Implement Diffi-Hellmen Key exchange Method

 Program:

/* This program calculates the Key for two persons

using the Diffie-Hellman Key exchange algorithm */

#include <stdio.h>

#include <math.h>

// Power function to return value of a ^ b mod P

long long int power(long long int a, long long int b,

                    long long int P)

{

    if (b == 1)

        return a;

    else

        return (((long long int)pow(a, b)) % P);

}

// Driver program

int main()

{

    long long int P, G, x, a, y, b, ka, kb;

    // Both the persons will be agreed upon the

    // public keys G and P

    P = 23; // A prime number P is taken

    printf("The value of P : %lld\n", P);

    G = 9; // A primitive root for P, G is taken

    printf("The value of G : %lld\n\n", G);

    // Alice will choose the private key a

    a = 4; // a is the chosen private key

    printf("The private key a for Alice : %lld\n", a);

    x = power(G, a, P); // gets the generated key

    // Bob will choose the private key b

    b = 3; // b is the chosen private key

    printf("The private key b for Bob : %lld\n\n", b);

    y = power(G, b, P); // gets the generated key

    // Generating the secret key after the exchange

    // of keys

    ka = power(y, a, P); // Secret key for Alice

    kb = power(x, b, P); // Secret key for Bob

    printf("Secret key for the Alice is : %lld\n", ka);

    printf("Secret Key for the Bob is : %lld\n", kb);

    return 0;

}

Output:

The value of P : 23
The value of G : 9

The private key a for Alice : 4
The private key b for Bob : 3

Secret key for the Alice is : 9
Secret Key for the Bob is : 9