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How to Encode and Decode Using the Vigenere Cipher...

The Vigenère cipher is a method of very weak 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 alphabet number, e.g. In a Caesar cipher shift of three, A would become D, B would become E, etc. A Vigènere is similar but more complex.

Steps

Encipher

1. Obtain a Vigènere Square (pictured at the end of this article) or create a Vigènere Square on your own.
2. Think of a key word that is shorter than the phrase or phrases you want to encipher. In this example we will use

LIME

3. Write your phrase down without spaces. For this example, we will use

WIKIHOWISTHEBEST

4. Under your phrase, write the keyword over and over again until you run out of space.

WIKIHOWISTHEBEST


LIMELIMELIMELIME

5. In this example, the word "lime" fits perfectly with the sentence, but when the keyword doesn't fit, you don't have to use the full word. i.e.

WIKIHOWISTHEBESTOFTHEBEST


LIMELIMELIMELIMELIMELIMEL

6. Go to the row of the first letter of the keyword in the Vigènere Square and find the first letter of your phrase. Then, follow that column up to the ciphered letter. In our example that would mean going to row L and finding W and going up to the cipher letter L.
7. Continue on in this fashion until your entire phrase is ciphered. Our example ends up as

LAYEWGKEHLVAQWGP


Decode

1. To decode ciphered text, do the reverse from above.
2. Find the row of the first letter of the ciphered text, and go over until you reach the column of the first letter of the keyword. That letter is the first letter of the enciphered phrase.
3. Continue on in this manner until you completely decipher the text.


Vigènere Square

















Tips

* Another Method of encryption is to find corresponding letter in row and column intersection. In this case "letter W and L is H" and so on. WIKIHOWISTHEBEST becomes HQWMSWIMDBTIMMEX.
* Double check to make sure you encipher and decipher correctly. An erroneously deciphered text can lead to misinterpretation.
* When giving this to someone else, they need to know the keyword to decipher the code, so secretly whisper it to them or use a predetermined Caesar cipher to encipher the keyword as well.
* There are online Vigènere Deciphers you can use to help decipher your code. Do a Google Search to find them.
* If you use a larger Vigènere square which includes both punctuation and spacing the cypher becomes more difficult to break also, especially when the "keyword" or "key phrase" is as long or longer than the message.
* Another method to further obfuscate your message is to Caesar cipher the original message using a predetermined value (Eg: Like ROT13) then run it through a Vigènere cipher, even if decoded without knowing the result was Caesar ciphered before the Vigènere cipher it still appears to be jibberish.
* The more often your "Keyword" or "Key Phrase" repeats the more easily patterns are detected in the encrypted text and the easier it is to break the cipher. A "Key" as long or longer than the length of the message is preferable.


Warnings

* This cipher is not fool-proof (no cipher is) and can be quickly cracked. By current standards, the Vigenere cipher is extremely weak. Do not use this for anything truly top-secret. For stronger encryption research RSA.