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Add secp256k1_fe_is_square_var function

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The implementation calls the secp256k1_modinvNN_jacobi_var code, falling back
to computing a square root in the (extremely rare) case it failed converge.
This commit is contained in:
Pieter Wuille
2023-01-04 16:05:34 -05:00
parent 1de2a01c2b
commit 6be01036c8
5 changed files with 73 additions and 0 deletions
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27
src/field_5x52_impl.h
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@@ -664,4 +664,31 @@ static void secp256k1_fe_inv_var(secp256k1_fe *r, const secp256k1_fe *x) {
#endif
}
static int secp256k1_fe_is_square_var(const secp256k1_fe *x) {
secp256k1_fe tmp;
secp256k1_modinv64_signed62 s;
int jac, ret;
tmp = *x;
secp256k1_fe_normalize_var(&tmp);
/* secp256k1_jacobi64_maybe_var cannot deal with input 0. */
if (secp256k1_fe_is_zero(&tmp)) return 1;
secp256k1_fe_to_signed62(&s, &tmp);
jac = secp256k1_jacobi64_maybe_var(&s, &secp256k1_const_modinfo_fe);
if (jac == 0) {
/* secp256k1_jacobi64_maybe_var failed to compute the Jacobi symbol. Fall back
* to computing a square root. This should be extremely rare with random
* input (except in VERIFY mode, where a lower iteration count is used). */
secp256k1_fe dummy;
ret = secp256k1_fe_sqrt(&dummy, &tmp);
} else {
#ifdef VERIFY
secp256k1_fe dummy;
VERIFY_CHECK(jac == 2*secp256k1_fe_sqrt(&dummy, &tmp) - 1);
#endif
ret = jac >= 0;
}
return ret;
}
#endif /* SECP256K1_FIELD_REPR_IMPL_H */