@cassie

use a private key from a test vector for a public key cryptosystem created in early 2000s. etch the public key and another public key to a type of stone that will survive the ages, perform diffie-hellman, use the output DH value as a feistel cipher key, encrypt the warning message, and etch it too. store the stone in one of the locations that was uncovered in the past hundred years (too early it might get destroyed or lost, recent enough and archaeological studies were rigorous enough to take inventory, not to mention _interest_ in a block etched with hexadecimal). carbon dating will confirm its authenticity, the message will verify its time traveling-sourced nature, the first public key will match to publicly known test vectors, the second will be easily detected as a public key, DH would be a fast follow, and cryptanalysts would quickly riff through options and identify the feistel cipher, decode the message, and consider the urgency.

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