Re: [PATCH v3 01/10] crypto: ecdsa - Convert byte arrays with key coordinates to digits

Stefan Berger
2024-02-29T14:58:42Z

On 2/29/24 04:11, Lukas Wunner wrote:
> On Fri, Feb 23, 2024 at 03:41:40PM -0500, Stefan Berger wrote:
>> +static inline void ecc_digits_from_bytes(const u8 *in, unsigned int nbytes,
>> + u64 *out, unsigned int ndigits)
>> +{
>> + unsigned int sz = ndigits << ECC_DIGITS_TO_BYTES_SHIFT;
>> + u8 tmp[ECC_MAX_DIGITS << ECC_DIGITS_TO_BYTES_SHIFT];
>> + unsigned int o = sz - nbytes;
>> +
>> + memset(tmp, 0, o);
>> + memcpy(&tmp[o], in, nbytes);
>> + ecc_swap_digits(tmp, out, ndigits);
>> +}
>
> Copying the whole key into tmp seems inefficient. You only need
> special handling for the first few bytes of "in" (6 bytes in the
> P521 case) and could use ecc_swap_digits() to convert the rest
> of "in" directly to "out" without using tmp.
>
> So it would be sufficient to allocate the first digit on the stack,
> memset + memcpy, then convert that to native byte order into "in[0]"
> and use ecc_swap_digits() for the rest.
>
> And the special handling would be conditional on "!o", so is skipped
> for existing curves.

Thanks. It looks like this now:

static inline void ecc_digits_from_bytes(const u8 *in, unsigned int nbytes,
u64 *out, unsigned int ndigits)
{
unsigned int o = nbytes & 7;
u64 msd = 0;
size_t i;

if (o == 0) {
ecc_swap_digits(in, out, ndigits);
} else {
for (i = 0; i < o; i++)
msd = (msd << 8) | in[i];
out[ndigits - 1] = msd;
ecc_swap_digits(&in[o], out, ndigits - 1);
}
}

Stefan

>
> Thanks,
>
> Lukas