Decorrelation is a design technique for (symmetric) cryptographic algorithms that provide provable security against any iterated attacks of a fixed order. Examples of this type of attack is Biham and Shamir's differential cryptoanalysis and Matsui's linear cryptoanalysis, both of which are iterated attacks of order 2.
Decorrelation is under the pending french patent WO9820643, applied for on 1996-11-04. It has a homepage on http://www.dmi.ens.fr/~vaudenay/decorrelation.html