Fixed range of random produced in BN_is_prime_fasttest_ex() #6547
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… rand < w-1. It was using 1<= rand < w (which is wrong by 1 on both ends)
paulidale
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Jun 21, 2018
levitte
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approval: review pending
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Should this be backported to 1.1.0? |
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The probability of hitting either of the end points is low: 2^(1-w). I think hitting the low end could reject a legitimate prime, which would be a slow down, but would otherwise be harmless. I'm not sure about the high end -- I'd need to check for being even and if not work through the mathematics. If this is back ported to 1.1.0, it should also go to 1.0.2. I don't see a need for either but would be easily convinced otherwise. |
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We don't claim FIPS compliance, so this seems like just a bugfix for master. |
richsalz
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Jun 21, 2018
richsalz
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approval: done
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I'm happy with just master. Looking more deeply, it seems that the only result at the limits is that a prime number can be declared not prime which shouldn't be harmful. |
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Merged to master only. |
levitte
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Jun 21, 2018
… rand < w-1. It was using 1<= rand < w (which is wrong by 1 on both ends) Reviewed-by: Paul Dale <paul.dale@oracle.com> Reviewed-by: Rich Salz <rsalz@openssl.org> (Merged from #6547)
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Thanks! |
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According to the FIPS 186-4 C.3.1 (4.1) The range is expected to be 1 < b < w-1.
It was using 1 <= b < w (which is wrong by 1 on both ends).
This is because rand_range(n) returns 0 <= b < w-1 (and then it added 1).
So instead use 2 + rand_range(w-3).
There is also a check for the number '3' added that early exits (in the case where trial_divisions is not used).
Also removed a redundant check for is_zero after subtracting 1 as it already checks <= 1
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