f Validate merkle inclusion proofs

Author: tnull

lightning-transaction-sync/src/electrum.rs

@@ -3,13 +3,16 @@ use crate::error::{TxSyncError, InternalError};
 
 use electrum_client::Client as ElectrumClient;
 use electrum_client::ElectrumApi;
+use electrum_client::GetMerkleRes;
 
 use lightning::util::logger::Logger;
 use lightning::{log_error, log_debug, log_trace};
 use lightning::chain::WatchedOutput;
 use lightning::chain::{Confirm, Filter};
 
 use bitcoin::{BlockHash, BlockHeader, Script, Txid};
+use bitcoin::hashes::Hash;
+use bitcoin::hashes::sha256d::Hash as Sha256d;
 
 use std::ops::Deref;
 use std::sync::Mutex;
@@ -263,7 +266,10 @@ where
 								debug_assert_eq!(prob_conf_height, merkle_res.block_height as u32);
 								match self.client.block_header(prob_conf_height as usize) {
 									Ok(block_header) => {
-										// TODO can we check the merkle proof here to be sure?
+										if !validate_merkle_proof(**txid, block_header.merkle_root.as_hash(), &merkle_res) {
+											log_error!(self.logger, "Retrieved Merkle block for txid {} doesn't match expectations. This should not happen. Please verify server integrity.", txid);
+											return Err(InternalError::Failed);
+										}
 										confirmed_txs.push(ConfirmedTx { tx: tx.clone(), block_header, block_height: prob_conf_height, pos: merkle_res.pos });
 									}
 									Err(e) => {
@@ -301,7 +307,10 @@ where
 												debug_assert_eq!(prob_conf_height, merkle_res.block_height as u32);
 												match self.client.block_header(prob_conf_height as usize) {
 													Ok(block_header) => {
-														// TODO can we check the merkle proof here to be sure?
+														if !validate_merkle_proof(txid, block_header.merkle_root.as_hash(), &merkle_res) {
+															log_error!(self.logger, "Retrieved Merkle block for txid {} doesn't match expectations. This should not happen. Please verify server integrity.", txid);
+															return Err(InternalError::Failed);
+														}
 														confirmed_txs.push(ConfirmedTx { tx: tx.clone(), block_header, block_height: prob_conf_height, pos: merkle_res.pos });
 													}
 													Err(e) => {
@@ -386,6 +395,28 @@ where
 
 }
 
+fn validate_merkle_proof(txid: Txid, merkle_root: Sha256d, merkle_res: &GetMerkleRes) -> bool {
+	let mut index = merkle_res.pos;
+	let mut cur = txid.as_hash();
+	for bytes in &merkle_res.merkle {
+		let mut reversed = Vec::with_capacity(32);
+		reversed.truncate(0);
+		reversed.extend(bytes.iter().rev());
+		let next_hash = Sha256d::from_slice(&reversed).unwrap();
+
+		let (left, right) = if index % 2 == 0 {
+			(cur, next_hash)
+		} else {
+			(next_hash, cur)
+		};
+
+		let data = [&left[..], &right[..]].concat();
+		cur = Sha256d::hash(&data);
+		index /= 2;
+	}
+	cur == merkle_root
+}
+
 impl<L: Deref> Filter for ElectrumSyncClient<L>
 where
 	L::Target: Logger,