QR code decoder

QR codes turn up as the payload of steganography challenges, as images recovered from PCAPs, and as the visible content of a file that looks like nothing else. Decoding one from an image file - rather than pointing a phone at a screen - is the awkward step this removes.

The format is also more robust than it looks, which is why partially destroyed codes are a common challenge shape rather than a dead end. QR carries Reed-Solomon error correction and a fixed structural skeleton, so a good deal of a code can be missing before it becomes genuinely unrecoverable.

When it will not scan

• Inverted colours. A white-on-black QR is valid but many decoders only try one polarity. ctfpal tries both.
• Missing finder patterns. The three corner squares can be reconstructed by hand in an image editor if a challenge removed them.
• Damage. QR carries Reed-Solomon error correction and recovers from up to 30% loss at the highest level - a visibly damaged code is often still readable.
• It is not a QR code. Aztec, Data Matrix, and PDF417 look similar at a glance and need different decoders.