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Steganography
“He who would keep a secret should keep it a secret that he hath a secret to keep.” Steganography has a long and honourable history as a tool for spies and secret agents. Invisible ink, certain letters in a book having pinpricks under them, the ‘dancing men’ in the Sherlock Holmes story of that name, even music, have all been used to encrypt information. Often this encryption has been successful. I remember an anecdote told by a former WW2 secret agent, where he relates being accosted by a Nazi officer who asked him, "what’s in the book?" "Music," he replied. The officer was clearly suspicious because he asked the agent to play the music. Having no choice, the agent obliged with a most hideous cacophony. The encryption algorithm he’d used hadn’t been designed to produce good music, after all. (You’ll be glad to hear that he was able to pass it off as ‘modern music’, which the Nazi was quite prepared to believe).
Steganography is a first line of defense. If nobody except you knows that your ciphertext is ciphertext, then nobody is going to try to crack it. But, once they do discover it, you’d better have a good algorithm, too, or you’re sunk.