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

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Date: Fri, 01 Mar 2024 22:26:29 +0200
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Stefan Berger" <stefanb@...ux.ibm.com>, "Lukas Wunner"
 <lukas@...ner.de>
Cc: <keyrings@...r.kernel.org>, <linux-crypto@...r.kernel.org>,
 <herbert@...dor.apana.org.au>, <davem@...emloft.net>,
 <linux-kernel@...r.kernel.org>, <saulo.alessandre@....jus.br>
Subject: Re: [PATCH v3 01/10] crypto: ecdsa - Convert byte arrays with key
 coordinates to digits

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);

BR, Jarkko