Can keys be hidden in text?
https://www.wired.com/story/fontcode-invisible-messages-steganography/
Abstract: We introduce FontCode, an information embedding technique for text documents.
https://www.cs.columbia.edu/cg/fontcode/
https://media.8ch.net/file_store/35c826eb3d2cc2751c787ae34778f150d2ce895ff6f2577ae3059acf004969a1.pdf