Dim dlg As New OpenFileDialog()
With dlg
   .Title = "Datei entschlüsseln..."
   .InitialDirectory = Environment.SpecialFolder.MyDocuments
   .Filter = "Verschlüsselte Dateien|*.sef|Alle Dateien|*.*"
End With
 
If dlg.ShowDialog() = DialogResult.OK Then
   Dim rm As New RijndaelManaged()
   Dim iv() As Byte
   Dim key() As Byte
 
   LoadKeys(iv, key)			
 
   Dim fsIn As New FileStream(dlg.FileName, FileMode.Open, _
   FileAccess.Read, FileShare.Read)
   Dim fsOut As New MemoryStream()
 
   Dim crypter As ICryptoTransform = rm.CreateDecryptor(iv, key)
   Dim cs As New CryptoStream(fsIn, crypter, CryptoStreamMode.Read)
   Dim buffer(4096) As Byte
   Dim read As Integer
 
   Do
      read = cs.Read(buffer, 0, 4096)
      fsOut.Write(buffer, 0, read)
   Loop While read > 0			
 
   Me.txtOutput.Text = Encoding.Unicode.GetString(fsOut.GetBuffer)
 
   fsOut.Close()
   cs.Close()
   fsIn.Close()
   rm.Clear()
End If
 
dlg.Dispose()

Private Sub SaveKeys(ByVal iv As Byte(), ByVal key As Byte())
   Dim fileIV As String = Application.StartupPath & "\iv.key"
   Dim fileKey As String = Application.StartupPath & "\key.key"
 
   My.Computer.FileSystem.WriteAllBytes(fileIV, iv, False)
   My.Computer.FileSystem.WriteAllBytes(fileKey, key, False)
End Sub
 
Private Sub LoadKeys(ByRef iv As Byte(), ByRef key As Byte())
   Dim fileIV As String = Application.StartupPath & "\iv.key"
   Dim fileKey As String = Application.StartupPath & "\key.key"
 
   iv = My.Computer.FileSystem.ReadAllBytes(fileIV)
   key = My.Computer.FileSystem.ReadAllBytes(filekey)
End Sub

Ralf