A second device comprising: a receiver ; a transmitter ; a master clock ; and a processor configured to: control the receiver to receive a signal from a first device , the signal including a component based on a slave clock of the first device ; authenticate the signal by comparing the component with a master signal, the master signal being based on the master clock ; if the signal is authenticated, perform an operation based on the signal; and if the signal is not authenticated, perform a further authentication process including controlling the transmitter to transmit a challenge signal to the first device and control the receiver to receive a challenge response from the first device , and if the further authentication process is successful, perform an operation based on either the signal or a further received signal received from the first device ; wherein the processor is configured to control the transmitter to transmit a resynchronization signal based on the master clock to the first device for the resynchronization of the slave clock either as at least a part of the challenge signal or as a further transmitted signal transmitted to the first device . a receiver ; a transmitter ; a master clock ; and a processor configured to: control the receiver to receive a signal from a first device , the signal including a component based on a slave clock of the first device ; authenticate the signal by comparing the component with a master signal, the master signal being based on the master clock ; if the signal is authenticated, perform an operation based on the signal; and if the signal is not authenticated, perform a further authentication process including controlling the transmitter to transmit a challenge signal to the first device and control the receiver to receive a challenge response from the first device , and if the further authentication process is successful, perform an operation based on either the signal or a further received signal received from the first device ; wherein the processor is configured to control the transmitter to transmit a resynchronization signal based on the master clock to the first device for the resynchronization of the slave clock either as at least a part of the challenge signal or as a further transmitted signal transmitted to the first device . control the receiver to receive a signal from a first device , the signal including a component based on a slave clock of the first device ; authenticate the signal by comparing the component with a master signal, the master signal being based on the master clock ; if the signal is authenticated, perform an operation based on the signal; and if the signal is not authenticated, perform a further authentication process including controlling the transmitter to transmit a challenge signal to the first device and control the receiver to receive a challenge response from the first device , and if the further authentication process is successful, perform an operation based on either the signal or a further received signal received from the first device ; wherein the processor is configured to control the transmitter to transmit a resynchronization signal based on the master clock to the first device for the resynchronization of the slave clock either as at least a part of the challenge signal or as a further transmitted signal transmitted to the first device . <EOS>