Vigenère Cipher with Example

In this tutorial, we are going to see What is Vigenère Cipher with Example?

The Vigenère cipher is a symmetric cryptosystem, which means that it uses the same key for encryption and decryption. The Vigenère cipher is very similar to the Caesar cipher, except that it uses a longer key to overcome the main problem of the Caesar cipher: the fact that a letter can be encoded in only one way. For that, we use a keyword instead of a simple character.
 

 
We first associate to each letter a corresponding number.
 

 
It is based on encoding a text with a word by adding to each of its letters the letter of another word called a key. The key is added to the text to be encrypted without limit, and then the ASCII code of each letter of the key is added to the text to be encrypted. For example, the text “hello world” with the key “go” will be encoded as follows:

Original text:
 

 
Key:
 

 
Encrypted text:
 

 
 
To decipher this message you just need to have the secret key and do the reverse decipherment, using a subtraction.

Although this encryption is much more secure than Caesar’s encryption, it can still be easily broken. Indeed, when the messages are much longer than the key, it is possible to locate the length of the key and to use for each sequence of the same length, the method consisting in calculating the frequency of appearance of the letters, allowing to determine the characters of the key one by one…

To avoid this problem, one solution is to use a key whose size is close to the text size in order to make a statistical study of the encrypted text impossible. This type of encryption system is called a one-time-key system. The problem with this type of method is the length of the encryption key (the longer the text to be encrypted, the larger the key must be), which prevents it from being memorized and implies a much greater probability of error in the key (a single error makes the text undecipherable…).
 
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