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Add functions to test if X coordinate is valid

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This commit is contained in:
Pieter Wuille
2022-11-04 16:17:56 -04:00
parent a597a5a9ce
commit 79e5b2a8b8
3 changed files with 63 additions and 1 deletions
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28
src/group_impl.h
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@@ -823,4 +823,32 @@ static int secp256k1_ge_is_in_correct_subgroup(const secp256k1_ge* ge) {
#endif
}
static int secp256k1_ge_x_on_curve_var(const secp256k1_fe *x) {
secp256k1_fe c;
secp256k1_fe_sqr(&c, x);
secp256k1_fe_mul(&c, &c, x);
secp256k1_fe_add_int(&c, SECP256K1_B);
return secp256k1_fe_is_square_var(&c);
}
static int secp256k1_ge_x_frac_on_curve_var(const secp256k1_fe *xn, const secp256k1_fe *xd) {
/* We want to determine whether (xn/xd) is on the curve.
*
* (xn/xd)^3 + 7 is square <=> xd*xn^3 + 7*xd^4 is square (multiplying by xd^4, a square).
*/
secp256k1_fe r, t;
#ifdef VERIFY
VERIFY_CHECK(!secp256k1_fe_normalizes_to_zero_var(xd));
#endif
secp256k1_fe_mul(&r, xd, xn); /* r = xd*xn */
secp256k1_fe_sqr(&t, xn); /* t = xn^2 */
secp256k1_fe_mul(&r, &r, &t); /* r = xd*xn^3 */
secp256k1_fe_sqr(&t, xd); /* t = xd^2 */
secp256k1_fe_sqr(&t, &t); /* t = xd^4 */
VERIFY_CHECK(SECP256K1_B <= 31);
secp256k1_fe_mul_int(&t, SECP256K1_B); /* t = 7*xd^4 */
secp256k1_fe_add(&r, &t); /* r = xd*xn^3 + 7*xd^4 */
return secp256k1_fe_is_square_var(&r);
}
#endif /* SECP256K1_GROUP_IMPL_H */