[Docs] [txt|pdf|xml] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

Versions: (draft-sheffer-ipsecme-dh-checks) 00 01 02 03 04 05 RFC 6989

ipsecme                                                       Y. Sheffer
Internet-Draft                                                  Porticor
Updates: 5996 (if approved)                                   S. Fluhrer
Intended status: Standards Track                                   Cisco
Expires: August 2, 2013                                 January 29, 2013


               Additional Diffie-Hellman Tests for IKEv2
                    draft-ietf-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

   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 August 2, 2013.

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



Sheffer & Fluhrer        Expires August 2, 2013                 [Page 1]

Internet-Draft                  DH Tests                    January 2013


   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.        Regular MODP Groups . . . . . . . . . . . . . . . . . . 3
   2.2.        MODP Groups with Small Subgroups  . . . . . . . . . . . 3
   2.3.        Elliptic Curve Groups . . . . . . . . . . . . . . . . . 4
   2.4.        Transition  . . . . . . . . . . . . . . . . . . . . . . 4
   3.          Security Considerations . . . . . . . . . . . . . . . . 5
   3.1.        DH Key Reuse and Multiple Peers . . . . . . . . . . . . 5
   3.2.        Groups not covered by this RFC  . . . . . . . . . . . . 5
   4.          IANA Considerations . . . . . . . . . . . . . . . . . . 5
   5.          Acknowledgements  . . . . . . . . . . . . . . . . . . . 6
   6.          References  . . . . . . . . . . . . . . . . . . . . . . 6
   6.1.        Normative References  . . . . . . . . . . . . . . . . . 6
   6.2.        Informative References  . . . . . . . . . . . . . . . . 6
   Appendix A. Appendix: Change Log  . . . . . . . . . . . . . . . . . 7
   A.1.        -00 . . . . . . . . . . . . . . . . . . . . . . . . . . 7
               Authors' Addresses  . . . . . . . . . . . . . . . . . . 7




























Sheffer & Fluhrer        Expires August 2, 2013                 [Page 2]

Internet-Draft                  DH Tests                    January 2013


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 3.1 for more details).  In particular, this is true for
   elliptic curve groups whose use is becoming ever more popular.  This
   document defines such tests for several types of DH groups.

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

   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
   keys are reused; this amount of leakage is insignificant.

   See Section 4 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]



Sheffer & Fluhrer        Expires August 2, 2013                 [Page 3]

Internet-Draft                  DH Tests                    January 2013


   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 anyways.

   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 4 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, 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).

   See Section 4 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, if 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.




Sheffer & Fluhrer        Expires August 2, 2013                 [Page 4]

Internet-Draft                  DH Tests                    January 2013


   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.

3.2.  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.


4.  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.






Sheffer & Fluhrer        Expires August 2, 2013                 [Page 5]

Internet-Draft                  DH Tests                    January 2013


           +-----------------------------+---------------------+
           |            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     | [current], Sec. 2.3 |
           +-----------------------------+---------------------+

   Note to RFC Editor: please replace [current] by the RFC number
   assigned to this document.

   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.


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.

   [RFC5903]  Fu, D. and J. Solinas, "Elliptic Curve Groups modulo a
              Prime (ECP Groups) for IKE and IKEv2", RFC 5903,



Sheffer & Fluhrer        Expires August 2, 2013                 [Page 6]

Internet-Draft                  DH Tests                    January 2013


              June 2010.

   [NIST-800-56A]
              National Institute of Standards and Technology (NIST),
              "Recommendation for Pair-Wise Key Establishment Schemes
              Using Discrete Logarithm Cryptography (Revised)", NIST PUB
              800-56A, March 2007.

   [Menezes]  Menezes, A. and B. Ostaoglu, "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.  -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










Sheffer & Fluhrer        Expires August 2, 2013                 [Page 7]

Internet-Draft                  DH Tests                    January 2013


   Scott Fluhrer
   Cisco Systems
   1414 Massachusetts Ave.
   Boxborough, MA  01719
   USA

   Email: sfluhrer@cisco.com












































Sheffer & Fluhrer        Expires August 2, 2013                 [Page 8]


Html markup produced by rfcmarkup 1.108, available from http://tools.ietf.org/tools/rfcmarkup/