PROBLEM TO BE SOLVED: To allow contributors and their viewers to exchange messages, while keeping anonymity.;SOLUTION: A message to be put on a bulletin board, a first return anonymity route information which indicates a first transmission route traced from a contributor transmission source, and the content of contribution data packets transmitted to him or herself which includes a first adjacent node identifier that is identification information on an adjacent node, on the first transmission route are registered to a database. When search conditions with respect to message, second return anonymity route information which indicates a second transmission route traced from a searcher transmission route, a second adjacent node identifier that is the identification information on an adjacent node on the second transmission route, and search data packets transmitted to him or herself, which includes transmission source public key for reply data encryption are received, the database is searched according to the search conditions. The detected message, the first return anonymity route information and the first adjacent node identifier are encrypted with the transmission source public key, to transmit it to a node that is indicated by the second adjacent node identifier.;COPYRIGHT: (C)2008,JPO&INPIT
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