Simple Diffie–Hellman Key Exchange Example in C.

Diffie-Hellman Key Exchange Method

Diffie–Hellman key exchange method allows two parties that have no prior knowledge of each other to jointly establish a shared secret key over an insecure channel. This key can then be used to encrypt subsequent communications using a symmetric key cipher.

EXAMPLE :-

#include<stdio.h>
long int power(int p,int q,int mod)
 {
  long long int s;
  if(q==1)
  return p;
  s=power(p,q/2,mod);
  if(q%2==0)
   return (s*s)%mod;
  else
   return (((s*s)%mod)*p)%mod;
 }
 long long int calculateKey(int p,int x,int n)
 {
  return power(p,x,n);
 }
 int main()
 {
  int n,g,x,p,y,q;  
  printf("Enter the value of n and g : "); 
  scanf("%d%d",&n,&g);    
  printf("Enter the value of x for the first person : ");
  scanf("%d",&x);  p=power(g,x,n);    
  printf("Enter the value of y for the second person : ");
  scanf("%d",&y);  q=power(g,y,n);
  printf("key for the first person is : %lld\n",power(q,x,n));
  printf("key for the second person is : %lld\n",power(p,y,n));
  return 0;
 }

OUTPUT

Enter the value of n and g : 5
23
Enter the value of x for the first person : 3
Enter the value of y for the second person : 5
key for the first person is : 2
key for the second person is : 2