Patent ID: 7995762

Claim:
A key issuing method for being performed by a user apparatus in a group signature system including said user apparatus and an issuer apparatus connected to said user apparatus through a network, comprising: reading an issuer public key from said issuer apparatus into a user storage through said network; receiving, from said issuer apparatus through said network, first confidential data including one or plural confidential texts which are produced by confidentializing said issuer public key using element data containing information of an element of a group in said issuer apparatus; performing a second confidential data generating process for generating second confidential data of a confidential text represented by a product of modulo-exponentiated element data corresponding to said confidential texts included in said first confidential data or a confidential text represented by the sum of said element data multiplied by a constant, using said issuer public key and said first confidential data; sending said second confidential data to said issuer apparatus through said network; receiving, from said issuer apparatus through said network, information generated in said issuer apparatus and based on said element data corresponding to said second confidential data; and generating a member public key which is a public key corresponding to said user apparatus and a member secret key which is a secret key corresponding to said user apparatus, using information based on said element data corresponding to said second confidential data, and writing said member public key and said member secret key into said user storage.