Group cipher communication method and group cipher communication system

A limited cipher communication system including a plurality of communication terminals each using an IC card and having an input/output interface to said IC card, and a communication network interconnecting said communication terminals, said IC cards having each a memory stored with a value different for individual IC cards to specify them and with secret values constituting a plurality of master keys which are common to a predetermined subset of said IC cards, said communication system comprising:

• in a terminal in which said IC card is inserted,
• means for generating and delivering a destination indicator for specifying a partner to be communicated with;
• means for selecting one of said plurality of master keys stored in said IC card inserted in this terminal on the basis of said destination indicator generated by this terminal or by that generated by an external terminal;
• key generation means for generating a group key by using the selected master key and said destination indicator;
• encipherment means for enciphering a transmission message to be transmitted to said partner using the group key generated from said key generation means;
• and
• decipherment means for deciphering the enciphered message received from said partner through said communication network using the group key from said key generation means.

In a limited broadcast cipher communication system for use in a communication system comprised of a plurality of communication terminals each using an IC card and having an input/output interface to said IC card, and a communication network interconnecting said communication terminals and in which a terminal starting communication generates a destination indicator for specifying a partner to be communicated with to transmit said destination indicator to other terminals, selects one of a plurality of master keys stored in an IC card inserted in said communication starting terminal on the basis of said destination indicator, generates a group key by using the selected master key, and performs encipherment and decipherment of a communication message by using the generated group key to exchange the communication message;

• and a terminal standing for a destination receives said destination indicator from said communication starting terminal, selects one of said plurality of master keys stored in an IC card inserted in said destination terminal on the basis of said destination indicator, generates the group key by using the selected master key, and performs encipherment and decipherment of said communication message by using said generated group key to exchange said communication message, said IC card used in said limited broadcast cipher communication system comprising:
• a CPU;
• an interface for data transfer to and from said terminal;
• a first memory having areas for storing a value specifying individual IC cards and secret values constituting a plurality of master keys which are common to a predetermined subset of said IC cards;
• a second memory for storing a group key generation program;
• and
• group key generation means including said CPU and said second memory, for selecting one master key from said first memory on the basis of said destination indicator and generating a group key by using the selected master key based on said group key generation program.