[hefty]

Maybe I'm taking your example a bit far, but it seems like you'd have to choose the hashing function carefully, correct?

To use your example, it must be 100% certain that a hash with 6 leading zeros is possible to generate with md5.

Also, I'm assuming you don't want clients spending too long on the problem, so it seems like you'd want to have a prediction of roughly how long it would take to compute the answer. Otherwise one client may get lucky after 10 iterations whist another may take 10 million. Are hashing functions predictable in that manner?

[grenoire]

Reliable hash functions must be able to map to all combinations of 256, 512, or however many bits they are using.