| 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.
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