Hmm, I think a better strategy might be embedding the ciphertext in the fur of cat pictures. Sending lots of cat pictures seems pretty normal for anyone. Might be possible to create a GAN that outputs a synthesized cat picture with a constraint of some ciphertext that can be decoded later. Or simple modulation might just work, if I can convince JPEG to not wreck it.
That is higher bandwidth, but for normal chat apps I would expect randomly applied compression in transit breaking things. Email could work though? And if you’re generating the JPEG or PNG yourself, you can put the cypher text in at whatever level you like, including highest entropy bits of the compressed data.
You’d have to be very careful to seem “normal”, as carelessly doing that can change the entropy in a detectable way even for the least significant bits — the least significant bits saved in something like JPEG is not the sensor noise, it’s the smallest stuff that humans pay attention to.
