ROT13 & Caesar

Rotate letters with ROT13, a Caesar shift or ROT47 — and break a shift you do not know.

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What this tool does

ROT13 & Caesar moves every letter a fixed number of places along the alphabet, wrapping Z round to A. ROT13 is the shift of thirteen that Usenet adopted for spoilers: thirteen is half of twenty-six, so running it a second time returns the original and one setting both hides and reveals. The Caesar option lets you pick any whole number from 1 to 25 and choose whether to shift forwards or back. Letters are the only thing either one touches — spaces, punctuation, digits and accented or non-Latin characters pass through unchanged, and upper and lower case are preserved, so a ciphered paragraph still has the shape of a paragraph. The ROT13.5 checkbox extends the rotation to digits, moving each on by five.

ROT47 is the wider variant. It rotates every printable ASCII character, from the exclamation mark through to the tilde, by forty-seven places, so digits and punctuation are scrambled along with the letters and the output looks nothing like prose. Space, tab and line break sit outside that range and are left alone, as is anything beyond ASCII. Like ROT13 it is its own inverse, because forty-seven is half of the ninety-four characters it covers, so the same setting undoes it.

Brute force is the option for text that somebody else rotated. It decodes the input with all twenty-five possible shifts and lays them out in a table of three columns: the shift, the decoded text, and an English-likeness figure. That figure is a blunt heuristic rather than cryptanalysis — it hunts for eighteen very common English words (the, and, that, with, from and their like) and adds the length of each one it finds, counting a word only when a plain space sits on either side of it, so a match hugged by a comma, a full stop or a line break is skipped. The note printed under the table names whichever shift scored highest, or reports that none of the twenty-five looked obviously like English instead of nominating one at random. Short input, prose in another language, or a sentence that happens to avoid those eighteen words all land on that second outcome.

None of this is encryption, and it is worth being blunt about that rather than leaving it implied. ROT13 and ROT47 have no key at all, a Caesar shift has twenty-five, and the brute-force option on this same page undoes any of them in one pass. Treat it as a way of putting text out of casual sight — a spoiler, a puzzle answer, an address you would rather a scraper did not lift — and not as a way of keeping something from anyone who cares to read it. When secrecy is the point, Text Encryption takes a passphrase and a real cipher. The rotation itself runs in this browser tab, so the message you paste is not uploaded, and that is the one privacy claim this tool can honestly make.

How to use it

  1. Paste the message. Type or paste it into the input box, or press Load sample. An empty box is refused rather than quietly returning nothing.
  2. Choose the cipher. ROT13, a Caesar shift, ROT47, or brute force to list every shift of text you did not encode yourself.
  3. Set the shift and direction. For a Caesar shift only: any whole number from 1 to 25, forwards to encode or back to decode. Anything outside that range is rejected, because 0 and 26 leave the text as it was.
  4. Decide whether digits move too. The ROT13.5 checkbox rotates each digit on by five. It appears for ROT13 and Caesar; ROT47 already covers digits, and brute force does not use it.
  5. Read the output, or copy it. For ROT13, Caesar and ROT47 the result pane holds the shifted text with a one-line note of the shift applied, and Copy output takes all of it. Brute force replaces that pane with the table of twenty-five candidates: decoded text beyond ninety characters is cut short with an ellipsis there, and Copy output hands over the table as CSV, so a long message has to be read in full by running the winning shift back through the Caesar setting.

Processing & privacy

Runs on your device. Your file is processed directly in this browser.

Common use cases

Posting a spoiler

ROT13 is the long-standing convention for film endings, riddle answers and punchlines, because a reader has to make a deliberate move to decode it.

Working through a puzzle or a CTF challenge

Brute force lays out all twenty-five shifts at once and flags the one that reads like English, which is quicker and less error-prone than trying them by hand.

Teaching how a substitution cipher works

Encode a sentence, decode it back with the same setting, then break it with brute force in the same sitting — a compact demonstration of why twenty-five keys is not a secret.

Keeping a string out of plain sight in a document

An email address or an answer key sitting in a shared file is hidden from a glance and from careless scraping, though not from anyone who recognises the pattern.

Questions

Is my text sent anywhere?
No. The rotation is done by script already loaded in this tab, and no request carries what you typed; once the page is open you can pull the network out and it still works. The page does ask the API for this tool's running use count, and that call carries the tool's name and nothing of your message.
How do I decode ROT13?
Run it through ROT13 again. Thirteen places forward, then thirteen more, brings each letter back where it started, so a single setting both encodes and decodes. ROT47 works the same way for the same reason.
Is a Caesar cipher secure?
No, and it is not offered as though it were. ROT13 and ROT47 have no key, a Caesar shift has twenty-five, and the brute-force option here tries all of them at once. Use Text Encryption when the requirement is that someone should not be able to read it.
What happens to numbers, punctuation and accented letters?
ROT13 and Caesar leave them exactly as they were, unless you tick ROT13.5, which rotates digits by five. Accented and non-Latin letters are never shifted. ROT47 changes every printable ASCII character but leaves spaces, tabs, line breaks and non-ASCII text alone.
Can brute force break anything else?
Only a Caesar shift, which includes ROT13, since ROT13 is simply shift thirteen. It cannot undo ROT47 or any keyed cipher, and its ranking only weighs a short list of common English words by length — short or non-English input produces no clear winner, and the tool reports that rather than guessing.
Why is the shift limited to 1 to 25?
A shift of twenty-six wraps the alphabet exactly once and hands back the input untouched, as does zero. Every distinct result lies between 1 and 25, which is also why brute force needs only twenty-five attempts to cover the whole key space.

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