Introduction

Cryptography is a way of conducting a protected communication between two parties using mathematical concepts and various rule-based calculations which in computer science terms are called algorithms. These algorithms transform ordinary plain text, information, messages in a series of text that is hard to decipher using a sequence of code so that it can only be accessed by those for whom it is intended to read and process. Let us understand how Vigenère cipher is used in cryptography. 

  1. What is Vigenère cipher
  2. Vigenère Cipher Encryption
  3. Vigenère Cipher Decryption
  4. Keyword method

1) What is Vigenère cipher  

In 1553, Giovan Battista Bellaso came up with the Vigenère cipher method for the first time. He designed Vigenère table/ Vigenère square also referred to as Tabula recta for the technique of encryption and decryption of plaintext sent by the sender. But what is Vigenère cipher? It is a special type of cryptography in which polyalphabetic substitution cipher is used which is like monoalphabetic substitution cipher except for one change. The alphabet that is taken as a cipher changes periodically while encoding the plaintext. This reduces the cipher less susceptible to attacks using cryptanalysis of letter frequencies.

Each plaintext symbol is encrypted using a cipher obtained from a polyalphabetic substitution.  The Vigenère table is made of alphabets written 26 different times in various rows and columns and with each revision, an alphabet is moved to its left compared to the previous alphabet.  The alphabet that is picked at each point depends on the keyword being repeated. Vigenère cipher algorithm was considered extremely difficult to break or let’s say unbreakable until many years.

2) Vigenère Cipher Encryption

The sender and the receiver must agree on setting up the initial key. Once both agree on the initial key then that key will be added at the start of the plaintext at the sender’s end. While encoding the sender will align the plaintext and the key beneath each other, the letter that appears in the plaintext will be considered as a reference letter for the row in the Vigenère table and the first letter present in the key will be considered as the reference column in the Vigenère table. The letter at which the selected row and column coincide is considered as the first letter of ciphertext. 

In the process of encryption, plaintext sent from the sender is encoded using a key to form a cipher. To find the cipher letter, we find the intersection between the column that is started by the plaintext letter and the row that is indexed by the key letter. 

3) Vigenère Cipher Decryption

In the process of decryption, we find the plaintext letter at the start of the column determined by the intersection of two diagonals; one containing the cipher letter and the row containing the key letter.  

1. Vigenère cipher Example

In the Vigenère cipher table which has a 26*26 matrix, encryption is done in the following way:

Let us assume the plaintext to be JAVATPOINT 

And the key to be BEST 

Let us perform Vigenère cipher encryption and decryption on this plaintext and key 

2. Vigenère cipher Encoder 

To generate a new key, we use the existing key. The existing key is repeated in a periodic circular manner until the length of the new key becomes equal to the length of the plaintext. 

As discussed above, to generate ciphertext, the first letters of plaintext and key are combined. Referring to the Vigenère table, the column of plaintext J and the row of the key B intersect at alphabet K, and hence K becomes the first letter of ciphertext. 

Likewise, go on repeating the same process further. The second letter of the plaintext is combined with the second letter of the key. To exhibit this again for understanding, the column of plaintext A from JAVATPOINT and the row of the key BEST- second letter E coincide at alphabet E in the Vigenère table, and hence E becomes the second letter of the ciphertext. Repeat the process further until the ciphertext becomes the same length as the plaintext. 

Upon completion, you will get this: 

Ciphertext = KENTUTGBOX

3. Vigenère cipher Decoder 

For Vigenère cipher decryption, let us first select the row where the key letter is located. Then find the ciphertext’s position in that row. Once this is done then the column tag of the respective ciphertext is the plaintext. Let us continue with the example at hand. 

In the above table, check out the row of the key that starts with letter B and the ciphertext starts with letter K. this ciphertext letter pops up in column J, and hence the first letter of the plaintext letter is J. 

Next in the row of the key is E and the ciphertext is E. this ciphertext letter pops up in column A and hence the first letter of the plaintext letter is A. 

 Repeat the process further until the ciphertext becomes the same length as the plaintext. 

Upon completion, you will get this: 

Plaintext = JAVATPOINT

4) Keyword method 

1. Vigenère cipher keyword encoder 

The Vigenère cipher keyword is like the key method but uses a keyword instead of using a single letter initial setting up key. The keyword chosen should be of more than one letter and is repeated. The sender writes the keyword repeatedly on the line underneath the plaintext in order to shape the key. For each column, the plaintext and these key pairs are encoded using the Vigenère table just like in the case of the key process.

2. Vigenère cipher keyword decoder

Write the key repeatedly first to decode a message encoded with the Vigenère keyword process. Write below the ciphertext. Using the Vigenère Square to decipher each pair of key-ciphertext letters the same way they were decoded with the autokey process.

The Vigenère cipher keyword is much safer than the key process, but it is still susceptible to attacks. Increasing the safety of the cipher can be achieved by using longer keywords.  is. For eg., if the keyword is if the plaintext, a sample of text already agreed upon the cipher is unbreakable if any message uses a different key.

While periodicity is avoided by running-key or autokey ciphers, two methods exist to cryptanalysis them. First, under the assumption that both the ciphertext and the key share the same frequency distribution of symbols, the cryptanalyst continues and applies statistical analysis.

Conclusion

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