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;
}