Abstract: An instrument for making secure messages exchanged between each of the actors of a network having a plurality of actors, each being capable of acting as a sender or as an addressee of a message that is encoded or sealed, the instrument being characterized in that it comprises, for each actor, a respective device (E1) including an integrated circuit with memory zones (2, 3) and a microprocessor (1) capable of executing at least one algorithm and of controlling access to said zones (2, 3) as a function of the nature of the operation requested thereof by the operator holding the device (E1), the operations being selected from the following: encoding (6, 7); calculating a key to be communicated (8); and decoding (9, 10); the memory zones (2 and 3) containing two types of base key that are masked, including a base key (T1) of a first type which is specific to each device (E1), and at least one base key (U) of a second type which is common to all of the devices.