Decryption is the act of transforming a ciphertext into a
The opposite process is encryption.
K be a set of keys,
M be a set of plaintext messages, and
C be a set of ciphertext messages.
is a decryption function or decryption transformation.
L. R. Knudsen, M. J. B. Robshaw, The Block Cipher Companion. Springer, 2011.