Vigenère Cipher Encoder, Decoder & Key Solver

Encrypt and decrypt messages using the polyalphabetic Vigenère Cipher. Includes automatic keyword analysis and tabular visualization.

Deep-Dive Educational Guide

Vigenère Cipher: Polyalphabetic Cryptanalysis, Kasiski Examination & Index of Coincidence

An exhaustive academic breakdown of the Vigenère Cipher: Bellaso origins, Tabula Recta matrix arithmetic in ℤ₂₆, the collapse of single-letter frequency analysis, the Kasiski test, Friedman’s Index of Coincidence, and runnable Python auto-crackers.

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How to Use Vigenère Cipher Encoder, Decoder & Key Solver

  1. Enter Plaintext and Keyword: Type your secret message and provide an alphabetic keyword (e.g., "SECRET" or "CIPHER").
  2. Polyalphabetic Shift Matching: The keyword repeats cyclically across your message length to assign a distinct Caesar shift to each individual letter.
  3. Inspect Vigenère Square Result: Review the encrypted or decrypted text and observe the Tabula Recta row-column intersections.

Frequently Asked Questions

How does the Vigenère Cipher differ from a Caesar Cipher?

While Caesar shifts every character by the same constant key, Vigenère uses a keyword to vary the shift per character, making simple frequency analysis much harder.

How is the Vigenère Cipher broken?

Historically, Vigenère was broken using Kasiski examination and the Friedman test (index of coincidence) to determine key length, followed by frequency analysis on each Caesar sub-cipher.