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

Stefan Berger
2024-03-18T18:38:49Z

On 3/18/24 01:47, Bharat Bhushan wrote:
>
>
>> -----Original Message-----
>> From: Stefan Berger <stefanb@linux.vnet.ibm.com>
>> Sent: Wednesday, March 13, 2024 12:06 AM
>> To: keyrings@vger.kernel.org; linux-crypto@vger.kernel.org;
>> herbert@gondor.apana.org.au; davem@davemloft.net
>> Cc: linux-kernel@vger.kernel.org; saulo.alessandre@tse.jus.br;
>> lukas@wunner.de; Bharat Bhushan <bbhushan2@marvell.com>;
>> jarkko@kernel.org; Stefan Berger <stefanb@linux.ibm.com>
>> Subject: [EXTERNAL] [PATCH v6 06/13] crypto: ecc - Implement
>> vli_mmod_fast_521 for NIST p521
>>
>> Prioritize security for external emails: Confirm sender and content safety
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>>
>> ----------------------------------------------------------------------
>> From: Stefan Berger <stefanb@linux.ibm.com>
>>
>> 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@linux.ibm.com>
>> ---
>> 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 */)

I think for NIST P521 9 can be pre-calculated. It will not change
anymore in the future.

>
>> +#define ECC_MAX_DIGITS DIV_ROUND_UP(521, 64) /* NIST P521 */
>
> /* NIST_P521 is max digits */
> #define ECC_MAX_DIGITS ECC_CURVE_ _DIGITS

In this case I think the DIV_ROUND_UP() along with the comment shows
that it needs to be updated if ever a larger curve comes along.

>
> Thanks
> -Bharat
>
>>
>> #define ECC_DIGITS_TO_BYTES_SHIFT 3
>>
>> --
>> 2.43.0
>