Vigenère cipher encoder and decoder

Encode and decode with a keyword.

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How to use the Vigenère cipher

  1. Type your message into Input and your keyword into Key.
  2. Choose Encode to scramble the message or Decode to read one.
  3. Copy the result. The person reading it needs the same key to decode it.

Why it was called unbreakable

A Caesar cipher uses one shift for the whole message, so the most common letter in the ciphertext usually stands for E. The Vigenère cipher uses a different shift for each letter of the key, which spreads the letter frequencies out and defeats that trick. That earned it the nickname le chiffre indéchiffrable, the indecipherable cipher. It was first described by Giovan Battista Bellaso in 1553 and only later credited to Blaise de Vigenère.

Encoding by hand

Write the key above the message, repeating it as many times as needed, and leave out spaces:

Key:     LEMONLEMONLE
Message: ATTACKATDAWN
Result:  LXFOPVEFRNHR

In each column, move the message letter forward by the key letter’s position in the alphabet, counting A as 0. A moved by L (11) gives L, and T moved by E (4) gives X. A Vigenère square, a 26 by 26 grid of shifted alphabets, lets you look the answers up instead of counting. To decode, move each letter backwards by the key letter instead.