It completely doable. Its unlikely there will be a collision in the first 6 characters of the md5 hash of 'AAAx' where x is [a-zA-Z0-9] and AAA is known. That

It completely doable. Its unlikely there will be a collision in the first 6 characters of the md5 hash of 'AAAx' where x is [a-zA-Z0-9] and AAA is known. That's only 62 hopes for collision out of a space of 2176782336 (36^6 ... right?) Think of it this way, if you capture the length of password, and the corresponding 6 hex characters, for every step of the password, that's quite a bit of information! (a quick check confirms no first 6 character collisions in a-z)