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SubjectRe: [RFC][PATCH] Pseudo-random number generator
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On Mon, 2006-12-04 at 11:15 -0500, Valdis.Kletnieks@vt.edu wrote:
> On Fri, 01 Dec 2006 14:19:15 +0100, Jan Glauber said:
> > New s390 machines have hardware support for the generation of pseudo-random
> > numbers. This patch implements a simple char driver that exports this numbers
> > to user-space. Other possible implementations would have been:
>
> > + for (i = 0; i < 16; i++) {
> > + entropy[0] = get_clock();
> > + entropy[1] = get_clock();
> > + entropy[2] = get_clock();
> > + entropy[3] = get_clock();
>
> By the time this loop completes, we'll have done 64 get_clock() - and if an
> attacker has a good estimate of what the system clock has in it, they'll be
> able to guess all 64 values, since each pass through the loop will have fairly
> predictable timing. So as a result, the pseudo-random stream will be a *lot*
> less random than one would hope for...

I completely agree. Filling the input buffer with timestamps looks quite
uncomfortable but was exactly what the hardware specification suggested.

At least for the initialisation of the PRNG I preferred get_random_bytes()
over get_clock to get a good initial seed. But get_random_bytes cannot
be used during normal operation since the PRNG read should not block.

> > + /*
> > + * It shouldn't weaken the quality of the random numbers
> > + * passing the full 16 bytes from STCKE to the generator.
> > + */
>
> As long as you realize that probably 12 or 13 or even more of those 16 bytes
> are likely predictable (depending exactly how fast the hardware clock ticks),
> and as a result the output stream will also be predictable.

Yes, if an attacker knows the initial clock value a brute-force attack
would be feasible to predict the output. But I don't know if the
hardware completely relies on the clock values or if there is any
internal state which is not visible by an attacker. I will try to find
out more details...

Jan


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