On 09/06/2010 09:49 PM, John Denker wrote:

If anybody can think of a practical attack against the randomness
of a thermal noise source, please let us know.  By "practical" I
mean to exclude attacks that use such stupendous resources that
it would be far easier to attack other elements of the system.

Blast it with RF for one.

Typically the natural thermal noise amounts to just a few millivolts, and so requires a relatively sensitive A/D converter. This makes it susceptible to injected "unnatural noise" overloading the conversion and changing most of the output bits to predictable values.

Using digital outputs from an enclosed module with enough shielding could probably prevent it. But there are plenty of environments which are too small (e.g., smart cards) or are potentially in the hands of the attacker for an extended period of time (smart cards, DRM devices, power meters, etc.).

- Marsh

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