Re: [PATCH v5 05/12] crypto: ecc - Implement vli_mmod_fast_521 for NIST p521

[Jarkko Sakkinen]

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@xxxxxxxxxxxxx>
> Tested-by: Lukas Wunner <lukas@xxxxxxxxx>
> ---
>  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.

> +			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