How to Solve a Vigenere Cipher
The Vigenere cipher takes a plain text sentence in conjunction with a keyword to create a cipher text. Without the keyword, it is almost impossible to decipher the code, which is why the Vigenere cipher was used for decades without being cracked. It is, however, possible to guess at the length of the keyword, which can make it possible to crack the code.
Instructions
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1
Search the ciphertext for repeated sequences of letters. Examining the length between the repeated sequences can give a clue about the length of the key word. Looking at the length of the sequence as a whole also can give a clue about how long the keyword might be. For example, a 95-character ciphertext can be divided by 5 and 19, which could imply that a 5-character keyword was used 19 times.
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Assign a value of L1, L2, L3 and so on to each character you believe to be in the keyword.
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3
Find the frequency with which each of the letters L1 - Ln (with n being the number of characters in the keyword) occurs as each letter in the alphabet. For example, maybe the L1 occurs as "A" four times, "B" two times, "C" one time, and so on.
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Compare the frequency of each character with the average frequency of each letter in the English language. For example, "E" occurs most frequently in the English language, so you should compare which letter L1 occurs as most frequently. If you assign a value from 1 to 26 to each letter in the alphabet starting with "A", the plain text is transformed into the ciphertext by adding the value of the plain text character and the value of the keyword character's value and subtracting 1. So if "E" turns up most frequently as "I," we can guess that L1 is "E." Repeat this process for each of the keyword's characters
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5
Use the keyword to decipher the ciphertext.
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