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Query string encryption for ASP.NET

, 7 May 2008
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Clear text query strings are a potential security threat for your web application. Thus, query strings should always be encrypted.

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
{
    /// <span class="code-SummaryComment"><summary></span>
    /// Http module that handles encrypted query strings.
    /// <span class="code-SummaryComment"></summary></span>
    public class CryptoQueryStringUrlRemapper : IHttpModule
    {
        #region IHttpModule Members

        /// <span class="code-SummaryComment"><summary></span>
        /// Initialize the http module.
        /// <span class="code-SummaryComment"></summary></span>
        /// <span class="code-SummaryComment"><param name="application">Application,</span>
        ///           that called this module.<span class="code-SummaryComment"></param></span>
        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

        /// <span class="code-SummaryComment"><summary></span>
        /// Event, that is called when the application acquires the request state.
        /// <span class="code-SummaryComment"></summary></span>
        /// <span class="code-SummaryComment"><param name="sender"></param></span>
        /// <span class="code-SummaryComment"><param name="e"></param></span>
        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.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Michael Ulmann
Architect Helvetic Solutions
Australia Australia
MCAD, MCPD Web Developer 2.0, MCPD Enterprise Developer 3.5
My company: www.helveticsolutions.com
Hopp Schwiiz Smile | :)

Comments and Discussions

 
GeneralThere is a major problem with this approach PinmemberOrionDR20-Mar-09 5:44 

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