Received-SPF: pass (google.com: domain of linux-kernel+bounces-89082-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Precedence: bulk
Mime-Version: 1.0
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset=UTF-8
Date: Fri, 01 Mar 2024 22:26:29 +0200
Subject: Re: [PATCH v3 01/10] crypto: ecdsa - Convert byte arrays with key
 coordinates to digits
From: "Jarkko Sakkinen" <jarkko@kernel.org>
To: "Stefan Berger" <stefanb@linux.ibm.com>, "Lukas Wunner"
 <lukas@wunner.de>
Cc: <keyrings@vger.kernel.org>, <linux-crypto@vger.kernel.org>,
 <herbert@gondor.apana.org.au>, <davem@davemloft.net>,
 <linux-kernel@vger.kernel.org>, <saulo.alessandre@tse.jus.br>
Message-Id: <CZIOY02QS2QC.LV0A0HNT7VKM@suppilovahvero>
References: <20240223204149.4055630-1-stefanb@linux.ibm.com>
 <20240223204149.4055630-2-stefanb@linux.ibm.com>
 <20240229091105.GA29363@wunner.de>
 <aabeec7b-618c-4d15-b033-4162b6e54f6a@linux.ibm.com>
In-Reply-To: <aabeec7b-618c-4d15-b033-4162b6e54f6a@linux.ibm.com>

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 n=
bytes,
> >> +					 u64 *out, unsigned int ndigits)
> >> +{
> >> +	unsigned int sz =3D ndigits << ECC_DIGITS_TO_BYTES_SHIFT;
> >> +	u8 tmp[ECC_MAX_DIGITS << ECC_DIGITS_TO_BYTES_SHIFT];
> >> +	unsigned int o =3D sz - nbytes;
> >> +
> >> +	memset(tmp, 0, o);
> >> +	memcpy(&tmp[o], in, nbytes);
> >> +	ecc_swap_digits(tmp, out, ndigits);
> >> +}
> >=20
> > 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.
> >=20
> > 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.
> >=20
> > 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 nbyte=
s,
>                                           u64 *out, unsigned int ndigits)
> {
>          unsigned int o =3D nbytes & 7;
>          u64 msd =3D 0;
>          size_t i;
>
>          if (o =3D=3D 0) {
>                  ecc_swap_digits(in, out, ndigits);
>          } else {
>                  for (i =3D 0; i < o; i++)
>                          msd =3D (msd << 8) | in[i];
>                  out[ndigits - 1] =3D msd;
>                  ecc_swap_digits(&in[o], out, ndigits - 1);

This would be more stream-lined IMHO:

        unsigned int o =3D nbytes & 7;
	unsigned int n =3D ndigits;
        u64 msd =3D 0;
        size_t i;

        if (o !=3D 0) {
                for (i =3D 0; i < o; i++)
                        msd =3D (msd << 8) | in[i];

                out[--n] =3D msd;
        }

        ecc_swap_digits(in, out, n);

BR, Jarkko