record/routing: set minimum padding size

record/blinded_data.go

@@ -10,6 +10,13 @@ import (
 	"github.com/lightningnetwork/lnd/tlv"
 )
 
+// AverageDummyHopPayloadSize is the size of a standard blinded path dummy hop
+// payload. In most cases, this is larger than the other payload types and so
+// to make sure that a sender cannot use this fact to know if a dummy hop is
+// present or not, we'll make sure to always pad all payloads to at least this
+// size.
+const AverageDummyHopPayloadSize = 51
+
 // BlindedRouteData contains the information that is included in a blinded
 // route encrypted data blob that is created by the recipient to provide
 // forwarding information.

record/blinded_data_test.go

@@ -196,6 +196,10 @@ func TestDummyHopBlindedDataEncoding(t *testing.T) {
 	encoded, err := EncodeBlindedRouteData(routeData)
 	require.NoError(t, err)
 
+	// Assert the size of an average dummy hop payload in case we need to
+	// update this constant in future.
+	require.Len(t, encoded, AverageDummyHopPayloadSize)
+
 	b := bytes.NewBuffer(encoded)
 	decodedData, err := DecodeBlindedRouteData(b)
 	require.NoError(t, err)