EB: Cryptology

The first class we learned about the Caesar Cipher and other shift ciphers, as well as learning some useful cryptology words.

• Plaintext - the meaningful English message
• Ciphertext - what you actually send; the secret coded message
• Encrypt - turn the plaintext into ciphertext
• Decrypt - turn the ciphertext back into plaintext

The students paired off and sent their partners encrypted messages to decrypt.

Today I gave the class several encrypted messages to break, of increasing degrees of difficulty. We worked them all out on the board. It was helpful that I forgot what the plaintext messages were!

Here's the first one:

MJQQT, HQFXX.

HTSLWFYZQFYNTSX TS GWJFPNSL YMNX HNUMJW!

- OJXXJ

This one is pretty easy. It looks like a letter, and I wrote it, so "OJXXJ" stands for "JESSE." The first word, "MJQQT," is "HELLO." Since I used a shift cipher, the rest follows from there.

For the next message, I didn't give them any helpful formatting.

KXNDRSCYXOKVCYFOBIXSMO

The most common English letter is "E," and the most common characters in this ciphertext are "X" and "O." I was still using a shift cipher, so there were really only two things to try. That didn't take long either.

Then, since they kept breaking my shift ciphers, I changed to a cipher that randomly mixed up the letters. I did give them back the formatting, though - I didn't want it to be impossible (or take more than the hour-long class)!

VCPT PT E DPYYRHRNV VMSR FY ZFDR

The letter "E" in the ciphertext is a one-letter word, so it must be either "I" or "A" (assuming I'm using grammatical English!). The class went with "A".

Then the two-letter words in the ciphertext, "PT" and "FY," must translate to two-letter English words that don't contain the letter "A." Also, the two-letter word "PT" is the second half of the four-letter word "VCPT." It was agreed that "THIS IS" was a reasonable guess at those first two words.

From there, they got the answer!

The picture shows, from left to right:

• The ciphertext-to-plaintext translation (we used this more for the first two puzzles)
• The ciphertext, with plaintext underneath
• A list of common 2-letter English words