#include <stdio.h>
#include <math.h>

/*
Example calculation of Diffie-Helman key exchange
Author: Tim Pierson, Dartmouth CS55, Winter 2021

compile: gcc diffie_helman.c -lm
note: add -lm to get math library, even though imported: https://stackoverflow.com/questions/12824134/undefined-reference-to-pow-in-c-despite-including-math-h

run: a.out
*/

void main() {
	int g = 5;  //prime, known to alice, bob, eve
	int p = 23; //prime, known to alice, bob, eve

	int a = 6; //random private, known only to alice
	int b = 15; //random private, known only to bob

	//alice knows g, p, and a
	//bob knows g, p, and b
	//eve knows g, p

	//alice sends to bob g^a mod p
	//bob sends to alice g^b mod p
	int a_to_b = (long long)(pow(g,a)) % p;
	printf("atob %i\n",a_to_b);
	int b_to_a = (long long)(pow(g,b)) % p;
	printf("btoa %i\n",b_to_a);
	//eve sees both a_to_b and b_to_a

	//alice calcualtes b_to_a^a % p
	//bob calculates a_to_b^b %p
	int alice = (long long)(pow(b_to_a,a)) % p;
	int bob = (long long)(pow(a_to_b,b)) % p;

	printf("Alice calculates %i\n",alice);
	printf("Bob calculates %i\n",bob);
}