDHParams params;
case ec_diffie_hellman:
ServerECDHParams;
}
} Parameters;
struct {
Parameters params;
digitally-signed struct {
ConnectionEnd entity;
Parameters params;
opaque handshake_messages<0..2^24-1>;
}
} ServerKeyExchange;
The new format includes explicit indicators of the entity (server),
the key exchange algorithm used, the parameters of the key exchange,
and the previously exchanged handshake messages. This modification
will be negotiated by using a new TLS extension to allow backwards
compatibility.
4. The extension
In order for a client to advertise its support for the new
ServerKeyExchange format we add a new extension
"new_server_key_exchange", with value TBD-BY-IANA, to the enumerated
ExtensionType defined in [RFC5246]. The "extension_data" field of
this extension is empty.
5. Server and client behavior
Clients, that wish to protect against cross-protocol attacks, SHOULD
include the extension of type "new_server_key_exchange" in the
(extended) client hello.
Servers that receive an extended client hello containing a
"new_server_key_exchange" extension, MAY accept the request for the
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new ServerKeyExchange format by including an extension of type
"new_server_key_exchange" in the extended server hello.
Servers compliant to this document, that did not receive the
extension MUST set the gmt_unix_time part of the Random value
included in ServerHello to zero. Because in cross-protocol attacks
the server's random value is redirected to the client, this is a way
for the server to indicate support for the extension even in the
presence of an adversary.
Clients compliant to this document, that advertised this extension
but didn't receive a corresponding extension from the server, MUST
check the gmt_unix_time part of the Random value included in
ServerHello message for the value zero. If the gmt_unix_time is zero
the client MUST abort the handshake with an "illegal_parameter" fatal
alert.
Note that this extension is applies to all versions of the TLS
protocol including TLS 1.2 [RFC5246] and SSL 3.0 [RFC6101].
6. Security considerations
This extension modifies the ServerKeyExchange message in order to
prevent attacks to the protocol similar in nature with the Wagner and
Schneier attack. In order for the protection to be applicable, both
the client and the server must support this extension.
Compliant servers that did not receive the extension from the client
are required to set the 4 bytes of the server's random value, that
encodes the time, as zero. This provides a tool to indicate support
for the extended format even in the presence of an adversary, but
comes at the cost of reducing the total randomness from the server
from 32 bytes to 28 bytes.
7. IANA Considerations
This document defines the TLS extension "new_server_key_exchange"
(value TBD-BY-IANA) whose value should be assigned from the TLS
ExtensionType Registry defined in [RFC5246].
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
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Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008.
8.2. Informative References
[WS-ATTACK] Wagner, D. and B. Schneier, "Analysis of the SSL
3.0 protocol", In Proceedings of the Second USENIX
Workshop on Electronic Commerce, USENIX Press ,
November 1996.
[CROSS-PROTOCOL] Mavrogiannopoulos, N., Vercauteren, F., Velichkov,
V., and B. Preneel, "A cross-protocol attack on the
TLS protocol", In Proceedings of the 19th ACM
Conference on Computer and Communications Security
(CCS 2012), ACM , October 2012.
[RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
Sockets Layer (SSL) Protocol Version 3.0",
RFC 6101, August 2011.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C.,
and B. Moeller, "Elliptic Curve Cryptography (ECC)
Cipher Suites for Transport Layer Security (TLS)",
RFC 4492, May 2006.
Authors' Addresses
Nikos Mavrogiannopoulos
KU Leuven ESAT/SCD/COSIC
Kasteelpark Arenberg 10, bus 2446
Leuven-Heverlee, B-3001
Belgium
EMail: nikos.mavrogiannopoulos@esat.kuleuven.be
Bart Preneel
KU Leuven ESAT/SCD/COSIC
Kasteelpark Arenberg 10, bus 2446
Leuven-Heverlee, B-3001
Belgium
EMail: bart.preneel@esat.kuleuven.be
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