You can just send encrypted text through a unencrypted channel, look, these are instructions for the next terrorist attack: vss8uATnfADT6Z/XN3yUcjq3R5e8P8HjIkm2GbS6e8CbOnQb5qx0112mmjgUezWbAMxSPegGo8TSMIj3rEEe9mG/JYzsDb9g2tm6zfeLEYA=
I mean, how does one prove that a message is anything but gibberish if they don't have the means to decode it?