PURPOSE: To obviate the storage of either of secrete data itself or enciphered secrete data in a device, and yet provide an effect similar to a case where the enciphered data is stored therein, by judging the data to b genuine secrete data through the proper decoding of a suitable cryptotext, using the secrete data as a cryptographic key.

CONSTITUTION: A code number CI gained from a decoding process is used as a cryptographic key, and a code number judgement sentence 36 enciphered using the number CI a key is decoded in a decoding circuit 34. Then, when a means 38 for judging proper decoding identifies that the text 36 is properly decoded, the number CI is judged to be genuine and belong to a user. When the decoding of the text 36 is judged to be improper, the code is again entered in the specified circuit and an IC card is regarded as invalid, if the code is over the capacity of the circuit. Regarding the plain text of the code number judgement sentence 36 enciphered using the code CI as a key, the decoding thereof is judged to be proper, for example, when text length is 8 bytes and the content of the sixth byte is equal to the content of the first byte multiplied by the content of the third byte.