Sunday, June 15, 2014

Arduino and wireless networking

The nRF24L01 module allows cheap networking for Arduinos. Other than VCC which it wants at 3v3 it uses SPI (4 wires) and a few auxiliary wires, one for an interrupt line. One of the libraries to drive these chips is the RF24. Packets can be up to 32 bytes in length, and by default attempts to send less than 32 bytes result in sending a whole 32 byte block. Attempts to send more than 32 bytes seem to result in only the first 32 being sent. There is a CRC which is a 2 byte and is on by default.

For doing some IoT things using a HMAC might be much better choice than just CRC. The major advantage being that one can tell that something coming over a wireless link was from the expected source. Of course, if somebody has physical access to the arduinos then they can clone the HMAC key, but one has to define what they want from the system. The HMAC provides a fairly good assurance that sensor data is coming from the arduino you think it is coming from rather than somebody trying to send fake data. Another rather cute benefit is that the system doesn't accept junk data attacks. If your HMAC doesn't match the packet doesn't get evaluated by the higher level software.

Using a Sha256 HMAC the return ping times go from 50 to 150ms. The doubling can be explained by the network traffic, as the HMAC is 32 bytes and will double the amount of traffic. I think perhaps the extra 50ms goes into hashing but I'll have to measure that more specifically to find out.

Once I clean up the code a little I'll probably push it to github. A new RF24HMAC class delegates to the existing RF24 class and sends a HMAC packet right after the user data packet for you. The interface is similar, I'm adding some writenum() calls which I might make more like nodejs buffer. Once you have added the user data packet call done() to send everything including the HMAC packet.

RF24 radio(9,10);
RF24HMAC radiomac( radio, "wonderful key" );
radiomac.writeu32( time );
bool ok = radiomac.done();

The receiving end boils down to getting a "packet" which is really just the 32 bytes of user data that was sent. The one readAuthenticatedPacket() call actually gets 2 packets off the wire, the user data packet and the HMAC packet. If the hmac calculated locally for the user data packet does not match the HMAC that was received from the other end then you get a null pointer back from readAuthenticatedPacket(). The data is either authenticated or you don't get to see any of it.

RF24HMAC radiomac( radio, "wonderful key" );
uint8_t* packetData = 0;
if( packetData = radiomac.readAuthenticatedPacket() )
     int di = 0;
     uint32_t v = readu32( packetData, di );
     got_time = v;
     printf("Got payload %lu...\n\r",got_time);

Oh yeah, I also found this rather cute code to output hex while sniffing around.

1 comment:

monkeyiq said...