Algorithms that output a unique 'fingerprint' for a given input of data (MDx, SHA1) Input data may be of almost any length No key is required - only data is needed as input (Though an IV can be used in some cases (! PHP)) Generated "fingerprint" is of a fixed length Deterministic Considered nigh-impossible to reconstruct the input data given the output data Also called one-way functions, message digests, signatures and hashes Unix, MySQL, htpasswd, ... password systems Hash user credentials together with preference information i.e. md5($user_name, $password, $mark_on_test) Data is not reversible - just comparable