Stream cipher
A stream cipher makes use of a much smaller and more convenient cryptographic key, for example 128 bits keys. Based on this key, it generates a pseudorandom keystream which can be combined with the plaintext digits in a similar way to the one-time pad encryption algorithm. However,  because the keystream is pseudorandom, and not truly random, the security associated with the one-time pad cannot be applied and it is quite possible for a stream cipher to be completely insecure.