To propose a technique for solving a key delivery problem. Both of a client and a server have a function of generating the same solution at the same date and time based on the same initial solution. The client sends identification information for identifying the client to the server (S1002). The client and the server generate the same solution with time synchronization based on the initial solution (S1003 and S2002). The client and the server perform encrypted communication using the same key as a common key (S1004 and S2003).