A step-by-step walkthrough. It plays on its own, or click the dots and arrows to move through it yourself.
Picture an infinitely detailed fractal: zoom in anywhere and you get a unique, never-repeating pattern, and if two people start from the same point and zoom the same way they always see the exact same picture. Both ends share that one starting seed, so each generates the same endless stream of keys on its own, nobody ever mails a key. A synchronized "stopwatch" (DASA) keeps them on the same frame, and because stopwatches drift there is a small grace window so a beat-off endpoint just nudges back into sync instead of being locked out.