Assuming that there are no weaknesses in SHA256, you'd have to calculate a set of 4.8x10^35 hashes to have a one in a million chance of seeing at least one coll

Assuming that there are no weaknesses in SHA256, you'd have to calculate a set of 4.8x10^35 hashes to have a one in a million chance of seeing at least one collision in that set[1]. If you could calculate (and store!) a trillion trillion (10^24) hashes per second, that would take about 15000 years. Needless to say, nobody has ever found a SHA256 collision. [1] http://en.wikipedia.org/wiki/Birthday_attack http://en.wikipedia.org/wiki/Birthday_attack