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

[Stefan Berger <stefanb@linux.ibm.com>]

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