Patent ID: 7155607

Claim:
A method for authentication using a challenge and response system between a first apparatus and a second apparatus, the first apparatus having a first converting unit that performs a first conversion, the second apparatus having a second converting unit that performs a second conversion, a relation between the first conversion and the second conversion being identical or reverse, the method comprising: an authenticating step of sending a piece of challenge data from the first apparatus to the second apparatus, sending a piece of response data from the second apparatus to the first apparatus, the piece of response data having been converted from the piece of challenge data by the second conversion, judging in the first apparatus, based on the first conversion, whether the piece of response data is converted from the piece of challenge data by the second conversion, and authenticating the second apparatus if a result of the judgment is affirmative; and a setting step of, when the result of the judgment in the authenticating step is affirmative, setting a first conversion and a second conversion to be used in a next authentication, instead of the first conversion and the second conversion used in a current authentication, respectively, a content of the first conversion and the second conversion for the next authentication being different from a content of the first conversion and the second conversion used in the current authentication, and a relation between the first conversion and the second conversion for the next authentication being the same as the relation between the first conversion and the second conversion used in the current authentication.