23 replies to this topic

[TcM]

If we make an md5 for a file and then brute force that md5 what string will the brute forcer generate?

[WingedPanther]

Brute forcing an md5 is finding another string that produces the same md5 hash. It could produce any of the infinitely many strings that have the common md5 hash.

[TcM]

So a file and a string can have the same md5?

[WingedPanther]

Yes.

[TcM]

I thought that md5 was unique for every string/file. So the string generated can be just a random string.. has nothing to do with the file name?

[John]

md5 is a 128-bit hash, which is generally represented as a 32-character hexadecimal string. I've read numerous documents on how the md5 algorithm works, and I still can't understand it, but the md5 hash is dependent on the entire file. However, there are only a certain amount of 32-character combinations / permutations that can be created, so there is a possibility [around 1 x 10^-100] that two files will have the same exact md5 hash [which is called an md5 collision].

There was an Asian professor [Wang I believe] who "cracked" the md5 and created an algorithm to create md5 collisions, about 10 years ago. I was doing some reading the other day, and I read that there is an algorithm to create a collision as fast as 31 seconds.

[TcM]

But what is the aim to create md5 collisions?

[Maurice_Z]

Prove that there is still a room for something better than md5 I guess. To get a better knowledge on security, to improve security. Something around this things I guess, since md5 is really widely used in passwords encrypting (Is this the proper word?).

[John]

TheComputerMaster said:

But what is the aim to create md5 collisions?

I can only think of malicious reasons to create collisions. If you know a password hash, and can create a collision, you "cracked" the password. Also many applications use md5 to verify a files integrity, creating a collision, could allow you to pack a virus with an exe and keep the exe's same checksum, potentially going undetected by anti-virus software.

[TcM]

So the file will force the Md5 checkers to generate an md5 that the malicious file wants.. right?

[John]

The md5 checkers will generate the same hashes for the "good" file and the "bad" file.

[TcM]

Wow, so how is this crack implemented.. is it an addon binded/bound with the "bad" file? or it's a piece of code you compile with the program?