PURPOSE: To make a common key generating time of the system almost independent of the increase in number of participants of the system by dividing a receiver terminal equipment into plural groups and devising the system so that only ID information by a group participant for generating a group common key is required in order that each receiver acquires a secret key.
CONSTITUTION: This system consists of plural groups 103,104 each comprising plural reception stations and an information service station 101. The information service station 101 generates a secret key for service information cryptography and uses the secret key to cipher the service information, and generates a common key of each group corresponding to a distribution destination information string comprising each subscriber identifier of each group, uses the common key to cipher the secret key and sends the service information and the secret key to be ciphered respectively by means of multiple address communication. Each reception station receives the information from the information service station 101 and generates a common key of each group from the distribution destination information string for each group and uses the common key to decode a ciphered secret key. When the subscriber identifier is not included in the distribution destination information string of the group, the generation of the common key of the group is inhibited.
TAKARAGI KAZUO
NAKAMURA TSUTOMU
