Rot-13 Cipher

A symmetric rotation cipher.

The English language consists of 26 letters (a-z) which is exactly divisible by 13. This allows for the equal association of the first half of the alphabet with the second half. Because of this fact, the English rot-13 cipher is special.

Notice that plaintext e is associated with r and ciphertext e is also associated with r. The association of plaintext to ciphertext is the same both ways, making for an identical encoding and decoding process. This describes what is called a symmetric cipher. Because of this, the rot-13 key can be simplified by writing the initial 13 letters in the first row and the next 13 letters in the second row.

Now we can use this simplified rot-13 key to encode and decode messages very quickly.

In general, rotation ciphers are not very secure. However, they are good to learn and will, at minimum, delay your adversaries from discovering your immediate plans. By the time they intercept and decode this message, lunch will be over!

Rot-13 is commonly used in online forums to hide punchlines, spoilers, and solutions to puzzles.

You will find a rot-n template linked in this series. Cut out the two pieces and make your own rot-n cipher disk like the one pictured below. Rotate the inner disk until n = 13 appears in the window.

Rot-13 setting

The ciphertext is found on the outer edge and the plaintext is found on the inner edge. Use the disk to verify the following:

 plaintext: cipher disk
ciphertext: pvcure qvfx

Now if you think this is super cool, you might enjoy one of my favorite websites of all time: rot13.com

Mission 3: Decode the message using the rot-13 cipher.

 plaintext: 
ciphertext: hfr gur sbepr

Mission 4: Encode the message using the rot-13 cipher.

 plaintext: resist the dark side 
ciphertext:

Next series post —Cracking the Code
Previous series post: — Rot-N Ciphers

Link to Mission Answers
→ Buy This Book Now $7.99