Abstract: An electronic key comprises a first binary counter and a first multiplexer or pulse code modulating a uniform clock pulse train to produce a first coded pulse train modulated with a given code. The key is connected to an electronic lock comprising: a second binary counter and a second multiplexer for pulse code modulating the same clock pulse train to produce a second coded pulse train modulated with the same code; an EXCLUSIVE-OR gate to which said first and second multiplexers are connected; a first NAND gate having one input connected to the output of the EXCLUSIVE-OR gate; a flip-flop or a binary counter having its clock input connected to the output of the NAND gate and an inital logical ONE output connected to a second input of the NAND gate; a second NAND gate having one input connected to the output of the flip-flop or counter and another input connected to the output of the second multiplexer; and a binary output counter having its clock input connected to the output of the second NAND gate.