I have this situation where a PC is connected, via RS-232, to a certain hardware module that controls industial machinery.
The hardware is supposed to receive commands to turn on or off these machines, and the same command may need to be sent multiple times in a span of a single minute. Now, I am very new to cryptography, so I will be programming this hardware with an already pre-existing library that supports AES-128, AES-192 and AES-256 in ECB and CBC modes.
The problem I'm facing is that a potential attacker can very easily gain access to the transmission line and just probe it for the message being sent, and then use this information to send commands that can compromise the industrial machinery.
Let's say that the PC encripts a message like "MACHINE1 ON" using AES-256 with a secret key that the hardware module knows. Since the command is always the same, using AES in ECB mode will result in the same ciphertext, meaning that the attacker can just repeat the same ciphertext in order to turn the machine on.
Now, CBC encryption makes use of an IV to randomize the ciphertext, but the problem is, how will both the PC and hardware module know the IV? If the PC generates a truly random IV and encrypts the command with it, there's no way the hardware module will be able to decrypt the message without it.
So it thought that maybe the PC should transmit the IV to the hardware, using ECB mode, and then transmit the command encrypted with that IV, but we're back to square one, because an attacker can just repeat both ciphertexts and the hardware will just accept it (is this considered a replay attack?).
I've read a little about MAC and how it allows to verify if the sender has not changed, but how does it apply to this scenario? Would it even help? If not, what techniques could one use in a situation such as this?
Keep in mind that there is very little variety in the messages that will be send to the hardware module. The hardware module is also capable of generating truly random numbers, and it replies to the commands with a simple "OK" or "Invalid command" messages. These should also be encrypted to ensure an attacker gets as little information as possible.
I also read that sometimes, the IV is sent at the beggining of the encrypted text, so that the receiving end knows that the first N bytes correspond to the IV and can decrypt the message. Isn't this also vulnerable to the same replay attack?
Wouldn't it be better to implement some kind of "handshaking", where the hardware is the one to generate the IV and send it to the PC, the PC modifies the IV in a deterministic way, and then sends the messages using that modified IV? The process would then repeat each time the PC needs to send a command, and as long as the hardware doesn't generate the same IV twice, everything would be safe, right?
I'm sorry if this question is all over the place, I'm very confused about criptography and I'm being asked to implement something that I have almost no clue as to how it works. Any help would be much appreciated.