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

[Lukas Wunner <lukas@wunner.de>]

On Fri, Mar 01, 2024 at 10:26:29PM +0200, 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);

Maybe eliminate the for-loop as well?

	unsigned int o = nbytes & 7;
	u64 msd = 0;

	if (o != 0) {
		/* if key length is not a multiple of 64 bits (NIST P521) */
		memcpy((u8 *)&msd + sizeof(msd) - o, in, o);
		out[--ndigits] = be64_to_cpu(msd);
		in += o;
	}

	ecc_swap_digits(in, out, ndigits);