From 8b78a55c711317ff9e60ab35dee3d66b1ede94dd Mon Sep 17 00:00:00 2001
From: Jane Lusby <jane@zfnd.org>
Date: Mon, 6 Jul 2020 13:16:21 -0700
Subject: [PATCH] Start work implementing sprout note commitment function

---
 zebra-chain/src/notes.rs        |  6 ++++++
 zebra-chain/src/notes/sprout.rs | 38 ++++++++++++++++++++++++---------
 zebra-chain/src/types/amount.rs |  4 ++++
 3 files changed, 38 insertions(+), 10 deletions(-)

diff --git a/zebra-chain/src/notes.rs b/zebra-chain/src/notes.rs
index 8f6cbf2ef..cdff16645 100644
--- a/zebra-chain/src/notes.rs
+++ b/zebra-chain/src/notes.rs
@@ -6,3 +6,9 @@ pub mod sprout;
 /// The randomness used in the Pedersen Hash for note commitment.
 #[derive(Copy, Clone, Debug, PartialEq)]
 pub struct NoteCommitmentRandomness(pub [u8; 32]);
+
+impl AsRef<[u8]> for NoteCommitmentRandomness {
+    fn as_ref(&self) -> &[u8] {
+        &self.0
+    }
+}
diff --git a/zebra-chain/src/notes/sprout.rs b/zebra-chain/src/notes/sprout.rs
index 7ebb99279..dff9a1d3e 100644
--- a/zebra-chain/src/notes/sprout.rs
+++ b/zebra-chain/src/notes/sprout.rs
@@ -1,33 +1,48 @@
 //!
 #![allow(dead_code)]
 
+use super::{memo::Memo, *};
+use crate::serde_helpers;
+use crate::serialization::{SerializationError, ZcashDeserialize, ZcashSerialize};
+use crate::types::amount::{Amount, NonNegative};
 use serde::{Deserialize, Serialize};
+use sha2::{Digest, Sha256};
 use std::{
     fmt,
     io::{self},
 };
 
-#[cfg(test)]
-use proptest::{collection::vec, prelude::*};
-
-use crate::serde_helpers;
-use crate::serialization::{SerializationError, ZcashDeserialize, ZcashSerialize};
-
-use super::{memo::Memo, *};
-
 ///
 pub struct Note {
     // TODO: refine type as a SHA-256d output derived from a spending key.
     paying_key: [u8; 32],
-    value: u64,
+    value: Amount<NonNegative>,
     // TODO: refine type as the input to the PRF that results in a nullifier.
     nullifier_seed: [u8; 32],
     note_commitment_randomness: NoteCommitmentRandomness,
 }
 
+impl Note {
+    pub fn note_commitment(&self) -> NoteCommitment {
+        let leading_byte: u8 = 0xB0;
+        let mut hasher = Sha256::default();
+        hasher.input([leading_byte]);
+        hasher.input(self.paying_key);
+        hasher.input(self.value.to_bytes());
+        hasher.input(self.nullifier_seed);
+        hasher.input(self.note_commitment_randomness);
+        let hash = hasher.result().into();
+        NoteCommitment { hash }
+    }
+}
+
+pub struct NoteCommitment {
+    hash: [u8; 32],
+}
+
 /// The decrypted form of encrypted Sprout notes on the blockchain.
 pub struct NotePlaintext {
-    value: u64,
+    value: Amount<NonNegative>,
     // TODO: refine type
     rho: [u8; 32],
     // TODO: refine as jub-jub appropriate in the base field.
@@ -82,6 +97,9 @@ impl ZcashDeserialize for EncryptedCiphertext {
     }
 }
 
+#[cfg(test)]
+use proptest::{collection::vec, prelude::*};
+
 #[cfg(test)]
 impl Arbitrary for EncryptedCiphertext {
     type Parameters = ();
diff --git a/zebra-chain/src/types/amount.rs b/zebra-chain/src/types/amount.rs
index 84565b88b..9831ddc76 100644
--- a/zebra-chain/src/types/amount.rs
+++ b/zebra-chain/src/types/amount.rs
@@ -21,6 +21,10 @@ impl<C> Amount<C> {
     {
         self.0.try_into()
     }
+
+    pub fn to_bytes(&self) -> [u8; 8] {
+        self.0.to_le_bytes()
+    }
 }
 
 impl<C> std::ops::Add<Amount<C>> for Amount<C>