invisible ink

A small instrument for hiding text inside text. Type into it; paste from it. The carrier looks ordinary because it is — the payload rides along as a run of zero-width Unicode characters between two frame markers, the way a smuggler rides the wheel well of a perfectly legal car.

Histiaeus of Miletus, stuck at Darius’s court and wanting to start a revolt, shaved a slave’s head, tattooed the message, waited for the hair to grow back, and sent the slave to Aristagoras with a single instruction at the end of the journey: shave my head and read. Herodotus, Book V. First documented covert channel; the security model is latency. The message is the body.

McLuhan flips the same coin: the medium is the message. Once you read the carrier as carrier, the carrier is the content. The inscription up top is one sentence in two registers — Histiaeus’s instruction in plain ink, McLuhan’s reply written underneath in a kind of ink the page won’t render.

what’s actually happening

Two zero-width characters do the work. U+200B (zero-width space) is bit 0; U+200C (zero-width non-joiner) is bit 1. A third, U+2060 (word joiner), brackets the payload so the decoder can find it inside arbitrary surrounding prose. Eight bits per UTF-8 byte. The whole codec lives in packages/stego — a hundred lines, no dependencies.

This is one method out of the hundred-plus that elder-plinius’s ST3GG implements across images, audio, packets, archives, and Unicode. ST3GG is AGPL-3.0; this is a fresh, much smaller implementation of just the zero-width Unicode flavor, so the site can stay where it is.

what it’s for

Pynchon. The Crying of Lot 49 runs on Oedipa’s question: is the network real or am I generating it? Every chi-square test is a paranoia test, and the muted post-horn scrawled on a bathroom wall is the same gesture as a run of U+200Bs in a paragraph that decodes to a sentence. The “n invisible characters / frame detected” line above the decode panel is on purpose. Most of the web does not have a frame in it. Some pages do.

Try the inscription. Then try pasting any passage on this site into the decoder. The fun half of Lot 49 is the half where Oedipa keeps finding it.