From 02b3bf4b59fd6c8fe912bd83e094d49ee20286cb Mon Sep 17 00:00:00 2001
From: Antioch Peverell <apeverell@protonmail.com>
Date: Wed, 5 Dec 2018 17:58:40 +0000
Subject: [PATCH] utxo_view hash compare (#2082)

* utxo_view validate_input now checks the data in the output MMR
* cleanup apply_input and validate_input
---
 chain/src/txhashset/txhashset.rs | 17 +++++------------
 chain/src/txhashset/utxo_view.rs | 25 ++++++++++++++-----------
 2 files changed, 19 insertions(+), 23 deletions(-)

diff --git a/chain/src/txhashset/txhashset.rs b/chain/src/txhashset/txhashset.rs
index ba123bc2..592fdb9b 100644
--- a/chain/src/txhashset/txhashset.rs
+++ b/chain/src/txhashset/txhashset.rs
@@ -910,16 +910,9 @@ impl<'a> Extension<'a> {
 		let commit = input.commitment();
 		let pos_res = self.batch.get_output_pos(&commit);
 		if let Ok(pos) = pos_res {
-			let output_id_hash = OutputIdentifier::from_input(input).hash_with_index(pos - 1);
-			if let Some(read_hash) = self.output_pmmr.get_hash(pos) {
-				// check hash from pmmr matches hash from input (or corresponding output)
-				// if not then the input is not being honest about
-				// what it is attempting to spend...
-				let read_elem = self.output_pmmr.get_data(pos);
-				let read_elem_hash = read_elem
-					.expect("no output at pos")
-					.hash_with_index(pos - 1);
-				if output_id_hash != read_hash || output_id_hash != read_elem_hash {
+			// First check this input corresponds to an existing entry in the output MMR.
+			if let Some(hash) = self.output_pmmr.get_hash(pos) {
+				if hash != input.hash_with_index(pos - 1) {
 					return Err(
 						ErrorKind::TxHashSetErr(format!("output pmmr hash mismatch")).into(),
 					);
@@ -933,10 +926,10 @@ impl<'a> Extension<'a> {
 				Ok(true) => {
 					self.rproof_pmmr
 						.prune(pos)
-						.map_err(|s| ErrorKind::TxHashSetErr(s))?;
+						.map_err(|e| ErrorKind::TxHashSetErr(e))?;
 				}
 				Ok(false) => return Err(ErrorKind::AlreadySpent(commit).into()),
-				Err(s) => return Err(ErrorKind::TxHashSetErr(s).into()),
+				Err(e) => return Err(ErrorKind::TxHashSetErr(e).into()),
 			}
 		} else {
 			return Err(ErrorKind::AlreadySpent(commit).into());
diff --git a/chain/src/txhashset/utxo_view.rs b/chain/src/txhashset/utxo_view.rs
index ceaf1353..f434931d 100644
--- a/chain/src/txhashset/utxo_view.rs
+++ b/chain/src/txhashset/utxo_view.rs
@@ -16,7 +16,7 @@
 
 use core::core::pmmr::ReadonlyPMMR;
 use core::core::{Block, Input, Output, Transaction};
-
+use core::ser::PMMRIndexHashable;
 use error::{Error, ErrorKind};
 use grin_store::pmmr::PMMRBackend;
 use store::Batch;
@@ -64,23 +64,26 @@ impl<'a> UTXOView<'a> {
 		Ok(())
 	}
 
+	// Input is valid if it is spending an (unspent) output
+	// that currently exists in the output MMR.
+	// Compare the hash in the output MMR at the expected pos.
 	fn validate_input(&self, input: &Input) -> Result<(), Error> {
-		let commit = input.commitment();
-		let pos_res = self.batch.get_output_pos(&commit);
-		if let Ok(pos) = pos_res {
-			if let Some(_) = self.pmmr.get_data(pos) {
-				return Ok(());
+		if let Ok(pos) = self.batch.get_output_pos(&input.commitment()) {
+			if let Some(hash) = self.pmmr.get_hash(pos) {
+				if hash == input.hash_with_index(pos - 1) {
+					return Ok(());
+				}
 			}
 		}
-		Err(ErrorKind::AlreadySpent(commit).into())
+		Err(ErrorKind::AlreadySpent(input.commitment()).into())
 	}
 
+	// Output is valid if it would not result in a duplicate commitment in the output MMR.
 	fn validate_output(&self, output: &Output) -> Result<(), Error> {
-		let commit = output.commitment();
-		if let Ok(pos) = self.batch.get_output_pos(&commit) {
+		if let Ok(pos) = self.batch.get_output_pos(&output.commitment()) {
 			if let Some(out_mmr) = self.pmmr.get_data(pos) {
-				if out_mmr.commitment() == commit {
-					return Err(ErrorKind::DuplicateCommitment(commit).into());
+				if out_mmr.commitment() == output.commitment() {
+					return Err(ErrorKind::DuplicateCommitment(output.commitment()).into());
 				}
 			}
 		}