**Block cipher - Wikipedia**
https://en.wikipedia.org/wiki/Block_cipher

Definition. A block cipher consists of two paired algorithms, one for encryption, E, and the other for decryption, D. Both algorithms accept two inputs: an input block of size n bits and a key of size k bits; and both yield an n-bit output block.The decryption algorithm D is defined to be the inverse function of encryption, i.e., D = E −1.More formally, a block cipher is specified by an ...

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