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Vigenere decipher process

In an earlier post we discussed a method to encrypt a string using Vigenere Cipher, in this post we see the method to decrypt the encrypted message.

The Vigenère cipher is a method of encrypting alphabetic text by using a series of interwoven Caesar ciphers based on the letters of a keyword. 
It is a form of polyalphabetic substitution.
Even though the vigenere cipher is easy to understand and implement, for three centuries it resisted all attempts to break it.


In order to encrypt a given message, a table of alphabets can be used, termed a tabula recta, The following is the image of the Table.


the above table consists of the alphabet written out 26 times in different rows, each alphabet shifted cyclically to the left compared to the previous alphabet. At different points in the encryption process, the cipher uses a different alphabet from one of the rows. The alphabet used at each point depends on a repeating keyword.
Now let us consider an example to understand the process of decryption,
For example, suppose that the plain text to be decrypted is:
SIXZBELUGVDQKPYZK 
Now we need a key that was used while encryption of the original mesage, Suppose the key is "LEMON", we repeat this key to match the length of our message
LEMONLEMONLEMONL
In order to decrypt the message we take the first 2 letters, one from the message and one from the key i.e 'S' and 'L' and in our table we take the column that starts with 'S' and in that column we go to the row that has 'L' and we replace our 'S' with the Starting letter of the row.
Message  :  SIXZBELUGVDQKPYZK 
Key          :  LEMONLEMONLEMONL
Cipher     :   HELLOTHISISMYBLOG

Let us now see how to calculate the index of the letter that need to replace the letter from the message, for our program we do not construct the table rather we use a simple formula that gives us index of cipher letter, the below formula is used in our code

position = (indexof(letter_in_message) - index(letter_in_key) + 26) % 26

so for our above example we get 
indexof('S') = 20
indexof('L') = 12
position = (20 - 12 + 26) % 26 = 8
and 8th letter is 'H', so we replace 'S' by 'H'.

Still confused, watch the following video to understand the process.
https://www.youtube.com/watch?v=oHcJ4QLiiP8

Algorithm
  1. Accept encrypted message and the key that was used for encryption.
  2. Create a empty string that will hold the decrypted message
  3. Make the length of key same as the message by appending it to itself as seen in above example
  4. Iterate over the message and apply the above formula to find the deciphered letter and append it to decrypted message string. 

C++ Program

Sample input and output to check the program


You might also be interested in 

Vigenere Cipher Algorithm
Counting Sort
Quick Sort using Recursion
Data Encryption using Caesar Cipher
Data Decryption using Caesar Cipher
LCM of 2 numbers
Anagram Strings

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