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.
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