Abstract: First and second applications are executed in a red domain, and a third application is executed in a black domain, with a lower security level, and via which a ciphered communication tunnel passes between the first and second applications. The first application transmits to the second application a nominal sequence of packets ordered according to their respective sizes; said nominal sequence is intercepted by the third application which, when it wishes to transmit information to the red domain, modifies said nominal sequence by deleting at least one packet, each deleted packet being dependent on said information; on reception of a sequence of packets supposed to be the nominal sequence of packets, the second application checks whether at least one packet has been deleted; and, if such is the case, the second application retrieves said information from the size of each packet thus deleted.