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

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Date: Fri, 1 Mar 2024 15:48:28 -0500
From: Stefan Berger <stefanb@...ux.ibm.com>
To: Jarkko Sakkinen <jarkko@...nel.org>, 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 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
>