Abstract: To enable control to be exercised over the use of equipment, a chip is built into the electronic circuitry of the equipment which chip has stored therein a series of pseudo-random numbers. When a crypto unit of the chip receives a first number generated by a controlling computer the pseudo-random number sequence is generated on the basis of an algorithm which is stored in the crypto unit of the chip. The first number is generated when data, including the serial number of the equipment, is entered into the controlling computer. The serial number of the equipment is entered in a read only memory of the chip so that the serial number of the equipment is a permanent and unalterable part of the electronic circuitry. Various uses to which equipment having its serial number entered in a chap thereof are also disclosed.