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

Stefan Berger
2024-03-02T21:20:05Z

On 3/2/24 09:00, Lukas Wunner wrote:
> 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);
>
Will take this for v5 with your Suggested-by:, ok?

Stefan