Cryptographic randomness from air turbulence in disk drives

Abstract

A computer disk drive’s motor speed varies slightly but irregularly, principally because of air turbulence inside the disk’s enclosure. The unpredictability of turbulence is well-understood mathematically; it reduces not to computational complexity, but to information losses. By timing disk accesses, a program can efficiently extract at least 100 independent, unbiased bits per minute, at no hardware cost. This paper has three parts: a mathematical argument tracing our RNG’s randomness to a formal definition of turbulence’s unpredictability, a novel use of the FFT as an unbiasing algorithm, and a “sanity check” data analysis.