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

[Lukas Wunner <lukas@wunner.de>]

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,

Lukas