Information Security – Class Activity: Alphabetic Rotation Cipher

The Alphabetic Rotation Cipher is often known as the Caesar cipher because it was used by Caesar in his secret correspondence. More accurately, Caesar used an alphabetic rotation in which the alphabet was shifted by three letters. The Rotation Cipher simply allows for more possibilities because any shift of the alphabet is a possible key.

For example, the following plaintext is encrypted using a Rotation Cipher in which the key = “E”. This means that A –> E, B –> F, etc, which is a shift of 4.

Plaintext:       SOME SECRET MESSAGE

Ciphertext:    WSQI  WIGVIX  QIWWEKI

The picture below shows a secret decoder ring to aid in encrypting and decrypting messages with the Rotation Cipher.

Rotation Cipher

The inner ring can be rotated according to the key being used. For example, if the key is “M”, rotate the inner ring so that “M” lines up with “A” on the outside. Then encrypt the message, replacing the outside letter with the inside letter (A with M, B with N, etc.). To decrypt, go the opposite way, replacing letters in the inside with letters on the outside (M with A, N with B, etc.)

Non-programming Activity

  • Have the students create their own decoder ring using paper. Then let them practice encrypting and decrypting messages.
  • Have the students choose a key and encrypt a short secret message. Then have them exchange ciphertexts and try to break one another’s ciphers using their decoder rings.
  • Discuss with them the strategies they used to figure out the secret message. In most cases, they will have used a brute-force method, trying all the possible keys until they found something that made sense. Ask them how many keys they had to try before finding the right one.
  • Discuss with them how many keys they would have to try in the worst case. That is, what if the key they needed was the last key? These are the kinds of things that cryptographers have to ask. In this case, the key space (the total number of possible keys) is only 26, making it possible for humans to break and trivial for a computer.
  • Finally, discuss with students ideas for how they might write a program to encrypt and decrypt messages using a rotation cipher.