Decorrelation theory
A framework for proving that a block cipher resists whole classes of attacks by measuring how far it sits from a truly random permutation.
Serge Vaudenay developed it from 1998 onward. The idea is to define a distance between the cipher and an ideal random permutation, taken over all pairs or tuples of inputs. If that distance is small enough, you can prove an upper bound on how well any differential or linear attack of a given order can do, without analysing each characteristic by hand.
Vaudenay designed the ciphers COCONUT98 and PEANUT around the principle, and the team behind the AES candidate DFC used it too. The irony arrived in 1999, when David Wagner broke COCONUT98 with the boomerang attack. The proof was correct; it just covered the attacks it was written for, and the boomerang stitches together two short differentials rather than using one long one. The theory survives as a tool. The promise of security against everything did not.