Scribd
Upload
21 views3 pages

Practical 2

This C program implements a Playfair cipher encryption and decryption algorithm. It includes functions to generate a 5x5 key table from a keyword with duplicates removed, search the table for byte positions, encrypt/decrypt by applying the cipher rule based on neighboring bytes' positions, and a main function to test it on a sample plaintext and ciphertext. Key preprocessing includes removing spaces and converting everything to lowercase before generating the table.

Uploaded by

hello
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as TXT, PDF, TXT or read online on Scribd
21 views3 pages

Practical 2

This C program implements a Playfair cipher encryption and decryption algorithm. It includes functions to generate a 5x5 key table from a keyword with duplicates removed, search the table for byte positions, encrypt/decrypt by applying the cipher rule based on neighboring bytes' positions, and a main function to test it on a sample plaintext and ciphertext. Key preprocessing includes removing spaces and converting everything to lowercase before generating the table.

Uploaded by

hello
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as TXT, PDF, TXT or read online on Scribd
You are on page 1/ 3

#include <stdio.

h>
#include <stdlib.h>
#include <string.h>
#define SIZE 30

void toLowerCase(char plain[], int ps)


{
int i;
for (i = 0; i < ps; i++) {
if (plain[i] > 64 && plain[i] < 91)
plain[i] += 32;
}
}

int removeSpaces(char* plain, int ps)


{
int i, count = 0;
for (i = 0; i < ps; i++)
if (plain[i] != ' ')
plain[count++] = plain[i];
plain[count] = '\0';
return count;
}

void generateKeyTable(char key[], int ks, char keyT[5][5])


{
int i, j, k, flag = 0, *dicty;

dicty = (int*)calloc(26, sizeof(int));

for (i = 0; i < ks; i++) {


if (key[i] != 'j')
dicty[key[i] - 97] = 2;
}
dicty['j' - 97] = 1;

i = 0;
j = 0;
for (k = 0; k < ks; k++) {
if (dicty[key[k] - 97] == 2) {
dicty[key[k] - 97] -= 1;
keyT[i][j] = key[k];
j++;
if (j == 5) {
i++;
j = 0;
}
}
}
for (k = 0; k < 26; k++) {
if (dicty[k] == 0) {
keyT[i][j] = (char)(k + 97);
j++;
if (j == 5) {
i++;
j = 0;
}
}
}
}
void search(char keyT[5][5], char a, char b, int arr[])
{
int i, j;

if (a == 'j')
a = 'i';
else if (b == 'j')
b = 'i';

for (i = 0; i < 5; i++) {


for (j = 0; j < 5; j++) {
if (keyT[i][j] == a) {
arr[0] = i;
arr[1] = j;
}
else if (keyT[i][j] == b) {
arr[2] = i;
arr[3] = j;
}
}
}
}

int mod5(int a)
{
if (a < 0)
a += 5;
return (a % 5);
}

void decrypt(char str[], char keyT[5][5], int ps)


{
int i, a[4];
for (i = 0; i < ps; i += 2) {
search(keyT, str[i], str[i + 1], a);
if (a[0] == a[2]) {
str[i] = keyT[a[0]][mod5(a[1] - 1)];
str[i + 1] = keyT[a[0]][mod5(a[3] - 1)];
}
else if (a[1] == a[3]) {
str[i] = keyT[mod5(a[0] - 1)][a[1]];
str[i + 1] = keyT[mod5(a[2] - 1)][a[1]];
}
else {
str[i] = keyT[a[0]][a[3]];
str[i + 1] = keyT[a[2]][a[1]];
}
}
}

void decryptByPlayfairCipher(char str[], char key[])


{
char ps, ks, keyT[5][5];

ks = strlen(key);
ks = removeSpaces(key, ks);
toLowerCase(key, ks);
ps = strlen(str);
toLowerCase(str, ps);
ps = removeSpaces(str, ps);

generateKeyTable(key, ks, keyT);

decrypt(str, keyT, ps);


}

int main()
{
char str[SIZE], key[SIZE];

strcpy(key, "pearlharbour");
printf("Enter key text: %s\n", key);

strcpy(str, "bwpnrsmualaw");
printf("Enter cipher text: %s\n", str);

decryptByPlayfairCipher(str, key);

printf("Deciphered text: %s\n", str);

return 0;
}

You might also like