Cipher

Let K be a set of keys and M be a set of plaintext messages. A cipher is a set of encryption functions {Ee:e∈K} and a set of decryption functions {Dd:d∈K} where for each encryption function Ee there exists a decryption function Dd such that for all m∈M:Dd(Ee(m))=m.

The pair (e,d) in the description above is known as a keypair.

References

L. R. Knudsen, M. J. B. Robshaw, The Block Cipher Companion. Springer, 2011.