Encrypt or decrypt text using the Vigenère cipher — a classic polyalphabetic substitution cipher invented in the 16th century.

Result: RIJVS UYVJN

Note: Only letters A-Z are processed. Non-alphabetic characters remain unchanged. Case is preserved.

What is the Vigenère Cipher?

The Vigenère cipher is a method of encrypting alphabetic text by using a simple form of polyalphabetic substitution. It was invented by the French diplomat Blaise de Vigenère in the 16th century, though it was incorrectly attributed to him — the cipher was actually first described by Giovan Battista Bellaso in 1553.

The cipher uses a keyword to shift letters of the plaintext. Each letter of the keyword determines the shift for the corresponding letter of the plaintext:

C_i = (P_i + K_i) mod 26

Where C_i is the ciphertext letter, P_i is the plaintext letter, and K_i is the keyword letter (A=0, B=1, ..., Z=25). The keyword is repeated to match the length of the plaintext.

The Vigenère cipher was considered "le chiffre indéchiffrable" (the unbreakable cipher) for centuries because it resists simple frequency analysis attacks. However, it was eventually broken in the 19th century by Charles Babbage and Friedrich Kasiski using the Kasiski examination and frequency analysis of repeated key patterns.

Key features of this calculator:

• Encryption — converts plaintext to ciphertext using the keyword
• Decryption — converts ciphertext back to plaintext using the same keyword
• Preserves case — uppercase stays uppercase, lowercase stays lowercase
• Non-alphabetic characters — spaces, numbers, and punctuation remain unchanged

The Vigenère cipher is still used today in recreational cryptography and as an educational example of polyalphabetic substitution ciphers.

How to Use This Calculator

1. Enter Text: Type or paste the text you want to encrypt or decrypt in the text area. Only letters A-Z and a-z will be processed.
2. Enter Key: Input the keyword in the key field. The key can contain only letters (A-Z).
3. Select Mode: Choose either “Encrypt” (to encode your text) or “Decrypt” (to decode your text).
4. Process: Click the “Process” button or press Enter on your keyboard.
5. Reset: Use the “Reset” button to restore default values (text: “HELLOWORLD”, key: “KEY”, mode: encrypt).
6. Interpretation: The result shows the processed text. For example, encrypting “HELLOWORLD” with key “KEY” gives “RIJVS UYVJN”. Decrypting the result with the same key returns the original text.