[openssl-dev] Partially- vs. full- reduced inputs to ecp_nistz256_neg

[Andy Polyakov]

>> * Fix ecp_nistz256_mul_by_2 and ecp_nistz256_mul_by_3 to fully reduce
>> their outputs.
>>
>> * Fix ecp_nistz256_add to fully reduce its output.
> 
> As for specifically addition see below. As for fixing mul_by_[23] and
> the fact that they use addition. There are two ways. a) Modify addition
> so that it *preserves* property of being fully reduced and leave
> mul_by_[23] as is. b) Let addition as is and add additional step to
> mul_by_[23]. The choice of approach can be platform-specific. For
> example on x86_64 a) is simpler and appears more efficient.

Consider attached diff.

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