Visual Basic

Hi, i am working in VB 2010 an a one-time pad software.
The problem is the following. I have a txt file with many continous characters like this "KHKJHDKJDHAKHDAKHDAKHAKDHADHAKJDHASJDHA", i need to read a variable number of this characters and after replace it with a same number of dots (Ex. "zxsd" replaced with "....").
Now, with this code i can read the characters from specific position:

Using stream = File.OpenRead("c:\key.txt")
stream.Seek(v, SeekOrigin.Begin) ' v = startup position for read in file key.txt 
Dim b = New Byte(a - 1) {} ' a= lenght of text in key.txt to be taken)
stream.Read(b, 0, a) 
Dim str = Encoding.ASCII.GetString(b)
txtPad.Text = str
End Using

But how i can do for replace text extracted with dots without delete any line?

Dim WithEvents Web  As SHDocVw.InternetExplorer		
Private Sub Form_Load()		
	Set Web = New SHDocVw.InternetExplorer	
	Web.Visible = True	
	Web.Navigate "http://incometaxindiaefiling.gov.in/e-Filing/UserLogin/LoginHome.html"	
End Sub		
Private Sub Web_DocumentComplete(ByVal pDisp As Object, URL As Variant)		
On Error GoTo aaa		
	msgbox "   URL: " & URL	
	if url = "http://incometaxindiaefiling.gov.in/e-Filing/UserLogin/LoginHome.html" then	
	Web.Document.getElementById("Login_userName").Value = "abcde1111h"	
	Web.Document.getElementById("Login_userName").onchange	
	Web.Document.getElementById("Login_password").Value = "123456789"	
	Web.Document.getElementById("dateField").Value = "15/09/1954"	
	end if		
Exit Sub		
aaa:		
MsgBox Err.Description & "   URL: " & URL		
End Sub	
Private Sub Web_OnQuit()		
	MsgBox "OnQuit fired"	
End Sub

Private Sub ttlLate()

        Dim datardrr As MySqlDataReader
        Try
            conn.Open()
            strSql = "SELECT dates between '" & dtpstart.Text & "' and '" & dtpend.Text & "', SUM(late) as SumTotalLate from tbl_attendance where employeeNumber='" & textempNoSrch.Text & "'"
           
            cmd = New MySqlCommand(strSql, conn)
            datardrr = cmd.ExecuteReader

            TextLateDeduct.Text = datardrr("SumTotalLate").ToString
            datardrr.Close()
           
            conn.Close()
        Catch ex As MySqlException
            MessageBox.Show(ex.Message)
        Finally
            conn.Dispose()
        End Try
    End Sub