better error handling in ASN.1 message parser

Hash2Pub/src/Hash2Pub/ASN1Coding.hs

@@ -284,12 +284,17 @@ encodeMessage
 
 parseMessage :: ParseASN1 FediChordMessage
 parseMessage = do
+    begin <- getNext
+    case begin of
+      Start Sequence -> return ()
+      x -> throwParseError $ "unexpected ASN.1 element " ++ show x
 -- request and response messages are distiguishable by their structure,
 -- see ASN.1 schema
     first <- getNext
     case first of
       Enumerated a -> parseRequest . toEnum . fromIntegral $ a
       IntVal i -> parseResponse i
+      other -> throwParseError $ "unexpected first ASN1 element: " ++ show other
 
 parseRequest :: Action -> ParseASN1 FediChordMessage
 parseRequest action = do
@@ -304,6 +309,11 @@ parseRequest action = do
             Leave -> parseLeaveRequest
             Stabilise -> parseStabiliseRequest
             Ping -> parsePingRequest
+    -- consume sequence end
+    end <- getNext
+    case end of
+      End Sequence -> return ()
+      x -> throwParseError $ "unexpected ASN.1 element " ++ show x
 
     return $ Request requestID sender parts part action payload
 
@@ -320,35 +330,48 @@ parseResponse responseTo = do
             Leave -> parseLeaveResponse
             Stabilise -> parseStabiliseResponse
             Ping -> parsePingResponse
+    -- consume sequence end
+    end <- getNext
+    case end of
+      End Sequence -> return ()
+      x -> throwParseError $ "unexpected ASN.1 element " ++ show x
 
     return $ Response responseTo senderID parts part action payload
 
 parseInteger :: ParseASN1 Integer
 parseInteger = do
-    IntVal parsed <- getNext
-    return parsed
+    i <- getNext
+    case i of
+        IntVal parsed -> return parsed
+        x -> throwParseError $ "Expected IntVal but got " ++ show x
 
 parseEnum :: Enum a => ParseASN1 a
 parseEnum = do
-    Enumerated en <- getNext
-    return $ toEnum . fromIntegral $ en
+    e <- getNext
+    case e of
+        Enumerated en -> return $ toEnum . fromIntegral $ en
+        x -> throwParseError $ "Expected Enumerated but got " ++ show x
 
 parseString :: ParseASN1 String
 parseString = do
-    ASN1String toBeParsed <- getNext
-    maybe (throwParseError "string parsing failed") return $ asn1CharacterToString toBeParsed
+    s <- getNext
+    case s of
+        ASN1String toBeParsed -> maybe (throwParseError "string parsing failed") return $ asn1CharacterToString toBeParsed
+        x -> throwParseError $ "Expected a ASN1String but got " ++ show x
 
 parseOctets :: ParseASN1 BS.ByteString
 parseOctets = do
-    OctetString bs <- getNext
-    return bs
+    os <- getNext
+    case os of
+        OctetString bs -> return bs
+        x -> throwParseError $ "Expected an OctetString but got " ++ show x
 
 parseNull :: ParseASN1 ()
 parseNull = do
-    -- ToDo: figure out how this fails on wrong inputs,
-    -- maybe a case distinction + throwParseError is better?
-    Null <- getNext
-    return ()
+    n <- getNext
+    case n of
+        Null -> return ()
+        x -> throwParseError $ "Expected Null but got " ++ show x
 
 parseNodeState :: ParseASN1 NodeState
 parseNodeState = do