On Tuesday, November 17, 2015 at 4:38:44 AM UTC-5, Jeffrey Walton wrote:
>
>
> My question is, how can we craft a test case to tickle that finding?
>>
>> I've looked at the issue and my conclusion is that the system is making a
>> mistake (may be wrong though).
>>
>> It assumes that t1 may get up to 63 in step 27, it then goes through a
>> standard-for-loop, noticing that j=0 < 1 = 64 - 63 in step 28 and 29,
>> meaning we get back to the start of the loop (step 30) but now we increment
>> to j=1 (step 31) and check if j = 1 < 1 = 64 - 63 (step 32), which is
>> obviously false, so t1 + j couldn't get 63 + 1 (which results undefined
>> behavior) but only 63 + 0 (which is allowed).
>>
> OK, thanks. I'm still not clear on it. Its not your analysis, my analysis
> or Coverity's analysis... Its simply error'ing on the side of caution.
>
> Here's what we are interested int:
>
> for (unsigned int j=0; j<WORD_BITS-t1; j++)
> temp ^= ((temp >> j) & 1) << (t1 + j);
>
> temp ^= ((temp >> j) & 1) << (t1 + j) reduces to:
>
> x << (t1 + j);
>
> Here's the equality we are interested in:
>
> (t1 + j) < WORD_BITS-t1
>
> And it must be less than WORD_BITS. So we should be able to assert on:
>
> 2 * t1 + j < WORD_BITS
>
> Does that sound about right?
>
This assert fires:
EC2N validation suite running...
Assertion failed: (2*t1+j < WORD_BITS), function MultiplicativeInverse,
file gf2n.cpp, line 720.
Abort trap: 6
We have to get to the bottom of this so we can clear the finding. I can't
let Dr. Guttman's Cryptlib exceed us
(http://scan.coverity.com/projects/cryptlib).
Dr. Guttman is a good guy. He's writes some of the tightest, cleanest code
I have seen. He also writes self-debugging code, which is rare in the open
source world. It probably explains why his defect rate is so low: his code
alerts him of problems, so he does not have to waste time tracking problems
down.
Jeff
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