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
+            // Apply padding if requested (iOS-compatible: selective padding for privacy)
+            if (padding) {
                 val optimalSize = MessagePadding.optimalBlockSize(result.size)
-            val paddedData = MessagePadding.pad(result, optimalSize)
+                return MessagePadding.pad(result, optimalSize)
+            }
             
-            return paddedData
+            return result
             
         } catch (e: Exception) {
             Log.e("BinaryProtocol", "Error encoding packet type ${packet.type}: ${e.message}")

app/src/test/java/com/bitchat/android/mesh/BLEPacketPaddingPolicyTest.kt

@@ -0,0 +1,133 @@
+package com.bitchat.android.mesh
+
+import com.bitchat.android.protocol.BinaryProtocol
+import com.bitchat.android.protocol.BitchatPacket
+import com.bitchat.android.protocol.MessagePadding
+import com.bitchat.android.protocol.MessageType
+import org.junit.Assert.assertArrayEquals
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNotNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class BLEPacketPaddingPolicyTest {
+    private val senderID = byteArrayOf(0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x00)
+    private val timestamp = 1_709_600_000_000uL
+
+    @Test
+    fun `BLE padding policy matches iOS packet type table`() {
+        val expected = mapOf(
+            MessageType.ANNOUNCE to false,
+            MessageType.MESSAGE to false,
+            MessageType.LEAVE to false,
+            MessageType.REQUEST_SYNC to false,
+            MessageType.FRAGMENT to false,
+            MessageType.FILE_TRANSFER to false,
+            MessageType.NOISE_ENCRYPTED to true,
+            MessageType.NOISE_HANDSHAKE to true
+        )
+
+        expected.forEach { (type, shouldPad) ->
+            assertEquals(
+                "${type.name} BLE padding policy must match iOS",
+                shouldPad,
+                BLEPacketPaddingPolicy.shouldPadForBLE(type.value)
+            )
+        }
+
+        assertFalse(
+            "Unknown packet types should be sent unpadded, matching iOS default",
+            BLEPacketPaddingPolicy.shouldPadForBLE(0x7Fu)
+        )
+    }
+
+    @Test
+    fun `public BLE packet types are encoded without PKCS7 padding tails`() {
+        val publicTypes = listOf(
+            MessageType.ANNOUNCE,
+            MessageType.MESSAGE,
+            MessageType.LEAVE,
+            MessageType.REQUEST_SYNC,
+            MessageType.FRAGMENT,
+            MessageType.FILE_TRANSFER
+        )
+
+        publicTypes.forEach { type ->
+            val packet = packet(type.value, payload = payloadEndingWithoutPaddingShape(type))
+            val raw = packet.toBinaryData(padding = false)!!
+            val encodedForBLE = packet.toBinaryData(
+                padding = BLEPacketPaddingPolicy.shouldPadForBLE(packet.type)
+            )!!
+
+            assertArrayEquals(
+                "${type.name} BLE encoding should be the unpadded frame",
+                raw,
+                encodedForBLE
+            )
+            assertFalse(
+                "${type.name} BLE encoding must not leave iOS-visible PKCS#7 tail bytes",
+                hasPkcs7PaddingTail(encodedForBLE)
+            )
+            assertNotNull(
+                "${type.name} unpadded BLE frame must remain decodable",
+                BinaryProtocol.decode(encodedForBLE)
+            )
+        }
+    }
+
+    @Test
+    fun `Noise BLE packet types remain padded and decodable`() {
+        val noiseTypes = listOf(MessageType.NOISE_HANDSHAKE, MessageType.NOISE_ENCRYPTED)
+
+        noiseTypes.forEach { type ->
+            val packet = packet(type.value, payload = "noise-${type.name}".toByteArray())
+            val raw = packet.toBinaryData(padding = false)!!
+            val encodedForBLE = packet.toBinaryData(
+                padding = BLEPacketPaddingPolicy.shouldPadForBLE(packet.type)
+            )!!
+
+            assertTrue("${type.name} BLE frame should be padded", encodedForBLE.size > raw.size)
+            assertTrue(
+                "${type.name} BLE frame should end with valid PKCS#7 padding",
+                hasPkcs7PaddingTail(encodedForBLE)
+            )
+            assertArrayEquals(
+                "${type.name} padding should strip back to the raw frame",
+                raw,
+                MessagePadding.unpad(encodedForBLE)
+            )
+            assertNotNull(
+                "${type.name} padded BLE frame must remain decodable",
+                BinaryProtocol.decode(encodedForBLE)
+            )
+        }
+    }
+
+    private fun packet(type: UByte, payload: ByteArray): BitchatPacket {
+        val version = if (type == MessageType.FILE_TRANSFER.value) 2u.toUByte() else 1u.toUByte()
+        return BitchatPacket(
+            version = version,
+            type = type,
+            senderID = senderID,
+            recipientID = null,
+            timestamp = timestamp,
+            payload = payload,
+            signature = null,
+            ttl = 7u,
+            route = null
+        )
+    }
+
+    private fun payloadEndingWithoutPaddingShape(type: MessageType): ByteArray {
+        return "ios-ble-policy-${type.name}-z".toByteArray()
+    }
+
+    private fun hasPkcs7PaddingTail(data: ByteArray): Boolean {
+        if (data.isEmpty()) return false
+        val paddingLength = data.last().toInt() and 0xFF
+        if (paddingLength <= 0 || paddingLength > data.size) return false
+        val start = data.size - paddingLength
+        return data.copyOfRange(start, data.size).all { (it.toInt() and 0xFF) == paddingLength }
+    }
+}

app/src/test/java/com/bitchat/android/protocol/BinaryProtocolTest.kt

@@ -1,6 +1,8 @@
 package com.bitchat.android.protocol
 
 import org.junit.Assert.assertEquals
+import org.junit.Assert.assertArrayEquals
+import org.junit.Assert.assertFalse
 import org.junit.Assert.assertNotNull
 import org.junit.Assert.assertNull
 import org.junit.Assert.assertTrue
@@ -368,6 +370,54 @@ class BinaryProtocolTest {
             medPayload.contentEquals(medDecoded.payload))
     }
 
+    @Test
+    fun `encoding with padding false returns exact unpadded frame`() {
+        val payload = "ios-compatible-unpadded".toByteArray()
+        val packet = makePacket(payload = payload)
+
+        val unpadded = BinaryProtocol.encode(packet, padding = false)!!
+        val padded = BinaryProtocol.encode(packet, padding = true)!!
+
+        assertEquals(
+            "v1 unpadded frame size must be header + sender + payload",
+            14 + 8 + payload.size,
+            unpadded.size
+        )
+        assertEquals("small padded frame should pad to 256 bytes", 256, padded.size)
+        assertFalse(
+            "padding=false must not append PKCS#7 bytes",
+            hasPkcs7PaddingTail(unpadded)
+        )
+        assertArrayEquals(
+            "padded frame must unpad to the exact raw frame",
+            unpadded,
+            MessagePadding.unpad(padded)
+        )
+        assertPacketEquals(packet, BinaryProtocol.decode(unpadded)!!)
+        assertPacketEquals(packet, BinaryProtocol.decode(padded)!!)
+    }
+
+    @Test
+    fun `BitchatPacket toBinaryData propagates padding flag`() {
+        val packet = makePacket(payload = "packet-helper-padding-flag".toByteArray())
+
+        val defaultEncoded = packet.toBinaryData()!!
+        val explicitlyPadded = packet.toBinaryData(padding = true)!!
+        val unpadded = packet.toBinaryData(padding = false)!!
+
+        assertArrayEquals(
+            "default helper must remain padded for backward compatibility",
+            explicitlyPadded,
+            defaultEncoded
+        )
+        assertTrue("default helper should produce a padded frame", defaultEncoded.size > unpadded.size)
+        assertArrayEquals(
+            "helper padding=false must match BinaryProtocol padding=false",
+            BinaryProtocol.encode(packet, padding = false)!!,
+            unpadded
+        )
+    }
+
     /**
      * Oversized packet bypasses padding
      *
@@ -1141,6 +1191,14 @@ class BinaryProtocolTest {
         return decoded!!
     }
 
+    private fun hasPkcs7PaddingTail(data: ByteArray): Boolean {
+        if (data.isEmpty()) return false
+        val paddingLength = data.last().toInt() and 0xFF
+        if (paddingLength <= 0 || paddingLength > data.size) return false
+        val start = data.size - paddingLength
+        return data.copyOfRange(start, data.size).all { (it.toInt() and 0xFF) == paddingLength }
+    }
+
     private fun assertPacketEquals(expected: BitchatPacket, actual: BitchatPacket) {
         assertEquals("version", expected.version, actual.version)
         assertEquals("type", expected.type, actual.type)