crypto: ecc - regularize scalar for scalar multiplication (3da2c1df) · Commits · 戴 / test

ecc_point_mult is supposed to be used with a regularized scalar, otherwise, it's possible to deduce the position of the top bit of the scalar with timing attack. This is important when the scalar is a private key. ecc_point_mult is already using a regular algorithm (i.e. having an operation flow independent of the input scalar) but regularization step is not implemented. Arrange scalar to always have fixed top bit by adding a multiple of the curve order (n). References: The constant time regularization step is based on micro-ecc by Kenneth MacKay and also referenced in the literature (Bernstein, D. J., & Lange, T. (2017). Montgomery curves and the Montgomery ladder. (Cryptology ePrint Archive; Vol. 2017/293). s.l.: IACR. Chapter 4.6.2.) Signed-off-by:

Vitaly Chikunov <vt@altlinux.org> Cc: kernel-hardening@lists.openwall.com Signed-off-by:

Herbert Xu <herbert@gondor.apana.org.au>

crypto/ecc.c

+12 −4

Original line number	Diff line number	Diff line
		@@ -842,15 +842,23 @@ static void xycz_add_c(u64 x1, u64 y1, u64 x2, u64 y2, u64 *curve_prime,

		static void ecc_point_mult(struct ecc_point *result,
		const struct ecc_point point, const u64 scalar,
		u64 initial_z, u64 curve_prime,
		u64 initial_z, const struct ecc_curve curve,
		unsigned int ndigits)
		{
		/* R0 and R1 */
		u64 rx[2][ECC_MAX_DIGITS];
		u64 ry[2][ECC_MAX_DIGITS];
		u64 z[ECC_MAX_DIGITS];
		u64 sk[2][ECC_MAX_DIGITS];
		u64 *curve_prime = curve->p;
		int i, nb;
		int num_bits = vli_num_bits(scalar, ndigits);
		int num_bits;
		int carry;

		carry = vli_add(sk[0], scalar, curve->n, ndigits);
		vli_add(sk[1], sk[0], curve->n, ndigits);
		scalar = sk[!carry];
		num_bits = sizeof(u64) * ndigits * 8 + 1;

		vli_set(rx[1], point->x, ndigits);
		vli_set(ry[1], point->y, ndigits);
		@@ -1014,7 +1022,7 @@ int ecc_make_pub_key(unsigned int curve_id, unsigned int ndigits,
		goto out;
		}

		ecc_point_mult(pk, &curve->g, priv, NULL, curve->p, ndigits);
		ecc_point_mult(pk, &curve->g, priv, NULL, curve, ndigits);
		if (ecc_point_is_zero(pk)) {
		ret = -EAGAIN;
		goto err_free_point;
		@@ -1100,7 +1108,7 @@ int crypto_ecdh_shared_secret(unsigned int curve_id, unsigned int ndigits,
		goto err_alloc_product;
		}

		ecc_point_mult(product, pk, priv, rand_z, curve->p, ndigits);
		ecc_point_mult(product, pk, priv, rand_z, curve, ndigits);

		ecc_swap_digits(product->x, secret, ndigits);

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