Avoid unnecessarily alloc'ing a new buffer when decrypting messages

When decrypting P2P messages, we already have a read buffer that we
read the message into. There's no reason to allocate a new `Vec` to
store the decrypted message when we can just overwrite the read
buffer and call it a day.

Author: TheBlueMatt

lightning/src/ln/peer_channel_encryptor.rs

@@ -169,6 +169,18 @@ impl PeerChannelEncryptor {
 		res.extend_from_slice(&tag);
 	}
 
+	fn decrypt_in_place_with_ad(inout: &mut [u8], n: u64, key: &[u8; 32], h: &[u8]) -> Result<(), LightningError> {
+		let mut nonce = [0; 12];
+		nonce[4..].copy_from_slice(&n.to_le_bytes()[..]);
+
+		let mut chacha = ChaCha20Poly1305RFC::new(key, &nonce, h);
+		let (inout, tag) = inout.split_at_mut(inout.len() - 16);
+		if chacha.decrypt_in_place(inout, tag).is_err() {
+			return Err(LightningError{err: "Bad MAC".to_owned(), action: msgs::ErrorAction::DisconnectPeer{ msg: None }});
+		}
+		Ok(())
+	}
+
 	#[inline]
 	fn decrypt_with_ad(res: &mut[u8], n: u64, key: &[u8; 32], h: &[u8], cyphertext: &[u8]) -> Result<(), LightningError> {
 		let mut nonce = [0; 12];
@@ -505,21 +517,20 @@ impl PeerChannelEncryptor {
 		}
 	}
 
-	/// Decrypts the given message.
+	/// Decrypts the given message up to msg.len() - 16. Bytes after msg.len() - 16 will be left
+	/// undefined (as they contain the Poly1305 tag bytes).
+	///
 	/// panics if msg.len() > 65535 + 16
-	pub fn decrypt_message(&mut self, msg: &[u8]) -> Result<Vec<u8>, LightningError> {
+	pub fn decrypt_message(&mut self, msg: &mut [u8]) -> Result<(), LightningError> {
 		if msg.len() > LN_MAX_MSG_LEN + 16 {
 			panic!("Attempted to decrypt message longer than 65535 + 16 bytes!");
 		}
 
 		match self.noise_state {
 			NoiseState::Finished { sk: _, sn: _, sck: _, ref rk, ref mut rn, rck: _ } => {
-				let mut res = Vec::with_capacity(msg.len() - 16);
-				res.resize(msg.len() - 16, 0);
-				Self::decrypt_with_ad(&mut res[..], *rn, rk, &[0; 0], msg)?;
+				Self::decrypt_in_place_with_ad(&mut msg[..], *rn, rk, &[0; 0])?;
 				*rn += 1;
-
-				Ok(res)
+				Ok(())
 			},
 			_ => panic!("Tried to decrypt a message prior to noise handshake completion"),
 		}
@@ -764,12 +775,11 @@ mod tests {
 
 		for i in 0..1005 {
 			let msg = [0x68, 0x65, 0x6c, 0x6c, 0x6f];
-			let res = outbound_peer.encrypt_buffer(&msg);
+			let mut res = outbound_peer.encrypt_buffer(&msg);
 			assert_eq!(res.len(), 5 + 2*16 + 2);
 
 			let len_header = res[0..2+16].to_vec();
 			assert_eq!(inbound_peer.decrypt_length_header(&len_header[..]).unwrap() as usize, msg.len());
-			assert_eq!(inbound_peer.decrypt_message(&res[2+16..]).unwrap()[..], msg[..]);
 
 			if i == 0 {
 				assert_eq!(res, hex::decode("cf2b30ddf0cf3f80e7c35a6e6730b59fe802473180f396d88a8fb0db8cbcf25d2f214cf9ea1d95").unwrap());
@@ -784,6 +794,9 @@ mod tests {
 			} else if i == 1001 {
 				assert_eq!(res, hex::decode("2ecd8c8a5629d0d02ab457a0fdd0f7b90a192cd46be5ecb6ca570bfc5e268338b1a16cf4ef2d36").unwrap());
 			}
+
+			inbound_peer.decrypt_message(&mut res[2+16..]).unwrap();
+			assert_eq!(res[2 + 16..res.len() - 16], msg[..]);
 		}
 	}
 
@@ -807,7 +820,7 @@ mod tests {
 		let mut inbound_peer = get_inbound_peer_for_test_vectors();
 
 		// MSG should not exceed LN_MAX_MSG_LEN + 16
-		let msg = [4u8; LN_MAX_MSG_LEN + 17];
-		inbound_peer.decrypt_message(&msg).unwrap();
+		let mut msg = [4u8; LN_MAX_MSG_LEN + 17];
+		inbound_peer.decrypt_message(&mut msg).unwrap();
 	}
 }

lightning/src/ln/peer_handler.rs

@@ -1402,17 +1402,18 @@ impl<Descriptor: SocketDescriptor, CM: Deref, RM: Deref, OM: Deref, L: Deref, CM
 									}
 									peer.pending_read_is_header = false;
 								} else {
-									let msg_data = try_potential_handleerror!(peer,
-										peer.channel_encryptor.decrypt_message(&peer.pending_read_buffer[..]));
-									assert!(msg_data.len() >= 2);
+									debug_assert!(peer.pending_read_buffer.len() >= 2 + 16);
+									try_potential_handleerror!(peer,
+										peer.channel_encryptor.decrypt_message(&mut peer.pending_read_buffer[..]));
+
+									let mut reader = io::Cursor::new(&peer.pending_read_buffer[..peer.pending_read_buffer.len() - 16]);
+									let message_result = wire::read(&mut reader, &*self.message_handler.custom_message_handler);
 
 									// Reset read buffer
 									if peer.pending_read_buffer.capacity() > 8192 { peer.pending_read_buffer = Vec::new(); }
 									peer.pending_read_buffer.resize(18, 0);
 									peer.pending_read_is_header = true;
 
-									let mut reader = io::Cursor::new(&msg_data[..]);
-									let message_result = wire::read(&mut reader, &*self.message_handler.custom_message_handler);
 									let message = match message_result {
 										Ok(x) => x,
 										Err(e) => {