A 128 bit random plaintext is more than secure enough... If you're trying to argue that the user puts in, say, 60 bits of entropy, and that the hashing algorit

A 128 bit random plaintext is more than secure enough... If you're trying to argue that the user puts in, say, 60 bits of entropy, and that the hashing algorithm is going to accidentally throw out 10 of those to result in 50 bits of entropy, I believe that any hashing algorithm that did that in a way that anyone can exploit with realistic computing power (a computer smaller than the size of the solar system) would be considered irredeemably broken.