Deciphering

I was going to tell you a lot of cool facts and historical stuff about the Vigenère Cipher, but I’m guessing no one else will find it as interesting as I do. If you’re interested in learning more, check out the book Cryptanalysis by Helen Fouché Gaines. It was originally written in the late 1930s, so all the ciphers are long obsolete, but are so much fun.

So, then, down to business. Here’s how to use the keys to decipher those three phrases.

How Vigenère Enciphering Works

This is what I did to scramble the original messages, not what you will need to do to solve them. But I am explaining this because the deciphering will make more sense if you know it.

The Vigenère Cipher uses an alphabetic table (on the last page of your puzzle packet) that works similarly to a multiplication table. When you want the multiple of 4 and 9, you find those two numbers in the top row and leftmost column, and you follow them to where they meet at 36.

When you are taking a plaintext message and enciphering it, you do the same with the first letter of your plaintext message and the first letter of your cipher key. If your message is “CHANGE OF PLANS” and your key is “FIN,” you would follow “C” and “F” to where they meet at “H.” Helen Fouché Gaines recommends using a piece of paper or a ruler under your desired row to make the letters easier to pick out. Moving on to second letters, you would follow “H” and “I” to where they meet at “P.” Third letters “A” and “N” come together at “N.” At this point, since the key is only three letters, it would repeat. The next pair of letters would be “N” and “F.”

CHANGE OF PLANS (plaintext)
FINFIN FI NFINF (key, repeated)
HPNSOR TN CQIAX (ciphertext)

How Vigenère deciphering works

Your turn. This is the method you will use to solve the phrases, and it is slightly different.

Back to the multiplication table. Let’s say you know that 4 times something equals 36, but you can’t for the life of you remember what that other number is. You could find 4 on the top row, follow it to 36, and then over to 9 on the leftmost column to find your second number.

That is how Vigenère deciphering works as well. Remember that the plaintext and the key both come from the edges, and the ciphertext is where they meet in the middle.

If you have a key and some ciphertext, you would find the first key letter on one of the outer edges, follow it to the first enciphered letter, and then over to the plaintext letter. Remember that ciphertext is found in the middle of the table, so you aren’t matching letters from the two edges, but from one edge and the middle. Repeat with second letters. As with enciphering, the key repeats after the text runs out.

Note that you have more keys than you do ciphers; not all of them will work. If you started deciphering one of the phrases and it doesn’t seem to be working, it’s probably not you. It’s the key.

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