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

[Stefan Berger]

On 3/1/24 15:26, Jarkko Sakkinen wrote:
> On Thu Feb 29, 2024 at 4:57 PM EET, Stefan Berger wrote:
> > 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);
>
> This would be more stream-lined IMHO:
>
>          unsigned int o = nbytes & 7;
> 	unsigned int n = ndigits;
>          u64 msd = 0;
>          size_t i;
>
>          if (o != 0) {
>                  for (i = 0; i < o; i++)
>                          msd = (msd << 8) | in[i];
>
>                  out[--n] = msd;
>          }
>
>          ecc_swap_digits(in, out, n);

You forgot to advance 'in'.

> BR, Jarkko