Vigenère Cipher Encoder/Decoder

The Vigenère cipher is a method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of a keyword. Unlike a simple Caesar cipher that shifts every letter by the same amount, the Vigenère cipher applies a different shift to each letter depending on the corresponding keyword character. This makes it a polyalphabetic cipher — the same plaintext letter can be encrypted to different ciphertext letters at different positions, making it resistant to simple frequency analysis.

Invented by Giovan Battista Bellaso in 1553 and later misattributed to Blaise de Vigenère, the cipher was considered unbreakable for over three centuries and earned the nickname "le chiffre indéchiffrable"(the indecipherable cipher). It was finally broken by Friedrich Kasiski in 1863 using a method that is now known as the Kasiski examination.

1. 1. Enter Keyword and Text: Type or paste your keyword and the text you want to encode or decode. Only letters A-Z are affected by the cipher — numbers, spaces, punctuation, and special characters pass through unchanged. Letter case is preserved in the output.
2. 2. Encode or Decode: For encoding, each plaintext letter is shifted forward by the position of the keyword letter (A=0, B=1, ..., Z=25). For decoding, each ciphertext letter is shifted backward by the keyword letter. The keyword repeats cyclically to cover the entire message length.
3. 3. Inspect the Results: The character mapping table shows every letter transformation step by step: the original letter, the keyword shift used, the numeric offset, and the resulting letter. The tabula recta (Vigenère square) visualizes the complete lookup table for all possible plaintext-key combinations.

• Polyalphabetic Substitution: Unlike monoalphabetic ciphers where each letter maps to exactly one other letter, the Vigenère cipher can map the same plaintext letter to multiple different ciphertext letters depending on position, determined by the repeating keyword pattern.
• The Encryption Formula: C_i = (P_i + K_{i mod len(K)}) mod 26. Each ciphertext character C_i is computed by taking the plaintext character P_i, shifting it forward by the shift value of the keyword character K at position i reduced modulo the keyword length.
• The Decryption Formula: P_i = (C_i - K_{i mod len(K)} + 26) mod 26. Decryption reverses the encryption by shifting each ciphertext character backward by the corresponding keyword character shift, wrapping around the alphabet using modular arithmetic.
• The Tabula Recta: The Vigenère square (tabula recta) is a 26×26 grid where each row is a Caesar shift of the alphabet. To encrypt, find the plaintext letter in the top row, the key letter in the left column, and read the ciphertext letter at the intersection. Decryption reverses this lookup.

Complete privacy: Our Vigenère cipher tool processes everything locally in your browser. Your plaintext, ciphertext, and keyword never leave your device. No data is uploaded, stored, or transmitted.

Not cryptographically secure: While the Vigenère cipher was revolutionary for its time, it is not secure by modern standards. It can be broken using the Kasiski examination or Friedman test, especially for longer messages. Modern encryption algorithms like AES should be used for actual security needs.

Educational purpose: This tool is designed primarily for education, historical interest, puzzles, and lightweight obfuscation. The visual tabula recta and character mapping make it an excellent teaching aid for understanding how polyalphabetic ciphers work.