Zero-correlation attack
An attack that finds linear approximations with exactly zero correlation, giving a way to tell the cipher apart from random.
Ordinary linear cryptanalysis relies on approximations that deviate from chance. A zero-correlation attack instead uses an approximation whose correlation is exactly zero over a chosen part of the cipher, so the summed statistic vanishes for right key guesses and not for wrong ones. With enough pairs the attacker can distinguish the cipher from a random permutation.
The technique was introduced in 2011 and applied mainly to reduced-round block ciphers. It is a distinguisher rather than a key recovery, but it is used to calibrate how many rounds a cipher needs in order to be considered secure.