Do we now learn that they fixed "always generate all 1s" with "never generate all 0s"??
EDIT: I've been unable to reproduce the problem on my CPU, FWIW. It's a Ryzen 5 3600.
EDIT2: OK, update, I can reproduce it with rdrand16, rdrand32 is fine but rdrand16 can never generate all 0s. So my CPU does have this problem!
return 4 # Determined by fair dice roll.Most of the console hacking talks are great, both informative and entertaining.
Basically I'm wondering if it's a bug in the version of the instruction that writes to a 16-bit reg, or a bug in the underlying RNG
*: missed a word the first time around
By your argument, it would not be a problem if the RNG never generated 0. So, it must follow that it would also not be a problem if it never generated {1, 2, 3, ..., 253}.
That means that our RNG now only generates the values 254 and 255. Which of the values is generated is unpredictable on any given call. However, 7 of the 8 output bits are now always fixed and so completely predictable. Can you imagine how an attacker could exploit that?
Failing to generate only the number 0 is a weaker version of the same class of flaw.
See section 7.3.17 of the Intel SDM, and how NIST SP800-90A (which the SDM refers to) defines "random number".
So it is not necessarily that it doesn't generate zero, they did not run enough times to increase the probability of actually generating a zero.
You definitely would expect a roughly equal number of 0s as any other of those numbers since it's uniformly distributed. And definitely not 0
They also write:
> Running the same programs on an Intel processor, and the 0's are there with no problem.
It is also possible that their code was generating too many zeros and the easiest fix was to discard them all.
I'm guessing you don't think there are people calling rdrand in a loop and throwing away the output with high probability except when it is 0, but I can't see how else you imagine people would be vastly more likely to use the output when it is 0?
