The coding of messages so as to render them unintelligible to other than authorized recipients. Many techniques are known for the conversion of the original message, known as plaintext, into its encrypted form, known as ciphertext, cipher, or code.
In a simple cipher system, for example, the sender and recipient hold identical copies of a secret key, and also an algorithm with which they each generate identical pseudorandom bit sequences. During encryption the sender modifies the plaintext string by combining it with the pseudorandom sequence to produce the ciphertext; the ciphertext is then transmitted. The recipient performs the reverse process with an identical pseudorandom sequence and the received ciphertext to recover the plaintext.
An alternative technique is to use a block cipher, in which the ciphertext corresponding to each block of, typically, 64 bits of plaintext is generated algorithmically using a key. In a symmetric block cipher the key used for decryption is closely related to that used for encryption, and both have to be kept secret. With asymmetric or public key encryption, the decryption key cannot be deduced from knowledge of the encryption key, which can thus be publicized to all intending message senders. See Data Encryption Standard, RSA encryption.