Communication system, server device, user device, method, and computer program

Abstract

The present invention provides a secure technique that allows two communication apparatus that perform encrypted communication to have a common initial solution. A large number of user apparatuses all have a function of generating the same solution under the same condition as far as the user apparatuses have the same initial solution, and can perform encrypted communication using solutions successively generated in synchronization from the same initial solution. All the user apparatuses and a server share the same initial solution and have a function of generating the same solution under the same condition and thus can generate synchronized solutions. The server generates synchronization information, which is information required to generate the initial solution but is not the initial solution itself (S2002), and transmits the synchronization information to at least one of two user apparatuses performing encrypted communication (S2003). The two user apparatuses generate the same initial solution based on the synchronization information (S1004). Then, the two user apparatuses perform encrypted communication (S1005).