import util for fr/bigint

rln/Cargo.toml

@@ -48,3 +48,5 @@ serde_json = "1.0.48"
 once_cell = "1.8"
 poseidon-rs = "0.0.8"
 sha2 = "0.10.1"
+
+ff = { package="ff_ce", version="0.11"}

rln/src/lib.rs

@@ -1,2 +1,4 @@
 pub mod ffi;
 pub mod public;
+pub mod identity;
+pub mod util;

rln/src/util.rs

@@ -0,0 +1,34 @@
+// Adapted from
+// https://github.com/worldcoin/semaphore-rs/blob/main/src/util.rs
+
+use ff::{PrimeField, PrimeFieldRepr};
+use num_bigint::{BigInt, Sign};
+use poseidon_rs::{Fr, FrRepr};
+
+pub fn fr_to_bigint(fr: Fr) -> BigInt {
+    let mut bytes = [0_u8; 32];
+    fr.into_repr().write_be(&mut bytes[..]).unwrap();
+    BigInt::from_bytes_be(Sign::Plus, &bytes)
+}
+
+pub fn bigint_to_fr(bi: &BigInt) -> Fr {
+    // dirty: have to force the point into the field manually, otherwise you get an
+    // error if bi not in field
+    let q = BigInt::parse_bytes(
+        b"21888242871839275222246405745257275088548364400416034343698204186575808495617",
+        10,
+    )
+    .unwrap();
+    let m = bi.modpow(&BigInt::from(1), &q);
+
+    let mut repr = FrRepr::default();
+    let (_, mut res) = m.to_bytes_be();
+
+    // prepend zeros
+    res.reverse();
+    res.resize(32, 0);
+    res.reverse();
+
+    repr.read_be(&res[..]).unwrap();
+    Fr::from_repr(repr).unwrap()
+}