Re: MD5 math question

[exon <exon () home se>]

Tim wrote:
> > Considering the fact that MD5 has been broken though, I'm fairly 
> > surprised it even came up to discussion. It's not exactly hard to find 
> > info or even collision-generators.
>
> See... People keep bringing this up, and it is true, MD5 has been
> shattered when it comes to collision resistance, but this is not the
> same as pre-image or second pre-image resistance.  I don't believe
> either of those are broken yet for MD5 (please link me if I am wrong),
> so I think this is still a valid discussion, and applicable other hash
> algorithms as well.

It's true that for password authentication there's no real need to 
switch to a different algorithm at this point. However, a flaw in the 
algorithm is still a flaw in the algorithm. The fact that there aren't 
any computationally feasible implementations that reverse-calculate a 
collision (i.e. by knowing only what hash it should collide with rather 
than which plain-text that resulted in the hash) doesn't change that.

One approach to take when brute-forcing passwords could be to simply add 
random bytes to a stream until it collides and then using a 
collision-generator to generate a collision short enough to not be 
discarded by the password validation mechanism. Those with deeper 
insights into the MD5 algorithm could probably come up with which bytes 
to add to make it collide faster, but it still means brute-forcing an 
MD5 password of considerable length is down from months to mere hours.

All this is ofcourse theory. I don't know enough cryptography to 
determine what is possible and what isn't, but since SHA1 hasn't been 
broken (yet) and there are enough open and free implementations of it to 
go around I'm a bit surprised to find that MD5 is considered for use in 
new applications.

/exon

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