How to Encode and Decode Using the Vigènere Cipher

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posted on 18 years ago — updated on 1 second ago

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The Vigenère cipher is a method of encryption that uses a series of different "Caesar ciphers" based on the letters of a keyword. In a Caesar cipher, each letter in the passage is moved a certain number of letters over, to be replaced by the corresponding letter. For example, this would mean that in a Caesar cipher shift of three: A would become D; B would become E; C would become F etc. A Vigenère cipher builds on this method by using multiple Caesar ciphers at different points in the message; this article shows you how to use it.[1]
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Steps

Encipher

Obtain a Vigenère Square (see Sample) or create a Vigenère Square on your own.

Think of a keyword that is shorter than the phrase or phrases you want to encipher. For this example we will use: LIME

Write down your message without spaces. For this example, we will use: WIKIHOWISTHEBEST

Write the keyword under your message, carefully lining each letter up with a letter from your message. Do this until you run out of message: WIKIHOWISTHEBEST LIMELIMELIMELIME

Cut the keyword to fit the phrase if needed. In the example used for this article, the word LIME fits perfectly but when the keyword is not a perfect fit, there is no need to use the full word. For example: WIKIHOWISTHEBESTOFTHEBEST LIMELIMELIMELIMELIMELIMEL

Go to the row of the first letter of the keyword in the Vigenère Square and go to the column of the first letter of the actual message and find the intersection point of the row and column. That's your cipher letter.

Continue on in this fashion until your entire phrase is ciphered. Using the sample Vigenère square given above, the first ciphertext letter is an H coming from row L and column W. So the ciphertext is: HQWMSWIMDBTIMMEX

Decipher

Perform the above steps in reverse to decode ciphered text.

Find the row of the first letter of the keyword. Look along it to find the first letter of the ciphered text in that row. Follow the column up to find the first letter of the encoded phrase at the top of the chart.