Caesar cipher, shift it, read it, break it.

A Caesar cipher is a way of scrambling a message by moving every letter a fixed number of places along the alphabet. With a shift of 3, A becomes D, B becomes E and so on, and the end of the alphabet wraps round to the start. This Caesar cipher encoder and decoder works with any shift from 1 to 25 or a negative one, has ROT13 on one button, shows a table of all 25 shifts, and charts the letters in your text so you can break a message by hand. It is a puzzle and learning tool and it is not secure.

How to use the Caesar cipher tool

1. Type or paste the message into Your text. The Result updates as you type.
2. Pick Encode to shift forward, or Decode to shift back by the same amount.
3. Set the Shift in the number box, typing it or using its arrows. Shift 3 and ROT13 are one click.
4. Under Options, choose whether to keep upper and lower case, keep spaces and punctuation, or put the letters in groups of five, which hides word lengths.
5. Press Copy on the result. Use the result as the text moves it into the box and flips the direction, so you can check it by going back. Copy link gives a link with your text and settings in it.

Worked examples

Count the letters from A as 0, so H is 7. With a shift of 3, HELLO becomes KHOOR: H (7) plus 3 is 10, which is K; E (4) plus 3 is 7, H; L (11) plus 3 is 14, O; the second L is O again; O (14) plus 3 is 17, R.

Near the end of the alphabet the count wraps at 26. X is 23, and 23 plus 3 is 26, which is 0 after the wrap, so X becomes A. Y becomes B and Z becomes C.

ROT13 shifts by 13. HELLO becomes URYYB: H (7) plus 13 is 20, U; E (4) plus 13 is 17, R; L (11) plus 13 is 24, Y; O (14) plus 13 is 27, which wraps to 1, B. Because 13 plus 13 is 26, running ROT13 twice gives back the original, so one setting both encodes and decodes.

A negative shift goes the other way. A shift of -3 does the same as a shift of 23, since -3 plus 26 is 23. The page says so under the shift box.

How to break a Caesar cipher

There are only 25 useful shifts, so the quick way is to try them all. Paste the coded message into Your text and look at Your text at all 25 shifts. Each row decodes the message with that shift. Read down for the row that is real words. When the text is long enough, the row that reads as English is marked, based on how many of its words are found in a word list. Press the row to put that shift in the tool.

The second method is the letter chart. In a Caesar cipher every plain letter always turns into the same coded letter, so the pattern of tall and short bars is the same as the plain text, only slid along. Find the tallest bar. If you think it stands for E, which is a common guess for English, pick E in Tallest bar stands for and the page works out the shift and offers to decode with it. It needs a longer message to be useful, because a few words do not have a reliable pattern. If the guess fails, try another letter or use the table.

Vigenere, the next step up

Switch to Vigenere key and type a key word. Each letter of the key is a shift: A is 0, B is 1 and so on. The first letter of your text uses the first key letter, the second uses the second, and the key repeats. The key moves on only for letters, so spaces and punctuation do not use it up. With the key LEMON, ATTACKATDAWN becomes LXFOPVEFRNHR: A (0) plus L (11) is L, T (19) plus E (4) is 23, X, T (19) plus M (12) is 31, which wraps to 5, F, and so on. This is the plain Vigenere cipher and it is also not secure. The table of 25 shifts is hidden in this mode, because there is no single shift to find, but the letter chart still works.

What is kept and what is not

• Only A to Z and a to z are shifted. Digits, accented letters such as é and other alphabets are left exactly as typed.
• Keep spaces and punctuation is on by default. Turn it off to get letters only, which is how many puzzles are written. Groups of five also removes everything that is not a letter.
• Turn off Keep upper and lower case to get capitals only.

Is a Caesar cipher secure?

No. With 25 possible shifts, trying all of them takes a moment, and this page does it for you. Vigenere is harder by hand but is also well known to be breakable. Use these ciphers for puzzles, escape rooms, geocaching clues, lessons and fun. For anything private, use real encryption.

If you want numbers instead of shifted letters, try letters to numbers. For dots and dashes, see the Morse code translator, and for spelling words out aloud, the phonetic alphabet translator.