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Encrypt files using Vigenere Cipher

In this post we discuss a method to encrypt your files using Vigenère Cipher.
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 encryption,
For example, suppose that the plain text to be encrypted is:
HELLOTHISISMYBLOG
Now we select a key that encrypts the given string, let the key be "LEMON", we repeat this key to match the length of our message
LEMONLEMONLEMONL
In order to encrypt the message we take the first 2 letter 1 from the message and one from the key i.e 'H' and 'L' and in our table we take the row that starts with H and go to the column that starts with L and we replace our 'H' with the corresponding letter.
Message  :  HELLOTHISISMYBLOG
Key          :  LEMONLEMONLEMONL
Cipher     :   SIXZBELUGVDQKPYZK 

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) + indexof(letter_in_key) ) % 26

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

Still confused, watch the following video to understand the process.
https://www.youtube.com/watch?v=SkJcmCaHqS0

Algorithm
  1. Accept message to be encrypted and the key.
  2. Create a empty string that will hold the encrypted 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 cipher letter and append it to encrypted message string. 
Advantages and Drawbacks

Crypt analysis does not give good results for against this cipher technique as each letter is replaced by some other letter each time, hence this makes it a very good cipher. but the disadvantage is the key length if the attacker guesses the length of the key then the repeating pattern might become simple to decode. 

C++ Program

Sample input and output to check the program

You might also be interested in 

Counting Sort
Quick Sort using Recursion
Data Encryption using Caesar Cipher
Data Decryption using Caesar Cipher
LCM of 2 numbers
Anagram Strings
Double Linked List

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