quadratic residue check

src/quadratic_residues.rs

+use num_bigint::{BigUint};
+
+// a is assumed to be co-prime
+// quadratic residue check
+
+pub fn is_quadratic_residue(a: &BigUint, p: &BigUint) -> bool {
+    a.modpow((p-1)/2, p) == 1
+}
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