Ok, I think I'm getting closer to understanding your point. > Right, and this is why we don't use them for arbitrary communication between networked computers,

Ok, I think I'm getting closer to understanding your point. > Right, and this is why we don't use them for arbitrary communication between networked computers, but for manufactured cars (or other cyber-physical devices) we have a trusted channel already! The factory that builds the car! So what exactly do you want to use the OTP for? Just for updates to the cars, or are there other forms of communication you'd want to see extra protected. Next, what exactly do you wan to ensure about a message? There are 3 properties we commonly seek to ensure (also referred to as C I A): * Confidentiality, i.e. the public can't read the messages * Integrity, i.e. the message that arrived is the message that was sent. * Authenticity, i.e. the message came from a trusted source A very simple OTP with xor-ing only gives Confidentiality. I suppose you could append some HMAC of the message, using part of the OTP for the key to ensure Authenticity and Integrity. Note that without this, I can flip any bits I want in your OTP encrypted message just by flipping random bits in the ciphertext. For something like a firmware update, that would be a problem. There is also the question of the order of the layering. Do you OTP-then-Encrypt or Encrypt-then-OTP? To be honest, it seems like the layering is useless. As using a full OTP with HMAC is already enough to give CIA. In the end, I'm still not sure of the significant advantages of an OTP over something like AES-GCM with a key shared between the device and the manufacturer. Especially because you need to include the HMAC, which introduces some of the complexity you seem to want to avoid. Really, the biggest downside of AES-GCM is that you need decent randomness for the nonce (though I believe you can deterministically derive the nonce from the message). The reason your choice for an OTP over symmetric encryption like AES-GCM really confuses me is because I don't see the benefit of the guaranteed Confidentiality of an OTP in this case. Really, Authenticity and Integrity are what matters here. I would also note that something like AES-GCM with a unique key per vehicle is already far from reality. In any case, you still need a way to keep bad guys from getting the secret key from your car. Moreover, it seems to me like any kind of engineering to create a decent 'secure enclave' could also be spent on verifying your symmetric encryption is actually 'done right'. Note that in this case, any push of data to a vehicle needs to a fully on-line connection between the factory and the car. No possibility for an intermediate CDN. With something like AES-GCM you could at least prepare a message for every car and deliver it asynchronously. This would ensure it is still possible for a mechanic to do a physical update in the case an internet connection has been fried. But for a true OTP, to ensure the pads remain sync, that cannot happen. I could see going for the AES-GCM route to protect against quantum decryption, but beyond that, I don't see the issue with using normal assymetric crypto.