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Keep results after point_add() and point_double() inside the finite field. Simplified point_is_negative_of().

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This commit is contained in:
Ondrej Mikle 2014-07-05 22:07:03 +02:00
parent d827b2c862
commit 323da2d434
1 changed files with 16 additions and 11 deletions
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27
ecdsa.c
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@ -54,14 +54,14 @@ void point_add(const curve_point *cp1, curve_point *cp2)
point_copy(cp1, cp2);
return;
}
if (point_is_negative_of(cp1, cp2)) {
point_set_infinity(cp2);
return;
}
if (point_is_equal(cp1, cp2)) {
point_double(cp2);
return;
}
if (point_is_negative_of(cp1, cp2)) {
point_set_infinity(cp2);
return;
}
bn_substract(&(cp2->x), &(cp1->x), &inv);
bn_inverse(&inv, &prime256k1);
@ -84,6 +84,8 @@ void point_add(const curve_point *cp1, curve_point *cp2)
bn_fast_mod(&yr, &prime256k1);
memcpy(&(cp2->x), &xr, sizeof(bignum256));
memcpy(&(cp2->y), &yr, sizeof(bignum256));
bn_mod(&(cp2->x), &prime256k1);
bn_mod(&(cp2->y), &prime256k1);
}
// cp = cp + cp
@ -124,6 +126,8 @@ void point_double(curve_point *cp)
bn_fast_mod(&yr, &prime256k1);
memcpy(&(cp->x), &xr, sizeof(bignum256));
memcpy(&(cp->y), &yr, sizeof(bignum256));
bn_mod(&(cp->x), &prime256k1);
bn_mod(&(cp->y), &prime256k1);
}
// res = k * p
@ -174,19 +178,20 @@ int point_is_equal(const curve_point *p, const curve_point *q)
}
// returns true iff p == -q
// expects p and q be valid points on curve other than point at infinity
int point_is_negative_of(const curve_point *p, const curve_point *q)
{
// if P == (x, y), then -P would be (x, -y) on this curve
bignum256 y_added;
if (!bn_is_equal(&(p->x), &(q->x))) {
return 0;
}
memcpy(&y_added, &(p->y), sizeof(bignum256));
bn_addmod(&y_added, &(q->y), &prime256k1);
return bn_is_zero(&y_added);
// we shouldn't hit this for a valid point
if (bn_is_zero(&(p->y))) {
return 0;
}
return !bn_is_equal(&(p->y), &(q->y));
}
// res = k * G