Re: [PATCH v4 04/12] crypto: ecc - Implement vli_mmod_fast_521 for NIST p521

Stefan Berger
2024-03-03T16:29:31Z

On 3/3/24 06:05, Lukas Wunner wrote:
> On Thu, Feb 29, 2024 at 09:19:59PM -0500, Stefan Berger wrote:
>> +static void vli_mmod_fast_521(u64 *result, const u64 *product,
>> + const u64 *curve_prime, u64 *tmp)
>> +{
>> + const unsigned int ndigits = 9;
>> + size_t i;
>> +
>> + for (i = 0; i < ndigits; i++)
>> + tmp[i] = product[i];
>> + tmp[8] &= 0x1ff;
>
> Hm, the other vli_mmod_fast_*() functions manually unroll those loops.
> Wondering if that would make sense here as well? It's also possible

Why would we do this? One could also argue why not use vli_set() instead
of the loop...

> to tell gcc to unroll a loop with a per-function...
>
> __attribute__((optimize("unroll-loops")))
>
> ...but I'm not sure about clang portability. >
>
>> @@ -941,6 +966,12 @@ static bool vli_mmod_fast(u64 *result, u64 *product,
>> + case 9:
>> + if (!strcmp(curve->name, "nist_521")) {
>> + vli_mmod_fast_521(result, product, curve_prime, tmp);
>> + break;
>> + }
>> + fallthrough;
>
> If you reorder patch 4 and 5, you could check for curve->nbits == 521 here,
> which might be cheaper than the string comparison.

Sure. I thought it's a nist spec for this particular curve, so compare
against 'nist' in the string. Though comparing against nbits also works,
of course.

>
>
>> -#define ECC_MAX_DIGITS (512 / 64) /* due to ecrdsa */
>> +#define ECC_MAX_DIGITS (576 / 64) /* due to NIST P521 */
>
> Maybe DIV_ROUND_UP(521, 64) for clarity?

Ok, will adjust.

Regards,
Stefan

>
> Thanks,
>
> Lukas
>