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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 this lead to collision? or, what is the chance of collision, assuming I’ll hash less than 65,536 (164) different elements?
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.