Patent ID: 7711113

Claim:
A private key generation apparatus in an ID-based signature and encryption system that comprises an encryption apparatus and a decryption apparatus and can use any character string as a public key, wherein: said private key generation apparatus comprises: a control and arithmetic unit, and a storage unit, wherein said control and arithmetic unit comprises: a private key generation and issuing means, which generates public parameters and a master key used in the entire system and stores the generated public parameters and master key into said storage unit, and uses said master key of said storage unit for generating a private key corresponding to a client's public key in response to a request of a client's computer, to issue the generated private key to said client's computer as a requester, where the private key is a most elemental private key initially-formed in the system; and a parameter publication means, which publishes said public parameters of said storage unit; and said private key generation and issuing means: adds g=e(P, P) (e is a bilinear mapping called a pairing) calculated in advance using a selected element P of a group of order q to said public parameters of said storage unit; and defines two elements P 1 and P 2 of said group as P 1 =s 1 P and P 2 =s 2 P, using random numbers s 1 and s 2 as a part of said master key, with s 1 and s 2 being included in a set Z q * of positive integers less than said order q and relatively prime with q, to calculate (s 1 +us 2 ) −1 P as the private key which is the most elemental private key initially-formed in the system.