Operation
Each letter is represented by a number modulo 26. (Often the simple scheme A = 0, B = 1, ..., Z = 25 is used, but this is not an essential feature of the cipher.) To encrypt a message, each block of n letters (considered as an n-component vector) is multiplied by an invertible n × n matrix, again modulus 26. To decrypt the message, each block is multiplied by the inverse of the matrix used for encryption.
The matrix used for encryption is the cipher key, and it should be chosen randomly from the set of invertible n × n matrices (modulo 26). The cipher can, of course, be adapted to an alphabet with any number of letters; all arithmetic just needs to be done modulo the number of letters instead of modulo 26.
Consider the message 'ACT', and the key below (or GYBNQKURP in letters):
Since 'A' is 0, 'C' is 2 and 'T' is 19, the message is the vector:
Thus the enciphered vector is given by:
which corresponds to a ciphertext of 'POH'. Now, suppose that our message is instead 'CAT', or:
This time, the enciphered vector is given by:
which corresponds to a ciphertext of 'FIN'. Every letter has changed. The Hill cipher has achieved Shannon's diffusion, and an n-dimensional Hill cipher can diffuse fully across n symbols at once.
Read more about this topic: Hill Cipher
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