Write it, hide it, hand it over
Nothing is hidden yet.
Held to the light, old invisible ink comes back warm and uneven, never all at once. This does the same with whatever you are holding: on a phone it reads the tilt, and on anything without a motion sensor it falls back to holding the pointer down, without being asked and without breaking. The message travels in the link itself — it is not stored on any server, because there is no server here to store it.