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

[Stefan Berger]

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
+ * 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) {
+			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
 
-- 
2.43.0