bolt4

commit 7e9196bd1f726947eea0aebd2179454fffa62304
parent dd4a5625a853a465d07feba5e6e5fee47c7b0776
Author: Jared Tobin <jared@jtobin.io>
Date:   Sat, 10 Oct 2026 13:43:48 +0400

lib: reject hop payloads under 2 bytes in construct

An all-Nothing HopPayload encodes to zero bytes. construct accepted
it and produced an onion that every receiver must reject (the spec
reserves payload lengths 0 and 1). Fail with the new
InvalidHopPayload error, carrying the hop index, instead.

Construct tests that used empty payloads now use a minimal one.

Diffstat:
M
lib/Lightning/Protocol/BOLT4/Construct.hs
 | 
6
+++++-
M
test/Main.hs
 | 
30
+++++++++++++++++++-----------

2 files changed, 24 insertions(+), 12 deletions(-)
diff --git a/lib/Lightning/Protocol/BOLT4/Construct.hs b/lib/Lightning/Protocol/BOLT4/Construct.hs
@@ -40,6 +40,7 @@ data Error
   | TooManyHops
   | PayloadTooLarge !Int
   | InvalidHopPubKey !Int
+  | InvalidHopPayload !Int  -- ^ hop index of a payload under 2 bytes
   deriving (Eq, Show)
 
 -- | Maximum number of hops in a route.
@@ -74,10 +75,13 @@ construct !sessionKey !hops !assocData
       let hopPubKeys = map hopPubKey hops
       (secrets, _) <- computeAllSecrets sessionKey ephPub hopPubKeys
 
-      -- Validate payload sizes
+      -- Validate payload sizes (receivers reject lengths 0 and 1)
       let payloadBytes = map (encodeHopPayload . hopPayload) hops
           payloadSizes = map payloadShiftSize payloadBytes
           totalSize = sum payloadSizes
+      case [ i | (i, p) <- zip [0 ..] payloadBytes, BS.length p < 2 ] of
+        i : _ -> Left (InvalidHopPayload i)
+        [] -> pure ()
       if totalSize > hopPayloadsSize
         then Left (PayloadTooLarge totalSize)
         else do
diff --git a/test/Main.hs b/test/Main.hs
@@ -512,8 +512,20 @@ testConstructErrorCases = testGroup "error cases" [
       case result of
         Left TooManyHops -> return ()
         _ -> assertFailure "Expected TooManyHops"
+  , testCase "rejects an empty hop payload" $ do
+      pub <- demand "parse_point" $
+        Secp256k1.parse_point (fromHex hop0PubKeyHex)
+      let emptyPayload = HopPayload Nothing Nothing Nothing
+                           Nothing Nothing Nothing []
+          hops = [Hop pub minimalPayload, Hop pub emptyPayload]
+      construct sessionKey hops "" @?= Left (InvalidHopPayload 1)
   ]
 
+-- | A small, valid hop payload.
+minimalPayload :: HopPayload
+minimalPayload =
+  HopPayload (Just 1000) (Just 500000) Nothing Nothing Nothing Nothing []
+
 testSharedSecretComputation :: TestTree
 testSharedSecretComputation =
   testCase "computes correct shared secrets (BOLT4 spec)" $ do
@@ -527,13 +539,11 @@ testSharedSecretComputation =
       Secp256k1.parse_point (fromHex hop3PubKeyHex)
     pub4 <- demand "parse_point" $
       Secp256k1.parse_point (fromHex hop4PubKeyHex)
-    let emptyPayload = HopPayload Nothing Nothing Nothing
-                         Nothing Nothing Nothing []
-        hops = [ Hop pub0 emptyPayload
-               , Hop pub1 emptyPayload
-               , Hop pub2 emptyPayload
-               , Hop pub3 emptyPayload
-               , Hop pub4 emptyPayload
+    let hops = [ Hop pub0 minimalPayload
+               , Hop pub1 minimalPayload
+               , Hop pub2 minimalPayload
+               , Hop pub3 minimalPayload
+               , Hop pub4 minimalPayload
                ]
         result = construct sessionKey hops ""
     case result of
@@ -557,10 +567,8 @@ testPacketStructure =
       Secp256k1.parse_point (fromHex hop0PubKeyHex)
     pub1 <- demand "parse_point" $
       Secp256k1.parse_point (fromHex hop1PubKeyHex)
-    let emptyPayload = HopPayload Nothing Nothing Nothing
-                         Nothing Nothing Nothing []
-        hops = [Hop pub0 emptyPayload,
-                Hop pub1 emptyPayload]
+    let hops = [Hop pub0 minimalPayload,
+                Hop pub1 minimalPayload]
         result = construct sessionKey hops ""
     case result of
       Left err ->