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
