function for hashing URL strings to check integrity?

Does anyone know of a function that assists with checking URL strings to make sure they haven't been monkeyed with? Ideally something that calculates the md5 hash of a string and then automatically verifies it when a user clicks a link. I've checked the usual places (hotscripts, devshed, etc) and didn't see anything, but I wanted to check with the group before rolling my own. Thanks. --kurt