diffie-hellman-small.js

function expmod( base, exp, mod ){
  if (exp === 0) return 1;
  if (exp % 2 === 0){
    return Math.pow( expmod( base, (exp / 2), mod), 2) % mod;
  }
  else {
    return (base * expmod( base, (exp - 1), mod)) % mod;
  }
}

var p = 21755381;
var g = 2;
var a = Math.floor((p-2)*Math.random());
var b = Math.floor((p-2)*Math.random());
//var a = 20518605; // value I stored from last time

var A = expmod(g, a, p);
var B = expmod(g, b, p);
console.log(a, " Alice private key");
console.log(A, " Alice public key");
console.log(b, " Bob  private key");
console.log(B, " Bob public key");

var alice_calculates = expmod(B, a, p);
var bob_calculates = expmod(A, b, p);

console.log(alice_calculates, bob_calculates);