crypto: ecc - regularize scalar for scalar multiplication

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>

diff --git a/crypto/ecc.c b/crypto/ecc.c
index 9d24e65227302b5f52bb75047bafcdf154d82dd3..ed1237115066b48b368b15688e6b2343db70d6ac 100644 (file)
--- a/crypto/ecc.c
+++ b/crypto/ecc.c
@@ -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);