Query string encryption for ASP.NET

• Download source code - 4.7 KB

Introduction

Using query strings to send data from the browser to the server is a widespread approach. Giving the visitor of a web application the opportunity of modifying query strings by transmitting them in clear text, is certainly a potential security threat.

Thus, I encourage developers to encrypt query strings, even if they do not contain confidential data. However, I am aware that it is still possible to alternate an encrypted query string, but with an appropriate exception handling, this is harmless.

Background

To keep this article simple, I used a contradictable encryption (DES encoding), though any cutting-edge encryption can be easily applied to the samples given.

Using the code

So, let's get down to business. The main part of the presented solution consists of a HttpModule which decrypts the query string and hence provides the page request with the ordinary unencrypted query strings:

using System;
using System.Web;
using System.Web.Configuration;

namespace SmartSoft.QueryStringEncryption
{
    /// <summary>
    /// Http module that handles encrypted query strings.
    /// </summary>
    public class CryptoQueryStringUrlRemapper : IHttpModule
    {
        #region IHttpModule Members

        /// <summary>
        /// Initialize the http module.
        /// </summary>
        /// <param name="application">Application,
        ///           that called this module.</param>
        public void Init(HttpApplication application)
        {
            // Attach the acquire request state event
            // to catch the encrypted query string
            application.AcquireRequestState += application_AcquireRequestState;
        }

        public void Dispose()
        {}
    
        #endregion

        /// <summary>
        /// Event, that is called when the application acquires the request state.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        public void application_AcquireRequestState(object sender, EventArgs e)
        {
            // Get http context from the caller.
            HttpApplication application = (HttpApplication) sender;
            HttpContext context = application.Context;

            // Check for encrypted query string
            string encryptedQueryString = context.Request.QueryString["request"];
            if (!string.IsNullOrEmpty(encryptedQueryString))
            {
                // Decrypt query strings
                string cryptoKey = WebConfigurationManager.AppSettings["CryptoKey"];
                string decryptedQueryString = 
                  CryptoQueryStringHandler.DecryptQueryStrings(encryptedQueryString, 
                                                               cryptoKey);
                context.Server.Transfer(
                  context.Request.AppRelativeCurrentExecutionFilePath + 
                  "?" + decryptedQueryString);
            }
        }
    }
}

As you might have noticed, if there is an encrypted query string for the current request, the module automatically terminates the execution of the current page and internally starts execution of a new request on the server.

The next step is to register the HttpModule in the web.config file:

<httpModules>
    <add name="CryptoQueryStringUrlRemapper" 
      type="SmartSoft.QueryStringEncryption.CryptoQueryStringUrlRemapper"/>
</httpModules>

Last but not least, do not forget to encrypt query strings before sending them back to the server:

private void PrepareSendButton()
{
    NameValueCollection queryStrings = new NameValueCollection();
    queryStrings.Add("param1", "Test1");
    queryStrings.Add("param2", "Test2");
    queryStrings.Add("param3", "Test3");

    // Encrypt query strings
    string encryptedString = CryptoQueryStringHandler.EncryptQueryStrings(
      queryStrings, WebConfigurationManager.AppSettings["CryptoKey"]);
    btnSendParams.PostBackUrl = string.Concat("~/Default.aspx?", encryptedString);
}

As outlined earlier in this article, the encryption class can be easily replaced by any other encryption class. A full running sample can be downloaded above.

Important issue

The method DecryptQueryStrings in the CryptoQueryStringHandler contains the following line :

return Encryption64.Decrypt(encryptedStrings.Replace(" ", "+"), key); 

For unknown reasons, the request replaces every '+' character in the query with an empty character.

History

• 30.04.2008 - First version (deleted -> was not possible to modify, why ever...).
• 01.05.2008 - Re-released updated article.
• 08.05.2008 - BeginRequest event in the HttpModule changed to AcquireRequestState in order to support Session data.