Caesar Cipher Encoder / Decoder (with ROT13)
Type text and pick a shift amount. Letters are rotated by that many positions in the alphabet (A-Z, a-z); other characters pass through unchanged. Use shift=13 for ROT13, shift=3 for the original Caesar.
Khoor, Zruog!
How it works
How the Caesar cipher works
Each letter in the input is replaced by a letter a fixed number of positions later in the alphabet. With shift +3, A becomes D, B becomes E, …, X becomes A (wrapping around). Non-letters pass through unchanged. Decoding is the same process with the opposite sign.
Named after Julius Caesar, who reportedly used a +3 shift for military messages. The shift size is the only secret. With only 26 possible shifts, it's trivially broken by trying them all — but it's an excellent teaching example for substitution ciphers.
ROT13 — the most common variant
ROT13 uses shift=13. Because 13 is half of 26, applying ROT13 twice returns the original text — encoding and decoding use the exact same operation. This made it popular on Usenet and forums for hiding spoilers and offensive jokes; readers can decode by clicking a 'show' button without needing a separate decoder.
ROT13 is not encryption in any meaningful sense. It's obfuscation — the equivalent of writing 'spoiler hidden below' so people who don't want to know can skip it.
Why this is fun, not secure
Modern attackers break a Caesar cipher in microseconds by frequency analysis (E is the most common letter in English; whatever maps to a frequent letter in the ciphertext is probably E). Don't use it for anything you care about.
Use it for: educational examples, puzzle games, ARGs (alternate reality games), hiding text in plain sight, ROT13 spoiler tags. Don't use it for: passwords, financial data, anything with consequences.
Frequently asked questions
›What's the difference between Caesar +3 and ROT13?
Just the shift amount. +3 is Caesar's original choice; 13 is exactly half the alphabet (so encoding and decoding are the same operation). Mathematically they're the same family of cipher.
›Why does shift +26 give the same text back?
26 letters in the alphabet — shifting by exactly 26 lands you on the original letter. Same for any multiple of 26.
›Why is ROT13 used for spoilers?
Because applying ROT13 twice returns the original. Forums and Usenet readers had built-in 'rot13 this text' buttons that worked for both directions. Convenient for hiding then revealing text.
›Does this handle non-Latin alphabets?
No — only A-Z and a-z. Cyrillic, Greek, CJK, and Arabic pass through unchanged. The Caesar cipher is intrinsically Latin-alphabet-specific.
›Can I use this for real security?
Absolutely not. With 25 possible shifts (excluding 0 and 26), a computer breaks it instantly. Use AES, RSA, or established cryptographic libraries for real security.
›What about Vigenère cipher?
A more advanced classical cipher using a keyword to vary the shift per letter. Still breakable by hand with enough text, but much harder than Caesar. We don't include it (yet).
›Why include negative shifts?
Convenience. Encoding with +3 and decoding with +3 is wrong; you need -3 to decode. Negative shifts let you decode without switching modes (or the toggle does it for you).
›Does the data leave my browser?
No. Encoding and decoding run locally; nothing is sent to a server.
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