# Vigenere Cipher: Everything Important You Need To Know in 2021

## 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 howÂ VigenÃ¨re cipher is used in cryptography.Â

## 1) What is VigenÃ¨re cipher Â

In 1553, Giovan Battista Bellaso came up with the VigenÃ¨re cipher method for the first time. He designed VigenÃ¨re table/ VigenÃ¨re square also referred to as Tabula recta for the technique of encryption and decryption of plaintext sent by the sender. ButÂ what is VigenÃ¨re cipher?Â 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. Â The VigenÃ¨re 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. Â The alphabet that is picked at each point depends on the keyword being repeated.Â VigenÃ¨re cipher algorithmÂ was considered extremely difficult to break or let’s say unbreakable until many years.

## 2) VigenÃ¨re 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 senderâ€™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 theÂ VigenÃ¨re tableÂ and the first letter present in the key will be considered as the reference column in the VigenÃ¨re table. The letter at which the selected row and column coincide is considered as the first letter of ciphertext.Â

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.Â

## 3) VigenÃ¨re 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. Â

### 1. VigenÃ¨re cipherÂ Example

In theÂ VigenÃ¨re cipher tableÂ which has a 26*26 matrix, encryption is done in the following way:

Let us assume the plaintext to be JAVATPOINTÂ

And the key to be BESTÂ

Let us perform VigenÃ¨re cipher encryption and decryption on this plaintext and keyÂ

### 2. VigenÃ¨re cipher EncoderÂ

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.Â

As discussed above, to generate ciphertext, the first letters of plaintext and key are combined. Referring to the VigenÃ¨re 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.Â

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 VigenÃ¨re 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.Â

Upon completion, you will get this:Â

Ciphertext = KENTUTGBOX

### 3. VigenÃ¨re cipher DecoderÂ

ForÂ VigenÃ¨re cipher decryption,Â 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.Â

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.Â

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.Â

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

Upon completion, you will get this:Â

Plaintext = JAVATPOINT

## 4) Keyword methodÂ

### 1. VigenÃ¨re cipher keyword encoderÂ

The VigenÃ¨re 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 VigenÃ¨re table just like in the case of the key process.

### 2. VigenÃ¨re cipher keyword decoder

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

TheÂ VigenÃ¨re cipher keywordÂ 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.Â  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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