Шифр Цезаря. Не шифровать знаки препинания и другие символы - Visual Basic .NET

Public Class Form1
    Const alfavit As String = "АаБбВвГгДдЕеЁёЖжЗзИиЙйКкЛлМмНнОоПпРрСсТтУуФфХхЦцЧчШшЩщЪъЫыЬьЭэЮюЯяAaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz"
    Const sdvig As Integer = 3
    Const osdvig As Integer = -3
 
    Private Function cs(ByVal s As String, ByVal sdvig As Integer) As String
        Dim i As Integer
        cs = ""
        For i = 1 To Len(s)
            cs &= cc(Mid(s, i, 1), sdvig)
        Next
    End Function
    Private Function ocs(ByVal s As String, ByVal sdvig As Integer) As String
        Dim i As Integer
        ocs = ""
        For i = 1 To Len(s)
            ocs &= cc(Mid(s, i, 1), osdvig)
        Next
    End Function
 
    Private Function cc(ByVal c As Char, ByVal sdvig As Integer) As String
        cc = ""
        If InStr(alfavit, c) <> 0 Then Return Mid(alfavit, ((InStr(alfavit, c) - 1 + 2 * sdvig + Len(alfavit)) Mod Len(alfavit) + 1), 1)
    End Function
 
    Private Sub ОткрытьToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles ОткрытьToolStripMenuItem.Click
 
        OpenFileDialog1.Filter = "Текстовый документ (*.txt)|*.txt|Документ Microsoft Word 97–2003 (*.doc)|*.doc|All files (*.*)|*.*"
 
        If OpenFileDialog1.ShowDialog() = System.Windows.Forms.DialogResult.OK Then
 
            TextBox1.Text = My.Computer.FileSystem.ReadAllText(OpenFileDialog1.FileName, System.Text.Encoding.Default)
        End If
 
    End Sub
 
    Private Sub ЗашифроватьToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles ЗашифроватьToolStripMenuItem.Click
 
        TextBox1.Text = cs(TextBox1.Text, sdvig)
    End Sub
 
    Private Sub СохранитьКакToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles СохранитьКакToolStripMenuItem.Click
        Dim myStream
        Dim saveFileDialog1 As New SaveFileDialog()
 
        saveFileDialog1.Filter = "txt files (*.txt)|*.txt|Документ Microsoft Word 97–2003 (*.doc)|*.doc|All files (*.*)|*.*"
        saveFileDialog1.FilterIndex = 2
        saveFileDialog1.RestoreDirectory = True
 
        If saveFileDialog1.ShowDialog() = DialogResult.OK Then
            myStream = saveFileDialog1.OpenFile()
            If (myStream IsNot Nothing) Then
                myStream.Close()
            End If
        End If
    End Sub
 
    Private Sub РасшифроватьToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles РасшифроватьToolStripMenuItem.Click
        TextBox1.Text = ocs(TextBox1.Text, sdvig)
    End Sub
 
    Private Sub ЗакрытьToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles ЗакрытьToolStripMenuItem.Click
        End
    End Sub
End Class