METHOD AND APPARATUS FOR DECRYPTING A COMMUNICATION

摘要

A decryption apparatus (109) comprises a key stream generator (111) generating a local decryption key stream. It furthermore comprises a synchronisation value receiver (201) receiving key stream synchronisation values. A synchronisation processor (203) implements a state machine which may operate in a synchronised state (303) wherein the communication is decrypted using the local key stream, a non-synchronised state (301) wherein the local key stream is not synchronised, or in an uncertain synchronisation state (305) wherein the communication is decrypted using the local key stream and wherein the local key stream is synchronised to each new received synchronisation value. The synchronisation processor (203) furthermore comprises a transition controller (213) operable to transition from the synchronised state to the non-synchronised state in response to a first criterion and to the uncertain synchronisation state in response to a second criterion. The first criterion preferably evaluates receive errors of the synchronisation values whereas the second criterion evaluates parameters not related to the encryption or decryption process. The apparatus may be used with a TETRA system, and should allow faster resynchronisation on cell handover.