Imports System.Security     ' SecureString
Imports System.Runtime.InteropServices ' dllImport
Imports System.Security.Principal ' Username
Imports System.Environment ' NewLine


<DllImport("advapi32.dll", SetLastError:=True, _
  CharSet:=CharSet.Unicode)>
  Shared Function LogonUser(ByVal username As String, _
    ByVal domain As String, ByVal password As IntPtr, _
    ByVal logonType As Integer, ByVal logonProvider As Integer, _
    ByRef token As IntPtr) As Boolean
  End Function

  Private loginName As String

   'Ein SecureString verschlüsselt den Passwortstring
   'und löscht ihn aus dem Speicher ...
  Private loginPasswordSecure As New Security.SecureString

  Private Function LoginValid(ByVal name As String) As Boolean

    Dim tokenHandle As IntPtr = IntPtr.Zero
    Const LOGON32_PROVIDER_DEFAULT As Integer = 0
    Const LOGON32_LOGON_INTERACTIVE As Integer = 2

    Try
      ' Zeiger auf den verschlüsselten Zeichenkettenstring ...
      tokenHandle = _
         Marshal.SecureStringToGlobalAllocUnicode(_
            loginPasswordSecure)
      If (LogonUser(name, "",
        Marshal.SecureStringToGlobalAllocUnicode(_
          loginPasswordSecure), LOGON32_LOGON_INTERACTIVE, _
          LOGON32_PROVIDER_DEFAULT, tokenHandle)) Then
         ' LoginName und Passwort stimmen überein
        Return True
      Else
        tbPassword.Text = String.Empty
         ' Mit Stopwatch die verstrichene Zeit exakt messen
        Dim sw As New Stopwatch()
        Dim count As Integer
        Console.Beep()
        Do While (count < 2)
          sw.Start()
          If sw.ElapsedMilliseconds > 100 Then
             Console.Beep()
             count = count + 1
          End If
        Loop

      End If
      Finally

      If tokenHandle <> IntPtr.Zero Then
        ' Der PasswordSecureString wird nicht mehr benötigt.
        ' Aus Sicherheitsgründen wird der SecureString ...
        ' Freigabe des Zeichenfolgezeigers
        Marshal.ZeroFreeGlobalAllocUnicode(tokenHandle)
      End If
    End Try

    Return False
  End Function