Use the key to generate the key stream z = z 1z2z3 ... and then use this key stream to encrypt the plaintext X=X0X 1X2 ... Conversely, the generated cipher is a sequence cipher, also known as stream cipher. The key stream generator F: zi=f(k, si) generates the key stream, where si is the state of the memory (storage unit) in the encryptor at time i, and k is the key. The development of sequence cipher scheme is an attempt to imitate the "one-time-one-secret" system.