feat: Implement iOS-compatible selective padding for BLE messages (#501)

* feat: Implement iOS-compatible selective padding for BLE messages

* regression tests for padding

---------

Co-authored-by: callebtc <93376500+callebtc@users.noreply.github.com>

app/src/main/java/com/bitchat/android/mesh/BLEPacketPaddingPolicy.kt

@@ -0,0 +1,18 @@
+package com.bitchat.android.mesh
+
+import com.bitchat.android.protocol.MessageType
+
+/**
+ * iOS-compatible BLE padding policy.
+ *
+ * Keep this aligned with iOS BLEOutboundPacketPolicy.padsBLEFrame(for:):
+ * only Noise frames are padded over BLE.
+ */
+object BLEPacketPaddingPolicy {
+    fun shouldPadForBLE(type: UByte): Boolean {
+        return when (MessageType.fromValue(type)) {
+            MessageType.NOISE_ENCRYPTED, MessageType.NOISE_HANDSHAKE -> true
+            else -> false
+        }
+    }
+}

app/src/main/java/com/bitchat/android/mesh/BluetoothPacketBroadcaster.kt

@@ -170,7 +170,9 @@ class BluetoothPacketBroadcaster(
         characteristic: BluetoothGattCharacteristic?
     ): Boolean {
         val packet = routed.packet
-        val data = packet.toBinaryData() ?: return false
+        // iOS-compatible: Use selective padding policy for BLE
+        val padForBLE = BLEPacketPaddingPolicy.shouldPadForBLE(packet.type)
+        val data = packet.toBinaryData(padding = padForBLE) ?: return false
         val isFile = packet.type == MessageType.FILE_TRANSFER.value
         if (isFile) {
             Log.d(TAG, "📤 Broadcasting FILE_TRANSFER: ${packet.payload.size} bytes")
@@ -261,7 +263,9 @@ class BluetoothPacketBroadcaster(
         characteristic: BluetoothGattCharacteristic?
     ) {
         val packet = routed.packet
-        val data = packet.toBinaryData() ?: return
+        // iOS-compatible: Use selective padding policy for BLE
+        val padForBLE = BLEPacketPaddingPolicy.shouldPadForBLE(packet.type)
+        val data = packet.toBinaryData(padding = padForBLE) ?: return
         val typeName = MessageType.fromValue(packet.type)?.name ?: packet.type.toString()
         val senderPeerID = routed.peerID ?: packet.senderID.toHexString()
         val incomingAddr = routed.relayAddress

app/src/main/java/com/bitchat/android/protocol/BinaryProtocol.kt

@@ -79,8 +79,8 @@ data class BitchatPacket(
         ttl = ttl
     )
 
-    fun toBinaryData(): ByteArray? {
-        return BinaryProtocol.encode(this)
+    fun toBinaryData(padding: Boolean = true): ByteArray? {
+        return BinaryProtocol.encode(this, padding = padding)
     }
 
     /**
@@ -198,7 +198,7 @@ object BinaryProtocol {
         }
     }
     
-    fun encode(packet: BitchatPacket): ByteArray? {
+    fun encode(packet: BitchatPacket, padding: Boolean = true): ByteArray? {
         try {
             // Try to compress payload if beneficial
             var payload = packet.payload
@@ -310,11 +310,13 @@ object BinaryProtocol {
             buffer.rewind()
             buffer.get(result)
             
-            // Apply padding to standard block sizes for traffic analysis resistance
-            val optimalSize = MessagePadding.optimalBlockSize(result.size)
-            val paddedData = MessagePadding.pad(result, optimalSize)
+            // Apply padding if requested (iOS-compatible: selective padding for privacy)
+            if (padding) {
+                val optimalSize = MessagePadding.optimalBlockSize(result.size)
+                return MessagePadding.pad(result, optimalSize)
+            }
             
-            return paddedData
+            return result
             
         } catch (e: Exception) {
             Log.e("BinaryProtocol", "Error encoding packet type ${packet.type}: ${e.message}")