A communication system has a first and a second communicating device operable to send and receive data units through a communication channel. Some of the data are encrypted using a security key. The first device comprises a first key generator generating a first embodiment of the key independently of a second embodiment of the key generated by a second generator of the second device, the second embodiment being generated independently of the first, which depends on parameter(s) characterizing a first transmission quality of the channel when receiving a first set of unencrypted data sent by the second device. The second embodiment depends on parameter(s) characterizing a second transmission quality of the channel when receiving a second set of unencrypted data sent by the first device, the first set being different from the second set.