The encryption and decryption mechanism of Vigenère cipher.

A  message is encrypted using a secret key and an encryption table(tabula recta)  in the Vigenère cipher.The 26 letters of the Latin alphabet, from A to Z, are usually repeated down the left side of each column and at the beginning of each row in the tabula recta. The 26 letters of the Latin alphabet are shifted one position to the right in a cyclic manner as the rows progress downwards in each row of the square. A moves down to the end once B has moved to the front. This goes on for the rest of the square.

 

Encrypt

Step 01: Firstly take this plaintext phrase and convert into upper case,for example lets take “institute of nation” and convert “INSTITUTE OF NATION”

 

Step 02: After finalizing the plaintext, the person encrypting would then pick a secret key, which would help encrypt and decrypt the message. For example secret key here is: BONSAI

Step 03: The next step is repeating the secret key enough times so its length matches the plain text

I

N

S

T

I

T

U

T

E

O

F

N

A

T

I

O

N

B

O

N

S

A

I

B

O

N

S

A

I

B

O

N

S

A

 

Step 04: the tradition by removing all spaces and punctuation, converting all letters to upper case, and dividing the result into 5-letter blocks(Once the two lines are split into five-letter groups)

plaintext

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

1

2

I

N

S

T

I

T

U

T

E

O

F

N

A

T

I

O

N

B

O

N

S

A

I

B

O

N

S

A

I

B

O

N

S

A

Step 05: start encrypting. Take one letter from the plaintext group and a letter from the secret key group (we’re going to start with I and B), and find the entry in the tabula recta where the row and column intersect. For this example, the first letter of the encrypted cipher text is J.(colums are use for text and rows are use for Keyword)

 

Decrypt

cipher text

Step01: Firstly, We can work backwards using the tabula recta. First repeat the secret key so its length matches the cipher text

J

B

F

L

I

B

V

H

R

G

F

V

B

H

V

G

N

B

O

N

S

A

I

B

O

N

S

A

I

B

O

N

S

A

 

Step02: Using the tabula recta, find the row that corresponds to the first letter in your secret key text- in our case, B In the B row, find the corresponding cipher text letter J. The vertical column where that cipher text letter is located reveals the plaintext letter I.