move limit of message parts to a global constant

Hash2Pub/FediChord.asn1

@@ -10,8 +10,8 @@ Request ::= SEQUENCE {
 	action			Action,
 	requestID		INTEGER,
 	sender			NodeState,
-	parts			INTEGER,	-- number of message parts
-	part			INTEGER,	-- part number of this message, starts at 1
+	parts			INTEGER (0..150),	-- number of message parts
+	part			INTEGER (0..150),	-- part number of this message, starts at 1
 	actionPayload	CHOICE {
 		queryIDRequestPayload		QueryIDRequestPayload,
 		joinRequestPayload			JoinRequestPayload,
@@ -27,8 +27,8 @@ Request ::= SEQUENCE {
 Response ::= SEQUENCE {
 	responseTo		INTEGER,
 	senderID		NodeID,
-	parts			INTEGER,
-	part			INTEGER,
+	parts			INTEGER (0..150),
+	part			INTEGER (0..150),
 	action			Action,
 	actionPayload	CHOICE {
 		queryIDResponsePayload	QueryIDResponsePayload,

Hash2Pub/src/Hash2Pub/ASN1Coding.hs

@@ -103,7 +103,6 @@ serialiseMessage maxBytesLength msg =
         actionPayload = payload msg
         encodedMsgs i = map (encodeASN1' DER . encodeMessage) $ messageParts i
         maxMsgLength msgs = maximum $ map BS.length msgs
-        maximumParts = 150
 
 -- | Deserialise a ASN.1 DER encoded bytesstring of a single 'FediChordMessage'.
 deserialiseMessage :: BS.ByteString

Hash2Pub/src/Hash2Pub/DHTProtocol.hs

@@ -11,6 +11,7 @@ module Hash2Pub.DHTProtocol
     , Action(..)
     , ActionPayload(..)
     , FediChordMessage(..)
+    , maximumParts
     )
         where
 
@@ -136,6 +137,11 @@ data ActionPayload =
                         }
     deriving (Show, Eq)
 
+-- | global limit of parts per message used when (de)serialising messages.
+-- Used to limit the impact of DOS attempts with partial messages.
+maximumParts :: Num a => a
+maximumParts = 150
+
 -- | dedicated data type for cache entries sent to or received from the network,
 -- as these have to be considered as unvalidated. Also helps with separation of trust.
 data RemoteCacheEntry = RemoteCacheEntry NodeState POSIXTime