Cardan Grilles: A Challenge

Throughout history, people have tried to find ways to pass hidden messages. The problem is that as soon as someone sees a conspicuous-looking note, they immediately try to figure out what the secret is; if you saw a piece of paper scrawled over with

Ehzduh wkh Lghv ri Pdufk

you’d want to know what you were missing out on. By encoding a message at all, the writer draws attention to his secret.

Some of the most effective schemes for passing secret messages work by hiding the fact that there’s a message there at all. The square below has a word hidden in it, although there are no obvious cues (clusters, spacing) that that is the case.

In order to see the secret word, you need to block out the rest. That is what Cardan grilles are for. A Cardan grille is a screen with holes that only allow the secret letters to show through. If I place this grille…

…on top of the letter grid above, we find a secret word.

You have to be careful with your Cardan grille, though. If it gets turned at all, it will block out the wrong letters. There are 3 different ways it might get turned:

a 90 degree rotation

a 180 degree rotation

a 270 degree rotation

There are also 4 different ways it could get flipped over (over the horizontal axis, the vertical axis, or either diagonal):

Altogether, there are 8 ways to orient the Cardan grille, and only 1 of them shows the correct secret word.

Suppose that a rogue party intercepted the letter grid and the Cardan grille, but didn’t know how to position it in order to find the hidden message. This interloper might discover the message through trial and error by testing each possible orientation (there are only 8, after all) and seeing which ones look like words. (The incorrect orientations, one would think, are likely to produce gibberish.)

Knowing this guess-and-check strategy, a devious encrypter might make the letter grid so that every orientation of the Cardan grille produces a legitimate word, leaving any potential snoops unsure about which word is the true secret. But that’s not easy to do. That’s where the challenge comes in:

Can you fill out the letter grid below so that each of the 8 orientations of my Cardan grille yields a valid English word?