Communication system based on shared cipher key, server unit for the same system, client unit for the same system, and method of sharing cipher key in communication system

摘要

A communication system which is capable of proper cipher communications with many and unspecific parties and of making all the entities share a common key and which does not require the change of its own secret information even if the public information is altered for the security purposes. In the communication system, a shared cipher key is changeable with only the change of a control variable of a server without the change of code figures of clients. Accordingly, the system operation to change the shared cipher key is possible at a short-time internal, which significantly improve the security of the cipher system. In addition, with the server control variable being set to a specific value, a client group belonging to a server can share the same cipher key. Thus, an in-group cipher is decipherable with only the server control in particular situations such as urgent situations.