Cryptography - Caesar Cipher

Step 1: Choose a number between 1 and 25. This is key. It will determine how each letter of the alphabet will change.

Step 2: Grab your cipher ring. Line it up so the top and bottom alphabet mirror each other. A above A, B above B, etc. The ring will look like this:

Step 3: Rotate the top alphabet ring a number of spaces equal to the key. This is known as a shift. For example, if the key is the number 1, then shift the top alphabet row to the right by 1 letter. The ring should then look like this:

Step 4: Now that the ring is shifted, do not rotate it any more. You can encrypt your message by looking at the message letter by letter. Find each letter on the top ring and change it to the letter on the bottom ring. For example, if letter A is on top of letter B, then A turns into B when encrypted. The direction of a shift in the Caesar cipher does matter. 

To decrypt a message using the cipher rings, steps 1-3 are the same. The only difference is you look at the encrypted message letter by letter. You find each letter on the bottom row and change it into the letter above it on the top row.

You will use the Caesar cipher ring to decrypt messages. Let’s start with a simple shift of 1. Remember, to decrypt the message, shift the top of the ring 1 space to the right where ‘A’ is on top of ‘B’. You will feel and hear a click with each rotation.

• The encrypted message is: HFODZCFS

• The key is: Rotate 1

After you rotate the ring, to decrypt the message, simply change the bottom letter into the top letter. 

Puzzle 1: What is the original message?

Not too bad, right? Let’s try a harder one. This next one has a shift of 3. Remember to reset ‘A’ on top of ‘A’ before shifting! Your cipher ring will look a bit different after the shift.

• The encrypted message is:  FRQILGHQWLDOLWB

• Key:  Rotate 3

Remember, to decrypt, change the bottom letters to top letters. 

Puzzle 2: What is the original message?

Great job! Let’s decrypt one more message knowing the key size.

The encrypted message is:  OQXOPEPQPEKJ

Key:  Rotate 22

Puzzle 3: What is the original message?

In brute-force decryption, you only have knowledge of the ciphertext, so you have to guess the value of the key until you get a legible plaintext that makes sense.

In normal cryptography, brute-force attacks are not effective unless you have a small key size. Luckily for you, the Caesar cipher can only encrypt a message in 25 different ways. That means in the worst case, you’ll have to try 25 different keys to decrypt the message. However, there is a trick to solve this message in only one or two attempts. Can you figure it out?

• The encrypted message is: A SE S UGVWTJWSCWJ

• Key:  Unknown

Puzzle 4: What is the original message?

Puzzle 5: What is the key (shift value)?

Follow the steps below to encrypt a message for another camper. Remember, when encrypting, you change letters from top to bottom. Only decryption goes from bottom to top.

Create your own appropriate original message. 

Puzzle 6: What is the message?

Decide on a key for the shift.

Puzzle 7: What is the key?

Encrypt the message using the key. 

Puzzle 8: What is the encrypted message?