Rekeying Method in secure Group in case of user-join and Communicating System using the same

The group key update method is disclosed in the secret group. How to update the group key according to the invention, a) the step of using the member authentication transmits the member's private key to the new member; b) generating a new group key using a key generating function; c) members comprising: using a private key encryption to transfer to the new members of the new group key; And d) comprises the step of transmitting the key conversion flag that indicates to the existing member that the group key is updated, where the key generation function group existing from the key generates a new group key, a new group using a conventional key group It characterized in that non-Dieter mini stick (deterministic) of the function to produce the key. According to the invention, the group keys are reduced amount of calculation and traffic at the time of update.