In a system comprising a broadcasting subsystem and a radiocommunication subsystem each connected to a mediation platform, said transmission being ciphered and arranged for being deciphered by using a key, the following steps are performed at the mediation platform, relatively to a user or group of users having terminals: determining one first and at least one second sets of values from said key, so that the knowledge of all the determined sets of values is required to retrieve the key; and controlling the broadcasting subsystem so that it transmits the first set of values to the user or group of users and the radiocommunication subsystem so that it transmits the at least one second set of values to the user or group of users.