MD5 Collision Source Code Released

SiliconEntity writes "The crypto world was shaken to its roots last year with the announcement of a new algorithm to find collisions in the still widely-used MD5 hash algorithm. Despite considerable work and commentary since then, no source code for finding such collisions has been published. Until today! Patrick Stach has announced the availability of his source code for finding MD5 collisions and MD4 collisions (Coral cache links provided to prevent slashdotting). MD4 collisions can be found in a few seconds (but nobody uses that any more), while MD5 collisions (still being used!) take 45 minutes on a 1.6 GHz P4. At last we will be able to implement various attacks which have been purely hypothetical until now. This more than anything should be the final stake in the heart of MD5, now that anyone can generate collisions whenever they want."

Comments

So what the hell do I do now?

[jeblucas]

I'm essentially crypto ignorant. About all I've known to do was verify MD5 hashes on downloads. Now that this is by-and-large pointless, how to check the veracity of things like Linux ISO's, video drivers, etc, ad inifintum?

WTF this source is useless

[RootsLINUX]

I guess someone thinks he's a little too cool to comment his code properly. Yeah, like "/* B2 */" tells me anything useful you insensitive clog!

So you found a collision, big deal

[goombah99]

Maybe someone could explain why collisions are a serious problem for MD5. Or at least in what instances they are. I can see that in some cases, such as password hashing this could be a problem. But for many other uses I don't see what harm a collision has. Maybe I misunderstand but as I understand it MD5s are normally used in a checksum manner to sign or provide a fingerprint of a document. If you have an original document and compute it's MD5 then it can match some certified MD5 check sum. If someone were to generate a fake document they coul dnot design it to match the MD5 fingerprint. They could create some bit of gibberish that did match it but not a document that was useful as a forgery.

I guess if one were trying to deliberately pedal gibberish, like say if you were the RIAA trying to destabilize a torrent net then that would be all you care about. But for more general issues I don't get it.

Or is it now possible to generate a collision that also contains some intentional content like a binary program or source code or bank statement. That woul dbe be bad.

It seems like even in the case of gibberish generation that some simple hacks could extend the useful life of this. For example, if you were to MD5 a whole document and then MD5 the concatenation of every other byte in the document, it woul dbe pretty hard to find two collisions that had that property simulateously. Sure I wont doubt there are better ways to hash something than adding hacks to MD5. I'm just saying that it seems like a simple hack might well be good enough to extend its useful lifespan for passwords and file shareing.

But I invite you to explain to me why I'm wrong.

You can spoof (almost) arbitrary documents!

[ChaosDiscord]

Maybe I misunderstand but as I understand it MD5s are normally used in a checksum manner to sign or provide a fingerprint of a document. If you have an original document and compute it's MD5 then it can match some certified MD5 check sum. If someone were to generate a fake document they coul dnot design it to match the MD5 fingerprint. They could create some bit of gibberish that did match it but not a document that was useful as a forgery.

Most document formats have lots of "dead space", parts you can pretty much modify at will without changing what the user actually sees. Comments in HTML or PostScript. Old junk data in Word documents. Executables can have just about anything you like added if you know your stuff. The MD5 attacks currently available only 128 "dead space" bytes to generate a collision. So far from being a gibberish document, one can generate almost any document you want. This page has a simple example with PostScript files. Both files have the same MD5 hash, but one is a relatively harmless letter of recommendation while the other is a grant of security clearance. Get your boss to sign your letter of recommendation digitally, swap in the security clearance file, and pass it on. This is a Big Deal and a Major Problem.

I may not have been clear enough...

[^BR]

Or you may have a brain tumor...

The generaly accepted definition for "cracking a password" is, given the encrypted password, being able to generate a password the once entered in the authentication system will generate the same encrypted form.

For the second time, this attack does not permit that. This attack permits build two byte streams that hashes to the same value. So in a password context, assuming the password system permits the entry permits more than 1024 bits of arbitrary data to be entered as password (I don't remind the details well, but I think this is the amount of data that you must be able to change between the two streams) one could generate to "passwords" (being that long they don't qualify for this name anymore IMHO) that would hash to the same value.

How does that amount to an attack on MD5 passwords again? Never at any point the attack had been being able given a unknown to him MD5 hash to produce a stream that would hash to the same value.

Re:Q and A

[CodeRx]

sha1(md5($password . '¥1i9k') . 'a-thirty-five-ch4racter-l0ng-str1ng' . md5($password))

This is a very bad password salting scheme and vulnerable to a dictionary attack. Once I have your database and salts, I can run a dictionary of common passwords through your scheme and crack any weak passwords.

You can make things much harder by having your salt change for each password - include the username for example. Now I have to run my entire dictionary through the sha/md5 function for each user. By doing this, you make the attack O(m*n) instead of O(m) (where m = the number of words in my dictionary and n = the number of users).

And as you mentioned in a follow up post, this code only generates documents with identical md5 sums, it does not generate a document with a given sum. So MD5 is broken for document signing and the like, but secure for password hashing for the time being.

SHA1 is not a good alternative in some cases

[chongo]

SHA1 is not a good alternative in some cases. For details on the cryptographic hash problem, see my paper:

SHA1 Cryptographic Hash Update

My paper talks about the general problem at a high level. It gives a summary of common opinions expressed at the NIST Cryptographic Hash conference. Moreover, it gives developers some specific cryptographic hash recommendations.

For the impatient, here is a summary for my recommendations for 2005-2006:

• Avoid non-standard cryptographic hashes
• Stop using MD5 now except for:
  • MD5 HMAC and MD5 hashed Passwords
  • Replace MD5 HMAC and MD5 hashed Passwords with SHA256 or SHA1 before end of 2007
• Existing applications that use SHA1, where possible, should changed to use SHA256 before the end of 2008
  • For interoperability with older applications and hardware, these applications may have to also support SHA1
  • If you must support both SHA256 and SHA1, take care so that a "man in the middle" cannot inappropriately downgrade
• Until a new Advanced Hash Standard (AHS) is adopted, new applications and hardware should be designed to use SHA256
  • For interoperability with older applications and hardware, these applications may have to also support SHA1
  • If you must support both SHA256 and SHA1, take care so that a "man in the middle" cannot inappropriately downgrade
• All new applications and protocols must be designed to be algorithm agile
• Existing applications and protocols should be modified to be algorithm agile by the end of 2008, if not sooner
• SHA384 or SHA512 may be used in place of SHA256 in the above examples
  • Keep in mind that SHA384 and SHA512 are slower and larger than SHA256 or SHA1
• Because it is possible that SHA1 will become unacceptably weak before 2008, and because SHA256 may become vulnerable to attack before Advanced Hash Standard (AHS) is adopted, a defense in depth approach must be taken

See the paper for mode details.