Network Working Group Y. Sheffer
Internet-Draft Porticor
Updates: 5996 (if approved) S. Fluhrer
Intended status: Standards Track Cisco
Expires: June 10, 2013 December 7, 2012
Additional Diffie-Hellman Tests for IKEv2
draft-sheffer-ipsecme-dh-checks-00
Abstract
This document adds a small number of mandatory tests required for the
secure operation of IKEv2 with elliptic curve groups. No change is
required to IKE implementations that use modular exponential groups,
other than a few rarely used so-called DSA groups. This document
updates the IKEv2 protocol, RFC 5996.
Status of this Memo
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This Internet-Draft will expire on June 10, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
1. Introduction
IKEv2 [RFC5996] consists of the establishment of a shared secret
using the Diffie-Hellman (DH) protocol, followed by authentication of
the two peers. Existing implementations typically use modular
exponential (MODP) DH groups, such as those defined in [RFC3526].
Recently there is growing interest in using IKEv2 with groups based
on elliptic curve cryptography. It turns out that using EC groups in
some scenarios requires that IKE peers implement additional tests.
This document defines these tests.
1.1. Conventions used in this document
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].
2. Group Membership Tests
This section describes the tests that need to be performed by IKE
peers receiving a Key Exchange (KE) payload. The tests are
RECOMMENDED for all implementations, but only REQUIRED for those that
reuse DH secret keys (as defined in [RFC5996], Sec. 2.12). The tests
are listed here according to the DH group being used.
2.1. Regular MODP Groups: Group 1, 2, 5, 14-18
These are currently the most commonly used groups; all these groups
have the property that (p-1)/2 is also prime; this section applies to
any such MODP group. The only check needed is a verification that
the peer's public value r is in the legal range (1 < r < p-1).
According to [Menezes], Sec 2.2, there is a possibility of leaking a
single bit of the secret exponent during reuse; this amount of
leakage is insignificant.
2.2. MODP Groups with Small Subgroups: Group 22, 23, 24
IKEv2 groups 22-24 [RFC5114] are modular exponential groups with
small subgroups; these all have (p-1)/2 composite. Sec. 2.1 of
[Menezes] describes some informational leakage from a small subgroup
attack on these groups. An IKE peer MUST check both that the peer's
public value is in range (1 < r < p-1) and that r**q = 1 mod p (where
q is the size of the subgroup, as listed in the RFC).
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2.3. Elliptic Curve Groups: Group 19-21, 25,26
IKEv2 groups 19-21, 25-26 are elliptic curve groups defined over a
field GF(p). According to [Menezes], Sec. 2.3, there is some
informational leakage possible. A receiving peer MUST check that its
peer's public value is valid; that is, it is not the point-at-
infinity, and that the x and y parameters from the peer's public
value satisfy the curve equation, that is, y**2 = x**3 + ax + b mod p
(where for groups 19, 20, 21, a=-3, and all other values of a, b and
p for the group are listed in the RFC).
2.4. Transition
Existing implementations of IKEv2 with ECDH groups MAY be modified to
include the tests described in the current document, provided they do
not reuse DH keys with multiple peers. The tests can be considered
as sanity checks, and will prevent the code having to handle inputs
that it may not have been designed to handle.
ECDH implementations that do reuse DH keys MUST be enhanced to
include the above tests.
3. Security Considerations
This entire document is concerned with the IKEv2 security protocol
and the need to harden it in some cases.
3.1. DH Key Reuse and Multiple Peers
This section describes the attack prevented by the tests defined
here.
Suppose that IKE peer Alice maintains IKE security associations with
peers Bob and Eve. Alice uses the same secret ECDH key for both SAs,
which is allowed with some restrictions. If Alice does not implement
these tests, Eve will be able to send a malformed public key, which
would allow her to efficiently determine Alice's secret key (as
described in Sec. 2 of [Menezes]). Since the key is shared, Eve will
be able to obtain Alice's shared IKE SA key with Bob.
4. IANA Considerations
This document does not have any IANA actions.
All the Diffie-Hellman groups mentioned here are managed by IANA.
Future documents that define new DH groups for use by IKEv2 MUST
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reference this document and describe which of the tests listed here
are applicable, or whether other tests are required.
5. Acknowledgements
We would like to thank Dan Harkins who initially raised this issue on
the ipsec mailing list.
The document was prepared using the lyx2rfc tool, created by Nico
Williams.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
6.2. Informative References
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003.
[RFC5114] Lepinski, M. and S. Kent, "Additional Diffie-Hellman
Groups for Use with IETF Standards", RFC 5114,
January 2008.
[Menezes] Menezes, A. and B. Ostaoglu, "On Reusing Ephemeral Keys In
Diffie-Hellman Key Agreement Protocols", December 2008, <
http://www.cacr.math.uwaterloo.ca/techreports/2008/
cacr2008-24.pdf>.
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Authors' Addresses
Yaron Sheffer
Porticor
10 Yirmiyahu St.
Ramat HaSharon 47298
Israel
Email: yaronf.ietf@gmail.com
Scott Fluhrer
Cisco Systems
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
Email: sfluhrer@cisco.com
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