Vigenere Cipher
Encrypt or decrypt text using the Vigenere cipher polyalphabetic encryption method. Enter text and a keyword to encode or decode messages instantly.
About This Calculator
The Vigenere Cipher Calculator lets you encrypt and decrypt text using the classic polyalphabetic substitution cipher invented by Giovan Battista Bellaso in 1553. Unlike simple Caesar ciphers that shift every letter by the same amount, the Vigenere cipher uses a repeating keyword to apply different shifts to each letter, making it exponentially harder to break.
Each letter of the keyword determines a shift amount (A=0, B=1, ..., Z=25) applied to the corresponding letter of the plaintext. The key repeats cyclically to cover messages of any length. Encryption follows Ci = (Pi + Ki) mod 26, and decryption reverses it with Pi = (Ci − Ki + 26) mod 26. Non-alphabetic characters pass through unchanged while preserving case.
The Vigenere cipher was considered unbreakable for over 300 years until Friedrich Kasiski published a method for determining the key length in 1863. This calculator includes a letter frequency chart that helps visualize how encryption transforms the statistical properties of the text.
This tool is perfect for cryptography students, puzzle enthusiasts, and anyone curious about classical encryption techniques. Use it to encode secret messages, decode historical ciphers, or learn how polyalphabetic encryption works.
Frequently Asked Questions
What is the Vigenere cipher?
The Vigenere cipher is a polyalphabetic encryption method that uses a keyword to shift each letter of the plaintext by a different amount. Each letter of the keyword determines the shift for the corresponding letter of the text, making it much harder to crack than simple Caesar ciphers.
How does the Vigenere cipher encryption work?
Encryption uses the formula Ci = (Pi + Ki) mod 26, where Pi is the plaintext letter number (A=0 to Z=25), Ki is the key letter number, and Ci is the ciphertext letter number. The key repeats cyclically to match the length of the plaintext.
How does the Vigenere cipher decryption work?
Decryption reverses the encryption using the formula Pi = (Ci - Ki + 26) mod 26, where Ci is the ciphertext letter number, Ki is the key letter number, and Pi is the recovered plaintext letter number. The same keyword is needed to decrypt the message.
Can I decrypt a Vigenere cipher without knowing the key?
Decrypting without the key is extremely difficult. Cryptanalysis methods like Kasiski examination and Friedman test can reveal the key length, followed by frequency analysis on each column. Short keys or dictionary-word keys can be brute-forced, but long random keys make decryption practically impossible.
Who invented the Vigenere cipher?
The cipher was invented by Giovan Battista Bellaso in 1553 and published in his book La cifra del Sig. It was later misattributed to Blaise de Vigenere who described an advanced version in his 1585 book Traite des chiffres. The cipher was considered unbreakable for centuries.
What is the Vigenere table or tabula recta?
The Vigenere table, also known as the tabula recta, is a 26x26 grid of letters where each row is a Caesar shift of the alphabet. To encrypt, find the plaintext letter in the top row and the key letter in the first column — their intersection gives the ciphertext letter.
Is this calculator free to use?
Yes, this Vigenere cipher calculator is completely free to use with no registration or hidden charges. You can encrypt and decrypt unlimited messages, and the URL saves your inputs for easy sharing.