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The first message carried across to a friend in good secrecy might be this work from the year 1641: "Mercury, Or The Secret and Swift Messenger: Shewing, how a Man May with Privacy and Speed Communicate His Thoughts to a Friend at Any Distance" by John Wilkins, who gives many more antique examples in this early master work on private data-communication, like using dolphins, ot inscribing a message on the shaven head of a slave and let his hair grow and next send him through the enemy lines. The reflective properties of the moon and their potential is mentioned as well, along with communication in the way 'the angels do', in other words telepathy. Given the present circumstances, this last one may be the most secure, also in our digital age.
You will find additional information on this tableau on the author's Flickr page.