help on visual basic 2008

[gladys_ny]

I need to create a project that will encrypt and decrypt the alphabet. if there is a space or unknown character the program should just leave it like that or blank .

"a" = 0
"b" = 1
"c"= 2 ...

"z" = 25
if the sum is greater than 26 then continues to
"a" = 26
"b" = 27
"c" = 28

I have tried to do it but it's not working my teacher said that we could use the Mod i don't know how to use
Please help


Option Explicit On
Option Strict On
Option Infer Off

Public Class Form1

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ChangeButton.Click

Dim plainlist As String
Dim key As String
Dim firstnumb As String
Dim secnum As String
Dim encryptedms As String
Dim newvalue As Integer
Dim alphavalue2 As Integer
Dim alphabet As String = "abcdefghijklmnopqrstuvwxyz"
Dim alphaValue As Integer
Dim counter As Integer = 0
Dim counter2 As Integer = 0



plainlist = plainTextBox.Text
key = keyTextBox.Text

counter = 0
counter2 = 0


Do While counter < plainlist.Length


firstnumb = plainlist.Substring(counter, 1)
alphaValue = alphabet.IndexOf(firstnumb)
MessageBox.Show(CStr(alphaValue))

secnum = key.Substring(counter2, 1)
alphavalue2 = alphabet.IndexOf(secnum)
MessageBox.Show(CStr(alphavalue2))



newvalue = alphabet.IndexOf(firstnumb) + alphabet.IndexOf(secnum)

MessageBox.Show(CStr(newvalue))


If alphaValue = -1 Then
encryptedms = firstnumb


End If

If alphaValue > 25 Then

alphaValue = alphaValue - 26

End If



If alphaValue = -1 Then

encryptedms = plainlist.Substring(counter, 1)

Else


encryptedms = alphabet.Substring(newvalue, 1)

End If

counter = counter + 1
counter2 = counter2 + 1


encryptLabel.Text = encryptLabel.Text & encryptedms

Loop

[EINSTEIN_007]

There is a microsoft article that describes how to use the cryptography classes that are provided by the Microsoft .NET Framework. You can use the cryptography classes to encrypt a text file to an unreadable state. Then, you can decrypt that text file back to its original format. For more information you can check this website.