Patent ID: 7016924

Claim:
A contactless integrated circuit (IC) card responding to a request sent from a reader/writer by using a response timing calculated on the basis of slot numbers included in the request and a random number, the contactless IC card comprising: a provisional random number generating unit operable to generate a plurality of provisional random numbers according to a part of calculations to be used for generating a random number; a provisional random number storage unit operable to store the plurality of provisional random numbers generated by the provisional random number generating unit; a random number generating unit operable to generate a random number on the basis of one of the stored provisional random numbers selected in sequential order at the time of receiving the request from the reader/writer; and a self-response timing judging unit operable to judge if there is time needed for generating and storing the plurality of provisional random numbers until timing to respond by the card itself, wherein the provisional random number generating unit generates the provisional random number if the self-response timing judging unit judges that there is the time needed for generating and storing the plurality of provisional random numbers until the response timing, and wherein the self-response timing judging unit judges if there is the time needed for generating and storing the plurality of provisional random numbers between receiving the request from the reader/writer and the response to the request.