Patent ID: 7822733

Claim:
A central server comprising: a retaining unit that generates and retains key data to be attached to a user search history and to be connected with an identifying information of a first user, when the first user has performed a search operation; a sending unit that sends the key data to a user terminal operated by the first user; and an identifying unit that identifies the first user by verifying matching between the key data retained by the retaining unit and key data included in the user search history of the first user, when a second user obtains information by referencing the user search history of the first user, wherein the key data is data pertaining to the user search history; the key data included in the user search history is encrypted with a private key which is pre-assigned to the first user; and when the second user refers to the user search history including the encrypted key data, the encrypted key data is further encrypted with a private key which is pre-assigned to the second user and the double encrypted key data is sent to the central server, wherein the identifying unit decrypts the double encrypted key and verifies the matching.