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There's a fairly specific subset of coding theory that I feel should exist, but I don't know what it's called or how to find it. It's best characterised by needing to obliquely embed subchannels of communication in human-readable text.
Here are some examples of problems that exist in this area:
1a) How do I pass arbitrary concealed data in a body of English text? Let's say I have a message of length n bits. What would be the most efficient way of obliquely encoding that message so that it passes for plain English text? For example, if I had the message 11001001, I could use a Markov text generator, and for each word, check and force the parity of a checksum of that word, picking eight words whose parities corresponded to [1, 1, 0, 0, 1, 0, 0, 1]. This wouldn't be very efficient, and it probably wouldn't be astonishingly comprehensible English, but it starts to address the problem.
1b) Say I have an existing body of well-formed English text, and want to somehow transform it so that it encodes the message 11001001. What variety of transformations would still yield well-formed English text?
2) Given a body of English text, how can I obliquely clue in a knowing observer (or a regex pattern) to the fact that this text is somehow distinguished and worthy of scrutiny for some predetermined purpose? Motivating case: I've just encoded the very important message 11001001 as a body of English text, using a method for solving problem #1 above. I want to do something to it so that my confederate will recognise it as containing this message. One way of doing this would be to wrap it in a known initiating and terminating phrase, a bit like ASCII-armour for a PGP key block. For example, I could begin with "Once upon a time..." and end in "...and they all lived happily ever after".
3) A group of conspirators want to adopt aliases on an anonymous forum. They want to be able to reliably recognise each other by their online aliases so they can collaborate, but don't want their affiliation to be apparent to other observers. They could, ahead of time, come up with one or more highly selective rules to which their aliases must conform. The identities of all conspirators would then be known to each other.
I think it is not very hard writing something which will encode a hidden phrase using odd and even counts. (But length is key).