Reject trailing bytes in protocol packets

tests/protocol_auth.rs

@@ -58,6 +58,21 @@ fn encrypted_packet_round_trips() {
     assert_eq!(plain, b"hello");
 }
 
+#[test]
+fn packet_decode_rejects_truncated_or_trailing_datagrams() {
+    let packet =
+        protocol::encode_plain(PacketKind::Ping, crypto::random_16(), 1, 0, b"hello").unwrap();
+
+    let truncated = &packet[..packet.len() - 1];
+    let err = protocol::decode(truncated).unwrap_err();
+    assert!(err.to_string().contains("truncated packet body"));
+
+    let mut trailing = packet.clone();
+    trailing.extend_from_slice(b"junk");
+    let err = protocol::decode(&trailing).unwrap_err();
+    assert!(err.to_string().contains("trailing packet bytes"));
+}
+
 #[test]
 fn plaintext_packet_never_decrypts_as_authenticated_body() {
     let key = crypto::random_32();