This week’s puzzle comes from an all-time classic, the introduction to 2007’s National Treasure 2: Book of Secrets.
The Playfair cipher is an incredibLe code system that is both difficult to decrypt without a computer and easily solvable by hand. You might be familiar with the Cryptoquip in the comics section of a newspaper, where you need to figure out which of the letters should be replaced with an E. Once you know that Q, for example, equals E, then that’s true for every Q in the problem. In the Playfair cipher, there are many letters that will stand for an E, making it much harder to crack! But if you know the system, decoding a message is a simple process.
Let’s start with a passphrase and a word to encrypt. Like in Book of Secrets, we’ll use DEATH for our passphrase, but we’ll just encrypt a single word: TREASURE.
First, we’re goinG to need a 5×5 grid. We’ll fill in our passphrase at the top, and then the rest of the letters in the alphabet in order all the way to the end. Since we only have 25 spaces, we’ll stick I and J together.
We also need tO split TREASURE into two-letter pairs:
TR EA SU RE
Take a pair and find both letters in the gRid. The two letters are always in the same row, the same column, or neither one.
If they’re in the same row, replace each letter with the letter immedIately to the right. For example, DA would become ET.
If they’re in the same column, replace each letter with the leTter immediately below it. For example, TS becomes GY.
If neitHer is true, then imagine the two letters as corners of a box. Replace each letter with the letter in the other corner of the same row. For example, EM becomes AL.
If your letter is at the bottom of a column or the far right of a row, simply loop around the grid. And if your word has an odd nuMber of letters, add an X to make a final letter pair.
So, our word TREASURE becomes TR EA SU RE which encryptS into AS AT UP QA.
Deciphering the code is simply taking the letter pairs and reversing the process. Try it out and make you get TREASURE again!
Then try decrypting this string: YL NR EX RQ AT QB ML NG OV LO OI DY
➡ The Hint
Don’t use DEATH. You already found the new passphrase hidden in this article, right?