RE: [EXTERNAL] [PATCH v6 06/13] crypto: ecc - Implement vli_mmod_fast_521 for NIST p521

[Bharat Bhushan]

> -----Original Message-----
> From: Stefan Berger <stefanb@xxxxxxxxxxxxxxxxxx>
> Sent: Wednesday, March 13, 2024 12:06 AM
> To: keyrings@xxxxxxxxxxxxxxx; linux-crypto@xxxxxxxxxxxxxxx;
> herbert@xxxxxxxxxxxxxxxxxxx; davem@xxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx; saulo.alessandre@xxxxxxxxxx;
> lukas@xxxxxxxxx; Bharat Bhushan <bbhushan2@xxxxxxxxxxx>;
> jarkko@xxxxxxxxxx; Stefan Berger <stefanb@xxxxxxxxxxxxx>
> Subject: [EXTERNAL] [PATCH v6 06/13] crypto: ecc - Implement
> vli_mmod_fast_521 for NIST p521
> 
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> ----------------------------------------------------------------------
> From: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> 
> 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>
> ---
>  crypto/ecc.c                  | 25 +++++++++++++++++++++++++
>  include/crypto/internal/ecc.h |  3 ++-
>  2 files changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/crypto/ecc.c b/crypto/ecc.c index 415a2f4e7291..99d41887c005
> 100644
> --- a/crypto/ecc.c
> +++ b/crypto/ecc.c
> @@ -902,6 +902,28 @@ static void vli_mmod_fast_384(u64 *result, const
> u64 *product,  #undef AND64H  #undef AND64L
> 
> +/*
> + * Computes result = product % curve_prime
> + * from "Recommendations for Discrete Logarithm-Based Cryptography:
> + *       Elliptic Curve Domain Parameters" section G.1.4
> + */
> +static void vli_mmod_fast_521(u64 *result, const u64 *product,
> +			      const u64 *curve_prime, u64 *tmp) {
> +	const unsigned int ndigits = ECC_CURVE_NIST_P521_DIGITS;
> +	size_t i;
> +
> +	/* Initialize result with lowest 521 bits from product */
> +	vli_set(result, product, ndigits);
> +	result[8] &= 0x1ff;
> +
> +	for (i = 0; i < ndigits; i++)
> +		tmp[i] = (product[8 + i] >> 9) | (product[9 + i] << 55);
> +	tmp[8] &= 0x1ff;

Can we get away from this hardcoding, like 9, 55, 0x1ff etc.
Or at least add comment about these.

> +
> +	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 +963,9 @@ static bool vli_mmod_fast(u64 *result, u64 *product,
>  	case ECC_CURVE_NIST_P384_DIGITS:
>  		vli_mmod_fast_384(result, product, curve_prime, tmp);
>  		break;
> +	case ECC_CURVE_NIST_P521_DIGITS:
> +		vli_mmod_fast_521(result, product, curve_prime, tmp);
> +		break;
>  	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
> ab722a8986b7..4e2f5f938e91 100644
> --- a/include/crypto/internal/ecc.h
> +++ b/include/crypto/internal/ecc.h
> @@ -33,7 +33,8 @@
>  #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_CURVE_NIST_P521_DIGITS  9

Maybe these can be defined as:
#define ECC_CURVE_NIST_P521_DIGITS  (DIV_ROUND_UP(521, 64) /* NIST P521 */)

> +#define ECC_MAX_DIGITS              DIV_ROUND_UP(521, 64) /* NIST P521 */

/* NIST_P521 is max digits */
#define ECC_MAX_DIGITS              ECC_CURVE_ _DIGITS

Thanks
-Bharat

> 
>  #define ECC_DIGITS_TO_BYTES_SHIFT 3
> 
> --
> 2.43.0