The shift cipher is a simple form of encryption that has been used since at least the days of the Roman Empire. Julius Caesar’s encrypted letters are the first widely known use of a shift cipher. A shift cipher is a type of substitution cipher that replaces each letter in a message with an adjacent letter from the alphabet one, two, three, or ten or more letters away.
If you answered that the ciphertext from the previous page is MJQQT, you are correct! See how the key of 5 is applied? Starting with H at the beginning of the plaintext, count 5 letters to the right in the alphabet and use that character (M) in the ciphertext version.
What about decryption? If the key is 5 and your ciphertext is MJQQT, count 5 letters earlier in the alphabet for each letter. For example, the plaintext H in HELLO is 5 letters to the left of the ciphertext letter M from MJQQT.
What happens with a key of 5 when you get to a letter like X? No problem, just start over at A. …but make sure to count the jump from Z to A as a letter. For example, if the plaintext letter is X, then the five character shift is Y Z A B C, and the ciphertext letter is C. In other words, when you run out of characters, restart at the beginning. This is called a circular shift.
Now that you can see how to encrypt characters with the Caesar cipher, let’s try it!
Next, show a friend how to decrypt it: