# Rot-N Cipher

## The rotation cipher — aka, the 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).

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*.

`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.

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*.

`— — — — — — — — — — — — — — — — — — — `

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.

` plaintext: game on at ten `

ciphertext:

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

` plaintext: `

ciphertext: dpao nylha wvdly jvtlz nylha ylzwvuzpipspaf

Next series post — Rot-13 Cipher

Previous series post:— Introduction to Substitution Ciphers

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