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rfc:rfc4894

Network Working Group P. Hoffman Request for Comments: 4894 VPN Consortium Category: Informational May 2007

  Use of Hash Algorithms in Internet Key Exchange (IKE) and IPsec

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The IETF Trust (2007).

Abstract

 This document describes how the IKEv1 (Internet Key Exchange version
 1), 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.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
 2.  Hashes in IKEv1 and IKEv2  . . . . . . . . . . . . . . . . . .  2
 3.  Hashes in IPsec  . . . . . . . . . . . . . . . . . . . . . . .  3
 4.  PKIX Certificates in IKEv1 and IKEv2 . . . . . . . . . . . . .  3
 5.  Choosing Cryptographic Functions . . . . . . . . . . . . . . .  3
   5.1.  Different Cryptographic Functions  . . . . . . . . . . . .  4
   5.2.  Specifying Cryptographic Functions in the Protocol . . . .  4
   5.3.  Specifying Cryptographic Functions in Authentication . . .  5
 6.  Suggested Changes  . . . . . . . . . . . . . . . . . . . . . .  6
   6.1.  Suggested Changes for the Protocols  . . . . . . . . . . .  6
   6.2.  Suggested Changes for Implementors . . . . . . . . . . . .  7
 7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
 8.  Informative References . . . . . . . . . . . . . . . . . . . .  8
 Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 10

Hoffman Informational [Page 1] RFC 4894 IKE and IPsec Hash Use May 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 the
 Authentication Header (AH) or Encapsulating Security Payload (ESP).

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 nonce values
 from both the initiator and the responder that the other party cannot
 predict in advance.  In IKEv1, the length of this nonce is at least
 64 bits; in IKEv2, it is at least 128 bits.  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 Hashed
 Message Authentication Codes (HMACs).  As described in
 [HMAC-reduction], MD5 used in HMACs is susceptible to forgery, and it
 is suspected that full SHA-1 used in HMAC is susceptible to forgery.
 There is no known reason for the person who creates legitimate
 integrity protection to want to spoof it.
 Both IKEv1 and IKEv2 have authentication modes that use digital
 signatures.  Digital signatures use hashes to make unique digests of
 the message being signed.  With the current known attacks, the only
 party that can create the two messages that collide is the IKE entity

Hoffman Informational [Page 2] RFC 4894 IKE and IPsec Hash Use May 2007

 that generates the message.  As shown in [Target-collisions], an
 attacker can create two different Public Key Infrastructure using
 X.509 (PKIX) certificates with different identities that have the
 same signatures.
 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 the trust anchors.  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
 key generator holds both private keys.
 IKEv1 can be used together with Network Access Translator (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).  As
 described in [HMAC-reduction], MD5 used in HMACs is susceptible to
 forgery, and it is suspected that full SHA-1 used in HMAC is
 susceptible to forgery.  There is no known reason for the person who
 creates legitimate packet authentication to want to spoof it.

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.

Hoffman Informational [Page 3] RFC 4894 IKE and IPsec Hash Use May 2007

 There are (at least) three distinct subtopics related to choosing
 cryptographic functions in protocols:
 o  The ability to pick between different 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. Different Cryptographic Functions

 Protocols that use cryptographic functions can either specify a
 single function, or can allow different 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 different hash functions in
 every significant place where hash functions are used (that is, in
 every place that has any susceptibility to a collision-reduction
 attack).

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 to its list, possibly

Hoffman Informational [Page 4] RFC 4894 IKE and IPsec Hash Use May 2007

 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
 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 is 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 Security
 Association (SA) establishment.

5.3. Specifying Cryptographic Functions in Authentication

 Passing public key certificates and signatures 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
 certification 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.  This can be non-PKIX certificates or other types of
 cryptographic identity-binding structures that may be used in the
 future.
 The question of specifying cryptographic functions is only relevant
 if one party has multiple certificates or signatures with different
 cryptographic functions.  In this section, the terms "proposer" and
 "chooser" have a different meaning than in the previous section.

Hoffman Informational [Page 5] RFC 4894 IKE and IPsec Hash Use May 2007

 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 or
 signatures, while other protocols only allow the proposer to send a
 single certificate or signature.  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).
 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 these areas 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 the 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 hoped 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.

Hoffman Informational [Page 6] RFC 4894 IKE and IPsec Hash Use May 2007

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
 of the known attacks and want to use "stronger" hash functions.
 Thus, implementors should strongly consider adding support for
 alternatives, particularly the AES-XCBC-PRF-128 [AES-PRF] and AES-
 XCBC-MAC-96 [AES-MAC] algorithms, as well as forthcoming algorithms
 based on the SHA-2 family [SHA2-HMAC].
 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.  Implementors of
 IKE that allow certificate authentication should strongly consider
 allowing the use of certificates that are signed with the SHA-256,
 SHA-384, and SHA-512 hash algorithms.  Similarly, those implementors
 should also strongly consider allowing the sending of multiple
 certificates for identification.

7. Security Considerations

 This entire document is about 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.

Hoffman Informational [Page 7] RFC 4894 IKE and IPsec Hash Use May 2007

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",
                      RFC 4270, November 2005.
 [HMAC-reduction]     Contini, S. and YL. Yin, "Forgery and Partial
                      Key-Recovery Attacks on HMAC and NMAC Using Hash
                      Collisions", Cryptology ePrint Report 2006/319,
                      September 2006.
 [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.

Hoffman Informational [Page 8] RFC 4894 IKE and IPsec Hash Use May 2007

 [PKI4IPsec]          Korver, B., "The Internet IP Security PKI
                      Profile of IKEv1/ISAKMP, IKEv2, and PKIX", Work
                      in Progress, April 2007.
 [SHA2-HMAC]          Kelly, S. and S. Frankel, "Using HMAC-SHA-256,
                      HMAC-SHA-384, and HMAC-SHA-512 With IPsec",
                      RFC 4868, May 2007.
 [Target-collisions]  Stevens, M., Lenstra, A., and B. de Weger,
                      "Target Collisions for MD5 and Colliding X.509
                      Certificates for Different Identities",
                      Cryptology ePrint Report 2006/360, October 2006.

Hoffman Informational [Page 9] RFC 4894 IKE and IPsec Hash Use May 2007

Appendix A. Acknowledgments

 Tero Kivinen helped with ideas in the first version of this document.
 Many participants on the SAAG and IPsec mailing lists contributed
 ideas in later versions.  In particular, suggestions were made by
 Alfred Hoenes, Michael Richardson, Hugo Krawczyk, Steve Bellovin,
 David McGrew, Russ Housley, Arjen Lenstra, and Pasi Eronen.

Author's Address

 Paul Hoffman
 VPN Consortium
 127 Segre Place
 Santa Cruz, CA  95060
 US
 EMail: paul.hoffman@vpnc.org

Hoffman Informational [Page 10] RFC 4894 IKE and IPsec Hash Use May 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.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

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Hoffman Informational [Page 11]

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