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Network Working Group                                         P. HoffmanRequest for Comments: 4270                                VPN ConsortiumCategory: Informational                                      B. Schneier                                           Counterpane Internet Security                                                           November 2005Attacks on Cryptographic Hashes in Internet ProtocolsStatus 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 Internet Society (2005).Abstract   Recent announcements of better-than-expected collision attacks in   popular hash algorithms have caused some people to question whether   common Internet protocols need to be changed, and if so, how.  This   document summarizes the use of hashes in many protocols, discusses   how the collision attacks affect and do not affect the protocols,   shows how to thwart known attacks on digital certificates, and   discusses future directions for protocol designers.1.  Introduction   In summer 2004, a team of researchers showed concrete evidence that   the MD5 hash algorithm was susceptible to collision attacks   [MD5-attack].  In early 2005, the same team demonstrated a similar   attack on a variant of the SHA-1 [RFC3174] hash algorithm, with a   prediction that the normally used SHA-1 would also be susceptible   with a large amount of work (but at a level below what should be   required if SHA-1 worked properly) [SHA-1-attack].  Also in early   2005, researchers showed a specific construction of PKIX certificates   [RFC3280] that use MD5 for signing [PKIX-MD5-construction], and   another researcher showed a faster method for finding MD5 collisions   (eight hours on a 1.6-GHz computer) [MD5-faster].   Because of these announcements, there has been a great deal of   discussion by cryptography experts, protocol designers, and other   concerned people about what, if anything, should be done based on theHoffman & Schneier           Informational                      [Page 1]

RFC 4270                   Attacks on Hashes               November 2005   news.  Unfortunately, some of these discussions have been based on   erroneous interpretations of both the news and on how hash algorithms   are used in common Internet protocols.   Hash algorithms are used by cryptographers in a variety of security   protocols, for a variety of purposes, at all levels of the Internet   protocol stack.  They are used because they have two security   properties: to be one way and collision free.  (There is more about   these properties in the next section; they're easier to explain in   terms of breaking them.)  The recent attacks have demonstrated that   one of those security properties is not true.  While it is certainly   possible, and at a first glance even probable, that the broken   security property will not affect the overall security of many   specific Internet protocols, the conservative security approach is to   change hash algorithms.  The Internet protocol community needs to   migrate in an orderly manner away from SHA-1 and MD5 -- especially   MD5 -- and toward more secure hash algorithms.   This document summarizes what is currently known about hash   algorithms and the Internet protocols that use them.  It also gives   advice on how to avoid the currently known problems with MD5 and   SHA-1, and what to consider if predicted attacks become real.   A high-level summary of the current situation is:   o  Both MD5 and SHA-1 have newly found attacks against them, the      attacks against MD5 being much more severe than the attacks      against SHA-1.   o  The attacks against MD5 are practical on any modern computer.   o  The attacks against SHA-1 are not feasible with today's computers,      but will be if the attacks are improved or Moore's Law continues      to make computing power cheaper.   o  Many common Internet protocols use hashes in ways that are      unaffected by these attacks.   o  Most of the affected protocols use digital signatures.   o  Better hash algorithms will reduce the susceptibility of these      attacks to an acceptable level for all users.2.  Hash Algorithms and Attacks on Them   A "perfect" hash algorithm has a few basic properties.  The algorithm   converts a chunk of data (normally, a message) of any size into a   fixed-size result.  The length of the result is called the "hashHoffman & Schneier           Informational                      [Page 2]

RFC 4270                   Attacks on Hashes               November 2005   length" and is often denoted as "L"; the result of applying the hash   algorithm on a particular chunk of data is called the "hash value"   for that data.  Any two different messages of any size should have an   exceedingly small probability of having the same hash value,   regardless of how similar or different the messages are.   This description leads to two mathematical results.  Finding a pair   of messages M1 and M2 that have the same hash value takes 2^(L/2)   attempts.  For any reasonable hash length, this is an impossible   problem to solve (collision free).  Also, given a message M1, finding   any other message M2 that has the same hash value as M1 takes 2^L   attempts.  This is an even harder problem to solve (one way).   Note that this is the description of a perfect hash algorithm; if the   algorithm is less than perfect, an attacker can expend less than the   full amount of effort to find two messages with the same hash value.   There are two categories of attacks.   Attacks against the "collision-free" property:   o  A "collision attack" allows an attacker to find two messages M1      and M2 that have the same hash value in fewer than 2^(L/2)      attempts.   Attacks against the "one-way" property:   o  A "first-preimage attack" allows an attacker who knows a desired      hash value to find a message that results in that value in fewer      than 2^L attempts.   o  A "second-preimage attack" allows an attacker who has a desired      message M1 to find another message M2 that has the same hash value      in fewer than 2^L attempts.   The two preimage attacks are very similar.  In a first-preimage   attack, you know a hash value but not the message that created it,   and you want to discover any message with the known hash value; in   the second-preimage attack, you have a message and you want to find a   second message that has the same hash.  Attacks that can find one   type of preimage can often find the other as well.   When analyzing the use of hash algorithms in protocols, it is   important to differentiate which of the two properties of hashes are   important, particularly now that the collision-free property is   becoming weaker for currently popular hash algorithms.  It is   certainly important to determine which parties select the material   being hashed.  Further, as shown by some of the early work,Hoffman & Schneier           Informational                      [Page 3]

RFC 4270                   Attacks on Hashes               November 2005   particularly [PKIX-MD5-construction], it is also important to   consider which party can predict the material at the beginning of the   hashed object.2.1.  Currently Known Attacks   All the currently known practical or almost-practical attacks on MD5   and SHA-1 are collision attacks.  This is fortunate: significant   first- and second-preimage attacks on a hash algorithm would be much   more devastating in the real world than collision attacks, as   described later in this document.   It is also important to note that the current collision attacks   require at least one of the two messages to have a fair amount of   structure in the bits of the message.  This means that finding two   messages that both have the same hash value *and* are useful in a   real-world attack is more difficult than just finding two messages   with the same hash value.3.  How Internet Protocols Use Hash Algorithms   Hash algorithms are used in many ways on the Internet.  Most   protocols that use hash algorithms do so in a way that makes them   immune to harm from collision attacks.  This is not by accident: good   protocol designers develop their protocols to withstand as many   future changes in the underlying cryptography as possible, including   attacks on the cryptographic algorithms themselves.   Uses for hash algorithms include:   o  Non-repudiable digital signatures on messages.  Non-repudiation is      a security service that provides protection against false denial      of involvement in a communication.  S/MIME and OpenPGP allow mail      senders to sign the contents of a message they create, and the      recipient of that message can verify whether or not the signature      is actually associated with the message.  A message is used for      non-repudiation if the message is signed and the recipient of the      message can later use the signature to prove that the signer      indeed created the message.   o  Digital signatures in certificates from trusted third parties.      Although this is similar to "digital signatures on messages",      certificates themselves are used in many other protocols for      authentication and key management.   o  Challenge-response protocols.  These protocols combine a public      large random number with a value to help hide the value when being      sent over unencrypted channels.Hoffman & Schneier           Informational                      [Page 4]

RFC 4270                   Attacks on Hashes               November 2005   o  Message authentication with shared secrets.  These are similar to      challenge-response protocols, except that instead of using public      values, the message is combined with a shared secret before      hashing.   o  Key derivation functions.  These functions make repeated use of      hash algorithms to mix data into a random string for use in one or      more keys for a cryptographic protocol.   o  Mixing functions.  These functions also make repeated use of hash      algorithms to mix data into random strings, for uses other than      cryptographic keys.   o  Integrity protection.  It is common to compare a hash value that      is received out-of-band for a file with the hash value of the file      after it is received over an unsecured protocol such as FTP.   Of the above methods, only the first two are affected by collision   attacks, and even then, only in limited circumstances.  So far, it is   believed that, in general, challenge-response protocols are not   susceptible, because the sender is authenticating a secret already   stored by the recipient.  In message authentication with shared   secrets, the fact that the secret is known to both parties is also   believed to prevent any sensible attack.  All key derivation   functions in IETF protocols take random input from both parties, so   the attacker has no way of structuring the hashed message.4.  Hash Collision Attacks and Non-Repudiation of Digital Signatures   The basic idea behind the collision attack on a hash algorithm used   in a digital-signature protocol is that the attacker creates two   messages that have the same hash value, causes one of them to be   signed, and then uses that signature over the other message for some   nefarious purpose.  The specifics of the attack depend on the   protocol being used and what the victim does when presented with the   signed message.   The canonical example is where you create two messages, one of which   says "I will pay $10 for doing this job" and the other of which says   "I will pay $10,000 for doing this job".  You present the first   message to the victim, get them to sign it, do the job, substitute   the second message in the signed authorization, present the altered   signed message (whose signature still verifies), and demand the   higher amount of money.  If the victim refuses, you take them to   court and show the second signed message.Hoffman & Schneier           Informational                      [Page 5]

RFC 4270                   Attacks on Hashes               November 2005   Most non-repudiation attacks rely on a human assessing the validity   of the purportedly signed message.  In the case of the hash-collision   attack, the purportedly signed message's signature is valid, but so   is the signature on the original message.  The victim can produce the   original message, show that he/she signed it, and show that the two   hash values are identical.  The chance of this happening by accident   is one in 2^L, which is infinitesimally small for either MD5 or   SHA-1.   In other words, to thwart a hash collision attack in a non-   repudiation protocol where a human is using a signed message as   authorization, the signer needs to keep a copy of the original   message he/she signed.  Messages that have other messages with the   same hash must be created by the same person, and do not happen by   accident under any known probable circumstances.  The fact that the   two messages have the same hash value should cause enough doubt in   the mind of the person judging the validity of the signature to cause   the legal attack to fail (and possibly bring intentional fraud   charges against the attacker).   Thwarting hash collision attacks in automated non-repudiation   protocols is potentially more difficult, because there may be no   humans paying enough attention to be able to argue about what should   have happened.  For example, in electronic data interchange (EDI)   applications, actions are usually taken automatically after   authentication of a signed message.  Determining the practical   effects of hash collisions would require a detailed evaluation of the   protocol.5.  Hash Collision Attacks and Digital Certificates from Trusted Third    Parties   Digital certificates are a special case of digital signatures.  In   general, there is no non-repudiation attack on trusted third parties   due to the fact that certificates have specific formatting.  Digital   certificates are often used in Internet protocols for key management   and for authenticating a party with whom you are communicating,   possibly before granting access to network services or trusting the   party with private data such as credit card information.   It is therefore important that the granting party can trust that the   certificate correctly identifies the person or system identified by   the certificate.  If the attacker can get a certificate for two   different identities using just one public key, the victim can be   fooled into believing that one person is someone else.Hoffman & Schneier           Informational                      [Page 6]

RFC 4270                   Attacks on Hashes               November 2005   The collision attack on PKIX certificates described in early 2005   relied on the ability of the attacker to create two different public   keys that would cause the body of the certificate to have the same   hash value.  For this attack to work, the attacker needs to be able   to predict the contents and structure of the certificate before it is   issued, including the identity that will be used, the serial number   that will be included in the certificate, and the start and stop   dates of the validity period for the certificate.   The effective result of this attack is that one person using a single   identity can get a digital certificate over one public key, but be   able to pretend that it is over a different public key (but with the   same identity, valid dates, and so on).  Because the identity in the   two certificates is the same, there are probably no real-world   examples where such an attack would get the attacker any advantage.   At best, someone could claim that the trusted third party made a   mistake by issuing a certificate with the same identity and serial   number based on two different public keys.  This is indeed   far-fetched.   It is very important to note that collision attacks only affect the   parts of certificates that have no human-readable information in   them, such as the public keys.  An attack that involves getting a   certificate with one human-readable identity and making that   certificate useful for a second human-readable identity would require   more effort than a simple collision attack.5.1.  Reducing the Likelihood of Hash-Based Attacks on PKIX Certificates   If a trusted third party who issues PKIX certificates wants to avoid   the attack described above, they can prevent the attack by making   other signed parts of the certificate random enough to eliminate any   advantage gained by the attack.  Ideas that have been suggested   include:   o  making part of the certificate serial number unpredictable to the      attacker   o  adding a randomly chosen component to the identity   o  making the validity dates unpredictable to the attacker by skewing      each one forwards or backwards   Any of these mechanisms would increase the amount of work the   attacker needs to do to trick the issuer of the certificate into   generating a certificate that is susceptible to the attack.Hoffman & Schneier           Informational                      [Page 7]

RFC 4270                   Attacks on Hashes               November 20056.  Future Attacks and Their Effects   There is a disagreement in the security community about what to do   now.  Even the two authors of this document disagree on what to do   now.   One of us (Bruce) believes that everyone should start migrating to   SHA-256 [SHA-256] now, due to the weaknesses that have already been   demonstrated in both MD5 and SHA-1.  There is an old saying inside   the US National Security Agency (NSA): "Attacks always get better;   they never get worse."  The current collision attacks against MD5 are   easily done on a single computer; the collision attacks against SHA-1   are at the far edge of feasibility today, but will only improve with   time.  It is preferable to migrate to the new hash standard before   there is a panic, instead of after.  Just as we all migrated from   SHA-0 to SHA-1 based on some unknown vulnerability discovered inside   the NSA, we need to migrate from SHA-1 to SHA-256 based on these most   recent attacks.  SHA-256 has a 256-bit hash length.  This length will   give us a much larger security margin in the event of newly   discovered attacks.  Meanwhile, further research inside the   cryptographic community over the next several years should point to   further improvements in hash algorithm design, and potentially an   even more secure hash algorithm.   The other of us (Paul) believes that this may not be wise for two   reasons.  First, the collision attacks on current protocols have not   been shown to have any discernible real-world effects.  Further, it   is not yet clear which stronger hash algorithm will be a good choice   for the long term.  Moving from one algorithm to another leads to   inevitable lack of interoperability and confusion for typical crypto   users.  (Of course, if any practical attacks are formulated before   there is community consensus of the properties of the cipher-based   hash algorithms, Paul would change his opinion to "move to SHA-256   now".)   Both authors agree that work should be done to make all Internet   protocols able to use different hash algorithms with longer hash   values.  Fortunately, most protocols today already are capable of   this; those that are not should be fixed soon.   The authors of this document feel similarly for new protocols being   developed: Bruce thinks they should start using SHA-256 from the   start, and Paul thinks that they should use SHA-1 as long as the new   protocols are not susceptible to collision attacks.  Any new protocol   must have the ability to change all of its cryptographic algorithms,   not just its hash algorithm.Hoffman & Schneier           Informational                      [Page 8]

RFC 4270                   Attacks on Hashes               November 20057.  Security Considerations   The entire document discusses security on the Internet.   The discussion in this document assumes that the only attacks on hash   algorithms used in Internet protocols are collision attacks.  Some   significant preimaging attacks have already been discovered   [Preimaging-attack], but they are not yet practical.  If a practical   preimaging attack is discovered, it would drastically affect many   Internet protocols.  In this case, "practical" means that it could be   executed by an attacker in a meaningful amount of time for a   meaningful amount of money.  A preimaging attack that costs trillions   of dollars and takes decades to preimage one desired hash value or   one message is not practical; one that costs a few thousand dollars   and takes a few weeks might be very practical.8.  Informative References   [MD5-attack]            X. Wang, D. Feng, X. Lai, and H. Yu,                           "Collisions for Hash Functions MD4, MD5,                           HAVAL-128 and RIPEMD", August 2004,                           <http://eprint.iacr.org/2004/199>.   [MD5-faster]            Vlastimil Klima, "Finding MD5 Collisions - a                           Toy For a Notebook", March 2005,                           <http://cryptography.hyperlink.cz/md5/MD5_collisions.pdf>.   [PKIX-MD5-construction] Arjen Lenstra and Benne de Weger, "On the                           possibility of constructing meaningful hash                           collisions for public keys", February 2005,                           <http://www.win.tue.nl/~bdeweger/CollidingCertificates/ddl-final.pdf>.   [Preimaging-attack]     John Kelsey and Bruce Schneier, "Second                           Preimages on n-bit Hash Functions for Much                           Less than 2^n Work", November 2004,                           <http://eprint.iacr.org/2004/304>.   [RFC3174]               Eastlake, D. and P. Jones, "US Secure Hash                           Algorithm 1 (SHA1)",RFC 3174,                           September 2001.   [RFC3280]               Housley, R., Polk, W., Ford, W., and D. Solo,                           "Internet X.509 Public Key Infrastructure                           Certificate and Certificate Revocation List                           (CRL) Profile",RFC 3280, April 2002.Hoffman & Schneier           Informational                      [Page 9]

RFC 4270                   Attacks on Hashes               November 2005   [SHA-1-attack]          Xiaoyun Wang, Yiqun Lisa Yin, and Hongbo Yu,                           "Collision Search Attacks on SHA1",                           February 2005,                           <http://theory.csail.mit.edu/~yiqun/shanote.pdf>.   [SHA-256]               NIST, "Federal Information Processing                           Standards Publication (FIPS PUB) 180-2,                           Secure Hash Standard", August 2002.Hoffman & Schneier           Informational                     [Page 10]

RFC 4270                   Attacks on Hashes               November 2005Appendix A.  Acknowledgements   The authors would like to thank the IETF community, particularly   those active on the SAAG mailing list, for their input.  We would   also like to thank Eric Rescorla for early material that went into   the first version, and Arjen Lenstra and Benne de Weger for   significant comments on the first version of this document.Authors' Addresses   Paul Hoffman   VPN Consortium   EMail: paul.hoffman@vpnc.org   Bruce Schneier   Counterpane Internet Security   EMail: schneier@counterpane.comHoffman & Schneier           Informational                     [Page 11]

RFC 4270                   Attacks on Hashes               November 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 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 AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Hoffman & Schneier           Informational                     [Page 12]