Caesar Cipher Encoder and Decoder

How to use the Caesar cipher tool

1. Paste or type your text in the input box, or press Example to load a sample sentence.
2. Set the Shift from 1 to 25. The default of 3 is the shift Julius Caesar used.
3. Choose Encode (shift forward) to make a secret message, or Decode (shift back) to read one when you know the shift.
4. Tick Shift digits too (0–9) if numbers should be hidden as well.
5. Use Copy, Download .txt or Use as input on the result. If you don’t know the shift, tick Show all 25 shifts (to crack a message) instead.

The result updates as you type. Nothing is sent to a server: the shifting runs in your browser.

How the shift works

Each letter moves the chosen number of places along the alphabet. Letters at the end of the alphabet wrap around to the start, so with shift 3, X becomes A, Y becomes B and Z becomes C. Decoding moves each letter back by the same number.

The Roman historian Suetonius describes the system in his life of Julius Caesar: to read Caesar’s secret letters, “substitute the fourth letter of the alphabet, namely D, for A, and so with the others”. In other words, a shift of 3.

Shifting forward by 25 gives the same result as shifting back by 1, because 25 + 1 = 26, a full turn of the alphabet. In the same way, decoding with shift 3 gives the same result as encoding with shift 23. A negative shift works too: encoding with −3 is the same as decoding with 3.

Exactly what changes and what doesn’t

• Only A–Z and a–z are shifted. Spaces, punctuation, symbols, line breaks and emoji stay in place, so the word lengths and sentence shape of the original are still visible.
• Case is kept. An uppercase letter gives an uppercase letter: iPhone 15 Pro with shift 3 becomes lSkrqh 15 Sur.
• Accented and non-Latin letters are not shifted. Café déjà vu, Straße, Привет 👍 with shift 3 becomes Fdié gémà yx, Vwudßh, Привет 👍. The é, à and ß stay as they are, and so do Cyrillic letters.
• Digits are left alone by default. With Shift digits too (0–9) ticked, each digit moves along 0–9 by the same shift and wraps from 9 to 0. Shift 7 turns Order #4471 into Vykly #1148; without the option it gives Vykly #4471. Because there are only ten digits, the digit shift is the remainder after dividing by 10. Shift 13 moves digits by 3 and shift 10 leaves them unchanged.
• Shift 0 or an empty box gives back the text unchanged.

If someone else encoded the message, ask whether they shifted the digits. Decoding digits that were never shifted (or the other way around) turns the numbers into different numbers, while the letters still decode correctly.

Cracking a message with all 25 shifts

A Caesar cipher has only 25 useful keys, so the quickest way to read a message without the key is to try all of them. Tick Show all 25 shifts and a table appears under the result. Each row shows the text shifted back by the number in the Shift back column, from 25 down to 1. Scan the rows for the one in plain English. Its Shift back number is the key the sender used.

For Wkh vhfuhw lv lq wkh pdsoh wuhh., part of the table looks like this:

Things to know about the table:

• It always shifts your input back, whatever the Direction setting. It follows the Shift digits too option: when that is ticked, digits in each row are shifted back by the same number as the letters (wrapping within 0–9), so for Vykly #1148 the Shift back 7 row reads Order #4471. When it is off, digits appear as they are in your input.
• Each row shows the first 200 characters of the text. That is plenty to spot the right row; then set that number as the shift, choose Decode and the full message appears in the result box.
• Short messages of one or two words can read as real words under more than one shift. Try a longer piece if two rows look plausible.

If no row makes sense, the message probably isn’t a Caesar cipher. A substitution cipher that mixes up the alphabet in any order can’t be cracked by shifting. For those, count how often each letter appears with the letter frequency counter and compare the counts with the most common letters in English.

ROT13

ROT13 is the Caesar cipher with a shift of 13. Because 13 is exactly half of 26, applying it twice gives back the original, so one operation both encodes and decodes: The butler did it. becomes Gur ohgyre qvq vg., and running that through ROT13 again gives The butler did it. Set the shift to 13 and either direction gives the same output.

ROT13 was used on Usenet newsgroups from the early 1980s to hide punchlines, puzzle answers and spoilers. Readers had to choose to decode the text, so nobody saw it by accident.

With Shift digits too ticked, shift 13 moves digits by 3, which does not undo itself. For text with numbers that should round-trip under ROT13, leave the digit option off.

When people use it

• Puzzles and escape rooms. A shifted clue is easy to make and fair to solve, and the cheat sheet beside the tool doubles as a decoder card.
• Teaching. The Caesar cipher shows in a few minutes how a key works, how wrap-around (modular arithmetic) works, and why 25 possible keys are too few.
• Spoilers and answers. ROT13 hides a quiz answer or plot twist in a post or email until the reader chooses to decode it.
• Capture-the-flag challenges. When a beginner challenge hides a flag with a letter shift, the all-shifts table finds it in one step.

The cipher only hides text from a casual glance. For data that has to stay private, use real encryption. For encodings that make text safe to transmit rather than secret, see what Base64 is or turn text into binary or Morse code.

Questions