#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<string.h>
int x, y, n, t, i, flag;
long int e[50], d[50], temp[50], j, m[50], en[50];
char msg[100];
int prime(long int);
void encryption_key();
long int cd(long int);
void encrypt();
void decrypt();
int main()
{
printf("\nENTER FIRST PRIME NUMBER\n");
scanf("%d", &x);
flag = prime(x);
if(flag == 0)
{
printf("\nINVALID INPUT\n");
exit(0);
}
printf("\nENTER SECOND PRIME NUMBER\n");
scanf("%d", &y);
flag = prime(y);
if(flag == 0 || x == y)
{
printf("\nINVALID INPUT\n");
exit(0);
}
printf("\nENTER MESSAGE OR STRING TO ENCRYPT\n");
scanf("%s",msg);
for(i = 0; msg[i] != NULL; i++)
m[i] = msg[i];
n = x * y;
t = (x-1) * (y-1);
encryption_key();
printf("\nPOSSIBLE VALUES OF e AND d ARE\n");
for(i = 0; i < j-1; i++)
printf("\n%ld\t%ld", e[i], d[i]);
encrypt();
decrypt();
return 0;
}
int prime(long int pr)
{
int i;
j = sqrt(pr);
for(i = 2; i <= j; i++)
{
if(pr % i == 0)
return 0;
}
return 1;
}
//function to generate encryption key
void encryption_key()
{
int k;
k = 0;
for(i = 2; i < t; i++)
{
if(t % i == 0)
continue;
flag = prime(i);
if(flag == 1 && i != x && i != y)
{
e[k] = i;
flag = cd(e[k]);
if(flag > 0)
{
d[k] = flag;
k++;
}
if(k == 99)
break;
}
}
}
long int cd(long int a)
{
long int k = 1;
while(1)
{
k = k + t;
if(k % a == 0)
return(k / a);
}
}
//function to encrypt the message
void encrypt()
{
long int pt, ct, key = e[0], k, len;
i = 0;
len = strlen(msg);
while(i != len)
{
pt = m[i];
pt = pt - 96;
k = 1;
for(j = 0; j < key; j++)
{
k = k * pt;
k = k % n;
}
temp[i] = k;
ct = k + 96;
en[i] = ct;
i++;
}
en[i] = -1;
printf("\n\nTHE ENCRYPTED MESSAGE IS\n");
for(i = 0; en[i] != -1; i++)
printf("%c", en[i]);
}
//function to decrypt the message
void decrypt()
{
long int pt, ct, key = d[0], k;
i = 0;
while(en[i] != -1)
{
ct = temp[i];
k = 1;
for(j = 0; j < key; j++)
{
k = k * ct;
k = k % n;
}
pt = k + 96;
m[i] = pt;
i++;
}
m[i] = -1;
printf("\n\nTHE DECRYPTED MESSAGE IS\n");
for(i = 0; m[i] != -1; i++)
printf("%c", m[i]);
printf("\n");
}
