> Sure they could: just disallow any "unknown" encrypted communication.
Even then, it's impossible to prevent arbitrary communication. You can always hide your message inside of allowed messages (steganography), or an even more basic albeit inefficient technique: just use the timing between allowed messages to encode your hidden message.
Even then, it's impossible to prevent arbitrary communication. You can always hide your message inside of allowed messages (steganography), or an even more basic albeit inefficient technique: just use the timing between allowed messages to encode your hidden message.