Caesar Cipher

What is the Caesar cipher?

The Caesar cipher is a substitution cipher that encrypts a message by shifting every letter a fixed number of places along the alphabet. It is named after the Roman general Julius Caesar, who reportedly used a shift of three to protect his private and military correspondence. Because each letter is always replaced by the same substitute, the Caesar cipher is the simplest and most widely taught example of classical cryptography, and it remains a perfect starting point for learning how encryption works.

Today the Caesar cipher turns up in puzzles, escape rooms, capture-the-flag challenges, and beginner programming exercises. Its most famous variant, ROT13, is still used across the web to hide spoilers, punchlines, and answers in plain sight.

How Caesar cipher encryption works

To encrypt, you pick a shift value, called the key, between 1 and 25. Each letter of your message then moves forward through the alphabet by that many positions, wrapping around from Z back to A when it runs off the end. Decryption reverses the process, shifting each letter backward by the same amount. Spaces, digits, and punctuation are normally left untouched, which is why the shape and length of the original message stay visible in the ciphertext.

For example, with a shift of three the letter A becomes D, B becomes E, and the word HELLO turns into KHOOR. To read the message, the recipient only needs to know the single shift value and move every letter three places back.

Worked example: a shift of three

Take the classic phrase ATTACK AT DAWN and apply Caesar's own shift of three. A maps to D, T maps to W, C maps to F, K maps to N, and so on, producing the ciphertext DWWDFN DW GDZQ. Notice that the spaces stay in place and repeated letters always encrypt to the same character, so the double T in ATTACK becomes a double W. Reversing the shift turns DWWDFN DW GDZQ straight back into ATTACK AT DAWN.

The Caesar cipher formula

Mathematically, each letter is treated as a number from 0 to 25, where A is 0 and Z is 25. Encryption is written as E(x) = (x + n) mod 26 and decryption as D(x) = (x - n) mod 26, where x is the letter's position and n is the shift. The mod 26 operation is what makes the alphabet wrap around, so shifting Y by three lands on B instead of running past the end of the alphabet.

ROT13, ROT5, and ROT47 explained

ROT13 is a Caesar cipher locked to a shift of thirteen. Because thirteen is exactly half of twenty-six, applying ROT13 twice returns the original text, so a single operation both encodes and decodes. ROT5 applies the same idea to the ten digits 0 through 9, which makes it handy for masking numbers, while ROT47 rotates all 94 printable ASCII characters together, scrambling letters, digits, and punctuation at once. Each of these is self-inverse, which is exactly what makes them so convenient for quickly hiding and revealing short pieces of text.

How to break a Caesar cipher

The Caesar cipher has only 25 possible keys, which makes it trivially easy to break. The most direct method is a brute-force attack: decode the message with every shift from 1 to 25 and read whichever result makes sense. A smarter approach is frequency analysis, which compares the letter distribution of each candidate against the typical frequencies of English, where E, T, and A are the most common letters, and picks the closest match automatically. The Crack mode in this tool does both for you, listing all 25 decodings and highlighting the most likely answer using a chi-squared score.

A short history of the Caesar cipher

The cipher takes its name from Julius Caesar, who according to the historian Suetonius shifted each letter three places to keep his messages private around 50 BC. His successor Augustus is said to have used a similar scheme with a shift of one. For centuries a simple letter shift was secure enough, mostly because so few people could read, let alone analyse a coded message. Its weakness was understood by the ninth century, when the Arab mathematician Al-Kindi described frequency analysis, the very technique that breaks the cipher in seconds today.