You're looking for two files f1 and f2 that hash to the same thing? And hoping that f1 is randomly generated by you, but f2 is someone else's original file whic

You're looking for two files f1 and f2 that hash to the same thing? And hoping that f1 is randomly generated by you, but f2 is someone else's original file which can be downloaded because of dedup? In general, solving for f1 and f2 (i.e., you get to control both) such that h(f1)=h(f2) is called finding collisions in hash functions and hash functions like SHA are considered collision-resistant. It is very difficult to find collisions. http://en.wikipedia.org/wiki/Collision_resistance http://en.wikipedia.org/wiki/Collision_resistance Of course, if f1=f2, then you've magically guessed the original file, so that is highly unlikely. What you're asking for is something stronger than collision resistance; you're asking for a second preimage, given a first. This is for all practical purposes (requires at least 2^120 or more computations) impossible for any well designed hash function.