Device authentication and encrypted communication system offering increased security

A system composed of a plurality of user devices, each storing unique secret information, a system device and a control unit. The control unit produces key capsule data by performing a digital signature conversion with message recovery on the unique secret information for each user device and distributes the unique secret information to the user devices. When device authentication and encrypted communication is performed, each user device transmits the key capsule data distributed by the control unit to the system device. The system device receives the data and recovers the unique secret information from the key capsule data by a digital signature verification conversion with message recovery, which uses the verification key distributed in advance by the control unit. Thus, the user device and the system device can share unique secret information and, using it as a key, perform device authentication and encrypted communication by performing encryption and decryption based on a secret key encryption algorithm.