Two Factor Authentication for the masses

2fa_1.cs

DateTime baseTime  = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)
long counter = GetCurrentTCounter();         
private long GetCurrentTCounter()         
{ 
  long seconds = (long)DateTime.UtcNow.Subtract(baseTime).TotalSeconds;          
  
  return seconds / interval; 
}

2fa_2.cs

private byte[] GetHMAC(long counter, byte[] Key)
{

  byte[] counterBytes = BitConverter.GetBytes(counter);

  if (BitConverter.IsLittleEndian)
  {
    Array.Reverse(counterBytes);
  }

  using (HMACSHA1 mac = new HMACSHA1(Key))
  {
    return mac.ComputeHash(counterBytes);
  }

}

2fa_3.cs

int mod = (int)Math.Pow(10, 6);

private int GetOTPValue(byte[] hash, int mod)
{
  int offset = hash.Last() & 0x0f;

  int binCode = (hash[offset] & 0x7f) << 24 |
      (hash[offset + 1] & 0xff) << 16 |
      (hash[offset + 2] & 0xff) << 8 |
      (hash[offset + 3] & 0xff);

  int otp = binCode % mod;

  return otp;

}

2fa_4.cs

otp.ToString().PadLeft(6, '0');