What is the Vigenère cipher?

It is a polyalphabetic substitution cipher that uses a repeating keyword to shift each plaintext letter by a different amount, making it harder to crack than a simple Caesar cipher.

What does the Vigenère square show?

The 26×26 grid maps every combination of plaintext letter (row) and key letter (column) to a ciphertext letter. Active cells are highlighted as you encrypt.

Can I decrypt without knowing the key?

The 'Crack It' tab shows letter frequency analysis. Comparing peaks to expected English frequencies can help deduce the key length and letters via the Kasiski/Index of Coincidence method.

Do ROT13 and Atbash need a key?

No. ROT13 is a fixed shift of 13 and Atbash mirrors the alphabet (A↔Z), so no key is required. They are also their own inverses.

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Vigenère Cipher

Encrypt and decrypt text with the Vigenère cipher and explore classical ciphers.

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About this tool

A full classical cipher toolkit featuring the Vigenère cipher with interactive tableau, Caesar cipher (shift 1–25), ROT13, and Atbash. The Vigenère mode visualises the 26×26 square, highlights active key letters, and shows a step-by-step encoding table. A built-in 'Crack It' frequency analysis tool gives hints for breaking unknown Vigenère keys.

How to use

1 Step 1: Select a cipher mode tab: Vigenère, Caesar, ROT13, or Atbash.
2 Step 2: Enter your plaintext (or ciphertext) and — for Vigenère/Caesar — provide the keyword or shift value.
3 Step 3: Click Encrypt or Decrypt. The result appears immediately along with a step-by-step character table.
4 Step 4: For Vigenère, use the 'Crack It' tab to run frequency analysis and compare letter distribution against expected English frequencies.

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