Let `K`

be a set of keys and `M`

be a set of plaintext messages.
A cipher is a set of encryption functions `{E`

and
a set of decryption functions _{e}:e∈K}`{D`

where
for each encryption function _{d}:d∈K}`E`

there exists a decryption function _{e}`D`

such that for all _{d}`m∈M:D`

.
_{d}(E_{e}(m))=m

The pair `(e,d)`

in the description above is known as a keypair.

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

Copyright © 2014 Barry Watson. All rights reserved.