Encryption system and encryption methods that are applicable to various keyless entry systems

Encryption system having a transmitter (1) sends an identification code to a keyless entry system used object, wherein the transmitter (1) comprising: a memory means (11) which stores the identification code; a first arithmetic means (12) encrypts the identification code outputted from the memory means (11); and a transmitting means (13), sends the encrypted identification code generated one of the first arithmetic means (12), wherein the first arithmetic means (12) divides both the identification code and an associated rolling code into a plurality of blocks, wherein the rolling code change its contents in response to each transmission of the identification code by the transmission means (13), a repetition number for an arithmetic encryption operation in a method determines an arithmetic operation and the arithmetic encryption operation repeated in each block in accordance with the repetition number, thereby to encrypt the identification code.