Attack MD5 with partial information? If I generate an MD5 hash from 2 pieces of information – i.e. by concatenating two strings – how easy is it to guess what that hash is, assuming an attacker has knowledge of only 1 piece of those info?
It all boils down to the amount of entropy in the second string. 

How is that second string generated? 

If a human generates it, you can be pretty sure that most of the time it will be easy to figure it out. If a computer generates it using a cryptographically strong random number generator, and the string has sufficient length (say more than 80 bits), it will not be feasible for an attacker to guess it.