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.

What is claimed is:  
     
         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.    
     
     
         2 . A validity check system as claimed in  claim 1 , further comprising: 
 a control sub-unit which has display control of said information browser device of said electronic data according to the check result.    
     
     
         3 . A validity check system as claimed in  claim 2 , wherein said control sub-unit further keeps said electronic data from being displayed on said information browser device after said check sub-unit detects a modification of said electronic data.  
     
     
         4 . A validity check unit for checking validity of electronic data, comprising: 
 a storing sub-unit which stores validity checkable information usable for checking the validity of said electronic data stored in an information publisher device; and    a check sub-unit which checks validity of said electronic data which is requested from an information browser device to said information publisher device, by using said validity checkable information and transmits a check result to said information browser device.    
     
     
         5 . A program product comprising computer readable program code which constructs, on a computer, a validity check unit which checks validity of electronic data, said validity check unit comprising: 
 a storing sub-unit which stores validity checkable information usable for checking validity of electronic data stored in an information publisher device; and    a check sub-unit which checks the validity of electronic data which was requested from an information browser device to said information publisher device, by using said validity checkable information, and transmits a check result to said information browser device.    
     
     
         6 . A validity check method for checking validity of electronic data, comprising the steps of: 
 specifying information that is usable for checking validity of electric data which is requested from an information browser device to an information publisher device for checking validity of the requested electronic data, among validity checkable information usable for checking the validity of each electronic data stored in said information publisher device; and    checking the validity of said electronic data using the information specified, and transmitting a validity check result of said electronic data to the information browser device.    
     
     
         7 . A validity check method as claimed in  claim 6 , further comprising the step of: 
 performing display control on said information browser device of said electronic data device according to the check result.    
     
     
         8 . A validity check method as claimed in  claim 7 , wherein said step of controlling display further comprises the steps of: 
 keeping said electronic data from being displayed in the information browser device after a modification of said electronic data is detected.