Private Key() As Byte = UTF8.GetBytes("<--fester string-->")
    Private KeyHash() As Byte = Nothing

    Public Function DecryptStream(ByVal UsedStream As Stream) As CryptoStream
        Dim cmd5Provider As New MD5CryptoServiceProvider
        Dim caesProvider As New RijndaelManaged
 
        Dim caesStream As CryptoStream
 
        KeyHash = cmd5Provider.ComputeHash(Key)
 
        With caesProvider
            .Key = KeyHash
            .GenerateIV()
            caesStream = New CryptoStream(UsedStream, .CreateDecryptor(.Key, _
              .IV), CryptoStreamMode.Read)
        End With
 
        Return caesStream
    End Function

    Public Function EncryptStream(ByVal UsedStream As Stream) As CryptoStream
        Dim cmd5Provider As New MD5CryptoServiceProvider
        Dim caesProvider As New RijndaelManaged
        Dim caesStream As CryptoStream
 
        KeyHash = cmd5Provider.ComputeHash(Key)
 
        With caesProvider
            .Key = KeyHash
            .GenerateIV()
            caesStream = New CryptoStream(UsedStream, .CreateEncryptor(.Key, _
              .IV), CryptoStreamMode.Write)
        End With
 
        Return caesStream
    End Function

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        Dim fStream As FileStream = File.Open("C:\img.jpg", FileMode.Create)
        Dim cStream As CryptoStream
 
        cStream = EncryptStream(fStream)
 
        Dim grabber As New ScreenGrabber(New Rectangle(New Point(0, 0), _
          PictureBox1.Size))
 
        PictureBox1.Image = grabber.GetScreenshot
 
        Dim info As ImageCodecInfo() = ImageCodecInfo.GetImageEncoders()
        Dim Params As EncoderParameters = New EncoderParameters(1)
        Params.Param(0) = New EncoderParameter(Encoder.Quality, 100L)
 
        PictureBox1.Image.Save(cStream, info(1), Params)
 
        cStream.Close()
        fStream.Close()
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button2.Click
        Dim fStream As FileStream = File.Open("C:\img.jpg", FileMode.Open)
        Dim cStream As CryptoStream
 
        cStream = DecryptStream(fStream)
 
        Try
            PictureBox1.Image = Image.FromStream(cStream)
        Catch ex As Exception
            MsgBox(ex.Message) ' Gibt "Ungültiger Parameter"
        End Try
 
        cStream.Close()
        fStream.Close()
    End Sub