you can encrypt your querystring value and then pass it and where you want to use, simply decrypt it. Also check these articles... how-to-encrypt-query-string-parameters-in-asp-net

http://www.codeproject.com/KB/web-security/QueryStringEncryptionNET.aspx

Basically a duplicate of: Security with QueryString values in Asp.net MVC

Bottom line - never trust input from the user especially over the web, always assume a user can and will tamper with the inputs.

You can compute and pass a hash of your parameters along with your querystring. On your landing page, compute the hash again and compare it with the query string hash, if both are not same it means the URL is tampered.

You can make functions inside a utility class like

const string secretKey = "%%YoUrSeCrEtKeY##";

     public static string CreateTamperProofUrl(string pageUrl)
    {
        try
        {
            return HttpUtility.UrlEncode(CreateDigest(pageUrl.Trim()));
        }
        catch (Exception)
        {
            throw;
        }
    }

    private static string CreateDigest(string pageUrl)
    {
        string urlToEncode = secretKey + pageUrl + secretKey;
        var hasher = new MD5CryptoServiceProvider();
        var encoder = new UTF8Encoding();

        byte[] hashedDataBytes = hasher.ComputeHash(encoder.GetBytes(urlToEncode));
        string signatureData = Convert.ToBase64String(hashedDataBytes);

        return signatureData;
    }

    public static bool IsValidDigest(string pageUrl, string receivedDigest)
    {
        if (receivedDigest == null)
        {
            return false;
        }

        string expectedDigest = CreateDigest(pageUrl);
        if (string.Compare(receivedDigest, expectedDigest) != 0)
        {
            return false;
        }
        else
            return true;
    }

On your landing page just check like this

    if (!Page.IsPostBack)
        {
            if (Request.QueryString["Digest"] != null)
            {
                // compare the digest
                string id = Request.QueryString["fid"];
                string digest = Request.QueryString["Digest"];

                if (Utility.IsValidDigest(id, digest))
                {
                    lblStatus.ForeColor = System.Drawing.Color.DarkGreen;
                    lblStatus.Text = "Valid digest received";
                }
                else
                {
                    lblStatus.ForeColor = System.Drawing.Color.Red;
                    lblStatus.Text = "Url is tampered!";
                }

            }
   }