I'm reading about LoRaWAN (specification). This network protocol has a particular way to encrypt data, detailed in the part 22.214.171.124. Here is a recap of how the encryption work, as I understood it.
First, a sequence A with blocks with a size of 16 bytes each
A1..An is created. The length of A depends of the length of the payload so that
len(payload) <= 16*n = len(A) < len(payload) + 16. Here are the values used for the block
0x01 | 0x00 | 0x00 | 0x00 | 0x00 | Dir | DevAddr | Counter | 0x00 | i
One block is 16-bytes long. The
Counter is incremented for each payload;
i is incremented for each block created (
1 <= i <= n).
1 depending if it's upward or downward, and the
DevAddr is fixed with the address of the end-device.
Now that the blocks are created, they are encrypted to get a sequence
S of blocks
Si = aes128_encrypt(K, Ai) for i = 1..n S = S1 | S2 | .. | Sn
And finally the payload is encrypted:
(pld | pad16) xor S truncation of the first len(payload) bytes
The message sent is created by concatenating a header (containing among other things the values of
Counter), the encrypted payload and a MIC (Message Integrity Check) generated with another AES key. The receiving device knows the key, so it can:
- check the MIC
- create the blocks
A1..Anby using the values indicated in the header
- encrypt them to get the sequence
- decipher the payload
Here are my questions
I find this implementation basic. More specifically, I'm under the impression that a specific bit of the payload is in the same position after the encryption. But I don't know if an attacker can use that to gain information?
Also, an attacker can know pretty much everything by intercepting messages, so he can probably know the sequence
A. Is there a way he can decrypt the payload or gain information about the key?