By one-way I mean md5 or sha1. They're extremely useful for one-way validation in cryptography/security. Applications of these are storing passwords (e.g. normally you should salt and hash passwords before storing them in a database) SHA is usually stronger (~160 bits) than md5 (~128 bits). Although, weaknesses have recently been found (~2005?) in these algorithms; it's easier than predicted to find hash collisions. NIST is sponsoring the creation of successor functions to these current one-way hashing algorithms.

http://en.wikipedia.org/wiki/SHA_hash_functions

http://developers.slashdot.org/developers/04/12/07/2019244.s...