A device for generating a nondeterministic response to a challenge (e.g. for use in a random number generator), the device comprises: a structure exhibiting a nondeterministic electrical output response to an electrical input, the device being arranged to facilitate a challenge of the structure to generate the nondeterministic response, by facilitating an electrical measurement of an output of the structure, the nondeterministic response being derivable from that measurement. The structure preferably exhibits negative differential resistance (e.g. resonant tunneling diode; memristor; Gunn diode). The electrical output response preferably is linked to a state-change in the structure, and/or comprises a change in electrical output from a first level to-or-beyond a second, threshold, level. The time taken for the electrical output response to be trigged may be measured, with the non-deterministic response derivable from the time measurement. The structure preferably exhibits quantum mechanical confinement, and is arranged to provide a unique response linked to the atomic makeup of the structure defining the quantum mechanical confinement, in response to the challenge.