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

Stefan Berger
2024-03-01T20:48:44Z

On 3/1/24 15:26, 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);

You forgot to advance 'in'.

>
> BR, Jarkko
>