If you compute a string that is say 32 characters long using just letters and numbers, then there are 62^32 possible combinations. This number is 58 digits long

If you compute a string that is say 32 characters long using just letters and numbers, then there are 62^32 possible combinations. This number is 58 digits long. In this case, extremely unlikely just means never unless you are being attacked by say Russians with secret technology and a room full of supercomputers. The world will almost certainly end before anyone guesses one of these numbers or before you generate two that are the same. You can look at the probability table for the Birthday Attack in this Wikipedia article and decide how unlikely you would like this event to be: http://en.wikipedia.org/wiki/Birthday_problem http://en.wikipedia.org/wiki/Birthday_problem Also from the article: "In theory, MD5, 128 bits, should stay within that range until about 820 billion documents, even if its possible outputs are many more."