Caesar Cipher Decoder and Encoder

To encrypt, enter your message and choose a shift value. Each letter moves forward by that many positions, wrapping around from Z to A, and the Alphabet row under the shift shows what every letter becomes. A negative shift moves letters backward, so -3 gives the same result as 23.

To decrypt, choose Decrypt, paste the ciphertext and enter the shift it was made with. Uppercase and lowercase letters keep their case, and spaces, numbers and punctuation are left as they are.

Choose Decrypt, paste the ciphertext on the left and enter the shift value that was used to encode it. If you don't know the shift, look at Decrypt Without the Key under the tool: it shows the most likely shift and the text it gives, with all 25 shifts listed below it. Press Use on any of them to set that shift.

A Caesar cipher has only 25 possible keys, so you can try them all. The tool decrypts the text with every shift, counts how often each letter appears in each result and compares those counts with how often letters appear in ordinary English, where E, T and A are common and J, Q and Z are rare. The comparison is a chi-squared test: the shift whose letter counts sit closest to English is ranked first.

This works well on a sentence or more. On a few words it can be fooled, because a short text may not have enough letters for the counts to look like English, and a wrong shift can happen to score better. It also expects English: text in another language, or text with few letters such as a code or a list of names, may rank the right shift lower. If the best guess reads as nonsense, read down the list.

Our Caesar cipher tool makes it easy to encode messages using the Caesar cipher. To encrypt a message, paste your text on the left, then choose a shift value to determine how many positions each letter should be shifted in the alphabet. The tool will automatically encode your message and display the results on the right.

The Caesar cipher is one of the oldest and simplest encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is shifted a certain number of places down or up the alphabet. The method is named after Julius Caesar, who is said to have used it to communicate with his generals.

The rules of Caesar Cipher are simple:

• Each letter in the alphabet is shifted a fixed number of positions. For example, with a shift of 3, A would be replaced by D, B would become E, and so on.
• Uppercase and lowercase letters are shifted separately. For example, with a shift of 3, A becomes D, but a becomes d.
• Non-alphabetic characters, such as numbers and punctuation, are left unchanged.
• There are only 25 useful shifts. A shift of 26 brings every letter back to itself, and a shift backward by 3 is the same as a shift forward by 23.

The Caesar cipher is one of the earliest known encryption techniques, dating back to ancient Rome. It is named after Julius Caesar, who is said to have used it to communicate with his generals. The method is a simple and effective way to encode messages, and it has been used throughout history for military, diplomatic, and personal communication.

ROT13 is the Caesar cipher with a shift of 13, so setting the shift to 13 here gives ROT13. Because 13 is half of 26, shifting by 13 twice brings every letter back where it started, which means encrypting and decrypting ROT13 are the same operation. With any other shift you need to know which direction to go: decrypting a shift of 3 means moving every letter back by 3.

Try our ROT13 Encoder/Decoder and compare for yourself.

The Caesar cipher has several practical and educational uses:

• Learning cryptography: It is one of the first ciphers taught in introductory courses because it clearly demonstrates the concept of substitution encryption.
• Puzzle games and escape rooms: Caesar ciphers are a popular element in treasure hunts, geocaching, and puzzle-based games where players must decode clues. Another beginner-friendly puzzle cipher is the A1Z26 cipher, which swaps letters for their alphabet number.
• Simple text obfuscation: While not secure for sensitive data, a Caesar shift can lightly obscure text such as spoilers or surprise messages. For a non-alphabetic encoding option, try the Morse code translator.
• Teaching programming: Implementing a Caesar cipher is a common beginner exercise for learning loops, character manipulation, and modular arithmetic.