The pseudorandom number generating system repeatedly performs simple transformation of a non-secure pseudorandom number sequence that may be generated quickly, and thus may quickly generate a highly secure pseudorandom number sequence having a long period. Furthermore, the encryption system and the decryption system do not generate a large encryption function difficult to be deciphered based on a shared key 122, but prepare multiple functions 126, which perform fast, different types of transformation, and select a combination of functions determined based on information of the shared key 122, and make the selected functions transform a text multiple times, thereby encrypt the text. Each of the functions is fast, and thus transformation by the entire combination is also fast. Furthermore, since the combination of functions and repetitive count can be changed, future improvement in specification is easy. Moreover, security is high since which functions are applied in what order is unknown.