No Stupid Questions

You'd need to arrange the code ahead of time with whoever you were communicating with. I can imagine a few ways to do it, the number of letters in the words, the arrangement of the spaces between words, first letter of first word, second letter of second word, up through five words and start over, prearranged code phrases of course but nothing that would work with an unwitting recipient.

Encode you message as the second letter in ever word. “Coded message” could be: “actually, your odd text added small fears as ester calls, egg deficiencies…”

In prison, you’d have plenty of time to choose more natural sounding words.

It could be anything, as long as your recipient has the cipher for your code. If they don't, then you better hope they're really good decrypters.

Your options depend on how much code you can agree on. Ask about imaginary relatives map each name to a meaning, so asking about the health of Aunt Judy could mean that the prison refuses your access to a lawyer and telling you thought about Uncle Sam could mean they keep you in solitary. Or whatever facts you want to communicate. Your revolutionary friends could respond in the same manner. The flaw is in being limited to previously agreed upon code words.

If you have access to a common book, you can refer to words by page, line and word number. You could embed the numbers as words in a story you are writing to your (imaginary) child. "Behind the seven hills, there were five houses with nine rooms each" can translate to "take the 9th word on page seven, line five". Obviously you need the exact same copy of the book for this to work, the Bible is quite common for this cipher.

The concept for this is called Steganography and that article has several examples, but most would be difficult without tools/references.

The easiest would be stuff like using certain letters of words or word lengths as mentioned in some other comments here.

Learn to write in Toki Pona? Its vocab is 120 words. Maybe someone out there could recognize that it's a language and go from there. Edit: the revolutionary would have to know the language first, if you mean without prior knowledge of anything, that's rough. it's too conspicuous anyway I got nothing

Occasionally, maybe once per paragraph, misspell a word intensionally. Your family, knowing how carefully you used to profread your own writing, will notice this as abnormal behavior. Either you captors have already damaged your menial health or you are trying to conceal a message. Gards reading you letters before posting them may be more used to bad writing among their detaines and not suspect anything deeper. Your family might reply using the same code, both acknowledging receipt of your coded message, and perhaps including a key for a more secure one.

Just use a contraband cellphone. That gives you access to powerful encryption.

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.

If you're important enough they won't let you send letters at all.

If you're unimportant, well anything you agreed on beforehand should be okay.

I'd probably go with metaphors and callbacks.

"Do you remember that one time we went to the pier?" and make up a story or something that can be re-interpreted. IDK if that's very good.