Re: [PATCH] CPU Jitter RNG: inclusion into kernel crypto API and /dev/random

[Clemens Ladisch <clemens@ladisch.de>]

Stephan Mueller wrote:
> Am Donnerstag, 7. November 2013, 02:03:57 schrieb Nicholas Mc Guire:
>> On Wed, 06 Nov 2013, Stephan Mueller wrote:
>>> Besides, how on earth shall an attacker even gain knowledge about the
>>> state of the CPU or disable CPU mechanisms? Oh, I forgot, your NSA
>>> guy. But if he is able to do that, all discussions are moot because
>>> he simply disables any noise sources by flipping a bit, reads the
>>> memory that is used to hold the state of the RNG or just overwrites
>>> the memory locations where data is collected, because the general
>>> protection mechanisms offered by the kernel and the underlying
>>> hardware are broken.
>>
>> No need to gain knowledge of the internal CPU state itt would be
>> sufficient to be able to put the CPU in a sub-state-space in which
>> the distribution is shifted. it may be enough to reduce the truely
>> random bits of some key only by a few bits to make it suceptible to
>> brute force attacks.
>
> Note, the proposed RNG contains an unbias operation (the Von-Neumann
> unbiaser) which is proven to remove any bias when it is established that
> the individual observations are independent. And the way the
> observations are generated ensures that they are independent.

"Independent" does not mean that your own code avoids reusing data from
the previous loop iteration; it means that the _entire_ process that
generates the bits is not affected by any memory of the past.

The observations are derived from the internal CPU state, which is *not*
reset between measurements.


Regards,
Clemens