A system comprising: a participant subsystem that is authorized to anonymously participate in a plurality of sessions using the same secret information provided by a manager subsystem, all of said secret information being transmitted to the participant prior to participation in a first of said plurality of sessions; and a reception subsystem for determining whether it is acceptable for the participant subsystem to participate in a session, characterized in that the participant subsystem comprises: an anonymous signing section for authorizing particular individual data using the secret information depending on session-related information to produce anonymous participation data with anonymous signature, wherein the reception subsystem comprises: an anonymous signature determining section for determining whether received data is anonymous participation data with anonymous signature authorized by the participant subsystem; and a sender match determining section for determining whether anonymous signatures of two different pieces of anonymous participation data representing two different contents of individual data that are received in a same session of said plurality of sessions are signed by an identical participant subsystem. a participant subsystem that is authorized to anonymously participate in a plurality of sessions using the same secret information provided by a manager subsystem, all of said secret information being transmitted to the participant prior to participation in a first of said plurality of sessions; and a reception subsystem for determining whether it is acceptable for the participant subsystem to participate in a session, characterized in that the participant subsystem comprises: an anonymous signing section for authorizing particular individual data using the secret information depending on session-related information to produce anonymous participation data with anonymous signature, wherein the reception subsystem comprises: an anonymous signature determining section for determining whether received data is anonymous participation data with anonymous signature authorized by the participant subsystem; and a sender match determining section for determining whether anonymous signatures of two different pieces of anonymous participation data representing two different contents of individual data that are received in a same session of said plurality of sessions are signed by an identical participant subsystem. an anonymous signing section for authorizing particular individual data using the secret information depending on session-related information to produce anonymous participation data with anonymous signature, wherein the reception subsystem comprises: an anonymous signature determining section for determining whether received data is anonymous participation data with anonymous signature authorized by the participant subsystem; and a sender match determining section for determining whether anonymous signatures of two different pieces of anonymous participation data representing two different contents of individual data that are received in a same session of said plurality of sessions are signed by an identical participant subsystem. an anonymous signature determining section for determining whether received data is anonymous participation data with anonymous signature authorized by the participant subsystem; and a sender match determining section for determining whether anonymous signatures of two different pieces of anonymous participation data representing two different contents of individual data that are received in a same session of said plurality of sessions are signed by an identical participant subsystem.