Playfair Encryption

Playfair Encryption implementation 

Playfair is one of the popular cryptographic software security algorithms. This technique encrypts pairs of letters at a time and generates more secure encrypted text compare to  the simple substitution cipher like Caesar.

Assumptions:

• Assume key matrix is given to us. 
• Read plain text(2 characters) from user.
• This program demonstrate four rules of the Playfair encryption algorithm.
• This program will process only 2 characters input. 
• You may extend to process n characters by repeating given logic.
• Add suitable exception for completing this program.

#include<stdio.h>

int main(){

 

  char arr[5][5]={"MONAR","CHYBD","EFGIK","LPQST","UVWXZ"};

  char pt[10];

 

  int i, j, r1=0, r2=0, c1=0, c2=0;

  printf("Playfair Keymatrix\n==================\n");

  for(i=0; i<5; i++)

  {

    for(j=0; j<5; j++)

    printf("%c ", arr[i][j]);

    printf("\n");

  }

 

  printf("Enter your plain text:");

  scanf("%s",pt);

  printf("Your plain text = %s", pt);

 

  for(i=0; i<5; i++)

  {

    for(j=0; j<5; j++)

    {

       if(arr[i][j] == pt[0])

       {

         r1=i; c1=j;

       }

       if(arr[i][j] == pt[1])

       {

         r2=i; c2=j;

       }

    }

  }

  if(r1==r2) //when both characters in same row

  {

    if(c2==4) //for char in last column

       printf("Ciphertext = %c%c \n", arr[r1][c1+1], arr[r2][0]);  

    else

       printf("Ciphertext = %c%c \n", arr[r1][c1+1], arr[r2][c2+1]);

  }

  if(c1==c2)//when both characters in same column

  {

    if(r2==4) //for char in last row

       printf("Ciphertext = %c%c \n", arr[r1+1][c1], arr[0][c2]);

    else

       printf("Ciphertext = %c%c \n", arr[r1+1][c1], arr[r2+1][c2]);

  }

  //when characters are not in a same row and column

  if(r1 != r2 && c1 != c2)

  {

    printf("\nCiphertext = %c%c \n", arr[r1][c2], arr[r2][c1]);

  }

  return 0;

}

Output of the program

Playfair Keymatrix

==================

M O N A R

C H Y B D

E F G I K

L P Q S T

U V W X Z

Enter your plain text:IN

Your plain text = IN

Ciphertext = GA

>>> Playfair Decryption Program