From 4c13088cf6b6e0c078c8d53c3eaaf779f1e46439 Mon Sep 17 00:00:00 2001
From: Ariel Gabizon <ariel.gabizon@gmail.com>
Date: Mon, 19 Sep 2016 18:48:01 -0600
Subject: [PATCH] Schnorr NIZKs.

---
 src/protocol/mod.rs  |  1 +
 src/protocol/nizk.rs | 64 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 65 insertions(+)
 create mode 100644 src/protocol/nizk.rs

diff --git a/src/protocol/mod.rs b/src/protocol/mod.rs
index b928332..fd284a4 100644
--- a/src/protocol/mod.rs
+++ b/src/protocol/mod.rs
@@ -36,6 +36,7 @@ use snark::*;
 
 mod secrets;
 mod spair;
+mod nizk;
 mod multicore;
 pub use self::secrets::*;
 use self::spair::*;
diff --git a/src/protocol/nizk.rs b/src/protocol/nizk.rs
new file mode 100644
index 0000000..7d6fc71
--- /dev/null
+++ b/src/protocol/nizk.rs
@@ -0,0 +1,64 @@
+use bn::*;
+use rand::Rng;
+
+/// Hash a group element with BLAKE2b and interpret it as an
+/// element of Fr.
+fn hash_group_to_fr<G: Group>(r: &G) -> Fr {
+    use bincode::SizeLimit::Infinite;
+    use bincode::rustc_serialize::encode;
+    use blake2_rfc::blake2b::blake2b;
+
+    let serialized = encode(r, Infinite).unwrap();
+
+    let mut hash = [0; 64];
+    hash.copy_from_slice(blake2b(64, &[], &serialized).as_bytes());
+
+    Fr::interpret(&hash)
+}
+
+#[derive(Clone, RustcEncodable, RustcDecodable)]
+pub struct Nizk<G: Group> {
+    r: G,
+    u: Fr
+}
+
+impl<G: Group> Nizk<G> {
+    /// Constructing the non-interactive schnorr proof for knowledge of log
+    /// of s*f in base f, i.e., knowledge of s
+    fn new<R: Rng>(f: G, s: Fr, rng: &mut R) -> Nizk<G> {
+        let a = Fr::random(rng);
+        let r = f * a;
+        let c = hash_group_to_fr(&r);
+        Nizk {
+            r: r,
+            u: a + c * s
+        }
+    }
+
+    /// Verify the Nizk
+    fn verify(&self, f: G, fs: G) -> bool {
+        let c = hash_group_to_fr(&self.r);
+        
+        (f * self.u) == (self.r + fs * c)
+    }
+}
+
+#[test]
+fn nizk_test() {
+    fn nizk_test_group<G: Group>() {
+        let rng = &mut ::rand::thread_rng();
+        for _  in 0..50 {
+            let f = G::random(rng);
+            let s = Fr::random(rng);
+            let fs = f * s;
+
+            let proof = Nizk::new(f, s, rng);
+            assert!(proof.verify(f, fs));
+            assert!(!proof.verify(f, f * Fr::random(rng)));
+            assert!(!proof.verify(f * Fr::random(rng), fs));
+        }
+    }
+
+    nizk_test_group::<G1>();
+    nizk_test_group::<G2>();
+}