What is a Caesar cipher?

A Caesar cipher is one of the oldest known encryption methods, named after Julius Caesar who reportedly used it for private messages. It shifts every letter of the alphabet a fixed number of positions, wrapping from Z back to A. With a shift of three, A becomes D, B becomes E, and Z becomes C. Because the same shift is applied to every letter, it is a simple substitution cipher and very easy to break by hand.

How do I decrypt a Caesar cipher if I know the shift?

Set the mode to Decrypt, dial the slider to the same shift that was used to encrypt, and paste the cipher text. The tool shifts each letter back by that amount, wrapping around the alphabet, and shows the original message instantly.

How do I crack a Caesar cipher when I do not know the shift?

Use the brute-force panel. There are only 25 possible non-trivial shifts, so the tool tries them all and ranks each candidate by how closely its letter frequencies match natural English. The most likely plaintext appears at the top, usually revealing the answer at a glance. You can copy any candidate row directly.

What is ROT13 and how is it related?

ROT13 is simply a Caesar cipher with a shift of 13. Because 13 is exactly half of 26, applying it twice returns the original text, which makes ROT13 its own inverse. Select the preset labelled 13 to use it. It is commonly used online to hide spoilers and punchlines rather than for real security.

Does the tool keep punctuation, numbers and capital letters?

Yes. Only the 26 letters A to Z are shifted. Spaces, digits, punctuation and symbols pass straight through unchanged, and the original upper or lower case of each letter is preserved so the structure of your message stays readable.

Is the Caesar cipher secure, and is my text private?

The Caesar cipher offers no real security because there are only 25 keys and it can be cracked in seconds, as this tool demonstrates. Treat it as a learning aid or a bit of fun, not protection for sensitive data. Your privacy here is fully protected though, because every calculation happens in your browser and no text is ever sent to a server.

Caesar Cipher Tool — Gera Tools

The Caesar cipher is the classic starting point for anyone learning cryptography. This tool lets you encrypt plain text, decrypt cipher text, and crack messages whose shift you do not know — all from a single page with a live, two-way preview. It is built for students working through a cryptography exercise, puzzle and escape-room fans, teachers preparing examples, and anyone who has stumbled across a scrambled message and wants the answer fast.

How it works

A Caesar cipher shifts every letter of the alphabet a fixed number of places. With a shift of three, the letter A is replaced by D, B by E, and so on, with the end of the alphabet wrapping around so that X becomes A, Y becomes B and Z becomes C. The same fixed shift is applied to every letter in the message, which is what makes it a simple substitution cipher. Mathematically each letter is converted to a number from 0 to 25, the shift is added, the result is taken modulo 26, and the number is turned back into a letter. Decryption does exactly the same thing but subtracts the shift instead of adding it, so encrypting and then decrypting with the same key returns your original text.

Only the 26 letters are affected. Spaces, digits, punctuation and other symbols are left untouched, and the upper or lower case of each letter is preserved. Drag the shift slider or tap a preset to change the key, and the output updates instantly as you type. A shift of 13 gives you ROT13, a special case that is its own inverse because applying it twice brings you back to the start.

The real power of the tool is the brute-force panel. Because there are only 25 meaningful keys, it tries every one of them at once and scores each result against the natural frequency of letters in English. The candidate that looks most like real English floats to the top of the table, which usually reveals the hidden message immediately even when you have no idea what shift was used. Every row can be copied on its own.

Example

Take the message Gera Tools encrypted with a shift of three. Each letter moves three places forward: G becomes J, e becomes h, r becomes u, a becomes d, so the cipher text is Jhud Wrrov. To reverse it, switch to Decrypt, keep the shift at three, and the original Gera Tools reappears.

Now imagine you received Jhud Wrrov but did not know the shift. Open the brute-force panel and the row for shift three rises to the top with the highest English-likeness score, handing you the plaintext without any guesswork.

Plain text	Shift	Cipher text
Gera Tools	1	Hfsb Uppmt
Gera Tools	3	Jhud Wrrov
Gera Tools	13	Tren Gbbyf
Gera Tools	23	Dbox Qllip

Every figure is calculated in your browser — no text is uploaded or stored.