ipsecme Y. Sheffer Internet-Draft Porticor Updates: 5996 (if approved) S. Fluhrer Intended status: Standards Track Cisco Expires: October 24, 2013 April 22, 2013 Additional Diffie-Hellman Tests for IKEv2 draft-ietf-ipsecme-dh-checks-03 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 October 24, 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 October 24, 2013 [Page 1] Internet-Draft DH Tests April 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. 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 . . . . . . . . . . . . . . . . . 5 4. Security Considerations . . . . . . . . . . . . . . . 6 4.1. DH Key Reuse and Multiple Peers . . . . . . . . . . . 6 4.2. DH Key Reuse: Variants . . . . . . . . . . . . . . . . 7 4.3. Groups not covered by this RFC . . . . . . . . . . . . 7 4.4. Behavior Upon Test Failure . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . 8 6. Acknowledgements . . . . . . . . . . . . . . . . . . . 8 7. References . . . . . . . . . . . . . . . . . . . . . . 9 7.1. Normative References . . . . . . . . . . . . . . . . . 9 7.2. Informative References . . . . . . . . . . . . . . . . 9 Appendix A. Appendix: Change Log . . . . . . . . . . . . . . . . . 10 A.1. -03 . . . . . . . . . . . . . . . . . . . . . . . . . 10 A.2. -02 . . . . . . . . . . . . . . . . . . . . . . . . . 10 A.3. -01 . . . . . . . . . . . . . . . . . . . . . . . . . 10 A.4. -00 . . . . . . . . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . 10 Sheffer & Fluhrer Expires October 24, 2013 [Page 2] Internet-Draft DH Tests April 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 4.1 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. In addition, this document describes another potential attack related to reuse of DH keys: a timing attack. This additional material is taken from [RFC2412]. 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 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 keys are reused; this amount of leakage is insignificant. See Section 5 for the specific groups covered by this section. Sheffer & Fluhrer Expires October 24, 2013 [Page 3] Internet-Draft DH Tests April 2013 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, 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 (mod p), and all other values of a, b and p for the group are listed in the RFC). See Section 5 for the specific groups covered by this section. Sheffer & Fluhrer Expires October 24, 2013 [Page 4] Internet-Draft DH Tests April 2013 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 can assume that the sender is either truly malicious or else it has a bug in its implementation. 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 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 Sheffer & Fluhrer Expires October 24, 2013 [Page 5] Internet-Draft DH Tests April 2013 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 ECC 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. 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 secret key (as Sheffer & Fluhrer Expires October 24, 2013 [Page 6] Internet-Draft DH Tests April 2013 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. 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 Sheffer & Fluhrer Expires October 24, 2013 [Page 7] Internet-Draft DH Tests April 2013 on any secret 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. 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. The document was prepared using the lyx2rfc tool, created by Nico Williams. 7. References Sheffer & Fluhrer Expires October 24, 2013 [Page 8] Internet-Draft DH Tests April 2013 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-04 (work in progress), April 2013. [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. [Kocher] Kocher, P., "Timing Attacks on Implementations of Diffie- Hellman, RSA, DSS, and Other Systems", December 1996, . [Menezes] Menezes, A. and B. Ustaoglu, "On Reusing Ephemeral Keys In Diffie-Hellman Key Agreement Protocols", December 2008, . [IANA-DH-Registry] Sheffer & Fluhrer Expires October 24, 2013 [Page 9] Internet-Draft DH Tests April 2013 IANA, "Internet Key Exchange Version 2 (IKEv2) Parameters, Transform Type 4 - Diffie-Hellman Group Transform IDs", Jan. 2005, . Appendix A. Appendix: Change Log Note to RFC Editor: please remove this section before publication. A.1. -03 o Added the Brainpool curves to the IANA registration table. A.2. -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.3. -01 o Corrected an author's name that was misspelled. o Added recipient behavior if a test fails, and the related security considerations. A.4. -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 October 24, 2013 [Page 10] Internet-Draft DH Tests April 2013 Scott Fluhrer Cisco Systems 1414 Massachusetts Ave. Boxborough, MA 01719 USA Email: sfluhrer@cisco.com Sheffer & Fluhrer Expires October 24, 2013 [Page 11]