crypto: ecdh - add public key verification test

According to SP800-56A section 5.6.2.1, the public key to be processed
for the ECDH operation shall be checked for appropriateness. When the
public key is considered to be an ephemeral key, the partial validation
test as defined in SP800-56A section 5.6.2.3.4 can be applied.

The partial verification test requires the presence of the field
elements of a and b. For the implemented NIST curves, b is defined in
FIPS 186-4 appendix D.1.2. The element a is implicitly given with the
Weierstrass equation given in D.1.2 where a = p - 3.

Without the test, the NIST ACVP testing fails. After adding this check,
the NIST ACVP testing passes.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 38 insertions, 4 deletions

diff --git a/crypto/ecc.c b/crypto/ecc.c
index 815541309a95..8facafd67802 100644
--- a/crypto/ecc.c
+++ b/crypto/ecc.c
@@ -1019,6 +1019,36 @@ out:
 	return ret;
 }
 
+/* SP800-56A section 5.6.2.3.4 partial verification: ephemeral keys only */
+static int ecc_is_pubkey_valid_partial(const struct ecc_curve *curve,
+				       struct ecc_point *pk)
+{
+	u64 yy[ECC_MAX_DIGITS], xxx[ECC_MAX_DIGITS], w[ECC_MAX_DIGITS];
+
+	/* Check 1: Verify key is not the zero point. */
+	if (ecc_point_is_zero(pk))
+		return -EINVAL;
+
+	/* Check 2: Verify key is in the range [1, p-1]. */
+	if (vli_cmp(curve->p, pk->x, pk->ndigits) != 1)
+		return -EINVAL;
+	if (vli_cmp(curve->p, pk->y, pk->ndigits) != 1)
+		return -EINVAL;
+
+	/* Check 3: Verify that y^2 == (x^3 + a·x + b) mod p */
+	vli_mod_square_fast(yy, pk->y, curve->p, pk->ndigits); /* y^2 */
+	vli_mod_square_fast(xxx, pk->x, curve->p, pk->ndigits); /* x^2 */
+	vli_mod_mult_fast(xxx, xxx, pk->x, curve->p, pk->ndigits); /* x^3 */
+	vli_mod_mult_fast(w, curve->a, pk->x, curve->p, pk->ndigits); /* a·x */
+	vli_mod_add(w, w, curve->b, curve->p, pk->ndigits); /* a·x + b */
+	vli_mod_add(w, w, xxx, curve->p, pk->ndigits); /* x^3 + a·x + b */
+	if (vli_cmp(yy, w, pk->ndigits) != 0) /* Equation */
+		return -EINVAL;
+
+	return 0;
+
+}
+
 int crypto_ecdh_shared_secret(unsigned int curve_id, unsigned int ndigits,
 			      const u64 *private_key, const u64 *public_key,
 			      u64 *secret)
@@ -1046,16 +1076,20 @@ int crypto_ecdh_shared_secret(unsigned int curve_id, unsigned int ndigits,
 		goto out;
 	}
 
+	ecc_swap_digits(public_key, pk->x, ndigits);
+	ecc_swap_digits(&public_key[ndigits], pk->y, ndigits);
+	ret = ecc_is_pubkey_valid_partial(curve, pk);
+	if (ret)
+		goto err_alloc_product;
+
+	ecc_swap_digits(private_key, priv, ndigits);
+
 	product = ecc_alloc_point(ndigits);
 	if (!product) {
 		ret = -ENOMEM;
 		goto err_alloc_product;
 	}
 
-	ecc_swap_digits(public_key, pk->x, ndigits);
-	ecc_swap_digits(&public_key[ndigits], pk->y, ndigits);
-	ecc_swap_digits(private_key, priv, ndigits);
-
 	ecc_point_mult(product, pk, priv, rand_z, curve->p, ndigits);
 
 	ecc_swap_digits(product->x, secret, ndigits);