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@xxxxxxxxxxxx>
Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx
---
Changes from v1:
- No code changes, only description updates to be more informative.
crypto/ecc.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/crypto/ecc.c b/crypto/ecc.c
index 8facafd67802..adcce310f646 100644
--- 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);
@@ -1004,7 +1012,7 @@ 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;
@@ -1090,7 +1098,7 @@ 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);
--
2.11.0
