a method of securely transferring data from a transmitter to a receiver which includes the steps of at the transmitter encrypting data which at least in part is based on timer information at the transmitter , to form a transmission word , transmitting the transmission word to the receiver , at the receiver decrypting the transmission word , validating the transmission word by comparing the transmitted timer information to predetermined information at the receiver ; and when a valid transmission word is received adjusting the said predetermined information .