Editing Confusion and diffusion (section)

==Theory==
In Shannon's original definitions, ''confusion'' refers to making the relationship between the [[ciphertext]] and the [[symmetric key]] as complex and involved as possible; ''diffusion'' refers to dissipating the statistical structure of [[plaintext]] over the bulk of [[ciphertext]]. This complexity is generally implemented through a well-defined and repeatable series of ''substitutions'' and ''permutations''. Substitution refers to the replacement of certain components (usually bits) with other components, following certain rules. Permutation refers to manipulation of the order of bits according to some algorithm. To be effective, any non-uniformity of plaintext bits needs to be redistributed across much larger structures in the ciphertext, making that non-uniformity much harder to detect.

In particular, for a randomly chosen input, if one flips the ''i''-th bit, then the probability that the ''j''-th output bit will change should be one half, for any ''i'' and ''j''—this is termed the [[Avalanche effect|strict avalanche criterion]]. More generally, one may require that flipping a fixed set of bits should change each output bit with probability one half.

One aim of confusion is to make it very hard to find the key even if one has a large number of plaintext-ciphertext pairs produced with the same key. Therefore, each bit of the ciphertext should depend on the entire key, and in different ways on different bits of the key. In particular, changing one bit of the key should change the ciphertext completely.