Page "Cipher" Paragraph 2

The operation of a cipher usually depends on a piece of auxiliary information, called a key ( or, in traditional NSA parlance, a cryptovariable ).

The encrypting procedure is varied depending on the key, which changes the detailed operation of the algorithm.

A key must be selected before using a cipher to encrypt a message.

Without knowledge of the key, it should be difficult, if not nearly impossible, to decrypt the resulting ciphertext into readable plaintext.