Digital data authentication method

Abstract

This invention provides a method for identifying a purchaser who purchased content from which an illegal copy was produced. A provider system encrypts a content purchased by the purchaser using a public key of a purchaser system and sends the encrypted content to the purchaser system. The purchaser system creates a digital signature of the content with the use of a private key of its own and embeds the created digital signature into the received content. When an illegal copy is found, the provider system verifies the digital signature, embedded in the illegal copy as a digital watermark, to identify the purchaser who purchased the content from which the illegal copy was produced.

1 . A validity check system having an information browser device that displays electronic data, an information publisher device that stores electronic data in a state that said electronic data can be displayed in said information browser device, and a validity check unit that checks validity of said electronic data, 
 said validity check unit comprising:    a storing sub-unit which stores validity checkable information usable for checking validity of said electronic data stored in said information publisher device; and    a check sub-unit which checks the validity of electronic data which was requested from said information browser device to said information publisher device, by using said validity checkable information and transmits the check result to said information browser device.