Playfair Cipher in C (Encryption & Decryption)

The Playfair cipher or Playfair square or Wheatstone-Playfair cipher is a manual symmetric encryption technique. The scheme was invented in 1854 by Charles Wheatstone, but bears the name of Lord Playfair for promoting its use.

The Playfair cipher uses a 5 by 5 table containing a key word or phrase. Memorization of the keyword and 4 simple rules was all that was required to create the 5 by 5 table and use the cipher.

C Program:

	#include <stdio.h>
	#include <conio.h>
	#include <string.h>
	#include <ctype.h>
	#define MX 5
	int choice;
	void playfair(char ch1, char ch2, char key[MX][MX]) {
	int i, j, w, x, y, z;
	for (i = 0; i < MX; i++) {
	for (j = 0; j < MX; j++) {
	if (ch1 == key[i][j]) {
	w = i;
	x = j;
	} else if (ch2 == key[i][j]) {
	y = i;
	z = j;
	}
	}
	}
	//printf("%d%d %d%d",w,x,y,z);
	if (w == y) {
	if(choice==1){
	x = (x + 1) % 5;
	z = (z + 1) % 5;
	}
	else{
	x = ((x - 1) % 5+5)%5;
	z = ((z - 1) % 5+5)%5;
	}
	printf("%c%c", key[w][x], key[y][z]);
	} else if (x == z) {
	if(choice==1){
	w = (w + 1) % 5;
	y = (y + 1) % 5;
	}
	else{
	w = ((w - 1) % 5+5)%5;
	y = ((y - 1) % 5+5)%5;
	}
	printf("%c%c", key[w][x], key[y][z]);
	}
	else {
	printf("%c%c", key[w][z], key[y][x]);
	}
	}
	void removeDuplicates(char str[]){
	int hash[256] = {0};
	int currentIndex = 0;
	int lastUniqueIndex = 0;
	while(*(str+currentIndex)){
	char temp = *(str+currentIndex);
	if(0 == hash[temp]){
	hash[temp] = 1;
	*(str+lastUniqueIndex) = temp;
	lastUniqueIndex++;
	}
	currentIndex++;
	}
	*(str+lastUniqueIndex) = '\0';
	}
	void main() {
	int i, j, k = 0, l, m = 0, n;
	char key[MX][MX], keyminus[25], keystr[10], str[25] = {
	0
	};
	char alpa[26] = {
	'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'
	};
	printf("\n1.Encryption\n2.Decryption\n\nChoice(1 or 2):");
	scanf("%d",&choice);
	if(choice!=1 && choice!=2){ printf("Invalid Choice"); return;}
	fflush(stdin);
	printf("\nEnter key:");
	gets(keystr);
	printf("Enter the text:");
	gets(str);
	removeDuplicates(keystr);
	n = strlen(keystr);
	//convert the characters to uppertext
	for (i = 0; i < n; i++) {
	if (keystr[i] == 'j') keystr[i] = 'i';
	else if (keystr[i] == 'J') keystr[i] = 'I';
	keystr[i] = toupper(keystr[i]);
	}
	//convert all the characters of plaintext to uppertext
	for (i = 0; i < strlen(str); i++) {
	if (str[i] == 'j') str[i] = 'i';
	else if (str[i] == 'J') str[i] = 'I';
	str[i] = toupper(str[i]);
	}
	// store all characters except key
	j = 0;
	for (i = 0; i < 26; i++) {
	for (k = 0; k < n; k++) {
	if (keystr[k] == alpa[i]) break;
	else if (alpa[i] == 'J') break;
	}
	if (k == n) {
	keyminus[j] = alpa[i];
	j++;
	}
	}
	//construct key keymatrix
	k = 0;
	for (i = 0; i < MX; i++) {
	for (j = 0; j < MX; j++) {
	if (k < n) {
	key[i][j] = keystr[k];
	k++;
	} else {
	key[i][j] = keyminus[m];
	m++;
	}
	printf("%c ", key[i][j]);
	}
	printf("\n");
	}
	// construct diagram and convert to cipher text
	printf("\nEntered text :%s\nOutput Text :", str);
	for (i = 0; i < strlen(str); i++) {
	if (str[i] == 'J') str[i] = 'I';
	if (str[i + 1] == '\0') playfair(str[i], 'X', key);
	else {
	if (str[i + 1] == 'J') str[i + 1] = 'I';
	if (str[i] == str[i + 1]) playfair(str[i], 'X', key);
	else {
	playfair(str[i], str[i + 1], key);
	i++;
	}
	}
	}
	if(choice==2) printf(" (Remove unnecessary X)");
	}

view raw PlayFair.c hosted with ❤ by GitHub

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