Environmentally dependent physically unclonable function device

A physically unclonable function (PUF) device comprising a plurality of conductors 2, at least some of which are arranged so that they interact electrically and/or magnetically with one another. A media 3 surrounds at least of portion of each of the conductors and further defines at least one cavity 10, where the cavity is structured to provide the device with an environmentally dependent characteristic. Circuitry 1 applies an electrical challenge signal to at least one of the conductors and receives an electrical output 5 from at least one of the other conductors to generate an identifying response to the challenge signal that is unique to the device. The device further comprises a temperature variation means such that the circuitry generates an identifying response which varies with the temperature of the device. The cavity may be an impermeable enclosure and the environmentally dependent characteristic may be permittivity and/or permeability. Alternatively the characteristic may be at least one of pressure, temperature, humidity, magnetic field, electric field, light/radiation levels or type of fluid in the environment surrounding the device. The temperature varying means may be provided by heating wires.