Schrödinger's QR code

Solving the "bored" puzzle I'd posted a few weeks back.

Jul 08, 2024

A month and a half back, I'd posted a puzzle for the bored ones. When designing this puzzle, I'd titled it "Schrödinger's QR code" in my mind, but revealing the name would've been a hint in itself. It was solved by Advait a week later with about 10 hrs of effort, which I thought was pretty impressive.

BIP39 again?

The mnemonic presented in the puzzle is a BIP39 encoded URL, which to those of you who've followed me from my Twitter days, is a standard routine for me. Some folks have asked me in the past why I do this, and the reason is pretty simple - often times our mind associates a concept too strongly with the context in which it was presented, e.g. BIP39 for most folks gets associated too strongly with wallet master phrases. By invoking it in a completely different context, it's my feeble attempt to generalise that concept a bit more.

It's really no different than say a Base64. The encoded URL in this case is a redirect to the main core of the puzzle.

The main puzzle

The redirected URL takes you to a hidden page with a QR code, and some guiding wisdom ("Your eyes can deceive you, don't trust them" - Obi-Wan Kenobi). If you were to open the link referred to in the QR code, you reach another hidden page that lets you know you're not at the right place, with some Harry Potter inspired quote to help you: "Erised stra ehru oy tub. Gnivie cedeb nac sedo crq."

You don't necessarily need to be a Harry Potter fan to decode the above. A good eye, or a quick online search should suffice to figure out the hint.

With the hint, you go back and take a look at the QR code again. But what's the mystery? Maybe it's some metadata inside the image? Maybe you could do a quick strings <qrcode.png> and confirm there's nothing hidden in there, at least in the metadata.

I'm guessing most of us have seen QR codes that are embellished with some logo. And if I were to ask you how does it work despite the embellishment, most of you would be able to answer it - that it has some sort of error correction baked into it.

That gives us a possible path forward. What if the error corrected version is the misdirect, while the uncorrected version contains the correct information? The question is rhetorical of course, given how I titled this post.

If you feel up to it, you can always deconstruct the actual answer by taking a look at the QR codes - a visual explainer post I'd written recently. The interactive tool I built for that post should allow you to explore for yourself - simply upload the wonderland QR code.

Closing Remarks

I thought it was an interesting idea to encode QR codes that have a deliberate misdirect into it - particularly if the image is going to be stored digitally, not physically. Given that almost every QR code reader error corrects by default, it becomes a sort of (poor?) steganography.

And writing this post has also reminded me that I still need to put out that post about the mathematics of Reed Solomon error correction 😄. It's been too many of puzzle posts recently anyway.