3.2.2. Using Existing Disk Drives

Disk drives have small random fluctuations in their rotational speed due to chaotic air turbulence [DAVIS, Jakobsson]. The addition of low-level disk seek-time instrumentation produces a series of measurements that contain this randomness. Such data is usually highly correlated, so significant processing is needed, as described in Section 5.2 below. Nevertheless, experimentation a decade ago showed that, with such processing, even slow disk drives on the slower computers of that day could easily produce 100 bits a minute or more of excellent random data.

Every increase in processor speed, which increases the resolution with which disk motion can be timed or increases the rate of disk seeks, increases the rate of random bit generation possible with this technique. At the time of this paper and with modern hardware, a more typical rate of random bit production would be in excess of 10,000 bits a second. This technique is used in random number generators included in many operating system libraries.

Note: the inclusion of cache memories in disk controllers has little effect on this technique if very short seek times, which represent cache hits, are simply ignored.