Caesar Cipher / ROT13

There are only twenty-five shifts worth trying, so step the number from 1 to 25 and read each result — the right one announces itself, because English is the only reading that will look like English. If you would rather not, count letters instead. In ordinary English text E is the most common letter by a wide margin, with T, A, O and I not far behind, so the distance from the most frequent ciphertext letter to E is usually the key. Short messages defeat counting, since a dozen letters are noise, and there a crib — a word you expect to appear — does better.

No, and it has not been for centuries. A key with twenty-five possible values falls to a person with a pencil, never mind a computer, and the ciphertext keeps enough structure that letter counting often finds the key on the first attempt. Treat it as obfuscation: it stops a passing eye, not an interested one. Anything that genuinely has to stay private wants modern encryption with a real key — an encrypted archive, an age or GPG file, an entry in a password manager — where the secrecy rests on a key nobody can guess rather than on a number between one and twenty-five.

ROT13 is a Caesar cipher with the shift fixed at 13. Because 13 is half of 26, applying it twice returns the original text, so one operation both scrambles and unscrambles. That property made it the usual way to hide punchlines, spoilers and puzzle answers in plain text on Usenet, and later on forums and mailing lists, where a reader could choose to decode rather than have the ending pushed at them. It was never offered as security. The ROT13 button sets the shift to 13 for you, and at that shift Encrypt and Decrypt give the same output.

Either a backward shift of 3 or a forward shift of 23 — the same move, because the alphabet wraps at 26 and 26 minus 3 is 23. Switching to Decrypt and leaving the number at 3 is the version that is harder to get wrong, since you type the key you were given instead of doing arithmetic on it. The forward-only equivalent matters where a tool offers one direction only, such as a spreadsheet formula or a throwaway script. It also explains why puzzle setters disagree about naming: a shift of 3 and a shift of 23 describe one cipher from opposite ends.

That depends on the tool, and it is worth knowing which kind you have. This one does the work in the page: the shift is arithmetic on character codes inside your browser, so there is no request to send and no server that could keep a copy. Your text lives in one piece of component state and the output is recomputed from it as you type — reload the page and both are gone. Neither the text nor the shift is written into the address bar or into browser storage, so there is no link carrying your message and nothing left for the next person at the machine.

Because a shift cipher is defined over the twenty-six letters and nothing else — there is no agreed answer for what a comma shifted by seven ought to become. Variants exist: ROT5 rotates the digits 0 to 9, ROT18 pairs that with ROT13 so letters and numbers both move, and ROT47 shifts nearly every printable ASCII character. None of them is the classical Caesar, and this page implements the letters-only version. The cost is that everything you did not encrypt is a clue. Word boundaries, message length, an apostrophe near the end of a word, a question mark — all survive, and all help a reader guess.

Not on those characters. Only plain Latin A to Z, upper and lower case, falls inside the code ranges the shift touches, so é, ü, ñ, ß and every Greek, Cyrillic, Arabic, Hebrew or CJK character passes through untouched. French or German text therefore comes out half scrambled and half readable, with the accented letters standing as landmarks that make the rest easier to guess. There is no universal fix, because a shift cipher needs a defined alphabet order and languages disagree about it — some treat an accented form as a variant of the base letter, others as a letter in its own right.

A Vigenère cipher uses a keyword instead of a single number: each letter of the keyword supplies the shift for one letter of the message, and the keyword repeats until the message runs out. The same plaintext letter therefore becomes different ciphertext letters depending on where it lands, which flattens the letter frequencies and defeats the counting that breaks a Caesar in seconds. A Caesar is the special case where the keyword is one letter long. If no single shift ever yields readable English, a repeating key is the usual reason — and its length, not the key itself, is what an attacker hunts for first.