Side-channel silent modular inverse

[Niels Möller]

Torbjorn Granlund <tg at gmplib.org> writes:

> * mpn_sec_add_1
> 
> I'd say use the obvious algorithm: Create vector of n-1 zeros and then
> the input limb arg at index 0, invoke mpn_add_n.

That's good enough, I guess, at the cost of some extra scratch space.

> * mpn_cnd_neg
>
> Create zero vector, invoke mpn_sub_n.

That doesn't make it conditional. And I see no obvious way to do
conditional negation on top of mpn_cnd_sub_n.

Regards,
/Niels

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