Private Sub Test()
 
        Dim data As String = "üöäÜÖÄß!§$%&/()=?`hilfe"
        Dim pw As String = "üöäßÜÖÄ*+'#;:"
 
        Dim ed As String = verschlüsseln(data, pw)
 
        Dim dd As String = entschlüsseln(ed, pw)
 
        If dd <> data Then
            MsgBox("Sch.., klappt nicht")
        Else
            MsgBox("Toll, klappt ganz gut")
        End If
 
    End Sub

Imports System.Security.Cryptography
Imports System.IO
Imports System.Text
 
Module Module1
 
    Public Function verschlüsseln(ByVal daten As String, ByVal passwort As _
      String) As String
 
        Dim rd As New RijndaelManaged
 
        Dim md5 As New MD5CryptoServiceProvider
        Dim key() As Byte = md5.ComputeHash(Encoding.UTF8.GetBytes(passwort))
 
        md5.Clear()
        rd.Key = key
        rd.GenerateIV()
 
        Dim iv() As Byte = rd.IV
        Dim ms As New MemoryStream
 
        ms.Write(iv, 0, iv.Length)
 
        Dim cs As New CryptoStream(ms, rd.CreateEncryptor, _
          CryptoStreamMode.Write)
        Dim data() As Byte = System.Text.Encoding.UTF8.GetBytes(daten)
 
        cs.Write(data, 0, data.Length)
        cs.FlushFinalBlock()
 
        Dim encdata() As Byte = ms.ToArray()
 
        cs.Close()
        rd.Clear()
        Return Convert.ToBase64String(encdata)
 
    End Function
 
    Public Function entschlüsseln(ByVal daten As String, ByVal passwort As _
      String) As String
        Dim rd As New RijndaelManaged
        Dim rijndaelIvLength As Integer = 16
        Dim md5 As New MD5CryptoServiceProvider
        Dim key() As Byte = md5.ComputeHash(Encoding.UTF8.GetBytes(passwort))
 
        md5.Clear()
 
        Dim encdata() As Byte = Convert.FromBase64String(daten)
        Dim ms As New MemoryStream(encdata)
        Dim iv(15) As Byte
 
        ms.Read(iv, 0, rijndaelIvLength)
        rd.IV = iv
        rd.Key = key
 
        Dim cs As New CryptoStream(ms, rd.CreateDecryptor, _
          CryptoStreamMode.Read)
 
        Dim data(ms.Length - rijndaelIvLength) As Byte
        Dim i As Integer = cs.Read(data, 0, data.Length)
 
        cs.Close()
        rd.Clear()
 
        Return System.Text.Encoding.UTF8.GetString(data, 0, i)
 
    End Function
 
End Module

Those people who think they know everything are a great annoyance to those of us who do - Isaac Asimov