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In Shannon's original definitions, ''confusion'' refers to making the relationship between the Key and the Ciphertext as complex and involved as possible; ''diffusion'' refers to the property that redundancy in the statistics of the Plaintext is "dissipated" in the statistics of the Ciphertext . Diffusion is associated with dependency of bits of the output on bits of the input. In a cipher with good diffusion, flipping an input bit should change each output bit with a probability of one half (this is termed the Strict Avalanche Criterion ). Substitution (a Plaintext symbol is replaced by another) has been identified as a mechanism for primarily confusion (see S-box ); conversely Transposition (rearranging the order of symbols) is a technique for diffusion, although other mechanisms are also used in modern practice, such as linear transformations (e.g. in Rijndael ). Product Cipher s use alternating Substitution and Transposition phases to achieve both confusion and diffusion respectively. SEE ALSO
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