Vigenere Cipher Converter - Encrypt and Decrypt Online
Free online Vigenere cipher converter for encrypting and decrypting text with a repeating keyword. Spaces, punctuation, and letter case are preserved.
Online Vigenere Cipher Converter
Use this Vigenere cipher translator to convert plaintext to ciphertext or reverse the process with a keyword. It is useful for learning and checking classical cipher examples; it is not a replacement for modern encryption.
How to Use the Vigenere Cipher Converter
Choose a mode
Select Encrypt for plaintext or Decrypt for existing Vigenere ciphertext.
Enter your text
Paste the message in the input box. Non-letter characters can remain in the text.
Enter the keyword
Type the alphabetic keyword used to create or reverse the repeating shifts.
Run and copy
Click Encrypt or Decrypt, review the result, then copy it for your next step.
What Is the Vigenere Cipher?
The Vigenere cipher is a classical polyalphabetic substitution cipher. Instead of shifting every letter by one fixed amount, it uses the letters of a repeating keyword to choose different Caesar-style shifts. The same keyword is required for decryption.
Vigenere Cipher Example
With the keyword LEMON, the keyword is repeated across the letters in ATTACKATDAWN. Using A=0, the encrypted result is LXFOPVEFRNHR. This converter skips spaces and punctuation when advancing the keyword.
ATTACKATDAWNLEMONLEMONLELXFOPVEFRNHRHow Vigenere Encryption Works
Each letter is mapped to a number from 0 to 25. Encryption adds the plaintext value and keyword value modulo 26; decryption subtracts the keyword value. This is why a short keyword can be repeated across a longer message.
C = (P + K) mod 26P = (C - K) mod 26
Common Uses
- Classical cryptography study：Learn how polyalphabetic substitution differs from a single-shift Caesar cipher.
- Puzzle and game clues：Check a Vigenere clue or decode a message when the keyword is known.
- Programming practice：Verify an implementation in JavaScript, Python, Java, or another language.
- Text transformation tests：Test key alignment while keeping spaces, punctuation, digits, and line breaks readable.
Other Classical Cipher Names
If you are comparing classical cipher tools, you may also encounter these names. They use different rules and should not be treated as interchangeable with Vigenere.
Vigenere Cipher FAQ
What is a Vigenere cipher translator?
It is a tool that applies the Vigenere cipher to encrypt plaintext or decrypt ciphertext with a repeating alphabetic keyword.
How do I decrypt a Vigenere cipher online?
Choose Decrypt, paste the ciphertext, enter the same keyword used for encryption, and click Decrypt. The keyword must match for the original text to be recovered.
Does the keyword advance over spaces and punctuation?
No. This converter advances the keyword only for English letters A-Z, so spaces, punctuation, numbers, and line breaks remain unchanged.
Does this tool preserve uppercase and lowercase?
Yes. Uppercase input remains uppercase and lowercase input remains lowercase. The keyword itself is treated case-insensitively.
Can I use a keyword with numbers or symbols?
The cipher uses alphabetic keyword characters. Any numbers or symbols in the keyword are ignored; a keyword with no A-Z letters is rejected.
Is Vigenere encryption secure for passwords or private data?
No. Vigenere is a historical teaching and puzzle cipher and can be broken with modern cryptanalysis. Use a modern, authenticated encryption method for real security.
Why does my decrypted text look incorrect?
Check that you selected Decrypt, copied the ciphertext completely, and entered the exact same keyword. A different keyword or a shifted starting position changes every later letter.