Implement RedJubjub

Co-authored-by: Sean Bowe <ewillbefull@gmail.com>

src/util.rs

@@ -1,3 +1,7 @@
+use blake2_rfc::blake2b::Blake2b;
+
+use jubjub::{JubjubEngine, ToUniform};
+
 pub fn swap_bits_u64(x: u64) -> u64
 {
     let mut tmp = 0;
@@ -7,6 +11,14 @@ pub fn swap_bits_u64(x: u64) -> u64
     tmp
 }
 
+pub fn hash_to_scalar<E: JubjubEngine>(persona: &[u8], a: &[u8], b: &[u8]) -> E::Fs {
+    let mut hasher = Blake2b::with_params(64, &[], &[], persona);
+    hasher.update(a);
+    hasher.update(b);
+    let ret = hasher.finalize();
+    E::Fs::to_uniform(ret.as_ref())
+}
+
 #[test]
 fn test_swap_bits_u64() {
     assert_eq!(swap_bits_u64(17182120934178543809), 0b1000001100011011110000011000111000101111111001001100111001110111);