Diffie Hellman Key Exchange Algorithm - Program in C

Diffie–Hellman establishes a shared secret that can be used for secret communications while exchanging data over a public network. The crucial part of the process is that Alice and Bob exchange their secret colors in a mix only. Finally this generates an identical key that is computationally difficult (impossible for modern supercomputers to do in a reasonable amount of time) to reverse for another party that might have been listening in on them. Alice and Bob now use this common secret to encrypt and decrypt their sent and received data. 

The simplest and the original implementation of the protocol uses the multiplicative group of integers modulo p, where p is prime, and g is a primitive root modulo p. Here is an example of the protocol, with non-secret values in blue, and secret values in red.

1. Alice and Bob agree to use a prime number p = 23 and base g = 5 (which is a primitive root modulo 23).
2. Alice chooses a secret integer a = 6, then sends Bob A = g^a mod p
   • A = 5⁶ mod 23 = 8
3. Bob chooses a secret integer b = 15, then sends Alice B = g^b mod p
   • B = 5¹⁵ mod 23 = 19
4. Alice computes s = B^a mod p
   • s = 19⁶ mod 23 = 2
5. Bob computes s = A^b mod p
   • s = 8¹⁵ mod 23 = 2
6. Alice and Bob now share a secret (the number 2).

/*
 this program calculates the Key
 for two persons
 using the Diffie Hellman Key exchange algorithm
*/
#include<stdio.h>
long long int power(int a,int b,int mod)
{
 long long int t;
 if(b==1)
  return a;
 t=power(a,b/2,mod);
 if(b%2==0)
  return (t*t)%mod;
 else
  return (((t*t)%mod)*a)%mod;
}
long long int calculateKey(int a,int x,int n)
{
 return power(a,x,n);
}
int main()
{
 int n,g,x,a,y,b;
 // both the persons will be agreed upon the common n and g
 printf("Enter the value of n and g : ");
 scanf("%d%d",&n,&g);

 // first person will choose the x
 printf("Enter the value of x for the first person : ");
 scanf("%d",&x);
 a=power(g,x,n);

 // second person will choose the y
 printf("Enter the value of y for the second person : ");
 scanf("%d",&y);
 b=power(g,y,n);

 printf("key for the first person is : %lld\n",power(b,x,n));
 printf("key for the second person is : %lld\n",power(a,y,n));
 return 0;
}