# Caesar Cipher Encoder and Decoder

> A Caesar cipher shifts every letter a fixed number of places through the alphabet. A shift of 13 is ROT13, which is its own inverse. It has 25 possible keys, offers no security at all, and exists today for puzzles, spoiler-hiding and teaching.

Source: https://rankcert.com/tools/caesar-cipher
Free, no signup. Updated: 2026-08-29

## How to use it

1. Type or paste your text.
2. Move the shift slider - 13 gives you ROT13.
3. If you are decoding an unknown shift, read down the list of all 25.

## Caesar shift reference

There are only 25 usable shifts, which is why trying them all breaks the cipher in seconds.

| Shift | A becomes | Name | Note |
| --- | --- | --- | --- |
| 1 | B | - | The classic one-step shift |
| 3 | D | Caesar's own | The shift Suetonius records Caesar using |
| 13 | N | ROT13 | Its own inverse - encoding twice returns the original |
| 23 | X | ROT23 | Same as shifting back by 3 |
| 25 | Z | - | Same as shifting back by 1 |
| 26 | A | - | No change - the alphabet wraps fully |

## Questions

### What is ROT13?

A Caesar cipher with a shift of 13. Because the alphabet has 26 letters, applying it twice returns the original - which is why it is used to hide spoilers rather than secrets.

### How do I crack a Caesar cipher?

Try all 25 shifts and read which one produces words. That is why this tool lists them all - with only 25 possibilities there is nothing to break.

### Is a Caesar cipher secure?

Not remotely. It has 25 possible keys and falls instantly to frequency analysis. It is a teaching tool and a party trick, never a security measure.

### What happens to numbers and punctuation?

They pass through unchanged. Only the 26 letters are shifted, and case is preserved.

### How do I decode a Caesar cipher without the key?

Try all 25 shifts and read which produces words - a brute force that takes seconds. For longer texts, letter frequency alone reveals the shift immediately.

### What is the difference between a Caesar cipher and ROT13?

ROT13 is a Caesar cipher with the shift fixed at 13. Because 13 is half of 26, encoding and decoding are the same operation.

### Why is a Caesar cipher not secure?

The key space is 25. Any computer tries every option instantly, and even by hand frequency analysis breaks it in minutes. It was adequate when most opponents could not read at all.

### What is a Vigenere cipher?

A Caesar cipher with a shift that changes per letter according to a keyword. It resisted analysis for three centuries and is still trivially broken today, but it is the natural next step up.

## Related tools

- [Morse Code Translator](https://rankcert.com/tools/morse-code-translator): Translate text to Morse code and back, covering all 26 letters, 10 digits and 18 punctuation marks. Runs in your browser with no signup.
- [Percentage Calculator](https://rankcert.com/tools/percentage-calculator): Work out X% of Y, what percentage one number is of another, and percentage increase or decrease between two values.
- [Roman Numeral Converter](https://rankcert.com/tools/roman-numeral-converter): Convert any number from 1 to 3999 into roman numerals and back, with validation that rejects malformed numerals like IIII. Instant, free, no signup.
- [Unit Converter](https://rankcert.com/tools/unit-converter): Convert between metric and imperial units for length, weight, volume, time, digital storage and temperature, with exact conversion factors.
- [Epoch Converter](https://rankcert.com/tools/epoch-converter): Convert Unix timestamps to human-readable dates and back, handling both seconds and milliseconds, with UTC, ISO 8601 and local output.
- [Subnet Calculator](https://rankcert.com/tools/subnet-calculator): Enter any IPv4 address in CIDR notation to get the network and broadcast addresses, usable host range, subnet mask, wildcard mask and host count.