Implementing Two Factor Authentication in ASP.NET MVC with Google Authenticator

using System;
using System.Runtime.Caching;

namespace TwoFactor
{
    public static class TimeBasedOneTimePassword
    {
        public static readonly DateTime UNIX_EPOCH = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

        private static MemoryCache _cache;

        static TimeBasedOneTimePassword()
        {
            _cache = new MemoryCache("TimeBasedOneTimePassword");
        }

        public static string GetPassword(string secret)
        {
            return GetPassword(secret, GetCurrentCounter());
        }

        public static string GetPassword(string secret, DateTime epoch, int timeStep)
        {
            long counter = GetCurrentCounter(DateTime.UtcNow, epoch, timeStep);

            return GetPassword(secret, counter);
        }

        public static string GetPassword(string secret, DateTime now, DateTime epoch, int timeStep, int digits)
        {
            long counter = GetCurrentCounter(now, epoch, timeStep);

            return GetPassword(secret, counter, digits);
        }

        private static string GetPassword(string secret, long counter, int digits = 6)
        {
            return HashedOneTimePassword.GeneratePassword(secret, counter, digits);
        }

        private static long GetCurrentCounter()
        {
            return GetCurrentCounter(DateTime.UtcNow, UNIX_EPOCH, 30);
        }

        private static long GetCurrentCounter(DateTime now, DateTime epoch, int timeStep)
        {
            return (long)(now - epoch).TotalSeconds / timeStep;
        }

        public static bool IsValid(string secret, string password, int checkAdjacentIntervals = 1)
        {
            // Keeping a cache of the secret/password combinations that have been requested allows us to
            // make this a real one time use system. Once a secret/password combination has been tested,
            // it cannot be tested again until after it is no longer valid.
            // See http://tools.ietf.org/html/rfc6238#section-5.2 for more info.
            string cache_key = string.Format("{0}_{1}", secret, password);

            if (_cache.Contains(cache_key))
            {
                throw new OneTimePasswordException("You cannot use the same secret/iterationNumber combination more than once.");
            }

            _cache.Add(cache_key, cache_key, new CacheItemPolicy { SlidingExpiration = TimeSpan.FromMinutes(2) });

            if (password == GetPassword(secret))
                return true;

            for (int i = 1; i <= checkAdjacentIntervals; i++)
            {
                if (password == GetPassword(secret, GetCurrentCounter() + i))
                    return true;

                if (password == GetPassword(secret, GetCurrentCounter() - i))
                    return true;
            }

            return false;
        }
    }
}