Two codes. One extra message.

Both symbols open the same address. One also carries a short message in bytes that an ordinary scanner stops reading before it reaches.

IMPLEMENTED MECHANISM / LIVE QR OUTPUT

Predict first

Both symbols return the same address to an ordinary scanner. Can a standards check still tell them apart?

Yes, reliably No, they are indistinguishable

The standard prescribes alternating 0xEC and 0x11 after the terminator, so any other value is a departure from a known pattern. This channel is easy to write into and just as easy to find.

BUILD / INSPECT

Put your own message underneath

Everything happens locally in this browser. No payload leaves the page.

Waiting for input

WHAT THE SCANNER SEES

—

WHAT CHANGED UNDERNEATH

Pad codewords after the terminator

WHAT A SECOND READER CAN RECOVER

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HOW AN ANALYST CAN NOTICE

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DATA CODEWORDS

The bytes themselves

payload + framing specified padding changed padding

Build a code above.

FINDING

Hidden from an ordinary reader does not mean hidden from an analyst.

The terminator tells a normal decoder that the overt message is complete. The remaining data capacity is filled with alternating 0xEC and 0x11 pad codewords. Replacing them can preserve the visible payload, but a standards-aware comparison exposes the change immediately.

Research basis: pad codewords as a carrier are documented in Tan et al. (2020) [BIB]. Regeneration-and-compare detection is described by Chen et al. (2023) [FULL]. HideAndSeen implements the same general pad-codeword mechanism; it does not claim to reproduce either paper byte for byte.

Read the research note