Like, say, you are a revolutionary who’s imprisoned, but you are allowed to write letters to your family. How would you covertly send a message to your fellow revolutionaries that only they can decode? (Your family are on your side will pass the letter along)
Obviously, it cannot appear to contain any obvious ciphertext, since the prison guards would just rip up your letter and not send it.
I mean it really depends on how much trouble you’re in. If you’re in deep they’re gonna be watching every thing you do no matter what. So any kind of message that looks suspect is gonna be investigated, even stuff like “Aunt Mary” or whatever is gonna have someone trying to figure it out.
The best way to do it is to exit the monitored communication entirely, either a burner phone with coded language, or having your communications come from someone else on the inside. That’s really the only way you’ll get around it, and even that can still be caught.