using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.Cryptography;
using System.Text;

namespace Chop.Game.Encryption
{
    public class DiffieHellman
    {
        #region Static vars
        public readonly int BIT_LENGTH = 30;
        #endregion

        #region Variables
        public BigInteger Prime { get; private set; }
        public BigInteger Generator { get; private set; }

        public BigInteger PrivateKey { get; private set; }
        public BigInteger PublicKey { get; private set; }

        public BigInteger PublicClientKey { get; private set; }

        public BigInteger SharedKey { get; private set; }
        #endregion

        #region Constructor
        public DiffieHellman()
        {
            this.InitDH();
        }

        public DiffieHellman(int b)
        {
            this.BIT_LENGTH = b;

            this.InitDH();
        }

        private void InitDH()
        {
            this.PublicKey = 0;
            while (this.PublicKey == 0)
            {
                this.Prime = BigInteger.genPseudoPrime(BIT_LENGTH, 10, new Random());
                this.Generator = BigInteger.genPseudoPrime(BIT_LENGTH, 10, new Random());

                this.PrivateKey = new BigInteger(GenerateRandomHexString(BIT_LENGTH), 16);

                if (this.Generator > this.Prime)
                {
                    BigInteger temp = this.Prime;
                    this.Prime = this.Generator;
                    this.Generator = temp;
                }

                this.PublicKey = this.Generator.modPow(this.PrivateKey, this.Prime);
            }
        }

        public DiffieHellman(BigInteger prime, BigInteger generator)
        {
            this.Prime = prime;
            this.Generator = generator;

            this.PrivateKey = new BigInteger(GenerateRandomHexString(BIT_LENGTH), 16);

            if (this.Generator > this.Prime)
            {
                BigInteger temp = this.Prime;
                this.Prime = this.Generator;
                this.Generator = temp;
            }

            this.PublicKey = this.Generator.modPow(this.PrivateKey, this.Prime);
        }
        #endregion

        #region SharedKey
        public void GenerateSharedKey(string ckey)
        {
            this.PublicClientKey = new BigInteger(ckey, 10);

            this.SharedKey = this.PublicClientKey.modPow(this.PrivateKey, this.Prime);
        }
        #endregion

        #region RandomHexString
        public static string GenerateRandomHexString(int len)
        {
            var rng = new RNGCryptoServiceProvider();
            byte[] bytes = new byte[len / 2];
            rng.GetBytes(bytes);

            BigInteger result = new BigInteger(bytes);

            if (result < 0)
            {
                result *= -1;
            }

            return result.ToString(16);
        }
        #endregion
    }
}