[PATCH v7 06/13] 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
the vli digit array provides enough elements to fit the larger integers
required by this curve.

Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
Tested-by: Lukas Wunner <lukas@xxxxxxxxx>
---
 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;
+
+	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
+#define ECC_MAX_DIGITS              DIV_ROUND_UP(521, 64) /* NIST P521 */
 
 #define ECC_DIGITS_TO_BYTES_SHIFT 3
 
-- 
2.43.0