Public Shared Function PotMod(a As Integer, p As Integer, n As Integer) As _
      Integer ' Berechnet (a^p) mod n
        Dim bits As List(Of Boolean) = GetBitsBackwards(p)
        Dim m As Integer = 1
        For i As Integer = bits.Count - 1 To 0 Step -1
            m = m * m Mod n
            If bits(i) Then m = m * a Mod n
        Next
        Return m
    End Function
 
    Private Shared Function GetBitsBackwards(a As Integer) As List(Of Boolean)
        ' gibt eine Bitliste zurück (rückwärts)
        Dim l As New List(Of Boolean)
        While a > 0
            l.Add(a Mod 2 = 1)
            a >>= 1
        End While
        Return l
    End Function