Encryption: Rasmatic/RFM69-Arduino-Library

The RFM69CW/RFM69HCW modules (based on the Semtech SX1231/SX1231H) have built in support for AES128 encryption. In this test harness I’m exploring the RFM69 AES128 implementation.

In the Arduino code I found the order of initialisation was critical. Because of the way the Rasmatic library is written the call to vRF69SetAesKey has to be after the vInitialize.

void setup() 
{
  Serial.begin(9600);

  pinMode(SENDER_DETECT_PIN, INPUT_PULLUP);  
  
  radio.Modulation     = FSK;
  radio.COB            = RFM69;
  radio.Frequency      = 915000;
  radio.OutputPower    = 10+18;          //10dBm OutputPower
  radio.PreambleLength = 16;             //16Byte preamble
  radio.FixedPktLength = false;          //packet in message which need to be send
  radio.CrcDisable     = false;          //CRC On
  radio.AesOn          = false;
  radio.SymbolTime     = 416000;         //2.4Kbps
  radio.Devation       = 35;             //35KHz for devation
  radio.BandWidth      = 100;            //100KHz for bandwidth
  radio.SyncLength     = 3;              //
  radio.SyncWord[0]    = 0xAA;
  radio.SyncWord[1]    = 0x2D;
  radio.SyncWord[2]    = 0xD4;

  // Highly secure 16byte fixed length key
  radio.AesKey[0] = 0x0;
  radio.AesKey[1] = 0x1;
  radio.AesKey[2] = 0x2;
  radio.AesKey[3] = 0x3;
  radio.AesKey[4] = 0x4;
  radio.AesKey[5] = 0x5;
  radio.AesKey[6] = 0x6;
  radio.AesKey[7] = 0x7;
  radio.AesKey[8] = 0x8;
  radio.AesKey[9] = 0x9;
  radio.AesKey[10] = 0xA;
  radio.AesKey[11] = 0xB;
  radio.AesKey[12] = 0xC;
  radio.AesKey[13] = 0xD;
  radio.AesKey[14] = 0xE;
  radio.AesKey[15] = 0xF;
  radio.AesOn = true ;

  radio.vInitialize();

  radio.vRF69SetAesKey();

When I first fired up the Arduino client on the Windows 10 IoT Core device I hadn’t configured the AES key but had enabled encryption.

19:21:25 Received 13 byte message =!{��>�_��5
19:21:26.114 RegIrqFlags 01000110
19:21:26 Received 13 byte message ���gǺm,0|��
19:21:26.273 Send-Done
19:21:26.453 RegIrqFlags 00001000
19:21:26.467 Transmit-Done
19:21:27.244 RegIrqFlags 01000110
19:21:27 Received 13 byte message w6�H�Y���#"#
19:21:28.373 RegIrqFlags 01000110
19:21:28 Received 13 byte message c�u�$mԙ���M{
...
Restart Arduino client
...
19:21:34.836 RegIrqFlags 01000110
19:21:34 Received 13 byte message ���gǺm,0|��
19:21:35.965 RegIrqFlags 01000110
19:21:35 Received 13 byte message w6�H�Y���#"#
19:21:36.429 Send-Done
19:21:36.610 RegIrqFlags 00001000
19:21:36.624 Transmit-Done
19:21:37.095 RegIrqFlags 01000110
19:21:37 Received 13 byte message c�u�$mԙ���M{
The program '[1560] backgroundTaskHost.exe' has exited with code -1 (0xffffffff).

When I restarted the Arduino client I got the same sequences of characters in the messages so it looks like the RFM69 encryption is most probably using electronic code book (ECB) rather than a mode with a changing initialisation vector(IV) e.g. cypher block chaining(CBC). (which wasn’t a surprise)

After modifying the Windows 10 IoT Core application to receive and transmit encrypted payloads

/*
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	 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
	 SOFTWARE

 */
namespace devMobile.IoT.Rfm69Hcw.Encryption
{
	using System;
	using System.Diagnostics;
	using System.Runtime.InteropServices.WindowsRuntime;
	using System.Text;
	using System.Threading.Tasks;
	using Windows.ApplicationModel.Background;
	using Windows.Devices.Gpio;
	using Windows.Devices.Spi;

	public sealed class Rfm69HcwDevice
	{
		private SpiDevice Rfm69Hcw;
		private GpioPin InterruptGpioPin = null;
		private const byte RegisterAddressReadMask = 0X7f;
		private const byte RegisterAddressWriteMask = 0x80;

		public Rfm69HcwDevice(int chipSelectPin, int resetPin, int interruptPin)
		{
			SpiController spiController = SpiController.GetDefaultAsync().AsTask().GetAwaiter().GetResult();
			var settings = new SpiConnectionSettings(chipSelectPin)
			{
				ClockFrequency = 500000,
				Mode = SpiMode.Mode0,
			};

			// Factory reset pin configuration
			GpioController gpioController = GpioController.GetDefault();
			GpioPin resetGpioPin = gpioController.OpenPin(resetPin);
			resetGpioPin.SetDriveMode(GpioPinDriveMode.Output);
			resetGpioPin.Write(GpioPinValue.High);
			Task.Delay(100);
			resetGpioPin.Write(GpioPinValue.Low);
			Task.Delay(10);

			// Interrupt pin for RX message & TX done notification 
			InterruptGpioPin = gpioController.OpenPin(interruptPin);
			resetGpioPin.SetDriveMode(GpioPinDriveMode.Input);

			InterruptGpioPin.ValueChanged += InterruptGpioPin_ValueChanged;

			Rfm69Hcw = spiController.GetDevice(settings);
		}

		private void InterruptGpioPin_ValueChanged(GpioPin sender, GpioPinValueChangedEventArgs args)
		{
			if (args.Edge != GpioPinEdge.RisingEdge)
			{
				return;
			}

			byte irqFlags = this.RegisterReadByte(0x28); // RegIrqFlags2
			Debug.WriteLine("{0:HH:mm:ss.fff} RegIrqFlags {1}", DateTime.Now, Convert.ToString((byte)irqFlags, 2).PadLeft(8, '0'));
			if ((irqFlags & 0b00000100) == 0b00000100)  // PayLoadReady set
			{
				// Read the length of the buffer
				byte numberOfBytes = this.RegisterReadByte(0x0);

				// Allocate buffer for message
				byte[] messageBytes = new byte[numberOfBytes];

				for (int i = 0; i < numberOfBytes; i++)
				{
					messageBytes[i] = this.RegisterReadByte(0x00); // RegFifo
				}

				string messageText = UTF8Encoding.UTF8.GetString(messageBytes);
				Debug.WriteLine("{0:HH:mm:ss} Received {1} byte message {2}", DateTime.Now, messageBytes.Length, messageText);
			}

			if ((irqFlags &amp; 0b00001000) == 0b00001000)  // PacketSent set
			{
				this.RegisterWriteByte(0x01, 0b00010000); // RegOpMode set ReceiveMode
				Debug.WriteLine("{0:HH:mm:ss.fff} Transmit-Done", DateTime.Now);
			}
		}

		public Byte RegisterReadByte(byte address)
		{
			byte[] writeBuffer = new byte[] { address &amp;= RegisterAddressReadMask };
			byte[] readBuffer = new byte[1];
			Debug.Assert(Rfm69Hcw != null);

			Rfm69Hcw.TransferSequential(writeBuffer, readBuffer);

			return readBuffer[0];
		}

		public byte[] RegisterRead(byte address, int length)
		{
			byte[] writeBuffer = new byte[] { address &amp;= RegisterAddressReadMask };
			byte[] readBuffer = new byte[length];
			Debug.Assert(Rfm69Hcw != null);

			Rfm69Hcw.TransferSequential(writeBuffer, readBuffer);

			return readBuffer;
		}

		public void RegisterWriteByte(byte address, byte value)
		{
			byte[] writeBuffer = new byte[] { address |= RegisterAddressWriteMask, value };
			Debug.Assert(Rfm69Hcw != null);

			Rfm69Hcw.Write(writeBuffer);
		}

		public void RegisterWrite(byte address, [ReadOnlyArray()] byte[] bytes)
		{
			byte[] writeBuffer = new byte[1 + bytes.Length];
			Debug.Assert(Rfm69Hcw != null);

			Array.Copy(bytes, 0, writeBuffer, 1, bytes.Length);
			writeBuffer[0] = address |= RegisterAddressWriteMask;

			Rfm69Hcw.Write(writeBuffer);
		}

		public void RegisterDump()
		{
			Debug.WriteLine("Register dump");
			for (byte registerIndex = 0; registerIndex <= 0x3D; registerIndex++)
			{
				byte registerValue = this.RegisterReadByte(registerIndex);

				Debug.WriteLine("Register 0x{0:x2} - Value 0X{1:x2} - Bits {2}", registerIndex, registerValue, Convert.ToString(registerValue, 2).PadLeft(8, '0'));
			}
		}
	}


	public sealed class StartupTask : IBackgroundTask
	{
		private const int ChipSelectLine = 1;
		private const int ResetPin = 25;
		private const int InterruptPin = 22;
		private Rfm69HcwDevice rfm69Device = new Rfm69HcwDevice(ChipSelectLine, ResetPin, InterruptPin);

		const double RH_RF6M9HCW_FXOSC = 32000000.0;
		const double RH_RFM69HCW_FSTEP = RH_RF6M9HCW_FXOSC / 524288.0;

		public void Run(IBackgroundTaskInstance taskInstance)
		{
			//rfm69Device.RegisterDump();

			// regOpMode standby
			rfm69Device.RegisterWriteByte(0x01, 0b00000100);

			// BitRate MSB/LSB
			rfm69Device.RegisterWriteByte(0x03, 0x34);
			rfm69Device.RegisterWriteByte(0x04, 0x00);

			// Frequency deviation
			rfm69Device.RegisterWriteByte(0x05, 0x02);
			rfm69Device.RegisterWriteByte(0x06, 0x3d);

			// Calculate the frequency accoring to the datasheett
			byte[] bytes = BitConverter.GetBytes((uint)(915000000.0 / RH_RFM69HCW_FSTEP));
			Debug.WriteLine("Byte Hex 0x{0:x2} 0x{1:x2} 0x{2:x2} 0x{3:x2}", bytes[0], bytes[1], bytes[2], bytes[3]);
			rfm69Device.RegisterWriteByte(0x07, bytes[2]);
			rfm69Device.RegisterWriteByte(0x08, bytes[1]);
			rfm69Device.RegisterWriteByte(0x09, bytes[0]);

			// RegRxBW
			rfm69Device.RegisterWriteByte(0x19, 0x2a);

			// RegDioMapping1
			rfm69Device.RegisterWriteByte(0x26, 0x01);

			// Setup preamble length to 16 (default is 3) RegPreambleMsb RegPreambleLsb
			rfm69Device.RegisterWriteByte(0x2C, 0x0);
			rfm69Device.RegisterWriteByte(0x2D, 0x10);

			// RegSyncConfig Set the Sync length and byte values SyncOn + 3 custom sync bytes
			rfm69Device.RegisterWriteByte(0x2e, 0x90);

			// RegSyncValues1 thru RegSyncValues3
			rfm69Device.RegisterWriteByte(0x2f, 0xAA);
			rfm69Device.RegisterWriteByte(0x30, 0x2D);
			rfm69Device.RegisterWriteByte(0x31, 0xD4);

			// RegPacketConfig1 Variable length with CRC on
			rfm69Device.RegisterWriteByte(0x37, 0x90);

			// Set the AES key and turn on AES RegPacketConfig2
			rfm69Device.RegisterWriteByte(0x3D, 0x03);
	
			rfm69Device.RegisterWriteByte(0x3E, 0x00);
			rfm69Device.RegisterWriteByte(0x3F, 0x01);
			rfm69Device.RegisterWriteByte(0x40, 0x02);
			rfm69Device.RegisterWriteByte(0x41, 0x03);
			rfm69Device.RegisterWriteByte(0x42, 0x04);
			rfm69Device.RegisterWriteByte(0x43, 0x05);
			rfm69Device.RegisterWriteByte(0x44, 0x06);
			rfm69Device.RegisterWriteByte(0x45, 0x07);
			rfm69Device.RegisterWriteByte(0x46, 0x08);
			rfm69Device.RegisterWriteByte(0x47, 0x09);
			rfm69Device.RegisterWriteByte(0x48, 0x0A);
			rfm69Device.RegisterWriteByte(0x49, 0x0B);
			rfm69Device.RegisterWriteByte(0x4A, 0x0C);
			rfm69Device.RegisterWriteByte(0x4B, 0x0D);
			rfm69Device.RegisterWriteByte(0x4C, 0x0E);
			rfm69Device.RegisterWriteByte(0x4D, 0x0F);
			
/*
			// Clear out the AES key
			rfm69Device.RegisterWriteByte(0x3E, 0x0);
			rfm69Device.RegisterWriteByte(0x3F, 0x0);
			rfm69Device.RegisterWriteByte(0x40, 0x0);
			rfm69Device.RegisterWriteByte(0x41, 0x0);
			rfm69Device.RegisterWriteByte(0x42, 0x0);
			rfm69Device.RegisterWriteByte(0x43, 0x0);
			rfm69Device.RegisterWriteByte(0x44, 0x0);
			rfm69Device.RegisterWriteByte(0x45, 0x0);
			rfm69Device.RegisterWriteByte(0x46, 0x0);
			rfm69Device.RegisterWriteByte(0x47, 0x0);
			rfm69Device.RegisterWriteByte(0x48, 0x0);
			rfm69Device.RegisterWriteByte(0x49, 0x0);
			rfm69Device.RegisterWriteByte(0x4A, 0x0);
			rfm69Device.RegisterWriteByte(0x4B, 0x0);
			rfm69Device.RegisterWriteByte(0x4C, 0x0);
			rfm69Device.RegisterWriteByte(0x4D, 0x0);
*/			

			rfm69Device.RegisterDump();

			while (true)
			{
				// Standby mode while loading message into FIFO
				rfm69Device.RegisterWriteByte(0x01, 0b00000100);

				byte[] messageBuffer = UTF8Encoding.UTF8.GetBytes("hello world " + DateTime.Now.ToLongTimeString());
				rfm69Device.RegisterWriteByte(0x0, (byte)messageBuffer.Length);
				rfm69Device.RegisterWrite(0x0, messageBuffer);

				// Transmit mode once FIFO loaded
				rfm69Device.RegisterWriteByte(0x01, 0b00001100);

				Debug.WriteLine("{0:HH:mm:ss.fff} Send-Done", DateTime.Now);

				Task.Delay(5000).Wait();
			}
		}
	}
}

I could see inbound messages from the transmit Arduino and Windows 10 device interleaved on the receive Arduino.

21:06:12.735 -> RX start
21:06:12.735 -> 0x0: 0x0
21:06:12.769 -> 0x1: 0x10
21:06:12.769 -> 0x2: 0x0
…
21:06:13.453 -> 0x3B: 0x0
21:06:13.453 -> 0x3C: 0x1
21:06:13.487 -> 0x3D: 0x1
21:06:15.218 -> MessageIn:hello world 9:06:15 PM
21:06:20.317 -> MessageIn:hello world 9:06:20 PM
21:06:24.559 -> MessageIn:Hello world:0
21:06:25.384 -> MessageIn:hello world 9:06:25 PM
21:06:28.009 -> MessageIn:Hello world:1
21:06:30.454 -> MessageIn:hello world 9:06:30 PM
21:06:31.455 -> MessageIn:Hello world:2
21:06:34.939 -> MessageIn:Hello world:3
21:06:35.596 -> MessageIn:hello world 9:06:35 PM
21:06:38.389 -> MessageIn:Hello world:4
21:06:40.666 -> MessageIn:hello world 9:06:40 PM
21:06:41.838 -> MessageIn:Hello world:5
21:06:45.316 -> MessageIn:Hello world:6
21:06:45.731 -> MessageIn:hello world 9:06:45 PM
21:06:48.761 -> MessageIn:Hello world:7
21:06:50.799 -> MessageIn:hello world 9:06:50 PM
21:06:52.214 -> MessageIn:Hello world:8

The next step will be merging and refactoring the test harness to extract the code for accessing the RFM69 registers into a separate class, then defining enumerations and constants for all the RFM69 settings.