A very simple and basic TOTP (Google) Authenticator Class in c#

Totp.cs

// Compatible with .Net Framework V2 and above
// Base32 code adapted from http://stackoverflow.com/questions/641361/base32-decoding
// Usage::
// From the url otpauth://totp/Google%3A{YOUR-EMAIL}?secret={YOUR-SECRET}&issuer=Google
// string base32secret = "{YOUR-SECRET}";
// string pin = Totp.GeneratePin(base32secret, 30, 6);

using System;
using System.Globalization;
using System.Security.Cryptography;

class Totp
{
    public static string GeneratePin(string base32secret, int interval, int pinLength)
    {
        if (IsBase32(base32secret) && (interval == 30 || interval == 60) && (pinLength == 6 || pinLength == 8))
            {
            byte[] secretBytes = Base32StringToBytes(base32secret);
            byte[] unixTimeBytes = BitConverter.GetBytes(((long)Math.Floor((DateTime.UtcNow - (new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc))).TotalSeconds)) / interval);
            if (BitConverter.IsLittleEndian)
                Array.Reverse(unixTimeBytes);
            byte[] hashBytes = new HMACSHA1(secretBytes).ComputeHash(unixTimeBytes);
            int Offset = hashBytes[hashBytes.Length - 1] & 0xF;
            byte[] bytes = new byte[4];
            Buffer.BlockCopy(hashBytes, Offset, bytes, 0, 4);
            if (BitConverter.IsLittleEndian)
                Array.Reverse(bytes);
            int integer = BitConverter.ToInt32(bytes, 0);
            int truncated = integer & 0x7FFFFFFF;
            int pin = truncated % 1000000;
            return pin.ToString(CultureInfo.InvariantCulture).PadLeft(pinLength, '0');
        }
        return String.Empty;
    }

    private static byte[] Base32StringToBytes(string input)
    {
        input = input.TrimEnd('=');
        int byteCount = input.Length * 5 / 8;
        byte[] returnBytes = new byte[byteCount];
        byte curByte = 0;
        byte bitsRemaining = 8;
        int mask = 0;
        int arrayIndex = 0;
        foreach (char c in input)
        {
            int cValue = CharToValue(c);
            if (bitsRemaining > 5)
            {
                mask = cValue << (bitsRemaining - 5);
                curByte = (byte)(curByte | mask);
                bitsRemaining -= 5;
            }
            else
            {
                mask = cValue >> (5 - bitsRemaining);
                curByte = (byte)(curByte | mask);
                returnBytes[arrayIndex++] = curByte;
                curByte = (byte)(cValue << (3 + bitsRemaining));
                bitsRemaining += 3;
            }
        }
        if (arrayIndex != byteCount)
            returnBytes[arrayIndex] = curByte;
        return returnBytes;
    }
    
    private static int CharToValue(char c)
    {
        int value = (int)c;
        if (value < 91 && value > 64)
            return value - 65;
        if (value < 56 && value > 49)
            return value - 24;
        if (value < 123 && value > 96)
            return value - 97;
        throw new Exception();
    }
    
    public static bool IsBase32(string b32)
    {
        Regex regex = new Regex(@"^[A-Z2-7]+=*$");
        Match match = regex.Match(b32);
        bool b = b32.Length % 8 == 0 && match.Success;
        return b32.Length % 8 == 0 && match.Success;
    }
}