Request.QueryString problem

I have a gridview with hyper-link function. When user click the hyper-link, system will encrypt the id and redirect to next page. 
Example: //localhost/TestSystem/details.aspx?codeid=LnZF9TpoCdgdY+enqSwrbQ==
when details page is loaded, it will auto decrypt the id.
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load paautoid = mdlDecrypt(Request.QueryString("paautoid").ToString) End Sub Now, the problem is the Request.QueryString("paautoid") pass the id is "LnZF9TpoCdgdY enqSwrbQ==" instead of  "LnZF9TpoCdgdY+enqSwrbQ==". The plus symbol is missing and when encrypt id have + symbol then will have problem to decrypt.

Protected Function mdlEncrypt(ByVal clearText As String) As String Dim EncryptionKey As String = "HPPRMMVP" Dim clea ...