linux-kernel - Re: [PATCH v5 05/12] crypto: ecc - Implement vli_mmod_fast

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Date: Thu, 7 Mar 2024 15:37:40 -0500
From: Stefan Berger <stefanb@...ux.ibm.com>
To: Jarkko Sakkinen <jarkko@...nel.org>, keyrings@...r.kernel.org,
        linux-crypto@...r.kernel.org, herbert@...dor.apana.org.au,
        davem@...emloft.net
Cc: linux-kernel@...r.kernel.org, saulo.alessandre@....jus.br, lukas@...ner.de
Subject: Re: [PATCH v5 05/12] crypto: ecc - Implement vli_mmod_fast_521 for
 NIST p521



On 3/7/24 14:11, Jarkko Sakkinen wrote:
> On Thu Mar 7, 2024 at 12:22 AM EET, Stefan Berger wrote:
>> Implement vli_mmod_fast_521 following the description for how to calculate
>> the modulus for NIST P521 in the NIST publication "Recommendations for
>> Discrete Logarithm-Based Cryptography: Elliptic Curve Domain Parameters"
>> section G.1.4.
>>
>> NIST p521 requires 9 64bit digits, so increase the ECC_MAX_DIGITS so that
>> arrays fit the larger numbers.
>>
>> Signed-off-by: Stefan Berger <stefanb@...ux.ibm.com>
>> Tested-by: Lukas Wunner <lukas@...ner.de>
>> ---
>>   crypto/ecc.c                  | 31 +++++++++++++++++++++++++++++++
>>   include/crypto/internal/ecc.h |  2 +-
>>   2 files changed, 32 insertions(+), 1 deletion(-)
>>
>> diff --git a/crypto/ecc.c b/crypto/ecc.c
>> index f53fb4d6af99..373660e7b19d 100644
>> --- a/crypto/ecc.c
>> +++ b/crypto/ecc.c
>> @@ -902,6 +902,31 @@ static void vli_mmod_fast_384(u64 *result, const u64 *product,
>>   #undef AND64H
>>   #undef AND64L
>>   
>> +/* Computes result = product % curve_prime
> 
> Missing empty comment line:
> 
> /*
>   *
> 
>> + * from "Recommendations for Discrete Logarithm-Based Cryptography:
>> + *       Elliptic Curve Domain Parameters" G.1.4
>> + */
>> +static void vli_mmod_fast_521(u64 *result, const u64 *product,
>> +				const u64 *curve_prime, u64 *tmp)
>> +{
>> +	const unsigned int ndigits = 9;
>> +	size_t i;
>> +
>> +	for (i = 0; i < ndigits; i++)
>> +		tmp[i] = product[i];
>> +	tmp[8] &= 0x1ff;
>> +
>> +	vli_set(result, tmp, ndigits);
>> +
>> +
>> +	for (i = 0; i < ndigits; i++)
>> +		tmp[i] = (product[8 + i] >> 9) | (product[9 + i] << 55);
>> +	tmp[8] &= 0x1ff;
>> +
>> +	vli_mod_add(result, result, tmp, curve_prime, ndigits);
>> +}
>> +
>> +
>>   /* Computes result = product % curve_prime for different curve_primes.
>>    *
>>    * Note that curve_primes are distinguished just by heuristic check and
>> @@ -941,6 +966,12 @@ static bool vli_mmod_fast(u64 *result, u64 *product,
>>   	case 6:
>>   		vli_mmod_fast_384(result, product, curve_prime, tmp);
>>   		break;
>> +	case 9:
>> +		if (curve->nbits == 521) {
> 
> Missing inline comment about the branching decision, and has a magic
> number.


Ok, will add comment to this and 6/12.
> 
>> +			vli_mmod_fast_521(result, product, curve_prime, tmp);
>> +			break;
>> +		}
>> +		fallthrough;
>>   	default:
>>   		pr_err_ratelimited("ecc: unsupported digits size!\n");
>>   		return false;
>> diff --git a/include/crypto/internal/ecc.h b/include/crypto/internal/ecc.h
>> index 4a556b41873e..de17bcdeb53a 100644
>> --- a/include/crypto/internal/ecc.h
>> +++ b/include/crypto/internal/ecc.h
>> @@ -33,7 +33,7 @@
>>   #define ECC_CURVE_NIST_P192_DIGITS  3
>>   #define ECC_CURVE_NIST_P256_DIGITS  4
>>   #define ECC_CURVE_NIST_P384_DIGITS  6
>> -#define ECC_MAX_DIGITS              (512 / 64) /* due to ecrdsa */
>> +#define ECC_MAX_DIGITS              DIV_ROUND_UP(521, 64) /* NIST P521 */
>>   
>>   #define ECC_DIGITS_TO_BYTES_SHIFT 3
>>   
> 
> BR, Jarkko