-Michael Heyman Wrote: Before we give up on using drive timings [as an entropy source], does anyone have evidence to verify this assertion [that SSD drives will have much less variation in read/write timing]? The reviews I have seen using tools like HD Tune and HD Tach seem to show timing noise reading and writing SSDs. I don't know where the noise comes from - it is probably not turbulence <grin/> - but it may be random enough that a long series of tests, say for a second or so (don't forget, these drives are fast), could provide a nice pool of unguessable bits. ======================
I probably should not be commenting, not being a real device guy. But, variations in temperature and time could be expected to change SSD timing. Temperature changes will probably change the power supply voltages and shift some of the thresholds in the devices. Oscillators will drift with changes in temperature and voltage. Battery voltages tend to go down over time and up with temperature. In addition, in some systems the clock frequency is purposely swept over something like a 0.1% range in order to smooth out the RF emissions from the device. (This can give a 20 or 30 dB reduction in peak emissions at a given frequency. There is, of course, no change in total emissions.) Combine all of these factors, and one can envision the SSD cycles taking varying numbers of system clock ticks and consequently the low order bits of a counter driven by a system clock would be "random." However, one would have to test this kind of entropy source carefully and would have to keep track of any changes in the manufacturing processes for both the SSD and the processor chip. Is there anyone out there who knows about device timing that can say more? Chuck Jackson --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to majord...@metzdowd.com