Patent ID: 6842859

Claim:
An authentication system comprising a first device and a second device, and transmitting an enciphered authentication data between said first device and said second device and determining whether a transmission of data to be transmitted between said first device and said second device is to be permitted or not, said first and second devices each including: encipher means for enciphering said authentication data while changing an encipherment rule for each unit transmission of data to be transmitted, wherein said encipher means is configured to change the encipherment rule enciphering said authentication data for each unit transmission according to a number of said transmissions; and transmitting means for transmitting said enciphered authentication data, wherein said first device further includes: prohibition command output means for determining whether or not a second authentication data, which is transmitted from said second to said first device and generated in said second device by an encipherment based on a first authentication data transmitted from said first device, matches with data generated by enciphering said first authentication data in said first device based on an encipherment rule corresponding to the same number of transmissions, and for supplying a prohibition command prohibiting the transmission of said data to be transmitted as an output when the determination result is negative.