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

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Date: Sat, 2 Mar 2024 16:19:49 -0500
From: Stefan Berger <stefanb@...ux.ibm.com>
To: Lukas Wunner <lukas@...ner.de>, Jarkko Sakkinen <jarkko@...nel.org>
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 3/2/24 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