Commit 3da2c1df by Vitaly Chikunov Committed by Herbert Xu

### 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 <[email protected]>
Cc: [email protected]
Signed-off-by: Herbert Xu <[email protected]>```
parent 8a5a79d5
 ... ... @@ -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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