What is a shift cipher​?

If you've ever wanted to send a secret message, a shift cipher is one of the simplest ways to hide its meaning. This ancient encryption technique works by replacing each letter with another that appears a fixed number of positions further along in the alphabet. The number of positions is called the encryption key, and it's the size of the shift used to disguise your text.

This simple idea of shifting the alphabet is what gives the cipher its name. When you line up the original alphabet with the shifted one, you get a complete cipher mapping that tells you exactly how to encrypt or decrypt any letter.

The shift cipher, also known as the Caesar cipher, was famously used by Julius Caesar to send confidential messages to his generals. Although it's straightforward by modern standards, it laid the foundation for all later encryption methods, serving as a valid starting point for cryptography.

In this article, you'll learn:

• What is a shift cipher?
• How to use a shift cipher decoder to reveal secret messages?
• How does a shift cipher wheel help you visualize the shifting process?
• What is the Caesar cipher?
• How to translate Caesar cipher text using simple shift rules?

We'll also explore how to translate Caesar cipher messages and understand why this ancient method still fascinates cryptography enthusiasts today. For practical tests, try our brilliant Caesar cipher shifter: an easy way to encode or decode text instantly and experiment with different alphabets.

A shift cipher works by taking each letter of your message and replacing it with another letter that appears a fixed number of positions further along in the alphabet. This number is the encryption key, and it tells you how far we "shift" the alphabet. This method belongs to a broader family called additive ciphers, in which each letter is encoded by adding a fixed key value to its alphabet position.

The most famous version of this method is the Caesar shift cipher, which always uses a fixed shift of three. In Caesar's system, $A$ moves to where $D$ normally is, $B$ becomes $E$, $C$ becomes $F$, and so on, which creates a simple shift that can be neatly shown by placing the plaintext alphabet above its shifted ciphertext counterpart. For example, with a key of 3, the word "FRIEND" becomes "IULHQG", because each letter moves forward by $3$ steps.

Any shift value creates a different shift cipher: the version where each letter becomes the one immediately after it is simply a shift of 1. Although examples usually use the Latin alphabet, shift ciphers work with any alphabet. If you'd like to explore other encoding systems, try our Vigenère cipher calculator or learn how to translate Morse code to English.

Encryption

Encryption using the shift cipher is very straightforward. We begin by creating the ciphertext alphabet, which is the regular alphabet shifted to the left by the number of positions specified by the key.

Mathematically, this process can be written as:

where:

• $C$ — Ciphertext letter number;
• $P$ — Plaintext letter number;
• $K$ — Number of positions to shift; and
• $\mathrm{mod}$ — Modulo operator.

The modulo operator makes the alphabet wrap around by keeping all shifts within the 26 letters in the Latin alphabet, so if a shift goes past Z, it loops back to A. You can learn more about this behavior in our guide on the uses of modulo.

Imagine encrypting the word "BOAT" with a shift of $3$ by creating a ciphertext alphabet that is rotated three places to the right, so $A$ → $D$, $B$ → $E$, $C$ → $F$, and so on. In practice, you then replace each plaintext letter with its shifted counterpart (this is exactly how an additive cipher works), giving $B$ → $E$, $O$ → $R$, $A$ → $D$, and $T$ → $W$, so the final result is "ERDW".

Decryption

To decrypt a shift cipher, we reverse the process using the formula:

This formula is what a shift cipher decoder uses to translate Caesar cipher text back into readable plaintext. It simply means we subtract the key from each ciphertext letter, and if the result goes below $0$, the modulo $26$ operation wraps it back to the end of the 26-letter alphabet, so we land on the correct plaintext letter.

Let's assume you receive the encoded message "MJCN?" and you know the key is $9$. By shifting each letter nine places backward in the alphabet, you get $M$ → $D$, $J$ → $A$, $C$ → $T$, and $N$ → $E$, while the question mark stays the same:

Did you notice how the modulo operator helped during decoding? When we reached $C = 2$ and subtracted the key ($2 − 9 = −7$), the result dropped below zero. Modulo 26 fixed this by looping the value back into the alphabet: −7 mod 26 = 19, which corresponds to $T$.

This example was easy because you already knew the key was $9$. If you don't know the key, you have to try every possible shift in the alphabet, but luckily, there are only as many shifts as there are letters minus one ($26 - 1 = 25$ in the Latin alphabet). To make this process easier, clever tools were invented: a shift cipher wheel lets you quickly rotate through all possible shifts until the hidden message becomes readable.