Include index as part of pmmr element hash (#748)

* storing index as part of pmmr element hash

* factor out hash_with_index into generic trait impl

* tighten up trait bounds for PMMRable

chain/src/sumtree.rs

@@ -30,7 +30,7 @@ use core::core::{Block, BlockHeader, Input, Output, OutputFeatures, OutputIdenti
                  OutputStoreable, TxKernel};
 use core::core::pmmr::{self, MerkleProof, PMMR};
 use core::core::hash::{Hash, Hashed};
-use core::ser::{self, PMMRable};
+use core::ser::{self, PMMRable, PMMRIndexHashable};
 
 use grin_store;
 use grin_store::pmmr::{PMMRBackend, PMMRFileMetadata};
@@ -141,7 +141,7 @@ impl SumTrees {
 				let output_pmmr: PMMR<OutputStoreable, _> =
 					PMMR::at(&mut self.utxo_pmmr_h.backend, self.utxo_pmmr_h.last_pos);
 				if let Some((hash, _)) = output_pmmr.get(pos, false) {
-					if hash == output_id.hash() {
+					if hash == output_id.hash_with_index(pos) {
 						Ok(hash)
 					} else {
 						Err(Error::SumTreeErr(format!("sumtree hash mismatch")))
@@ -348,14 +348,14 @@ impl<'a> Extension<'a> {
 	fn apply_input(&mut self, input: &Input, height: u64) -> Result<(), Error> {
 		let commit = input.commitment();
 		let pos_res = self.get_output_pos(&commit);
-		let output_id_hash = OutputIdentifier::from_input(input).hash();
 		if let Ok(pos) = pos_res {
+			let output_id_hash = OutputIdentifier::from_input(input).hash_with_index(pos);
 			if let Some((read_hash, read_elem)) = self.utxo_pmmr.get(pos, true) {
 				// 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...
 				if output_id_hash != read_hash
-					|| output_id_hash != read_elem.expect("no output at position").hash()
+					|| output_id_hash != read_elem.expect("no output at position").hash_with_index(pos)
 				{
 					return Err(Error::SumTreeErr(format!("output pmmr hash mismatch")));
 				}

core/src/core/pmmr.rs

@@ -40,7 +40,7 @@ use std::marker::PhantomData;
 use core::hash::{Hash, Hashed};
 use ser;
 use ser::{Readable, Reader, Writeable, Writer};
-use ser::PMMRable;
+use ser::{PMMRable, PMMRIndexHashable};
 use util;
 use util::LOGGER;
 
@@ -360,7 +360,7 @@ where
 	/// the same time if applicable.
 	pub fn push(&mut self, elmt: T) -> Result<u64, String> {
 		let elmt_pos = self.last_pos + 1;
-		let mut current_hash = elmt.hash();
+		let mut current_hash = elmt.hash_with_index(elmt_pos);
 		let mut to_append = vec![(current_hash, Some(elmt))];
 		let mut height = 0;
 		let mut pos = elmt_pos;
@@ -917,6 +917,7 @@ mod test {
 	use ser::{Error, Readable, Writeable};
 	use core::{Reader, Writer};
 	use core::hash::Hash;
+	use ser::{PMMRable, PMMRIndexHashable};
 
 	/// Simple MMR backend implementation based on a Vector. Pruning does not
 	/// compact the Vec itself.
@@ -1169,7 +1170,7 @@ mod test {
 		fn len() -> usize {
 			16
 		}
-	}
+}
 
 	impl Writeable for TestElem {
 		fn write<W: Writer>(&self, writer: &mut W) -> Result<(), Error> {
@@ -1305,28 +1306,28 @@ mod test {
 
 		// one element
 		pmmr.push(elems[0]).unwrap();
-		let node_hash = elems[0].hash();
-		assert_eq!(pmmr.root(), node_hash,);
+		let node_hash = elems[0].hash_with_index(1);
+		assert_eq!(pmmr.root(), node_hash);
 		assert_eq!(pmmr.unpruned_size(), 1);
 		pmmr.dump(false);
 
 		// two elements
 		pmmr.push(elems[1]).unwrap();
-		let sum2 = elems[0].hash() + elems[1].hash();
+		let sum2 = elems[0].hash_with_index(1) + elems[1].hash_with_index(2);
 		pmmr.dump(false);
 		assert_eq!(pmmr.root(), sum2);
 		assert_eq!(pmmr.unpruned_size(), 3);
 
 		// three elements
 		pmmr.push(elems[2]).unwrap();
-		let sum3 = sum2 + elems[2].hash();
+		let sum3 = sum2 + elems[2].hash_with_index(4);
 		pmmr.dump(false);
 		assert_eq!(pmmr.root(), sum3);
 		assert_eq!(pmmr.unpruned_size(), 4);
 
 		// four elements
 		pmmr.push(elems[3]).unwrap();
-		let sum_one = elems[2].hash() + elems[3].hash();
+		let sum_one = elems[2].hash_with_index(4) + elems[3].hash_with_index(5);
 		let sum4 = sum2 + sum_one;
 		pmmr.dump(false);
 		assert_eq!(pmmr.root(), sum4);
@@ -1334,33 +1335,34 @@ mod test {
 
 		// five elements
 		pmmr.push(elems[4]).unwrap();
-		let sum3 = sum4 + elems[4].hash();
+		let sum3 = sum4 + elems[4].hash_with_index(8);
 		pmmr.dump(false);
 		assert_eq!(pmmr.root(), sum3);
 		assert_eq!(pmmr.unpruned_size(), 8);
 
 		// six elements
 		pmmr.push(elems[5]).unwrap();
-		let sum6 = sum4 + (elems[4].hash() + elems[5].hash());
+		let sum6 = sum4 + (elems[4].hash_with_index(8) + elems[5].hash_with_index(9));
 		assert_eq!(pmmr.root(), sum6.clone());
 		assert_eq!(pmmr.unpruned_size(), 10);
 
 		// seven elements
 		pmmr.push(elems[6]).unwrap();
-		let sum7 = sum6 + elems[6].hash();
+		let sum7 = sum6 + elems[6].hash_with_index(11);
 		assert_eq!(pmmr.root(), sum7);
 		assert_eq!(pmmr.unpruned_size(), 11);
 
 		// eight elements
 		pmmr.push(elems[7]).unwrap();
 		let sum8 =
-			sum4 + ((elems[4].hash() + elems[5].hash()) + (elems[6].hash() + elems[7].hash()));
+			sum4 + ((elems[4].hash_with_index(8) + elems[5].hash_with_index(9))
+				+ (elems[6].hash_with_index(11) + elems[7].hash_with_index(12)));
 		assert_eq!(pmmr.root(), sum8);
 		assert_eq!(pmmr.unpruned_size(), 15);
 
 		// nine elements
 		pmmr.push(elems[8]).unwrap();
-		let sum9 = sum8 + elems[8].hash();
+		let sum9 = sum8 + elems[8].hash_with_index(16);
 		assert_eq!(pmmr.root(), sum9);
 		assert_eq!(pmmr.unpruned_size(), 16);
 	}

core/src/core/transaction.rs

@@ -938,6 +938,8 @@ pub struct OutputIdentifier {
 	pub commit: Commitment,
 }
 
+
+
 impl OutputIdentifier {
 	/// Build a new output_identifier.
 	pub fn new(features: OutputFeatures, commit: &Commitment) -> OutputIdentifier {
@@ -973,6 +975,13 @@ impl OutputIdentifier {
 	}
 }
 
+/// Ensure this is implemented to centralize hashing with indexes
+impl PMMRable for OutputIdentifier {
+	fn len() -> usize {
+		1 + secp::constants::PEDERSEN_COMMITMENT_SIZE + SWITCH_COMMIT_HASH_SIZE
+	}
+}
+
 impl Writeable for OutputIdentifier {
 	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ser::Error> {
 		writer.write_u8(self.features.bits())?;

core/src/ser.rs

@@ -25,7 +25,7 @@ use byteorder::{BigEndian, ByteOrder, ReadBytesExt};
 use keychain::{BlindingFactor, Identifier, IDENTIFIER_SIZE};
 use consensus;
 use consensus::VerifySortOrder;
-use core::hash::Hashed;
+use core::hash::{Hashed, Hash};
 use core::transaction::{SwitchCommitHash, SWITCH_COMMIT_HASH_SIZE};
 use util::secp::pedersen::Commitment;
 use util::secp::pedersen::RangeProof;
@@ -549,11 +549,23 @@ impl Writeable for [u8; 4] {
 }
 
 /// Trait for types that can serialize and report their size
-pub trait PMMRable: Readable + Writeable + Hashed + Clone {
+pub trait PMMRable: Readable + Writeable + Clone {
 	/// Length in bytes
 	fn len() -> usize;
 }
 
+/// Generic trait to ensure PMMR elements can be hashed with an index
+pub trait PMMRIndexHashable {
+	/// Hash with a given index
+	fn hash_with_index(&self, index: u64) -> Hash;
+}
+
+impl<T: PMMRable> PMMRIndexHashable for T {
+	fn hash_with_index(&self, index: u64) -> Hash {
+		(index, self).hash()
+	}
+}
+
 /// Useful marker trait on types that can be sized byte slices
 pub trait AsFixedBytes: Sized + AsRef<[u8]> {
 	/// The length in bytes

store/tests/pmmr.rs

@@ -37,13 +37,14 @@ fn pmmr_append() {
 	backend.sync().unwrap();
 
 	// check the resulting backend store and the computation of the root
-	let node_hash = elems[0].hash();
+	let node_hash = elems[0].hash_with_index(1);
 	assert_eq!(backend.get(1, false).expect("").0, node_hash);
 
-	let sum2 = elems[0].hash() + elems[1].hash();
-	let sum4 = sum2 + (elems[2].hash() + elems[3].hash());
-	let sum8 = sum4 + ((elems[4].hash() + elems[5].hash()) + (elems[6].hash() + elems[7].hash()));
-	let sum9 = sum8 + elems[8].hash();
+	let sum2 = elems[0].hash_with_index(1) + elems[1].hash_with_index(2);
+	let sum4 = sum2 + (elems[2].hash_with_index(4) + elems[3].hash_with_index(5));
+	let sum8 = sum4 + ((elems[4].hash_with_index(8) + elems[5].hash_with_index(9))
+		+ (elems[6].hash_with_index(11) + elems[7].hash_with_index(12)));
+	let sum9 = sum8 + elems[8].hash_with_index(16);
 
 	{
 		let pmmr: PMMR<TestElem, _> = PMMR::at(&mut backend, mmr_size);