substr md5 collision

Question

I need a 4-character hash. At the moment I am taking the first 4 characters of a md5() hash. I am hashing a string which is 80 characters long or less. Will thi

Answer 1

Surprisingly high indeed. As you can see from this graph of an approximate collision probability (formula from the wikipedia page), with just a few hundred elements your probability of having a collision is over 50%.

Note, of course, if you're facing the possibility of an attacker providing the string, you can probably assume that it's 100% - scanning to find a collision in a 16-bit search space can be done almost instantaneously on any modern PC. Or even any modern cell phone, for that matter.