Couldn’t be done something like a reversed book encryption? Something that is plain text and perfectly readable but that you can use to decrypt a message?

Wouldn’t binary transfer in base64 be undescifrable as they could be files in a proprietary format?

Shan ara chronicles has just that and I searched it in tvtropes: https://tvtropes.org/pmwiki/pmwiki.php/Series/TheShannaraChronicles#:~:text=The Shannara Chronicles is an American post-apocalyptic fantasy drama television

The Reveal: Not that the characters particularly care, but Safehold turns out to be the 3000 year old remains of San Francisco