using System;
using System.Security.Cryptography;
using System.Text;

namespace atakanozbancom.Models.classes
{
    public static class PasswordHasher
    {
        private const string Prefix = "pbkdf2";
        private const int DefaultIterations = 100000;
        private const int SaltSize = 16;
        private const int HashSize = 32;

        public static string Hash(string password)
        {
            if (password == null)
                throw new ArgumentNullException("password");

            var salt = new byte[SaltSize];
            using (var rng = RandomNumberGenerator.Create())
            {
                rng.GetBytes(salt);
            }

            var hash = Pbkdf2(password, salt, DefaultIterations, HashSize);
            return string.Format(
                "{0}${1}${2}${3}",
                Prefix,
                DefaultIterations,
                Convert.ToBase64String(salt),
                Convert.ToBase64String(hash));
        }

        public static bool IsHashed(string stored)
        {
            return !string.IsNullOrWhiteSpace(stored)
                && stored.StartsWith(Prefix + "$", StringComparison.Ordinal);
        }

        public static bool Verify(string password, string stored)
        {
            if (string.IsNullOrEmpty(password) || string.IsNullOrEmpty(stored))
                return false;

            if (!IsHashed(stored))
                return FixedTimeEquals(password, stored);

            var parts = stored.Split('$');
            if (parts.Length != 4 || !string.Equals(parts[0], Prefix, StringComparison.Ordinal))
                return false;

            int iterations;
            if (!int.TryParse(parts[1], out iterations) || iterations < 1000)
                return false;

            byte[] salt;
            byte[] expected;
            try
            {
                salt = Convert.FromBase64String(parts[2]);
                expected = Convert.FromBase64String(parts[3]);
            }
            catch
            {
                return false;
            }

            var actual = Pbkdf2(password, salt, iterations, expected.Length);
            return FixedTimeEquals(actual, expected);
        }

        private static byte[] Pbkdf2(string password, byte[] salt, int iterations, int length)
        {
            using (var derive = new Rfc2898DeriveBytes(password, salt, iterations, HashAlgorithmName.SHA256))
            {
                return derive.GetBytes(length);
            }
        }

        private static bool FixedTimeEquals(byte[] a, byte[] b)
        {
            if (a == null || b == null || a.Length != b.Length)
                return false;

            var diff = 0;
            for (var i = 0; i < a.Length; i++)
                diff |= a[i] ^ b[i];

            return diff == 0;
        }

        private static bool FixedTimeEquals(string a, string b)
        {
            if (a == null || b == null || a.Length != b.Length)
                return false;

            var diff = 0;
            for (var i = 0; i < a.Length; i++)
                diff |= a[i] ^ b[i];

            return diff == 0;
        }
    }
}