Caesar & ROT13 cipher. Encode, decode, crack.
Between letters of a word, e.g. 8-5-12-12-15.
Between words. Decoding reads any digits it finds.
This cipher is self-inverse: encoding and decoding are the same operation.
Everything runs on your device. Your text never leaves the browser.
Brute-force Caesar
All 25 shifts of the input, scored by English letter frequency (χ², lower is more likely English). Top 3 are highlighted. Click a row to copy its full text.
Classic ciphers, cracked live
The Caesar cipher rotates each letter a fixed number of places through the alphabet, ROT13 is simply a Caesar shift of 13, which makes it its own inverse. ROT47 extends the idea to all printable ASCII, Atbash mirrors the alphabet (A↔Z), and A1Z26 swaps letters for their positions. None of them are secure by modern standards, there are only 25 possible Caesar keys, which is exactly what the brute-force panel demonstrates by decoding all of them at once and ranking each candidate against English letter frequencies. Great for puzzles, geocaching, CTF warm-ups and teaching the basics of cryptanalysis.
How to use the Caesar and ROT13 cipher tool
This tool encodes and decodes five classic ciphers: Caesar, ROT13, ROT47, Atbash and A1Z26. The output rebuilds as you type, and a brute force panel tries all 25 Caesar shifts on your input and ranks them by how much they look like English. It suits puzzles, geocaching, CTF warm ups and teaching basic cryptanalysis.
Step by step
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Pick a cipher
Choose Caesar, ROT13, ROT47, Atbash or A1Z26 in the top row. Caesar is selected when the page opens. ROT13, ROT47 and Atbash are their own inverse, so the Encode and Decode buttons are grayed out for them and a note says so.
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Choose a direction
For Caesar and A1Z26, press Encode to hide text or Decode to reveal it. Decoding a Caesar message applies the same shift in reverse.
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Set the shift or the separators
For Caesar, drag the Shift slider from -25 to +25. It starts at +13. For A1Z26, set the Number separator that goes between the letters of a word (default -) and the Word separator that goes between words (default a slash with a space on each side).
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Type or paste your text
Enter text in Input. Output updates live, with a character count above each box. Press Sample to load a test sentence or Clear to empty the input.
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Adjust case and symbols
Untick Preserve case to turn the output into capitals. Untick Keep non-letters to drop spaces, digits and punctuation from the output. Preserve case is grayed out for ROT47, because ROT47 turns letters into other characters and forcing capitals would break the decode.
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Crack an unknown shift
If you do not know the Caesar key, look at the Brute-force Caesar panel below the main card. It decodes your input with every shift from 1 to 25. Tick Sort by likelihood to put the best guesses first, and click any row to copy its full text.
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Copy, save or chain
Press Copy result, or Download .txt to save the output as a text file. Press Use output as input to move the result into the input box; for Caesar and A1Z26 this also flips between Encode and Decode. Copy all rows and Download rows export the whole brute force list.
Reading the result
- With the default Caesar shift of +13, the Sample sentence starting "Attack at dawn!" encodes to "Nggnpx ng qnja!". Letters move, while spaces and punctuation stay put when Keep non-letters is ticked.
- Each brute force row shows the shift, a chi squared score and the start of the decoded text. A lower score means the letter mix is closer to normal English.
- The three lowest scores get a likely #1, #2 or #3 badge. A row shows a dash instead of a score when the text has fewer than 4 letters, because there is too little to judge.
- In A1Z26 decoding, any number outside 1 to 26 becomes a question mark, and the tool lists the numbers it could not convert above the boxes.
Tips and common mistakes
- A brute force row with shift 3 is the text shifted back by 3, so it is what you get by decoding a Caesar message that was encoded with +3.
- The frequency score needs a sentence or more to be reliable. On a few words, check the top three rows by eye rather than trusting the first.
- ROT13 is a Caesar shift of 13, so encoding twice returns the original text. The same goes for ROT47 and Atbash.
- In A1Z26 encode mode, untick Keep non-letters only if you want digits and symbols dropped. The tool then lists what it skipped.
- None of these ciphers protect secrets. The brute force panel shows that a Caesar message falls in one pass.
Privacy and limits
All encoding, decoding and scoring runs in your browser. Your text is not uploaded and not saved, so clearing the box or closing the tab removes it. Copying uses your browser's clipboard, and downloads are made on your device.
Five ciphers, one workbench.
- Encrypt or decrypt with a custom Caesar shift from −25 to +25, updated live as you drag the slider.
- Apply ROT13, ROT47 or Atbash with a single click, the tool knows they're self-inverse, so there's no mode to get wrong.
- Encode and decode A1Z26 (letters as numbers) with custom letter and word separators; decoding reads any digits it finds.
- Crack unknown Caesar ciphers with the brute-force panel: all 25 shifts scored by English letter frequency, top 3 flagged.
- Control the output, preserve or flatten letter case and keep or strip non-letter characters.
- Copy, download or chain: copy the result, save it as a .txt file, or feed the output back into the input with one click.
Frequently asked questions.
Is the Caesar / ROT13 cipher tool free?
Is it private? Is my text uploaded anywhere?
Which ciphers does it support?
How does the brute-force panel rank the shifts?
What's the difference between ROT13 and ROT47?
Is a Caesar cipher secure for sensitive data?
Published · Last updated .
