SHOP APPARATUS AND PURCHASER APPARATUS

A purchaser apparatus stores “anonymous order information including an order ID and purchaser identity verifying information” and “purchaser identity proving secret information”. A shop apparatus stores “anonymous order information including an order ID and purchaser identity verifying information”. If necessary, a purchaser sends a request including an order ID and zero knowledge proving information that a purchaser knows the purchaser identity proving secret information, from a purchaser apparatus to a shop apparatus. A shop apparatus verifies the zero knowledge proving information, based on purchaser identity verifying information retrieved from an order ID included in the request. Therefore, the unlinkability for past use is eliminated if necessary, and inconvenience caused by the unlinkability is eliminated.