According to an aspect of the invention, there is provided a device for generating a unique response to a challenge, the device comprising: a plurality of structures, each structure being able to change from a first distinct state, to a second distinct state, in response to an appropriate input challenge; the device being arranged to facilitate a challenge of the plurality of structures in combination, by changing an input to the plurality of structures in combination, to cause each structure of the plurality of structures to change from the first distinct state, to the second distinct state; as part of the challenge, the device being arranged to facilitate a measurement of an output of the plurality of structures in combination, in response to the input; wherein the unique response is at least indicative of a sequence in which the change in state takes place for each of the plurality of structures, in response to the input.