A
stream cipher is a
symmetric key cipher where plaintext digits are combined with a
pseudorandom cipher digit stream (
keystream). In a stream cipher each
plaintext digit is encrypted one at a time with the corresponding digit of the keystream, to give a digit of the ciphertext stream. Since encryption of each digit is dependent on the current state of the cipher, it is also known as
state cipher. In practice, a digit is typically a
bit and the combining operation an
exclusive-or (XOR)(see
cipher).