MD5 To Be Considered Harmful Someday

Effugas writes "I've completed an applied security analysis (pdf) of MD5 given Xiaoyun Wang et al's collision attack (covered here and here). From an applied perspective, the attack itself is pretty limited -- essentially, we can create 'doppelganger' blocks (my term) anywhere inside a file that may be swapped out, one for another, without altering the final MD5 hash. This lets us create any number of binary-inequal files with the same md5sum. But MD5 uses an appendable cascade construction -- in other words, if you happen to find yourself with two files that MD5 to the same hash, an arbitrary payload can be applied to both files and they'll still have the same hash. Wang released the two files needed (but not the collision finder itself). A tool, Stripwire, demonstrates the use of colliding datasets to create two executable packages with wildly different behavior but the same MD5 hash. The faults discovered are problematic but not yet fatal; developers (particularly of P2P software) who claim they'd like advance notice that their systems will fail should take note."

Comments

Exploit?

[Limburgher]

So does this mean that it's possible to find a useful MD5-equivalent file for any file? Just because someone alters a file does not mean they have done anything destructive. Would one be able to take a binary, make a change of some sort, and then run a tool to determine the block of data to add to the binary to both allow the change to take effect and cancel out the MD5 change? How complex would it be to construct this tool?

Re:damn

[WolfWithoutAClause]

There will ALWAYS be collisions with any kind of hashing algorythm.

Yes, but a good hash makes it *extremely* difficult to find them. MD5 is looking pretty mediocre right now.