Security system

A challenge-response security system suitable for a vehicle 10 is disclosed. The system comprises a security control unit 12 which communicates with a remote transponder 14. The security control unit 12 is arranged in use to send out a challenge signal when it determines that the transponder 14 is likely to be in the proximity of the vehicle 10. So as to reduce the likelihood of challenge response pairs being bugged during the time it takes to run the information bus 22 up to its operating condition, a batch of challenge signals are pre-generated by a random number generator 16 forming part of the security control unit 12 and are pre-loaded into a memory 26 of the door module 18. When a door handle 24 is operated, the door module 18 sends the pre-loaded challenge signals out in sequence to the transponder 14. This also helps reduce the propagation delay which might exist if a user had to wait for the information bus 22 to become fully operational.