Generating a unique hardware identifier using clock signals

A unique identifier for a hardware computer apparatus, such as a personal computer (PC), is generated by measuring the relative deviations of clock signals produced by individual components of the apparatus e.g. by measuring the relative deviations of audio, video, CPU and system clock signals. The step of measuring the relative deviations may comprise measuring the deviations of clock signals relative to a reference clock signal, which may be the CPU or system clock signal. The unique identifier may be stored locally or at an external storage device for later comparison with a re-generated identifier. Several applications are disclosed including use of the identifier in a digital rights management (DRM) system. In one embodiment the identifier may be used as an encryption key.