Question

C Programming - RSA encryption

Hi there, I'm struggling with writing a C program to encrypt and decrypt a string using the RSA algorithm (C90 language only).

It can contain ONLY the following libraries: stdio, stdlib, math and string.

I want to use the following function prototypes to try and implement things:

/*Check whether a given number is prime or not*/
int checkPrime(int n)

/*to find gcd*/
int gcd(int a, int h)

void Encrypt();

void Decrypt();

int getPublicKeys();

int getPrivateKeys();

An attempt so far, but it doesn't work and can't encrypt words:

#include <stdio.h>

#include <string.h>

#include <math.h>

#include <stdlib.h>

/*Check whether a given number is prime or not*/
int checkPrime(int n)
{
   int i, j;
   j = sqrt(n);
   for (i=2; i<=j; i++)
   {
   if(n%i==0)
       return 0;
   }

/*Finding the greatest common denominator*/
int gcd(int a, int h)
{
   int temp;
   while (1)
   {
       temp = a % h;
       if (r == 0)
       return h;
       a = h;
       h = temp;
   }
}

void Encrypt(int n, int e)
{
   double msg;
   printf("\nMessage to be encrypted: ");
   scanf("%lf", &msg);

   double c = pow(msg, e);
   c = fmod(c, n);
   printf("\nData that's encrypted: %lf", c);
}

void Decrypt(int n, int d)
{
   double msg;
   printf("\nMessage to be decrypted: ");
   scanf("%lf", &msg);

   double c = pow(msg, d);
   c = fmod(c, n);
   printf("\nData decrypted (Original message): %lf, c);
}

void getPublicKeys(int p, int q, int *n, int *e)
{
   *n = p * q;
   *e = 2;
   int phi = (p - 1) * (q - 1);

   while ((*e) < phi)
   {
       if (gcd((*e), phi) != 1)
           (*e)++;
       else
           break;
   }
}

int getPrivateKeys(int p, int q, int e)
{
   int k = 2;
   int phi = (p - 1) * (q - 1);
   int d = (1 + (k * phi)) / e;
   return d;
}

/*Driver Function*/
int main()
{
   int p;
   int q;

   printf("\nChoose two random prime numers p&q: ");
   scanf("%d %d",&p,&q);
   if (checkPrime(p) || checkPrime(q))
   {
       printf("One or more of the numbers are not prime\nExiting...\n");
       return 0;
   }

   int n, e;
   getPublicKeys(p, q, &n, &e);

   int d = getPrivateKeys(p, q, e);
   printf("%d %d %d\n", n, e, d);
   printf("Keys Generated...\n\n");

   printf("-----------MENU-----------\n");
   printf("1. Encrypt a message\n");
   printf("2. Decrypt a message\n");
   printf("3. Exit the program\n");

   while (1)
   {
       int ch;
       printf("Enter your choice: ");
       scanf("%d", &ch);
       if (ch == 1)
           Encrypt(n, e);
       else if (ch == 2)
           Decrypt(n, d);
       else if (ch == 3)
           break;
       else
           printf("Wrong choice\n");
   }
   return 0;
}

Please don't do something like this: http://www.trytoprogram.com/c-examples/c-program-to-encrypt-and-decrypt-string/. I'm trying to incorporate this into a header file and the use of global variables and a fix sized encryption and decryption for a char[50] long is making it impossible. Something a little easier would be better. Appreciate any help

Answer 1

#include <stdio.h>

#include <string.h>

#include <math.h>

#include <stdlib.h>

/*Check whether a given number is prime or not*/
int checkPrime(int n)
{
int i, j;
j = sqrt(n);
for (i=2; i<=j; i++)
{
if(n%i==0)
{
return 0;
}
}
return 1;

}
/*Finding the greatest common denominator*/
int gcd(int a, int b)
{
// Everything divides 0
if (a == 0)
return b;
if (b == 0)
return a;

// base case
if (a == b)
return a;

// a is greater
if (a > b)
return gcd(a-b, b);
return gcd(a, b-a);
}

void Encrypt(double n, double e)
{
double msg;
printf("Message to be encrypted: ");
scanf("%lf", &msg);

double c = pow(msg, e);
c = fmod(c, n);
printf("Data that's encrypted: %lf", c);
}

void Decrypt(double n, double d)
{
double msg;
printf("Message to be decrypted: ");
scanf("%lf", &msg);

double c = pow(msg, d);
c = fmod(c, n);
printf("Data decrypted (Original message): %lf", c);
}

void getPublicKeys(double p, double q, double *n, double *e,double *phi)
{
*n = p * q;
*e = 2;
*phi = (p - 1) * (q - 1);

while (*e < *phi)
{
if (gcd(*e, *phi) != 1)
(*e)++;
else
break;
}
}

int getPrivateKeys(double phi,double e)
{
int k = 2;
double d = (1 + (k * phi)) / e;
return d;
}

/*Driver Function*/
int main()
{
double p,q,n,e,phi,d;
printf("Choose two random prime numers p&q: ");
scanf("%lf %lf",&p,&q);
if (!checkPrime(p)&&!checkPrime(q))
{
printf("One or more of the numbers are not prime Exiting...");
return 0;
}
getPublicKeys(p, q, &n, &e, &phi);

d = getPrivateKeys(phi, e);
printf("%lf %lf %lf\n", n, e, d);
printf("Keys Generated...\n\n");

printf("-----------MENU-----------\n");
printf("1. Encrypt a message\n");
printf("2. Decrypt a message\n");
printf("3. Exit the program\n");

while (1)
{
int ch;
printf("Enter your choice: ");
scanf("%d", &ch);
if (ch == 1)
Encrypt(n, e);
else if (ch == 2)
Decrypt(n, d);
else if (ch == 3)
break;
else
printf("Wrong choice\n");
}
return 0;
}

printf( MENU---\n); printf("1. Encrypt a message\n"); printf("2. Decrypt a message n"); 104 105 106 input Choose two random prime numers p&q: 3 21.000000 5.000000 5.000000 ys Generated.. . MENU 1. Encrypt a message 2. Decrypt a message 3. Exit the program Enter your choice: 1 Message to be encrypt ed: 12 Data that's encrypted: 3.000000Enter your choice:2 Message to be decrypted: 3 Data decrypted (Original message) 12.000000Enter your choice: 3 Program finished with exit code0 Press ENTER to exit console act 19:38 ENG INTL 30-05-2019