Rot-N Cipher

A rotation cipher, rot-3 in particular, is often referred to as a Caesar Cipher.

A substitution cipher is a method used to encode a message by replacing each letter with a symbol, drawing, number or another letter. This is one of the earliest and simplest types of ciphers and comes in many forms. The rot-n cipher, short for rotation by n letters, can be performed with the aid of a cipher disk (pictured below).

Vintage rotation cipher disk.

By rotating the inner disk, its letters align with the letters of the outer disk. We can use the resulting letter association to create a cipher.

This rotation is like shifting the alphabet. For example, doing this for the lowercase English letters where n = 3, that is shifting the alphabet 3 letters to the right, we see that the letter a is associated with d, b is associated with e, and so on.

In the first row, when you get to letter w, wrap the alphabet around associating the letter x with a, y with b, and z with c. This produces the following rot-3 key.

We can use this key to encode the following famous quote by Roman Emperor Julius Caesar.

I came, I saw, I conquered.

The original message is called the plaintext and the encoded message is called the ciphertext. Capitalization, punctuation, and spacing are typically ignored. However, we will keep the spacing for now.

To encode this message, find i in the first row of the key, then replace that with the letter l found in the second row. Next, see that we should replace c with f.

Rot-3 Key
plaintext:  i came i saw i conquered
ciphertext: l f

Continue in this way by replacing the plaintext letters with the associated ciphertext letters.

--------------------------------------
plaintext: i came i saw i conquered
ciphertext: l fdph l vdz l frqtxhuhg
--------------------------------------

To decode a rot-n message, apply the process in reverse. Let’s next decode the following rot-7 message:

The first step is to construct a rot-7 key. Begin by shifting the letter a in the alphabet 7 letters to the right and then fill in the rest in alphabetic order.

Wrap the remaining letters around, starting with t obtaining the following key.

Notice that a rot-n key can be presented with two rows as shown above or on a cipher disk as pictured here.

Rot-7 Setting

Rot-N Cut-Out Template

Because we are starting with ciphertext, look to the second row first when making letter substitutions. Here z is associated with s and l associated with e.

Rot-7 key
— — — — — — — — — — — — — — — — — — — 
plaintext: se
ciphertext: zlclu pz aol sbjrf ubtily
— — — — — — — — — — — — — — — — — — —

Complete the plaintext by continuing this process using the rot-7 key.

— — — — — — — — — — — — — — — — — — — 
plaintext: seven is the lucky number
ciphertext: zlclu pz aol sbjrf ubtily
— — — — — — — — — — — — — — — — — — —

To deter your adversaries from catching on, you can change the rotation number n often. In fact, Julius Caesar used a similar 3-letter rotation cipher in his confidential communications. For this reason, ciphers that involve a shifting of the alphabet are often called Caesar Ciphers.

Mission 1: Use the rot-3 cipher to encode the message.

Rot-3 Key
 plaintext: game on at ten 
ciphertext:

Mission 2: Use the rot-7 cipher to decode the message.

Rot-7 Key
  plaintext: 
ciphertext: dpao nylha wvdly jvtlz nylha ylzwvuzpipspaf

Next series post — Rot-13 Cipher
Previous series post:— Introduction to Substitution Ciphers

Link to Mission Answers
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