Add grouphash

src/grouphash/mod.rs

@@ -0,0 +1,30 @@
+// 64 zeros, substitute with random future determined string like a blockhash, or randomness beacom
+const U: [u8; 64] = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
+
+// option to return None or point
+fn grouphash<E: JubjubEngine>(tag: &[u8], params: &E::Params) -> Option<montgomery::Point<E, PrimeOrder>> {
+    // Check to see that scalar field is 255 bits
+    assert! (E::Fr::NUM_BITS == 255);
+
+    // Perform hash, get random 32-byte string
+    let mut h = Blake2s::new_keyed(&[], 32);
+    h.process(&U);
+    h.process(tag);
+    let h = h.fixed_result();
+
+    // Take first unset first bit of hash
+    let sign = (h[0] >> 7) == 1;
+    h[0] &= 0b01111111;
+
+    // cast to prime field representation
+    let mut x0 = <E::Fr as PrimeField>::Repr::default();
+    x0.read_be(&h[..]).unwrap();
+
+    match E::Fr::from_repr(x0) {
+        Ok(x0) => {
+            let tmp = montgomery::Point::get_for_x(x0, sign, params).mul_by_cofactor(params);
+            if tmp == mongomery::Point.zero() { None } else { Some(tmp) };
+        }
+        Err(_) => None
+    }
+}

src/lib.rs

@@ -6,4 +6,4 @@ extern crate rand;
 
 pub mod jubjub;
 pub mod circuit;
-
+pub mod grouphash;