In cryptography, a stream cipher is a symmetric key cipher where plaintext bits are combined with a pseudorandom cipher bit stream (keystream) using an exclusive-or (xor) operation. In a stream cipher the Plaintext digits are encrypted one at a time, and the transformation of successive digits varies during the encryption state. An alternative name is a state cipher, as the encryption of each digit is dependent on the current state. In practice, the digits are typically single bits or bytes.