About Caesar Cipher
The Caesar cipher shifts each letter by a chosen number of places, so with a shift of 3 an A becomes a D and the word CAESAR becomes FDHVDU. It is the oldest substitution cipher in continuous recorded use, and its structure — a single number standing in for the whole key — makes it the clearest possible illustration of how classical encryption works.
This implementation accepts shifts from -25 to 25, which means negative values are supported and decode in the other direction; a shift of -3 undoes a shift of 3. Case is preserved throughout, and characters that are not letters are copied through untouched, so the enciphered text keeps the spacing and punctuation of the original.
It is worth being explicit about its standing: with only 25 meaningful keys, the cipher is broken by trying all of them, which takes a person a couple of minutes with a shift slider and a computer no time at all. It is a teaching tool and a puzzle mechanism, not a way to protect information.
How to use Caesar Cipher
- 1 Enter the text in the Text field.
- 2 Set the Shift value between -25 and 25 — the default of 3 is the shift Julius Caesar is said to have used.
- 3 Read the result, or reverse it by setting the shift to the same value negated.
Frequently asked questions
How do I decrypt text encrypted with a shift of 3?
Use a shift of -3, or equivalently a shift of 23. Negative values are supported and move the letters the other way. Because the shift is the entire key, decoding is just encoding in reverse.
How many possible keys are there?
Twenty-five that change anything — shifts of 0 and 26 leave the text as it was. That is a search space small enough to exhaust by hand, which is why the cipher has been obsolete for serious use for centuries.
Why does my punctuation stay the same?
Because only letters are shifted. Digits, spaces and punctuation have no position in the alphabet to move along, so they are copied through unchanged — which preserves the shape of the original sentence and makes longer messages easier to recognise.
Is it still used anywhere real?
Not for protecting information. It survives in puzzle books, escape rooms and teaching material, and occasionally as a deliberately weak layer in a toy protocol. Any modern use should rely on proper encryption instead.
Free, online, and private
Free to use
Caesar Cipher is completely free. There is no account to create, no login, and no hidden charge or usage limit.
Runs online in your browser
It is a free online tool that runs entirely in your browser. Nothing to download or install — open the page and start using it right away.
Your data stays private
All processing happens locally on your device. Your input is never uploaded, stored, or tracked, so it never leaves your browser.
How Caesar Cipher behaves in detail
These are the specifics of this implementation — the exact ranges, defaults and fixed settings you are working with.
| Parameter | Value |
|---|---|
| Shift field | number input, default 3, declared from -25 to 25 |
| Characters shifted | A-Z and a-z only; every other character is copied through |
| Wrapping | double modulo, so negative shifts work — a shift of -1 turns A into Z |
| Output case | preserved, so capitals stay capitals |
| Non-numeric shift | falls back to 3 |
Compiled from the implementation of this tool and checked against it by itoolnet.
What Caesar Cipher does not do
Knowing the edges of a tool saves more time than another list of its features. These are the cases where it will not do what you might expect.
- Only the twenty-six unaccented Latin letters move. Digits, spaces, punctuation and accented letters such as é are copied unchanged, so the substitution is partial and the untouched characters stand out against the shifted ones.
- The shift is reduced modulo 26 inside the cipher, so the declared -25 to 25 field window is not what keeps the result valid — a shift of 30 behaves exactly like 4, and a negative value is handled by the same wrap rather than being rejected.
- There is no all-shifts view and no automatic detection. Recovering a message means guessing the shift and reading the output, and the tool gives you no frequency data or candidate list to work from.
Tested and documented by itoolnet, the team that builds and maintains this site.