Caesar Cipher Encoder & Decoder

Shift every letter by a fixed number — the oldest cipher still in daily use for puzzles and spoilers. Any shift, ROT13, or brute-force all 25 at once.

Written by Alex Rivera · Last verified: 2026-09-10

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What Is the Caesar Cipher?

Every letter shifts by a fixed number of positions in the alphabet. With a shift of 3, A becomes D, B becomes E, and Z wraps around to C — that is the whole cipher.

Julius Caesar used a shift of 3 to protect Roman military messages around 58 BC, which makes this one of the oldest documented ciphers. “HELLO” with a shift of 3 becomes KHOOR; shifting back by 3 restores it. Letters keep their case, and anything that is not a letter — digits, spaces, punctuation, emoji — passes through unchanged.

One honest limit: the Caesar cipher is not security. With only 25 possible shifts, anyone can try them all in seconds (the Brute force mode above does exactly that). Use it for puzzles, teaching and hiding spoilers — never for real secrets. For the distinction between public encodings and keyed ciphers, see our article on how the same bits can mean different things and what is binary code for how text becomes bytes in the first place.

ROT13: The Self-Inverse Variant

ROT13 is the Caesar cipher with a shift of 13 — exactly half the alphabet. That makes it self-inverse: applying it twice restores the original, so one button both encodes and decodes. “caesar” becomes pnrfne, and pnrfne becomes “caesar” again. This symmetry is why ROT13 became the standard for hiding forum spoilers, puzzle answers and geocaching hints: no key to remember, and nobody reads it by accident.

Common Mistakes

  • Mistake: expecting secrecy. Correction: 25 possible shifts is not a keyspace — it is a to-do list. Use Caesar for puzzles and spoilers; use real encryption for anything that matters.
  • Mistake: shifting the wrong direction. Encoding shifts forward, decoding shifts back — KHOOR only becomes HELLO when you shift back by 3. Correction: if the output looks like more ciphertext, flip between Encode and Decode.
  • Mistake: assuming non-letters shift too. Correction: only A–Z move; numbers, spaces and punctuation stay put. If your output looks scrambled around the letters, check for a tool that mangles them — this one preserves them.

Practice

1. Decode KHOOR with shift 3. K→H, H→E, O→L, O→L, R→O — HELLO.

2. What is ROT13 of “caesar”? Each letter moves halfway around the alphabet: pnrfne. Apply ROT13 again to check — you should get “caesar” back.

Caesar Cipher FAQ

Is the Caesar cipher secure?

No — not at all. There are only 25 possible shifts, so anyone can try them all in seconds. The Caesar cipher is for puzzles, teaching and hiding spoilers, never for protecting real secrets.

What is ROT13?

ROT13 is the Caesar cipher with a shift of 13. Because the alphabet has 26 letters, applying ROT13 twice restores the original text — one operation both encodes and decodes. It is the classic way to hide spoilers and puzzle answers in forums.

How do I decode a Caesar cipher without knowing the shift?

Try all 25 possible shifts — one of them will read as plain English. The Brute force mode above lists every shift at once, so you can spot the real message at a glance instead of guessing.

Does the Caesar cipher work with numbers and punctuation?

They pass through unchanged. Only the 26 letters A–Z shift; digits, spaces, punctuation and emoji stay exactly where they are, and letter case is preserved — “Hello!” becomes “Khoor!”, not a garbled string.

Who invented the Caesar cipher?

Julius Caesar, who used a shift of 3 to protect Roman military messages around 58 BC. It is one of the oldest documented ciphers — and one of the first ever broken, which is exactly why it is still taught.

What is the difference between an encoding and a cipher?

An encoding (like UTF-8 or Base64) is a public rule anyone can reverse; a cipher uses a key. The Caesar cipher’s “key” is just the shift number — and with only 25 options, it offers obfuscation, not secrecy.