Upon a license-data transmitter verifying a certificate C[KPdx] (the license-data receiver and the license-data transmitter will be referred to as “x” and “y”, respectively), the license-data transmitter transmits challenge information Ep(KPdx, Kcy) to the license-data receiver. In response to the challenge information, the license-data receiver transmits session information Es(Kcy, Ksx//KPpy) to the license-data transmitter. The license-data transmitter provides encrypted license data Es(Ksx, Ep(KPpx, LIC)) in a form encrypted using these two keys Ksx and KPpx thus received. With the cryptosystem according to the present invention employing EC-DH as the public key cryptosystem, the transmitter provides the second or subsequent license data without transmission of the challenge information and without updating the shared key.