To combine the advantages of block cipher and stream cipher by making the number of bits in a key stream block larger than the number of bits in a plaintext block.

A key stream generator KS generates an intermediate key in the system of a pseudo-random number sequence S in accordance with a cryptographic key K. The KS is a pseudo-random number generator used for stream cipher like WAKE, SEAL or RC4, for example. The KS can be the block cipher of a counter mode or a hash function with a counter. The intermediate key S is inputted to a mix function MX. The mix function MX receives a plain sentence message P, connects it with the pseudo-random number sequence S and generates a cipher text C. The number of bits in a key stream block is larger than the number of bits in the plaintext block.