1. A proximity device comprising: a radio interface providing remote access to information stored persistently on the device, and to circuitry for generating a volatile response on the device; a storage for persistent information coupled to the radio interface for storing persistent values on the device; circuitry coupled to the radio interface for generating a volatile response, wherein the circuitry for generating the volatile response includes a signal delay component configurable to form a plurality signal delay paths, each delay path being configurable such that a signal propagation characteristic of the delay path depends on at least one of a persistent value stored in the device and a challenge value received at the device over the radio interface, the signal propagation characteristic of the delay path depending on fabrication characteristics of the device that are substantially unique to the device from amongst other devices fabricated according to a common design, a comparison component coupled to the signal delay component, for comparing the signal propagation characteristics of multiple of the signal delay paths, and an accumulation component coupled to the comparison component for accumulating values output from the comparison component to determine the volatile response.