A system including at least two parts or stations wherein a transaction or connection between any two or more of the parts or stations is conducted or established by means of an access code, the access code being available to an accessed part or station and requiring an identical access code to be provided to an accessing part or station at the time of conducting the transaction or establishing the connection. The system is characterised in that the access code is one of a plurality of codes provided to the accessed part or station and available to the accessing part or station. The system is further characterised in that the access code is selected from the plurality of codes at the time of conducting the transaction or establishing the connection, such that no two transactions are conducted or no two connections are established with the same access codes.