What you want is a system based on one-time-pads which is easy to setup and maintain. So long as you have a means of encrypting your message with a pad that the recipient knows you would be good. Historically those one time pads are obscure books that both parties own and a common method of encryption. So long as your pad is secure one time pads are essentially unbreakable. The means that the code is transmitted doesn’t add or detract from the security of a one time pad.