Wednesday, May 25, 2016

Decoding a LoraWAN payload send by your ESP8266 gateway



If you are receiving a LoraWAN packet on you LoraWAN gateway using the ESP8266 module most probably  you will forward it to a back-end server to be processed by your application. But the message is encoded and encrypted so this is the procedure to do it right.


rxpk update: {"rxpk":[{"tmst":1060664170,"chan":0,"rfch":0,"freq":868.100000,"stat":1,"modu":"LORA","datr":"SF7BW125","codr":"4/5","lsnr":12,"rssi":-28,"size":31,"data":"QGIH4AIAqgABvJNVF4DpUapp/xQN1REVnI+jYoR6Ig=="}]}




Step1. Base64

The "data":"QGIH4AIAqgABvJNVF4DpUapp/xQN1REVnI+jYoR6Ig==" need to be decoded using base64 and the result will be:

64, 98, 7, -32, 2, 0, -86, 0, 1, -68, -109, 85, 23, -128, -23, 81, -86, 105, -1, 20, 13, -43, 17, 21, -100, -113, -93, 98, -124, 122, 34





Step 2. Build the block A(i) according to the 4.3.3.1 

For each data message, the algorithm defines a sequence of Blocks Ai  for i = 1..k 
with k = ceil(len(pld) / 16). In our case the k=2 because the len(pld)=31 so there are 2 blocks that need to be constructed.

An A Block will be like this one:

-----------------------------------------------------------------------------------------------------------
                          | Dir | Device Address     | Fcnt Up/Down       |      | Block number     
-----------------------------------------------------------------------------------------------------------
0x01 | 0x00 0x00 0x00 0x00| 0x00| 0x62 0x07 0xE0 0x02| 0xAA 0x00 0x00 0x00| 0x00 | 0x01



-----------------------------------------------------------------------------------------------------------
                          | Dir | Device Address     | Fcnt Up/Down       |      | Block number     
-----------------------------------------------------------------------------------------------------------
0x01 | 0x00 0x00 0x00 0x00| 0x00| 0x62 0x07 0xE0 0x02| 0xAA 0x00 0x00 0x00| 0x00 | 0x02


Where Dir is direction and it is 0 for uplink and 1 for downlink



Step 3. Encrypt each A Block

The blocks Ai are encrypted to get a sequence S of blocks S(i) :  S(i) = aes128_encrypt(K, Ai) for i = 1..k

So, on this step each A block will be encrypted ( yes encrypted !!!) AES128 with the same
key used on node side. 

A ciphertext will be like:

c7b11d72ec853e8853dd4362a77d71ad



Step 4. XOR the cipher with payload.

S = S1 | S2 | .. | Sk

Don't forget that encryption and decryption of the payload is done by truncating (pld | pad16) XOR S to the first len(pld) octets.


At the end the text {"Hello":"World1"} will be available for your application. If at the node side this text is encrypted your app will need to do a decryption to get it.





3 comments:

  1. Hi, can you give the sample code to decode the payload?

    ReplyDelete
  2. thank dude it's work!
    little suggestion payload size less than 16 byte
    pad zeros on LSB to make it work.
    1864ea1974ee4cbd26d0be3ac41d6edd cipher
    700186751b8b4f4165 payload
    700186751b8b4f416500000000000000 zero pad
    68656c6c6f6503fc43d0be3ac41d6edd result
    68656c6c6f data (hello)

    ReplyDelete