I had an idea about my device: offer plans to build it yourself. Buy your components, and build it. Reading about Anom today makes me upset that simple multiplication on a device could have prevented all this. Seeing also that so much money is put into security... I don't understand why nobody has built such thing yet.
My device has the convenience to work immediately with no previous data/key shared between the protagonists, with an en/decryption key by key (you can chat live and decrypt any word real time). But a scheme with shared data/key and the whole message encrypted and then decrypted is much simpler to realize and as safe (just less cool)
How do you plan to accomplish that without key exchange? One of the most important elements of secure communication is key exchange, there are even algorithms developed to do just that. If you are not going to do that, then you must set a pre shared key when you ship this so called device. Even if you don't do that, and want users to generate keys themselves, what are you going to do then? How are you going to input the public key of other party into the device securely? (Assuming you are talking about asymmetric encryption)