path: root/crypto/ecc.c
diff options
authorVitaly Chikunov <>2018-11-11 20:40:02 +0300
committerHerbert Xu <>2018-11-16 14:11:04 +0800
commit3da2c1dfdb802b184eea0653d1e589515b52d74b (patch)
tree2f5689d4508f993fc477ce5dbdb35c6e76d6ff8b /crypto/ecc.c
parent8a5a79d5556b822143b4403fc46068d4eef2e4e2 (diff)
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 <> Cc: Signed-off-by: Herbert Xu <>
Diffstat (limited to 'crypto/ecc.c')
1 files changed, 12 insertions, 4 deletions
diff --git a/crypto/ecc.c b/crypto/ecc.c
index 9d24e6522730..ed1237115066 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 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);