Caesar Cipher Encoder & Decoder

Apply a known shift to English letters

A Caesar cipher replaces each letter with another letter a fixed number of positions away in the alphabet. This tool encrypts or decrypts ASCII English letters using a shift from 0 through 25; the default shift is 3. Uppercase and lowercase remain in their respective cases. Spaces, digits, punctuation, Chinese characters, emoji, and other non-ASCII letters pass through unchanged.

For a letter position x, where A/a is 0 and Z/z is 25, and a shift k, encryption is (x + k) mod 26. Decryption reverses it with (x - k + 26) mod 26. Modulo 26 wraps letters around the alphabet, so with shift 3, Z becomes C. The Boston University ciphering exercise describes this wraparound and reverse-shift idea.

Encryption and decryption are opposite directions

Select Encrypt to move letters forward by the chosen shift. Select Decrypt to move them backward by that same amount. For example, with shift 3, Hello, Zz! 123 encrypts to Khoor, Cc! 123. Decrypting that result with shift 3 restores the original text. Shift 0 leaves the input unchanged; shift 25 moves one place backward during encryption.

What happens when the shift is unknown

The page applies the one shift you select. It does not list all candidate shifts, rank likely plaintext, or automatically crack an unknown message. If you are solving a puzzle, you can try candidate values manually and inspect the outputs. If no result makes sense, the message may use a different cipher or character alphabet.

Encode or decode a short message

1. Choose the direction

   Select Encrypt to apply the forward shift or Decrypt to reverse a known shift.

2. Set an integer from 0 to 25

   Type a shift or adjust it with the plus and minus controls. Use the same number for decryption that was used to encrypt.

3. Enter the text

   Type or paste the message. The result updates as the text, mode, or shift changes.

4. Check a wraparound letter

   For shift 3, verify that Z becomes C and z becomes c. Spaces, punctuation, numbers, and other characters should remain in place.

5. Verify by reversing the operation

   Paste the output back as input, switch direction, and keep the same shift. A matching result should restore the original ASCII letters.

Why a different tool may give a different result

This conversion uses the 26-letter English alphabet and preserves every character outside A–Z and a–z. A cipher that shifts digits, uses a custom alphabet, strips punctuation, or normalizes case will not necessarily match.

Questions about shifts and results

Do not use a Caesar shift for secrecy

This is a classical cipher for learning and puzzles, not a way to protect sensitive information.

• A fixed shift has very few possible values and is easy to reverse. Do not use it for passwords, private messages, personal information, or financial data.
• Unchanged spaces, punctuation, numbers, and non-English characters can reveal the original message structure.
• The output confirms only that the selected letter shift was applied. It does not prove that the input was a Caesar ciphertext or that a decoded sentence is correct.