Caesar Cipher Decoder
Encode, decode and translate the Caesar shift cipher instantly. Pick any shift from 0 to 25, flip between encode and decode, or — when you do not know the key — turn on show all 26 shifts to brute-force the message and read off the plaintext. Case is preserved and non-letters pass through unchanged. Everything runs in your browser; nothing is uploaded.
Runs entirely in your browser — nothing is uploaded or sent to a server. The Caesar cipher is an educational classical cipher, not encryption; never use it to protect anything that actually needs to stay secret.
What this tool is
- A free, in-browser Caesar (shift) cipher tool: type text, pick a shift from 0 to 25, and encode or decode instantly.
- No key? Use the "show all 26 shifts" view to brute-force a cipher — every possible decryption is listed so you can spot the readable one.
- Standard A–Z shift: upper and lower case are preserved and anything that is not a letter (spaces, digits, punctuation) passes through untouched.
- Everything runs client-side in JavaScript — nothing you type is ever sent to a server.
- The Caesar cipher is an educational classical cipher with only 25 real keys. It is trivially broken and is NOT secure encryption.
How the Caesar cipher works
The Caesar cipher — also called a shift cipher — replaces every letter with the one a fixed number of positions further along the alphabet. With a shift of 3, ABC becomes DEF, and the end of the alphabet wraps around so XYZ becomes ABC. To decode, you shift back by the same amount. Because every letter moves by the same distance, the key is just a single number between 1 and 25.
Encoding and decoding
- Encode. Add the shift to each letter's position, modulo 26. Shift 3 turns "HELLO" into "KHOOR".
- Decode. Subtract the same shift. Shift 3 turns "KHOOR" back into "HELLO". This tool's Decode mode does the subtraction for you.
- Pass-through. Spaces, digits and punctuation are left exactly as they are, and upper/lower case is preserved — only A–Z and a–z are shifted.
Why it is trivial to break
A Caesar cipher has only 25 usable keys. That is a vanishingly small keyspace — a computer (or a patient human) can try every single shift in moments. The brute-force view in this tool does exactly that: it prints all 26 candidate decryptions so you can pick out the one that reads as real language. Even without trying every key, the cipher preserves the frequency of each letter, so frequency analysis — matching the most common cipher letter to E, the most common letter in English — usually reveals the shift from a single short message.
Security note: The Caesar cipher is an educational classical cipher, not encryption. With only 25 keys and intact letter frequencies it is broken instantly, so it must never be used to protect passwords, messages or any data that genuinely needs to stay confidential. Use it for puzzles, CTFs, teaching and games — and reach for a modern authenticated cipher such as AES-GCM when you need real security.
Frequently asked questions
What is a Caesar cipher?
A Caesar cipher is one of the oldest and simplest substitution ciphers. Each letter in the message is shifted a fixed number of places down the alphabet: with a shift of 3, A becomes D, B becomes E, and so on, wrapping around so X, Y, Z become A, B, C. It is named after Julius Caesar, who is reported to have used a shift of 3 to protect military messages. Because the whole alphabet is rotated by the same amount, it is also called a shift cipher.
How do I decode a Caesar cipher without knowing the shift?
Because there are only 25 possible non-trivial shifts, you can simply try all of them — this is called a brute-force attack. Turn on the "show all 26 shifts" view in this tool and it lists every possible decryption at once; scan the list for the one that reads as plain language. For longer texts you can also use frequency analysis: in English the letter E is the most common, so the shift that maps the most frequent cipher letter back to E is usually the key.
Is the Caesar cipher secure?
No. The Caesar cipher has only 25 usable keys, so an attacker can try every one of them in a fraction of a second and read your message. It also leaves letter-frequency patterns intact, so even without brute force it falls instantly to frequency analysis. It offers no real security and should only be used for puzzles, teaching, CTF challenges and games — never to protect anything that genuinely needs to stay private. For real confidentiality use a modern authenticated cipher such as AES-GCM.
What is the difference between a Caesar cipher and ROT13?
ROT13 is simply a Caesar cipher with a fixed shift of 13. Because 13 is exactly half of 26, applying ROT13 twice returns the original text, which makes it its own inverse — the same operation both encodes and decodes. A general Caesar cipher lets you choose any shift from 1 to 25, so ROT13 is one specific member of the Caesar family. The Vigenère cipher goes further still, using a keyword so that different letters are shifted by different amounts.