Guess 100 bits of md5 hash
A 3 minute running time on my laptop (running Java) already gave me a bit string where 99 bits are identical. It might be that you're overthinking this.

This sounds like a classical proof of work that you're supposed to do. As it is unlikely that there are any shortcuts I guess you'll have to break it brute-force. And as is it a gaming site, the faster GPU probably wins.

MD5 still has pre-image resistance of over $2^{123}$, with the specific attack mentioned here. You won't get far with just finding collisions as the pre-image seems to be (unknown) and given.



Just computing on a laptop gives, comparing with a hash of all zero's:

100 : 00000000000000000000000625b4aa69 -&gt; 0740a0a42a0074920018000008308212


only just over 26,000,000,000 (16 billion for the US, 16 milliard for the rest of us) hashes required; the hexadecimal representation of the value as 16 byte integer is used as input.

Took some time but nothing major, unoptimized, single threaded Java code, running on a downclocked i7 laptop. On a fast machine with semi-optimized, multi-threaded code expect a 16 times improvement minimum.