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HttpSecureCookie, A Way to Encrypt Cookies with ASP.NET 2.0

, 3 Apr 2006 Ms-PL
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Discussing how to encode and tamper-proof text and cookies using the MachineKey, by using reflection.

Introduction

I really have some good laughs when I tamper with cookies on my machine and watch the results when it is submitted back to the site. On the other hand, I don’t want any one to do the same to the cookies that I make!

Cookies, most of the times, shouldn’t be in plain text, at least, they should be tamper-proof! Revealing the content of your cookies might give curious and malicious people an idea about your application’s architecture, and that might help hacking it.

ASP.NET encodes and hashes its authorization ticket, making it secure and tamper-proof. However, the methods used to secure authorization cookies are inaccessible from outside the .NET framework libraries, so you can’t protect your own cookie using these methods; you need to protect it yourself using your own encryption key, encoding and hashing algorithms. HttpSecureCookie works around this by accessing the same methods ASP.NET uses for cookie authorization.

Of course, you shouldn’t save valuable information in your cookies, but if you have to, then this library is at your disposal.

Background

Before you start using this code, if you do not know what MachineKey is, I highly recommend checking this MSDN article: How To: Configure MachineKey in ASP.NET 2.0.

ASP.NET uses the System.Web.Security.CookieProtectionHelper internal class to decode and encode the content of a cookie before submitting it to the client. This class is based on the MachineKey. I wonder why Microsoft kept this class internal!?

To be able to access this internal class, I had to use reflection to be able to access the Decode and Encode methods of CookieProtectionHelper.

Eric Newton has a similar and good article on CP: Encrypting cookies to prevent tampering. However, that code is made for .NET 1.1 and it doesn't work with .NET 2.0 (but it does with some modifications); moreover, its resulting cipher text is in binary format versus being in encrypted format, and I don't know if this is a security risk. Also, I am accessing a higher level class System.Web.Security.CookieProtectionHelper than the one used by that article, System.Web.Configuration.MachineKey, to obtain the cryptography service, and that saved me time by not writing some low level code.

There is also another available method for encoding cookies, by using the FormsAuthenticationTicket and FormsAuthentication.Encrypt; for more information, check the section "Creating the Forms Authentication Cookie" on Explained: Forms Authentication in ASP.NET 2.0. However, I believe, the method mentioned in this article is more flexible.

Obtaining Reference to the CookieProtectionHelper Class via Reflection

To be able to access System.Web.Security.CookieProtectionHelper, I had to create a wrapper class CookieProtectionHelperWrapper which uses reflection to obtain a reference to the underlying methods and exposes the same methods as of the original class:

public static class CookieProtectionHelperWrapper {

    private static MethodInfo _encode;
    private static MethodInfo _decode;

    static CookieProtectionHelperWrapper() {
        // obtaining a reference to System.Web assembly
        Assembly systemWeb = typeof(HttpContext).Assembly;
        if (systemWeb == null) {
            throw new InvalidOperationException(
                "Unable to load System.Web.");
        }
        // obtaining a reference to the internal class CookieProtectionHelper
        Type cookieProtectionHelper = systemWeb.GetType(
                "System.Web.Security.CookieProtectionHelper");
        if (cookieProtectionHelper == null) {
            throw new InvalidOperationException(
                "Unable to get the internal class CookieProtectionHelper.");
        }
        // obtaining references to the methods of CookieProtectionHelper class
        _encode = cookieProtectionHelper.GetMethod(
                "Encode", BindingFlags.NonPublic | BindingFlags.Static);
        _decode = cookieProtectionHelper.GetMethod(
                "Decode", BindingFlags.NonPublic | BindingFlags.Static);

        if (_encode == null || _decode == null) {
            throw new InvalidOperationException(
                "Unable to get the methods to invoke.");
        }
    }

    public static string Encode(CookieProtection cookieProtection, 
                                byte[] buf, int count) {
        return (string)_encode.Invoke(null, 
                new object[] { cookieProtection, buf, count });
    }

    public static byte[] Decode(CookieProtection cookieProtection, 
                                string data) {
        return (byte[])_decode.Invoke(null, 
                new object[] { cookieProtection, data });
    }

}

MachineKeyCryptography: A Cryptography Class Based on MachineKey

MachineKeyCryptography is a static class that provides text ciphering and tamper-proofing services, it provides higher level access to CookieProtectionHelperWrapper. So, if you want to cipher any text based on the machine key, this class is the right one to use.

public static string Encode(string text, CookieProtection cookieProtection) {
    if (string.IsNullOrEmpty(text) || cookieProtection == CookieProtection.None) {
        return text;
    }
    byte[] buf = Encoding.UTF8.GetBytes(text);
    return CookieProtectionHelperWrapper.Encode(cookieProtection, buf, buf.Length); 
}

public static string Decode(string text, CookieProtection cookieProtection) {
    if (string.IsNullOrEmpty(text)) {
        return text;
    }
    byte[] buf;
    try {
        buf = CookieProtectionHelperWrapper.Decode(cookieProtection, text);
    }
    catch(Exception ex) {
        throw new InvalidCypherTextException(
            "Unable to decode the text", ex.InnerException);
    }
    if (buf == null || buf.Length == 0) {
        throw new InvalidCypherTextException(
            "Unable to decode the text");
    }
    return Encoding.UTF8.GetString(buf, 0, buf.Length);
}

HttpSecureCookie Class

This static class will handle the service of securing the content of a cookie. Also, it provides a service to clone a cookie. This class uses MachineKeyCryptography internally to provide crypting services:

public static class HttpSecureCookie {

    public static HttpCookie Encode(HttpCookie cookie) {
        return Encode(cookie, CookieProtection.All);
    }

    public static HttpCookie Encode(HttpCookie cookie, 
                  CookieProtection cookieProtection) {
        HttpCookie encodedCookie = CloneCookie(cookie);
        encodedCookie.Value = 
          MachineKeyCryptography.Encode(cookie.Value, cookieProtection);
        return encodedCookie;
    }

    public static HttpCookie Decode(HttpCookie cookie) {
        return Decode(cookie, CookieProtection.All);
    }

    public static HttpCookie Decode(HttpCookie cookie, 
                  CookieProtection cookieProtection) {
        HttpCookie decodedCookie = CloneCookie(cookie);
        decodedCookie.Value = 
          MachineKeyCryptography.Decode(cookie.Value, cookieProtection);
        return decodedCookie;
    }

    public static HttpCookie CloneCookie(HttpCookie cookie) {
        HttpCookie clonedCookie = new HttpCookie(cookie.Name, cookie.Value);
        clonedCookie.Domain = cookie.Domain;
        clonedCookie.Expires = cookie.Expires;
        clonedCookie.HttpOnly = cookie.HttpOnly;
        clonedCookie.Path = cookie.Path;
        clonedCookie.Secure = cookie.Secure;

        return clonedCookie;
    }
}

Using the Code

Using HttpSecureCookie is easy; for a complete demo, please check the sample application. To encode a cookie:

HttpCookie cookie = new HttpCookie("UserName", "Terminator");
cookie.Expires = DateTime.Now.AddDays(1);
HttpCookie encodedCookie = HttpSecureCookie.Encode(cookie);
Response.Cookies.Add(encodedCookie);

To decode an encoded cookie:

HttpCookie cookie = Request.Cookies["UserName"];
lblDisplayBefore.Text = cookie.Value;
HttpCookie decodedCookie = HttpSecureCookie.Decode(cookie);

To use HttpSecureCookie on a web farm, you need to set the correct MachineKey configuration in Web.Config. For more information, check the "Web Farm Deployment Considerations" section on How To: Configure MachineKey in ASP.NET 2.0. To generate a machine key, please check How to create keys by using Visual C# .NET for use in Forms authentication.

Limitations

This library uses reflection, so it might break with the next version of .NET. Also, it doesn't work with .NET 1.1.

Reflection might have some performance implication; however, I used an assembly that is already loaded, "System.Web.dll", and I am only using reflection once across the life time of the application, to gain extra performance.

Conclusion

If you don't want a sophisticated cookie encryption service, if you don't want to mind the encryption key, and if you don't want to create your own encryption algorithm, then this library is for you!

Comments and suggestions are welcome. Please vote if you like this article (or if you didn't).

History

  • 1 April 2006 - First version.

License

This article, along with any associated source code and files, is licensed under The Microsoft Public License (Ms-PL)

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About the Author

Adam Tibi
Architect
United Kingdom United Kingdom
Passionate about refining software practices, promoting self-motivated teams and orchestrating agile projects that hit the deadline.
Lives in London, UK and works as a .NET architect consultant in the City.
 
Need a pro service to help your organisation? Contact me via my website www.AdamTibi.net.
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Comments and Discussions

 
QuestionBug on 4.5 framework ? Pinmemberdiego mesa tabares5-Oct-12 7:29 
AnswerRe: Bug on 4.5 framework ? PinmemberAdam Tibi5-Oct-12 17:19 
GeneralRe: Bug on 4.5 framework ? PinmemberJordi Corominas11-Jul-13 22:10 
BugRe: Bug on 4.5 framework ? Pinmemberheroes003-Dec-13 21:30 
GeneralRe: Bug on 4.5 framework ? PinmemberAdam Tibi4-Dec-13 2:19 
SuggestionInternal class and reflection Pinmembervasekz11-Feb-12 8:33 
GeneralRe: Internal class and reflection PinmemberAdam Tibi13-Feb-12 11:41 
GeneralRe: Internal class and reflection PinmemberJasonJoh3-Aug-12 1:44 
GeneralNot update cookie instantly Pinmemberashutosh kumar jha20-May-11 1:15 
GeneralNot working reliably in a web farm, even though machineKeys are synced Pinmemberfrankosaurus3-Dec-10 17:03 
GeneralRe: Not working reliably in a web farm, even though machineKeys are synced Pinmemberfrankosaurus5-Dec-10 15:10 
GeneralWas working great - now throwing errors PinmemberChuck Bevitt4-Nov-10 14:42 
I've used this for a couple of years on my web apps; it's been working great. Suddenly, a month or so ago, it started occasionally throwing errors. This only happens 5 to 15 times a day, on a site with thousands of hits per day.
 
Before anyone asks, yes, the web site is on a server farm and I do specify the machine key in the web config file. I haven't made any changes that could account for this.
 
Here are a couple of the errors from our error notification emails, which unwind the inner exceptions:
 
Error Ticket is: VSIIS1_11-04-2010___3
 
Message(0): /Login.aspx, IP: 209.99.19.8, Raw User Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; InfoPath.2; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)
 
Stack Trace(0):
 
Source(0): Error.aspx
 
Message(1): Exception of type 'System.Web.HttpUnhandledException' was thrown.
 
Stack Trace(1): at System.Web.UI.Page.HandleError(Exception e)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest()
at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
at System.Web.UI.Page.ProcessRequest(HttpContext context)
at ASP.login_aspx.ProcessRequest(HttpContext context)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
 
Source(1): System.Web
 
Message(2): Unable to decode the text
 
Stack Trace(2): at AdamTibi.Web.Security.MachineKeyCryptography.Decode(String text, CookieProtection cookieProtection)
at AdamTibi.Web.Security.HttpSecureCookie.Decode(HttpCookie cookie, CookieProtection cookieProtection)
at AdamTibi.Web.Security.HttpSecureCookie.Decode(HttpCookie cookie)
at Global.get_AccessAccountID()
at Global.SetPreferenceCookies()
at _Default.Authenticate(String cUser, String cPass, String& cFailureText)
at _Default.Login1_Authenticate(Object sender, AuthenticateEventArgs e)
at System.Web.UI.WebControls.Login.OnAuthenticate(AuthenticateEventArgs e)
at System.Web.UI.WebControls.Login.AttemptLogin()
at System.Web.UI.WebControls.Login.OnBubbleEvent(Object source, EventArgs e)
at System.Web.UI.Control.RaiseBubbleEvent(Object source, EventArgs args)
at System.Web.UI.WebControls.Button.OnCommand(CommandEventArgs e)
at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)
at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
 
Source(2): AdamTibi.Web.Security
 
Message(3): Unable to validate data.
 
Stack Trace(3): at System.Web.Configuration.MachineKeySection.EncryptOrDecryptData(Boolean fEncrypt, Byte[] buf, Byte[] modifier, Int32 start, Int32 length, IVType ivType, Boolean useValidationSymAlgo, Boolean signData)
at System.Web.Configuration.MachineKeySection.EncryptOrDecryptData(Boolean fEncrypt, Byte[] buf, Byte[] modifier, Int32 start, Int32 length, IVType ivType, Boolean useValidationSymAlgo)
at System.Web.Security.CookieProtectionHelper.Decode(CookieProtection cookieProtection, String data)
 
Source(3): System.Web
 

 
Error Ticket is: VSIIS2_11-04-2010___2
 
Message(0): /Login.aspx, IP: 206.216.34.171, Raw User Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.648)
 
Stack Trace(0):
 
Source(0): Error.aspx
 
Message(1): Exception of type 'System.Web.HttpUnhandledException' was thrown.
 
Stack Trace(1): at System.Web.UI.Page.HandleError(Exception e)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest()
at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
at System.Web.UI.Page.ProcessRequest(HttpContext context)
at ASP.login_aspx.ProcessRequest(HttpContext context)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
 
Source(1): System.Web
 
Message(2): Unable to decode the text
 
Stack Trace(2): at AdamTibi.Web.Security.MachineKeyCryptography.Decode(String text, CookieProtection cookieProtection)
at AdamTibi.Web.Security.HttpSecureCookie.Decode(HttpCookie cookie, CookieProtection cookieProtection)
at AdamTibi.Web.Security.HttpSecureCookie.Decode(HttpCookie cookie)
at Global.get_AccessAccountID()
at Global.SetPreferenceCookies()
at _Default.Authenticate(String cUser, String cPass, String& cFailureText)
at _Default.Login1_Authenticate(Object sender, AuthenticateEventArgs e)
at System.Web.UI.WebControls.Login.OnAuthenticate(AuthenticateEventArgs e)
at System.Web.UI.WebControls.Login.AttemptLogin()
at System.Web.UI.WebControls.Login.OnBubbleEvent(Object source, EventArgs e)
at System.Web.UI.Control.RaiseBubbleEvent(Object source, EventArgs args)
at System.Web.UI.WebControls.Button.OnCommand(CommandEventArgs e)
at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)
at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
 
Source(2): AdamTibi.Web.Security
 
Message(3): Length of the data to decrypt is invalid.
 
Stack Trace(3): at System.Security.Cryptography.RijndaelManagedTransform.TransformFinalBlock(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount)
at System.Security.Cryptography.CryptoStream.FlushFinalBlock()
at System.Web.Configuration.MachineKeySection.EncryptOrDecryptData(Boolean fEncrypt, Byte[] buf, Byte[] modifier, Int32 start, Int32 length, IVType ivType, Boolean useValidationSymAlgo)
at System.Web.Security.CookieProtectionHelper.Decode(CookieProtection cookieProtection, String data)
 
Source(3): mscorlib
 

I thought at first that we were starting to get hits from mobile devices that were causing the problem; adding the raw user agent to the error notification proved that that is not the case.
 
My best guess is that some Microsoft patch may have caused the problem.
 
It's running on IIS6 on Windows Server 2003 (virtual machines) and ASP.Net 2.0.
 
If anyone has any ideas, I'd really appreciate it. Remember, the problem only occurs a few times each day - usually it works fine.
 
Anyway, thanks for the library - it's worked great up to now.
GeneralRe: Was working great - now throwing errors Pinmembermoabi K15-May-11 18:55 
GeneralRe: Was working great - now throwing errors PinmemberChuck Bevitt16-May-11 7:18 
GeneralRe: Was working great - now throwing errors Pinmemberdiego mesa tabares29-Sep-11 12:58 
GeneralI'm pretty sure I'm missing something, but... Pinmemberreinux1-Nov-10 22:26 
GeneralRe: I'm pretty sure I'm missing something, but... Pinmemberfrankosaurus5-Dec-10 15:06 
GeneralRe: I'm pretty sure I'm missing something, but... Pinmemberreinux5-Dec-10 15:46 
GeneralThanks for the props Pinmemberericnewton769-Jul-10 8:12 
GeneralMedium Trust PinmemberSpiff Dog1-Mar-10 11:40 
GeneralRe: Medium Trust PinmemberAdam Tibi2-Mar-10 1:12 
GeneralThanks for the insight Adam...I've actually implemented your code in my project PinmemberZiad J.khan26-Nov-09 11:45 
GeneralRe: Thanks for the insight Adam...I've actually implemented your code in my project PinmemberAdam Tibi30-Nov-09 3:20 
Generalerror : padding is invalid... Pinmemberjamel197416-Sep-08 5:39 
GeneralRe: error : padding is invalid... PinmemberAdam Tibi16-Sep-08 12:54 

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