RSA C Program

// C program for RSA asymmetric cryptographic 
// algorithm. For demonstration values are 
// relatively small compared to practical 
// application 
#include<stdio.h> 
#include<math.h>

// Returns gcd of a and b 
int gcd(int a, int h) 
{ 
int temp; 
while (1) 
{ 
temp = a%h; 
if (temp == 0) 
return h; 
a = h; 
h = temp; 
} 
}

// Code to demonstrate RSA algorithm 
int main() 
{ 
// Two random prime numbers 
double p = 3; 
double q = 7;

// First part of public key: 
double n = p*q;

// Finding other part of public key. 
// e stands for encrypt 
double e = 2; 
double phi = (p-1)*(q-1); 
while (e < phi) 
{ 
// e must be co-prime to phi and 
// smaller than phi. 
if (gcd(e, phi)==1) 
break; 
else
e++; 
}

// Private key (d stands for decrypt) 
// choosing d such that it satisfies 
// d*e = 1 + k * totient 
int k = 2; // A constant value 
double d = (1 + (k*phi))/e;

// Message to be encrypted 
double msg = 20;

printf("Message data = %lf", msg);

// Encryption c = (msg ^ e) % n 
double c = pow(msg, e); 
c = fmod(c, n); 
printf("\nEncrypted data = %lf", c);

// Decryption m = (c ^ d) % n 
double m = pow(c, d); 
m = fmod(m, n); 
printf("\nOriginal Message Sent = %lf", m);

return 0; 
}