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

Lukas Wunner
2024-03-02T14:00:03Z

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);