(1) The core algorithm used to encrypt data. A cipher transforms regular data (plaintext) into a coded set of data (ciphertext) that is not reversible without a key. For example, AES and DES are examples of secret key block ciphers. The complete encryption algorithm is the cipher plus the technique used to apply the cipher to the message, which can be a very intricate series of steps. See cryptography, encryption algorithm, block cipher and plaintext.