From 75cad3bb0a7cd2d8e6fd839e1a339fa085fa4fd4 Mon Sep 17 00:00:00 2001
From: Deirdre Connolly <deirdre@zfnd.org>
Date: Sun, 9 Aug 2020 21:08:38 -0400
Subject: [PATCH] Impl TryFrom's for Diversifier and use those to construct
 Sapling NoteCommitments

---
 zebra-chain/src/commitments/sapling.rs | 16 +++++++++----
 zebra-chain/src/keys/sapling.rs        | 32 ++++++++++++++++++--------
 2 files changed, 33 insertions(+), 15 deletions(-)

diff --git a/zebra-chain/src/commitments/sapling.rs b/zebra-chain/src/commitments/sapling.rs
index 6f2089d8..5a1195bb 100644
--- a/zebra-chain/src/commitments/sapling.rs
+++ b/zebra-chain/src/commitments/sapling.rs
@@ -82,7 +82,7 @@ impl NoteCommitment {
         diversifier: Diversifier,
         transmission_key: TransmissionKey,
         value: Amount<NonNegative>,
-    ) -> (CommitmentRandomness, Self)
+    ) -> Option<(CommitmentRandomness, Self)>
     where
         T: RngCore + CryptoRng,
     {
@@ -95,9 +95,15 @@ impl NoteCommitment {
         // Jubjub repr_J canonical byte encoding
         // https://zips.z.cash/protocol/protocol.pdf#jubjub
         //
-        // The `From<Diversifier>` impls for the `jubjub::*Point`s handles
+        // The `TryFrom<Diversifier>` impls for the `jubjub::*Point`s handles
         // calling `DiversifyHash` implicitly.
-        let g_d_bytes = jubjub::AffinePoint::from(diversifier).to_bytes();
+        let g_d_bytes: [u8; 32];
+        if let Ok(g_d) = jubjub::AffinePoint::try_from(diversifier) {
+            g_d_bytes = g_d.to_bytes();
+        } else {
+            return None;
+        }
+
         let pk_d_bytes = <[u8; 32]>::from(transmission_key);
         let v_bytes = value.to_bytes();
 
@@ -107,10 +113,10 @@ impl NoteCommitment {
 
         let rcm = CommitmentRandomness(generate_trapdoor(csprng));
 
-        (
+        Some((
             rcm,
             NoteCommitment::from(windowed_pedersen_commitment(rcm.0, &s)),
-        )
+        ))
     }
 
     /// Hash Extractor for Jubjub (?)
diff --git a/zebra-chain/src/keys/sapling.rs b/zebra-chain/src/keys/sapling.rs
index d1d922f8..860e7d0c 100644
--- a/zebra-chain/src/keys/sapling.rs
+++ b/zebra-chain/src/keys/sapling.rs
@@ -629,19 +629,31 @@ impl From<Diversifier> for [u8; 11] {
     }
 }
 
-impl From<Diversifier> for jubjub::AffinePoint {
-    /// Get a diversified base point from a diversifier value in
-    /// affine representation
-    fn from(d: Diversifier) -> jubjub::AffinePoint {
-        jubjub::ExtendedPoint::from(d).into()
+impl TryFrom<Diversifier> for jubjub::AffinePoint {
+    type Error = &'static str;
+
+    /// Get a diversified base point from a diversifier value in affine
+    /// representation.
+    fn try_from(d: Diversifier) -> Result<Self, Self::Error> {
+        if let Ok(extended_point) = jubjub::ExtendedPoint::try_from(d) {
+            Ok(extended_point.into())
+        } else {
+            Err("Invalid Diversifier -> jubjub::AffinePoint")
+        }
     }
 }
 
-impl From<Diversifier> for jubjub::ExtendedPoint {
-    /// Get a diversified base point from a diversifier value in
-    /// extended representation
-    fn from(d: Diversifier) -> jubjub::ExtendedPoint {
-        diversify_hash(d.0).unwrap()
+impl TryFrom<Diversifier> for jubjub::ExtendedPoint {
+    type Error = &'static str;
+
+    fn try_from(d: Diversifier) -> Result<Self, Self::Error> {
+        let possible_point = diversify_hash(d.0);
+
+        if let Some(point) = possible_point {
+            Ok(point)
+        } else {
+            Err("Invalid Diversifier -> jubjub::ExtendedPoint")
+        }
     }
 }