What is the Caesar cipher?

The Caesar cipher is one of the oldest and simplest encryption techniques. Each letter in the plaintext is shifted a fixed number of positions down the alphabet. Julius Caesar reportedly used a shift of 3, turning A into D, B into E, and so on. It is a substitution cipher - the same letter is always replaced by the same other letter.

How do I decode a Caesar cipher?

To decode, use the same shift value that was used for encoding, then click "Decode." The decoder shifts each letter backward by that amount. If you do not know the shift, try all 25 possibilities - ROT13 (shift 13) is the most common variant you will encounter online.

What is ROT13 and how does it relate to Caesar cipher?

ROT13 is the Caesar cipher with a shift of 13. Because the English alphabet has 26 letters, applying ROT13 twice returns the original text - making it self-inverse. ROT13 is commonly used to hide spoilers and puzzle answers online. Our dedicated ROT13 tool applies this specific shift.

Does the cipher preserve punctuation and spaces?

Yes. Only letters (A–Z and a–z) are shifted. Numbers, spaces, punctuation, and all non-alphabetic characters pass through unchanged. Case is preserved: lowercase letters remain lowercase and uppercase remain uppercase.

Is Caesar cipher secure?

No. The Caesar cipher provides essentially zero security by modern standards. There are only 25 possible shifts, so it can be brute-forced in seconds. A frequency analysis attack also works because the most common letters in English (E, T, A, O) remain the most common after shifting. Use it for puzzles and learning, never for sensitive data.

Is my text processed on the server?

No. All encoding and decoding happens entirely in your browser using JavaScript. Your text is never sent to any server.

Caesar Cipher Encoder & Decoder

Encode and decode text with the Caesar cipher online. Choose any shift 1–25. Free Caesar cipher encoder/decoder, no signup required.

🔒 Your text stays in your browser - nothing is sent to our servers

Shift:3

Input

Encoded Output

How to Use

Three steps to get started

Choose encode or decode

Select "Encode" to encrypt plaintext or "Decode" to decrypt a Caesar-encoded message.

Set the shift

Use the shift slider to choose a value from 1 to 25. Shift 13 is ROT13, the most common variant.

Paste and copy

Paste your text into the input and copy the result. Use the same shift value for encoding and decoding.

About This Tool

The Caesar cipher: history and how it works

The Caesar cipher is a monoalphabetic substitution cipher that encrypts a message by replacing each letter with the letter a fixed number of positions further along the alphabet. That fixed number is the key, and because the English alphabet has 26 letters there are only 25 useful keys - a shift of 0 or 26 leaves the text unchanged.

The name comes from Suetonius, who recorded in The Twelve Caesars around 121 AD that Julius Caesar protected his military correspondence by writing D for A, E for B, and so on: a right shift of 3. His nephew Augustus reportedly used a shift of 1. Mathematically the operation is simply modular addition, usually written E(x) = (x + n) mod 26 for encryption and D(x) = (x − n) mod 26 for decryption, where letters are numbered A=0 through Z=25.

The algorithm works by treating the alphabet as a circular ring. With a shift of 3, the mapping becomes:

Plain:  A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Cipher: D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

To decrypt, simply shift in the opposite direction by the same amount - or equivalently, shift forward by 26 − n.

Modern uses

Puzzles and escape rooms - Caesar ciphers are a staple of puzzle hunts
ROT13 on forums - shift 13 hides spoilers on Reddit and old Usenet posts
Cryptography education - the simplest cipher to understand and implement
CTF challenges - beginner capture-the-flag competitions frequently include Caesar-encoded flags

How it is broken

Two attacks defeat it, and both are trivial. Brute force simply tries all 25 shifts and reads whichever output is English - the keyspace is so small that a person can do it by hand in a few minutes. Frequency analysis, first described by the ninth-century Arab polymath al-Kindi, needs only the ciphertext: since every A becomes the same letter every time, the letter distribution is preserved and merely rotated. E is about 12.7% of English text and T about 9.1%, so finding the most common ciphertext letter and measuring its distance from E usually reveals the key from a single sentence.

It helps to know what the Caesar cipher is not. Unlike the Vigenère cipher, which uses a repeating keyword so the same plaintext letter can encrypt differently in different positions, Caesar applies one shift everywhere - it is the degenerate case of Vigenère with a one-letter key. Unlike a general substitution cipher with an arbitrary 26-letter mapping and its 26! possible keys, Caesar allows only rotations. And unlike Base64 or ROT13, which are encodings anyone can reverse without a secret, Caesar at least has a key, even if a useless one.

Treat it as a teaching tool and a puzzle mechanic, never as protection. Encoding and decoding both run in JavaScript on this page, so no message you paste is uploaded or stored - though if a message genuinely needs to stay secret, reach for modern authenticated encryption such as AES-GCM instead.

FAQ

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