Network Working Group P. Hoffman Internet-Draft VPN Consortium Expires: July 27, 2007 January 23, 2007 Use of Hash Algorithms in IKE and IPsec draft-hoffman-ike-ipsec-hash-use-04.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on July 27, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract This document describes how the IKEv1, IKEv2, and IPsec protocols use hash functions, and explains the level of vulnerability of these protocols to the reduced collision resistance of the MD5 and SHA-1 hash algorithms. Hoffman Expires July 27, 2007 [Page 1] Internet-Draft IKE and IPsec Hash Use January 2007 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Hashes in IKEv1 and IKEv2 . . . . . . . . . . . . . . . . . . 3 3. Hashes in IPsec . . . . . . . . . . . . . . . . . . . . . . . 4 4. PKIX Certificates in IKEv1 and IKEv2 . . . . . . . . . . . . . 4 5. Choosing Cryptographic Functions . . . . . . . . . . . . . . . 4 5.1. Multiple Cryptographic Functions . . . . . . . . . . . . . 5 5.2. Specifying Cryptographic Functions in the Protocol . . . . 5 5.3. Specifying Cryptographic Functions in Authentication . . . 6 6. Suggested Changes . . . . . . . . . . . . . . . . . . . . . . 7 6.1. Suggested Changes for the Protocols . . . . . . . . . . . 7 6.2. Suggested Changes for Implementors . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 8. Informative References . . . . . . . . . . . . . . . . . . . . 8 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9 Appendix B. PKIX issues . . . . . . . . . . . . . . . . . . . . . 9 Appendix C. Changes between versions . . . . . . . . . . . . . . 10 C.1. Changes between -00 and -01 . . . . . . . . . . . . . . . 10 C.2. Changes between -01 and -02 . . . . . . . . . . . . . . . 11 C.3. Changes between -02 and -03 . . . . . . . . . . . . . . . 11 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . . . . 12 Hoffman Expires July 27, 2007 [Page 2] Internet-Draft IKE and IPsec Hash Use January 2007 1. Introduction Recently, attacks on the collision-resistance properties of MD5 and SHA-1 hash functions have been discovered; [HashAttacks] summarizes the discoveries. The security community is now reexamining how various Internet protocols use hash functions. The goal of this reexamination is to be sure that the current usage is safe in the face of these new attacks, and whether protocols can easily use new hash functions when they become recommended. Different protocols use hash functions quite differently. Because of this, the IETF has asked for reviews of all protocols that use hash functions. This document reviews the many ways that three protocols (IKEv1 [IKEv1], IKEv2 [IKEv2], and IPsec [ESP] and [AH]) use hash functions. In this document, "IKEv1" refers to only "Phase 1" of IKEv1 and the agreement process. "IKEv2" refers to the IKE_SA_INIT and IKE_AUTH exchanges. "IPsec" refers to IP encapsulated in either AH or ESP. The intended status of this document is an Informational RFC that has been reviewed by the IETF. 2. Hashes in IKEv1 and IKEv2 Both IKEv1 and IKEv2 can use hash functions as pseudo-random functions (PRFs). The inputs to the PRFs always contain values from both the initiator and the responder that the other party cannot predict in advance; because of this, the use of hash functions in IKEv1 and IKEv2 are not susceptible to any known collision-reduction attack. IKEv1 also uses hash functions on the inputs to the PRF. The inputs are a combination of values from both the initiator and responder, and thus the hash function here is not susceptible to any known collision-reduction attack. In IKEv2, hashes are used as integrity protection for all messages after the IKE_SA_INIT Exchange. These hashes are used in HMACs, and are thus not susceptible to any known collision-reduction attack. Both IKEv1 and IKEv2 have authentication modes that use digital signatures. Digital signatures use hashes to make unique fingerprints of the message being signed. Such signatures are not susceptible to collision attacks because they are not intended to have any non-repudiation or third-party-verification functionality. Hoffman Expires July 27, 2007 [Page 3] Internet-Draft IKE and IPsec Hash Use January 2007 IKEv1 has two modes, "public key encryption" and "revised public key encryption", that use hashes to identify the public key used. The hash function here is used simply to reduce the size of the identifier. In IKEv2 with public-key certificates, a hash function is used for similar purposes, both for identifying the sender's public key and in identifying the trust roots. Using a collision- reduction attack, an individual could create two public keys that have the same hash value. This is not considered to be a useful attack because the same person holds both private keys. IKEv1 can be used together with NAT traversal support, as described in [NAT-T]; IKEv2 includes this NAT traversal support. In both of these cases, hash functions are used to obscure the IP addresses used by the initiator and/or the responder. The hash function here is not susceptible to any known collision-reduction attack. 3. Hashes in IPsec AH uses hash functions for authenticating packets; the same is true for ESP when ESP is using its own authentication. For both uses of IPsec, hash functions are always used in hashed MACs (HMACs). HMACs are not susceptible to any known collision-reduction attack. 4. PKIX Certificates in IKEv1 and IKEv2 Some implementations of IKEv1 and IKEv2 use PKIX certificates for authentication. Any weaknesses in PKIX certificates due to particular ways hash functions are used, or due to weaknesses in particular hash functions used in certificates, will be inherited in IKEv1 and IKEv2 implementations that use PKIX-based authentication. 5. Choosing Cryptographic Functions Recently, there has been more discussion in the IETF about the ability of one party in a protocol to tell the other party which cryptographic functions the first party prefers the second party to use. The discussion was spurred in part by [Deploying]. Although that paper focuses on hash functions, it is relevant to other cryptographic functions as well. There are (at least) three distinct subtopics related to choosing cryptographic functions in protocols: Hoffman Expires July 27, 2007 [Page 4] Internet-Draft IKE and IPsec Hash Use January 2007 o The ability to pick between multiple cryptographic functions instead of having just one specified in the protocol o If there are multiple functions, the ability to agree on which function will be used in the main protocol o The ability to suggest to the other party which kinds of cryptographic functions should be used in the other party's public key certificates 5.1. Multiple Cryptographic Functions Protocols that use cryptographic functions can either specify a single function, or can allow multiple functions. Protocols in the first category are susceptible to attack if the specified function is later found to be too weak for the stated purpose; protocols in the second category can usually avoid such attacks, but at a cost of increased protocol complexity. In the IETF, protocols that allow a choice of cryptographic functions are strongly preferred. IKEv1, IKEv2, and IPsec already allow multiple hash functions in every place where hash functions are used. 5.2. Specifying Cryptographic Functions in the Protocol Protocols that allow a choice of cryptographic functions need to have a way for all parties to agree on which function is going to be used. Some protocols, such as secure electronic mail, allow the initiator to simply pick a set of cryptographic functions; if the responder does not understand the functions used, the transmission fails. Other protocols allow for the two parties to agree on which cryptographic functions will be used. This is sometimes called "negotiation", but the term "negotiation" is inappropriate for protocols in which one party (the "proposer") lists all the functions it is willing to use, and the other party (the "chooser") simply picks the ones that will be used. When a new cryptographic function is introduced, one party may want to tell the other party that they can use the new function. If it is the proposer who wants to use the new function, the situation is easy: the proposer simply adds the new function in its list, possibly removing other parallel functions that the proposer no longer wants to use. On the other hand, if it is the chooser who wants to use the new function and the proposer didn't list it, the chooser may want to signal the proposer that they are capable of using the new function or the chooser may want to say that it is only willing to use the new function. If a protocol wants to handle either of these cases, it has to have a way for the chooser to specify this information to the Hoffman Expires July 27, 2007 [Page 5] Internet-Draft IKE and IPsec Hash Use January 2007 proposer in its acceptance and/or rejection message. It is not clear from a design standpoint how important it might be to let the chooser specify the additional functions it knows. As long as the proposer offers all the functions it wants to use, there is no reason for the chooser to say "I know one you don't know". The only place where the chooser being able to signal the proposer with different functions is in protocols where listing all the functions might be prohibitive, such as where they would add additional round trips or significant packet length. IKEv1 and IKEv2 allow the proposer to list all functions. Neither allows the chooser to specify which functions that were not proposed it could have used, either in a successful or unsuccessful SA establishment. 5.3. Specifying Cryptographic Functions in Authentication Passing public key certificates used in authentication creates additional issues for protocols. When specifying cryptographic functions for a protocol, it is an agreement between the proposer and the chooser. When choosing cryptographic functions for public key certificates, however, the proposer and the chooser are beholden to functions used by the trusted third parties, the certificate authorities (CAs). It doesn't really matter what either party wants the other party to use, since the other party is not the one issuing the certificates. In this discussion, the term "certificate" does not necessarily mean a PKIX certificate. Instead, it means any message that binds an identity to a public key, where the message is signed by a trusted third party. The question of specifying cryptographic functions is only relevant if one party has multiple certificates with different cryptographic functions. In this section, the terms "proposer" and "chooser" have a different meaning than in the previous section. Here, both parties act as proposers of the identity they want to use and the certificates with which they are backing up that identity, and both parties are choosers of the other party's identity and certificate. Some protocols allow the proposer to send multiple certificates, while other protocols only allow the proposer to send a single certificate. Some protocols allow the proposer to send multiple certificates but advise against it, given that certificates can be fairly large (particularly when the CA loads the certificate with lots of information). Hoffman Expires July 27, 2007 [Page 6] Internet-Draft IKE and IPsec Hash Use January 2007 IKEv1 and IKEv2 allow both parties to list all the certificates that they want to use. [PKI4IPsec] proposes to restrict this by saying that all the certificates for a proposer have to have the same identity. 6. Suggested Changes In investigating how protocols use hash functions, the IETF is looking at (at least) two areas of possible changes to individual protocols: how the IETF might need to change the protocols, and how implementors of current protocols might change what they do. This section describes both of this with respect to IKEv1, IKEv2, and IPsec. 6.1. Suggested Changes for the Protocols Protocols might need to be changed if they rely on the collision- resistance of particular hash functions. They might also need to be changed if they do not allow for agreement of hash functions because it is expected that the "preferred" hash function for different users will change over time. IKEv1 and IKEv2 already allow for the agreement of hash functions for both IKE and IPsec, and thus do not need any protocol change. IKEv1 and IKEv2, when used with public key authentication, already allow each party to send multiple PKIX certificates, and thus do not need any protocol change. There are known weaknesses in PKIX with respect to collision- resistance of some hash functions. Because of this, it is expected that there will be changes to PKIX fostered by the PKIX Working Group. Some of the changes to PKIX may be usable in IKEv1 and IKEv2 without having to change IKEv1 and IKEv2. Other changes to PKIX may require changes to IKEv1 and IKEv2 in order to incorporate them, but that will not be known until the changes to PKIX are finalized. 6.2. Suggested Changes for Implementors As described in earlier sections, IKE and IPsec themselves are not susceptible to any known collision-reduction attacks on hash functions. Thus, implementors do not need to make changes such as prohibiting the use of MD5 or SHA-1. The mandatory and suggested algorithms for IKEv2 and IPsec are given in [IKEv2Algs] and [IPsecAlgs]. Note that some IKE and IPsec users will misunderstand the relevance Hoffman Expires July 27, 2007 [Page 7] Internet-Draft IKE and IPsec Hash Use January 2007 of the known attacks and want to use "stronger" hash functions. Thus, implementors should strongly consider adding support for alternatives to hash functions, particularly the AES-XCBC-PRF-128 [AES-PRF] and AES-XCBC-MAC-96 [AES-MAC] algorithms. Implementations of IKEv1 and IKEv2 that use PKIX certificates for authentication may be susceptible to attacks based on weaknesses in PKIX. It is widely expected that PKIX certificates in the future will use hash functions other than MD5 and SHA-1. Implementers of IKE that allow certificate authentication should strongly consider allowing the use of certificates that are signed with the SHA-256 hash algorithm. Similarly, those implementers should also strongly consider allowing the sending of multiple certificates for identification. 7. Security Considerations This entire document is about security, namely, the security implications of reduced collision-resistance of common hash algorithms for the IKE and IPsec protocols. The Security Considerations section of [HashAttacks] gives much more detail about the security of hash functions. 8. Informative References [AES-MAC] Frankel, S. and H. Herbert, "The AES-XCBC-MAC-96 Algorithm and Its Use With IPsec", RFC 3566, September 2003. [AES-PRF] Hoffman, P., "The AES-XCBC-PRF-128 Algorithm for the Internet Key Exchange Protocol (IKE)", RFC 4434, February 2006. [AH] Kent, S., "IP Authentication Header", RFC 4302, December 2005. [Deploying] Bellovin, S. and E. Rescorla, "Deploying a New Hash Algorithm", NDSS '06, February 2006. [ESP] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [HashAttacks] Hoffman, P. and B. Schneier, "Attacks on Cryptographic Hashes in Internet Protocols", Hoffman Expires July 27, 2007 [Page 8] Internet-Draft IKE and IPsec Hash Use January 2007 draft-hoffman-hash-attacks-04 (work in progress), June 2005. [IKEv1] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [IKEv2] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. [IKEv2Algs] Schiller, J., "Cryptographic Algorithms for use in the Internet Key Exchange Version 2", RFC 4307, December 2005. [IPsecAlgs] Eastlake, D., "Cryptographic Algorithm Implementation Requirements For ESP And AH", RFC 4305, December 2005. [NAT-T] Kivinen, T., Swander, B., Huttunen, A., and V. Volpe, "Negotiation of NAT-Traversal in the IKE", RFC 3947, January 2005. [PKI4IPsec] Korver, B., "The Internet IP Security PKI Profile of IKEv1/ISAKMP, IKEv2, and PKIX", draft-ietf-pki4ipsec-ikecert-profile-08 (work in progress), February 2006. Appendix A. Acknowledgements Tero Kivinen helped with ideas in the first draft of this document. Many participants on the SAAG and IPsec mailing lists contributed ideas in later drafts. Appendix B. PKIX issues IMPORTANT NOTE: This section will be removed in a future version of this Internet Draft, certainly before it becomes an RFC. It is left here to help whomever will be writing the document on PKIX hash use. As described in [HashAttacks], if a certificate authority (CA) issues certificates where the requesting party can predict the serial number and expiration date of the to-be-issued certificate, the requesting party can get a certificate that has a "shadow" certificate that has the same identity but different signing keys. This is not considered to be a useful attack in IKEv1 or IKEv2: the relying party views the attacker as the same entity because the identity is the same in both Hoffman Expires July 27, 2007 [Page 9] Internet-Draft IKE and IPsec Hash Use January 2007 certificates. There is speculation that this attack could be extended to allow a requesting party to get a certificate that has a "shadow" certificate that has a different identity (and possibly a different signing key). To date, there have been no examples of this in the cryptographic literature, and there have not even been any papers showing whether or not such an attack is even possible under the limitations of the current collision-reducing attacks. However, if such an attack is possible, it could have significant effects on protocols that rely on PKIX certificates. There are two possibilities of such a future attack. If the second identity is valid but is a bunch of noise, the only implementations affected by the attack are ones whose policy is "accept anyone with a certificate signed by the CA"; this policy is rarely used in practice. If the second identity is valid and is of another user who might normally be authenticated in the implementation, the attack is obviously, much more worrisome. Appendix C. Changes between versions (This section is to be removed by the RFC Editor.) C.1. Changes between -00 and -01 Added that IKEv2 uses hash functions for its own integrity protection, based on a suggestion by Michael Richardson. Added description of use of digital signatures in IKEv1 and IKEv2, based on a suggestion from Hugo Krawczyk. Removed the description of specific PKIX attacks and remedies, and replaced it with pointers to the future PKIX work. Added a section in the suggested changes to the protocols pointing out that some changes that come from the PKIX world will require no changes to IKE, but others might. Added the long section on "Choosing Cryptographic Functions" based on a suggestion from Steve Bellovin. Added references for AES-XCBC-PRF-128 and AES-XCBC-MAC-96, based on a suggestion from David McGrew. Strengthened the PKIX discussion that is now in an appendix, but will later be removed when the PKIX document is started. Hoffman Expires July 27, 2007 [Page 10] Internet-Draft IKE and IPsec Hash Use January 2007 C.2. Changes between -01 and -02 A bunch of editorial fixes from Alfred Hoenes. Updated the references to the new RFCs. C.3. Changes between -02 and -03 None; just a freshening. Author's Address Paul Hoffman VPN Consortium 127 Segre Place Santa Cruz, CA 95060 US Email: paul.hoffman@vpnc.org Hoffman Expires July 27, 2007 [Page 11] Internet-Draft IKE and IPsec Hash Use January 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Hoffman Expires July 27, 2007 [Page 12]