Decoding Encrypted Query string - asp.net

Question

I am using the method described in the following LINK and I am using the following code to encrypt:
'Page1.aspx
Protected Sub butEncrypt_Click(sender As Object, e As EventArgs) Handles butEncrypt.Click
Dim QueryString As String = "type=Int&pk=" & _primaryKey
QueryString = Tools.encryptQueryString(QueryString)
Response.Redirect(/SearchResults.aspx?Val=" & QueryString)
End Sub
and then finally de-encrypt:
'SearchResults.aspx
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If (Not IsPostBack) Then
If Not String.IsNullOrEmpty(HttpContext.Current.Request(CIAppGlobals.GlobalVar.Val)) Then
Dim qs As String = Request.QueryString(CIAppGlobals.GlobalVar.Val)
qs = Tools.decryptQueryString(qs)
Dim Values As String() = qs.Split(CChar("&"))
_imageType = String.Empty
_primaryKey = 0
For Each value As String In Values
Dim data As String() = value.Split(CChar("="))
Select Case data(0).ToUpper
Case "TYPE"
_imageType = data(1)
Case "PK"
_primaryKey = CInt(data(1))
End Select
Next
Else
_imageType = HttpContext.Current.Request("type")
_primaryKey = CInt(HttpContext.Current.Request("pk"))
End If
End If
End Sub
My question is should I being using a different method to extract the decoded query string values other than what I am doing? Thanks in advance for your constructive responses.
Solution
After looking at Darin's response I have decided to incorporate it into my project, here is my updated code:
'Page1.aspx
Protected Sub butEncrypt_Click(sender As Object, e As EventArgs) Handles butEncrypt.Click
Dim query = HttpUtility.ParseQueryString(String.Empty)
query("type") = "Int"
query("pk") = CStr(_primaryKey)
Dim QueryString As String = Tools.encryptQueryString(query.ToString())
Response.Redirect(/SearchResults.aspx?Val=" & QueryString)
End Sub
I still want to encrypt the query string because I want to prevent users from changing the Query String Values manually

Answer 1

You are incorrectly building the query string in the first place. You are using string concatenations and not properly encoding them. What if _primaryKey contains a & or = characters? You could use the ParseQueryString method to properly build a query string:
Dim query = HttpUtility.ParseQueryString(String.Empty)
query("type") = "Int"
query("pk") = _primaryKey
Dim queryString = query.ToString()
The same method could be used for parsing the decoded query string:
Dim values = HttpUtility.ParseQueryString(qs)
Dim type = query("type")
Dim primaryKey = query("pk")
' work with the type and primaryKey values
Never use string concatenations and splitting when dealing with urls. Always use the right tool for the right job.
That's as far as creating/parsing query strings is concerned. As far as encrypting/decryption the values is concerned, you haven't shown/told us anything about the Tools class that you are using so I cannot provide you with any constructive comments about it.
You know that the best encryption is to never send the actual value to the client. So you could store it in some backend storage on the server and then use an unique id in the url. This id could be used on the target page to fetch the original value. This way you don't need to be encrypting/decrypting anything.