Patent ID: 7302572

Claim:
An authentication method for authenticating a portable information recording medium ( 100 ) as valid when external equipment ( 200 ) accesses said portable information recording medium, comprising: an operation defining step for determining a first key (α), a second key (β), an encoding operation, and a decoding operation so that encoded data (C) is obtained by executing said encoding operation using said first key (α) for arbitrary authentication data (R) and data identical to said authentication data (R) is obtained by executing said decoding operation using said second key (β); a medium preparing step in which said first key (α) is stored in the portable information recording medium ( 100 ), and a processing function for executing said encoding operation is prepared for the portable information recording medium; a random number transmitting step (S 1 and S 2 ) in which said external equipment ( 200 ) generates a random number or numbers, and said random number or numbers are transmitted to the portable information recording medium ( 100 ) as authentication data (R); an authentication data storing step (S 6 ) in which said transmitted authentication data (R) is received and stored in a predetermined storing location ( 130 ) inside the portable information recording medium ( 100 ), wherein a plural number n of storing locations ( 130 ) at which authentication data (R) can be stored are prepared in the portable information recording medium ( 100 ), and only latest n pieces of authentication data (R) are stored in the authentication data storing step (S 6 ), and applying rewrite processing to a storing location at which an oldest authentication data (R) has been written; a judging step (S 4 and S 5 ) in which it is investigated whether or not a newly transmitted authentication data (R) agrees with previously stored authentication data (R) in the portable information recording medium ( 100 ), and only when it is determined that the newly transmitted authentication data (R) disagrees with any of the previously stored authentication data (R), it is judged that encoding is permitted; an encoding step (S 7 and S 8 ) in which, only when encoding is permitted in said judging step, the encoding operation using the first key (α) is executed for the transmitted authentication data (R), and resulted encoded data (C) is returned to the external equipment ( 200 ); a decoding step (S 9 and S 10 ) in which the decoding operation using the second key (β) is executed in the external equipment ( 200 ) for the encoded data (C) that has been returned from the portable information recording medium ( 100 ); and an authenticating step (S 11 through S 14 ) in which, when data identical to the authentication data (R) transmitted in said random number transmitting step is obtained as a result of said decoding operation, the portable information recording medium ( 100 ) is authenticated as valid.