Internet DRAFT - draft-ietf-ipsecme-dh-checks
draft-ietf-ipsecme-dh-checks
ipsecme Y. Sheffer
Internet-Draft Porticor
Updates: 5996 (if approved) S. Fluhrer
Intended status: Standards Track Cisco
Expires: December 6, 2013 June 4, 2013
Additional Diffie-Hellman Tests for IKEv2
draft-ietf-ipsecme-dh-checks-05
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 6, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . 3
2. Group Membership Tests . . . . . . . . . . . . . . . . 3
2.1. Sophie Germain Prime MODP Groups . . . . . . . . . . . 3
2.2. MODP Groups with Small Subgroups . . . . . . . . . . . 4
2.3. Elliptic Curve Groups . . . . . . . . . . . . . . . . 4
2.4. Transition . . . . . . . . . . . . . . . . . . . . . . 5
2.5. Protocol Behavior . . . . . . . . . . . . . . . . . . 5
3. Side-Channel Attacks . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . 6
4.1. DH Key Reuse and Multiple Peers . . . . . . . . . . . 7
4.2. DH Key Reuse: Variants . . . . . . . . . . . . . . . . 7
4.3. Groups not covered by this RFC . . . . . . . . . . . . 7
4.4. Behavior Upon Test Failure . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . 9
Appendix A. Appendix: Change Log . . . . . . . . . . . . . . . . . 10
A.1. -05 . . . . . . . . . . . . . . . . . . . . . . . . . 10
A.2. -04 . . . . . . . . . . . . . . . . . . . . . . . . . 10
A.3. -03 . . . . . . . . . . . . . . . . . . . . . . . . . 10
A.4. -02 . . . . . . . . . . . . . . . . . . . . . . . . . 10
A.5. -01 . . . . . . . . . . . . . . . . . . . . . . . . . 10
A.6. -00 . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . 11
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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].
IKEv2 does not require that any tests be performed by a peer
receiving a public Diffie-Hellman key from the other peer. This is
fine for the common case of MODP groups. For other DH groups, when
peers reuse DH values across multiple IKE sessions, the lack of tests
by the recipient results in a potential vulnerability (see
Section 4.1 for more details). In particular, this is true for
Elliptic Curve (EC) groups whose use is becoming ever more popular.
This document defines such tests for several types of DH groups.
In addition, this document describes another potential attack related
to reuse of DH keys: a timing attack. This additional material is
taken from [RFC2412].
This document updates [RFC5996] by adding security requirements that
apply to many of the protocol's implementations.
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 private keys (as defined in [RFC5996], Sec. 2.12). The
tests apply to the recipient of a KE payload, and describe how it
should check the received payload. They are listed here according to
the DH group being used.
2.1. Sophie Germain Prime MODP Groups
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. Each recipient MUST verify that the peer's
public value r is in the legal range (1 < r < p-1). According to
[Menezes], Sec 2.2, even with this check there remains the
possibility of leaking a single bit of the secret exponent when DH
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keys are reused; this amount of leakage is insignificant.
See Section 5 for the specific groups covered by this section.
2.2. MODP Groups with Small Subgroups
[RFC5114] defines modular exponential groups with small subgroups;
these are modular exponential groups with comparatively small
subgroups, and all have (p-1)/2 composite. Sec. 2.1 of [Menezes]
describes some informational leakage from a small subgroup attack on
these groups, if the DH private value is reused.
This leakage can be prevented if the recipient performs a test on the
peer's public value, however this test is expensive (approximately as
expensive as what reusing DH private values saves). In addition, the
NIST standard [NIST-800-56A] requires that test (see section
5.6.2.4), hence anyone needing to conform to that standard will need
to implement the test anyway.
Because of the above, the IKE implementation MUST choose between one
of the following two options:
o It 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). DH private values MAY then be
reused. This option is appropriate if conformance to
[NIST-800-56A] is required.
o It MUST NOT reuse DH private values (that is, the DH private value
for each DH exchange MUST be generated from a fresh output of a
cryptographically secure random number generator), and it MUST
check that the peer's public value is in range (1 < r < p-1).
This option is more appropriate if conformance to [NIST-800-56A]
is not required.
See Section 5 for the specific groups covered by this section.
2.3. Elliptic Curve Groups
IKEv2 can be used with elliptic curve groups defined over a field
GF(p) [RFC5903] [RFC5114]. 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, the x and y
parameters from the peer's public value satisfy the curve equation,
y^2 = x^3 + ax + b mod p (where for groups 19, 20, 21, a=-3 (mod p),
and all other values of a, b and p for the group are listed in the
RFC).
We note that an additional check to ensure that the public value is
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not the point at infinity is not needed, because IKE (in Sec. 7 of
[RFC5903]) does not allow for encoding this value.
See Section 5 for the specific groups covered by this section.
2.4. Transition
Existing implementations of IKEv2 with ECDH groups may be modified to
include the tests described in the current document, even if they do
not reuse DH keys. 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.
2.5. Protocol Behavior
The recipient of a DH public key that fails one of the above tests
must assume that the sender is either truly malicious or else it has
a bug in its implementation. The behavior defined below attempts to
balance resistance to attackers that are trying to disrupt the IKE
exchange, against the need to help a badly implemented peer by
providing useful error indications.
If this error happens during the IKE_SA_INIT exchange, then the
recipient MUST drop the message that contains an invalid KE payload,
and MUST NOT use that message when creating the IKE SA.
If the implementation implements the DoS-resistant behavior proposed
in Sec. 2.4 of [RFC5996], it may simply ignore the erroneous request
or response message, and continue waiting for a later message
containing a legitimate KE payload.
If DoS-resistant behavior is not implemented, and the invalid KE
payload was in the IKE_SA_INIT request, the implementation MAY send
an INVALID_SYNTAX error notification back, and remove the in-progress
IKE SA; if the invalid KE payload was in the IKE_SA_INIT response,
then the implementation MAY simply delete the half created IKE SA,
and re-initiate the exchange.
If the invalid KE payload is received during the CREATE_CHILD_SA
exchange (or any other exchange after the IKE SA has been
established) and the invalid KE payload is in the request message,
the Responder MUST reply with an INVALID_SYNTAX error notification
and drop the IKE SA. If the invalid KE payload is in a response, the
Initiator getting this reply MUST immediately delete the IKE SA by
sending an IKE SA Delete notification as a new exchange. In this
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case the sender evidently has an implementation bug, and dropping the
IKE SA makes it easier to detect.
3. Side-Channel Attacks
In addition to the small-subgroup attack, there is also a potential
timing attack on IKE peers when they are reusing Diffie-Hellman
secret values. This is a side-channel attack, which means that it
may or may not be a vulnerability in certain cases, depending on
implementation details and the threat model.
The remainder of this section is quoted from [RFC2412], Sec. 5, with
a few minor clarifications. This attack still applies to IKEv2
implementations, and both to MODP groups and ECDH groups. We also
note that more efficient countermeasures are available for EC groups
represented in projective form, but these are outside the scope of
the current document.
Timing attacks that are capable of recovering the exponent value used
in Diffie-Hellman calculations have been described by Paul Kocher
[Kocher]. In order to nullify the attack, implementors must take
pains to obscure the sequence of operations involved in carrying out
modular exponentiations.
One potential method to foil these timing attacks is to use a
"blinding factor". In this method, a group element, r, is chosen at
random, and its multiplicative inverse modulo p is computed, which
we'll call r_inv. r_inv can be computed by the Extended Euclidean
Method, using r and p as inputs. When an exponent x is chosen, the
value r_inv^x is also calculated. Then, when calculating (g^y)^x,
the implementation will calculate this sequence:
A = r*g^y
B = A^x = (r*g^y)^x = (r^x)(g^(xy))
C = B*r_inv^x = (r^x)(r^(-1*x))(g^(xy)) = g^(xy)
The blinding factor is only necessary if the exponent x is used more
than 100 times (estimate by Richard Schroeppel).
4. Security Considerations
This entire document is concerned with the IKEv2 security protocol
and the need to harden it in some cases.
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4.1. DH Key Reuse and Multiple Peers
This section describes one variant of the attack prevented by the
tests defined above.
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 private 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.
4.2. DH Key Reuse: Variants
Private DH keys can be reused in different ways, with subtly
different security implications. For example:
1. DH keys are reused for multiple connections (IKE SAs) to the same
peer, and for connections to different peers.
2. DH keys are reused for multiple connections to the same peer
(e.g. when the peer is identified by its IP address) but not for
different peers.
3. DH keys are reused only when they had not been used to complete
an exchange, e.g. when the peer replies with an
INVALID_KE_PAYLOAD notification.
Both the small subgroup attack and the timing attack described in
this document apply at least to options #1 and #2.
4.3. Groups not covered by this RFC
There are a number of group types that are not specifically addressed
by this RFC. A document that defines such a group MUST describe the
tests required by that group.
One specific type of group would be an even-characteristic elliptic
curve group. Now, these curves have cofactors greater than 1; this
leads to a possibility of some information leakage. There are
several ways to address this information leakage, such as performing
a test analogous to the test in section 2.2, or adjusting the ECDH
operation to avoid this leakage (such as "ECC CDH", where the shared
secret really is hxyG). Because the appropriate test depends on how
the group is defined, we cannot document it in advance.
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4.4. Behavior Upon Test Failure
The behavior recommended in Section 2.5 is in line with generic error
treatment during the IKE_SA_INIT exchange, Sec. 2.21.1 of [RFC5996].
The sender is not required to send back an error notification, and
the recipient cannot depend on this notification because it is
unauthenticated, and may in fact have been sent by an attacker trying
to DoS the connection. Thus, the notification is only useful to
debug implementation errors.
On the other hand, the error notification is secure in the sense that
no secret information is leaked. All IKEv2 Diffie-Hellman groups are
publicly known, and none of the tests defined here depend on any
private key. In fact the tests can all be performed by an
eavesdropper.
The situation when the failure occurs in the Create Child SA exchange
is different, since everything is protected by an IKE SA. The peers
are authenticated, and error notifications can be relied on. See
Sec. 2.21.3 of [RFC5996] for more details on error handling in this
case.
5. IANA Considerations
This document requests that IANA should add a column named "Recipient
Tests" to the IKEv2 DH Group Transform IDs Registry
[IANA-DH-Registry].
This column should initially be populated as per the following table.
+------------------------------------+---------------------+
| Number | Recipient Tests |
+------------------------------------+---------------------+
| 1, 2, 5, 14, 15, 16, 17, 18 | [current], Sec. 2.1 |
| 22, 23, 24 | [current], Sec. 2.2 |
| 19, 20, 21, 25, 26, 27, 28, 29, 30 | [current], Sec. 2.3 |
+------------------------------------+---------------------+
Note to RFC Editor: please replace [current] by the RFC number
assigned to this document.
Groups 27-30 have been recently defined in
[I-D.merkle-ikev2-ke-brainpool].
Future documents that define new DH groups for IKEv2 are REQUIRED to
provide this information for each new group, possibly by referring to
the current document.
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6. Acknowledgements
We would like to thank Dan Harkins who initially raised this issue on
the ipsec mailing list. Thanks to Tero Kivinen and Rene Struik for
their useful comments. Much of the text in Section 3 is taken from
[RFC2412] and we would like to thank its author, Hilarie Orman.
The document was prepared using the lyx2rfc tool, created by Nico
Williams.
7. References
7.1. 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.
7.2. Informative References
[RFC2412] Orman, H., "The OAKLEY Key Determination Protocol",
RFC 2412, November 1998.
[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.
[RFC5903] Fu, D. and J. Solinas, "Elliptic Curve Groups modulo a
Prime (ECP Groups) for IKE and IKEv2", RFC 5903,
June 2010.
[I-D.merkle-ikev2-ke-brainpool]
Merkle, J. and M. Lochter, "Using the ECC Brainpool Curves
for IKEv2 Key Exchange",
draft-merkle-ikev2-ke-brainpool-06 (work in progress),
April 2013.
[NIST-800-56A]
National Institute of Standards and Technology (NIST),
"Recommendation for Pair-Wise Key Establishment Schemes
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Using Discrete Logarithm Cryptography (Revised)", NIST PUB
800-56A, March 2007.
[Kocher] Kocher, P., "Timing Attacks on Implementations of Diffie-
Hellman, RSA, DSS, and Other Systems", December 1996,
<http://www.cryptography.com/timingattack/paper.html>.
[Menezes] Menezes, A. and B. Ustaoglu, "On Reusing Ephemeral Keys In
Diffie-Hellman Key Agreement Protocols", December 2008, <h
ttp://www.cacr.math.uwaterloo.ca/techreports/2008/
cacr2008-24.pdf>.
[IANA-DH-Registry]
IANA, "Internet Key Exchange Version 2 (IKEv2) Parameters,
Transform Type 4 - Diffie-Hellman Group Transform IDs",
Jan. 2005, <http://www.iana.org/assignments/
ikev2-parameters/ikev2-parameters.xml#ikev2-parameters-8>.
Appendix A. Appendix: Change Log
Note to RFC Editor: please remove this section before publication.
A.1. -05
o Resolved IESG members' comments.
A.2. -04
o Implemented Sean's AD review, and removed the inapplicable
requirement on the point at infinity.
A.3. -03
o Added the Brainpool curves to the IANA registration table.
A.4. -02
o Based on Tero's review: Improved the protocol behavior, and
mentioned that these checks apply to Create Child SA. Added a
discussion of DH timing attacks, stolen from RFC 2412.
A.5. -01
o Corrected an author's name that was misspelled.
o Added recipient behavior if a test fails, and the related security
considerations.
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A.6. -00
o First WG document.
o Clarified IANA actions.
o Discussion of potential future groups not covered here.
o Clarification re: practicality of recipient tests for DSA groups.
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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