A method of encrypting information to be transmitted by a transmitting terminal in a communication system providing secure communications between a plurality of terminals employing an encryption protocol that uses a shared encryption key in each of the plurality of terminals, the method including modifying the share encryption key in the transmitting terminal to provide a modified encryption key and encrypting the information to be transmitted using the modified encryption key, characterised in that the share encryption key is modified by combining with the shared encryption key identifier code used in the communication system to identify the transmitting terminal. A corresponding method of decryption and a system and mobile radio terminal using the method are also described.
