Vigenere Cipher

Situation. Your a member of Team A. You are tasked to serve as a spy. You will be spying Team B. Team B trusted you with a very important message to give to their other camp. It may be a message for the upcoming attack on Team A. Being a spy, you open it, but all you find are jumbled letters. What the hell did you just find? A CIPHER!

In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption — a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment.

When people look at encrypted messages, it’s just like random letters, or in some cases, numbers. That’s the purpose of ciphers. This article will be focusing on a kind of cipher called Vigenere Cipher. It earned the title  le chiffre indéchiffrable (French for ‘the unbreakable cipher’).

The Vigènere 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 Vigènere cipher builds on this method by using multiple Caesar ciphers at different points in the message. This can be mixed though, according to the preference of the one making the message.

Simple enough? What about this?

This square is your best friend if you want to learn about the Vigenere Cipher. It’s called the Vigenere Square(duh). So… how to do this?

The following steps are from wikiHow. Link is provided at the end of the article.

1. Obtain a Vigènere Square or create a Vigènere Square on your own.Duh. (Here’s a link on how to make your own Vigenere square: http://www.wikihow.com/Create-a-Vigenere-Square)

2.Think of a keyword that is shorter than the phrase or phrases you want to encipher. Let’s use the word MAGIC.

3. Write down your message without spaces. My name will do. IANDOMINICPERIDONUESTRO

4. Write the keyword under your message, carefully lining each letter up with a letter from your message.

I   A   N   D   O   M   I   N   I   C   P   E   R   I   D   O   N   U   E   S   T   R   O

M A  G    I    C   M   A  G    I  C  M   A  G   I   C   M  A   G    I   C   M  A   G

5. Go to the row of the first letter of the keyword in the Vigènere Square and find the first letter of your phrase. Follow that column up to the ciphered letter. In the example, that would mean going to row M and finding I and going up to the cipher letter W.

6. Continue on in this fashion until your entire phrase is ciphered. The example ends up as: WAHVMAIHAAXELABCNOWQHRI

I   A   N   D   O   M   I   N   I   C   P   E   R   I   D   O   N   U   E   S   T   R   O

M A  G    I    C   M   A  G    I  C  M   A  G   I   C   M  A   G    I   C   M  A   G

W A   H   V  M   A    I   H   A A  X   E    L  A   B  C   N    O   W Q   H   R    I

 

IF YOU RECIEVE THIS CODED MESSAGE, HOW CAN YOU READ IT?

1. Find the column of the first letter of the ciphered text, and go over until you reach the row of the first letter of the keyword. It’s like using a Multiplication table. In the example, go to Column W. Find row M(first letter of the keywrd MAGIC). Find the intersection of the row and column (letter I).

2. Continue on in this manner until you completely decipher the text.

W A   H   V  M   A    I   H   A A  X   E    L  A   B  C   N    O   W Q   H   R    I

M A  G    I    C   M   A  G    I  C  M   A  G   I   C   M  A   G    I   C   M  A   G

I   A   N   D   O   M   I   N   I   C   P   E   R   I   D   O   N   U   E   S   T   R   O

Here are some examples you can work on:

 ANAXHFNDWAAAXKOXEAAQW       Key: GEISHA

YABZQAAHTKMUUES                              Key: TALTOS

PATIZKUA                                                         Key: SEE

 

Sources:

http://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher

http://en.wikipedia.org/wiki/Cipher

http://www.wikihow.com/Encode-and-Decode-Using-the-Vig%C3%A8nere-Cipher

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