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Posted 3 Apr 2006
Licenced Ms-PL

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

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


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.


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 CodeProject: 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(
        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);

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.


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.


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).


  • 1st April, 2006 - First version


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


About the Author

Adam Tibi
United Kingdom United Kingdom
Passionate about refining software practices, promoting self-motivated teams and orchestrating agile projects.
Lives in London, UK and works as a .NET architect consultant in the City.


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Comments and Discussions

GeneralRe: Padding is invalid and cannot be removed. Pin
Adam Tibi24-Apr-08 22:32
professionalAdam Tibi24-Apr-08 22:32 
QuestionClarification Pin
stixoffire1-Apr-08 22:57
memberstixoffire1-Apr-08 22:57 
Generalthank you Pin
Rob van der Veer25-Feb-08 4:38
memberRob van der Veer25-Feb-08 4:38 
GeneralRe: thank you Pin
Adam Tibi25-Feb-08 22:04
professionalAdam Tibi25-Feb-08 22:04 
QuestionHow do you delete the cookie? Pin
Four-Fish10-Dec-07 6:11
memberFour-Fish10-Dec-07 6:11 
QuestionInvalidCypherTextException can't find? Pin
dianlongliu27-Nov-07 14:18
memberdianlongliu27-Nov-07 14:18 
AnswerRe: InvalidCypherTextException can't find? Pin
Adam Tibi27-Nov-07 23:45
professionalAdam Tibi27-Nov-07 23:45 
GeneralRe: InvalidCypherTextException can't find? Pin
dianlongliu28-Nov-07 3:30
memberdianlongliu28-Nov-07 3:30 

行 100: catch (Exception ex)
行 101: {
行 102: throw new InvalidCypherTextException("Unable to decode the text", ex.InnerException);//there is the error
行 103: }
行 104: if (buf == null || buf.Length == 0)

错误 2 找不到类型或命名空间名称“InvalidCypherTextException”(是否缺少 using 指令或程序集引用?) C:\Documents and Settings\Administrator\My Documents\Visual Studio 2005\WebSites\cookie\encrypt.aspx.cs 106 23 C:\...\cookie\
the name space can't be found
and 错误 MachineKeyCryptography” can 't find

GeneralRe: InvalidCypherTextException can't find? Pin
Adam Tibi28-Nov-07 23:03
professionalAdam Tibi28-Nov-07 23:03 
GeneralRe: InvalidCypherTextException can't find? Pin
dianlongliu29-Nov-07 14:55
memberdianlongliu29-Nov-07 14:55 
GeneralRe: InvalidCypherTextException can't find? Pin
dianlongliu29-Nov-07 15:00
memberdianlongliu29-Nov-07 15:00 
GeneralRe: InvalidCypherTextException can't find? Pin
Adam Tibi29-Nov-07 23:39
professionalAdam Tibi29-Nov-07 23:39 
GeneralRe: InvalidCypherTextException can't find? Pin
dianlongliu30-Nov-07 3:10
memberdianlongliu30-Nov-07 3:10 
QuestionPadding exception Pin
DaberElay9-Sep-07 22:07
memberDaberElay9-Sep-07 22:07 
AnswerRe: Padding exception Pin
Adam Tibi9-Sep-07 22:30
professionalAdam Tibi9-Sep-07 22:30 
AnswerRe: Padding exception Pin
Boxcarwilli15-Sep-07 19:37
memberBoxcarwilli15-Sep-07 19:37 
AnswerRe: Padding exception Pin
rsmith6661-Oct-07 15:33
memberrsmith6661-Oct-07 15:33 
Generalthank u Pin
Member 396127727-Mar-07 1:03
memberMember 396127727-Mar-07 1:03 
GeneralRe: thank u Pin
Adam Tibi28-Mar-07 9:19
professionalAdam Tibi28-Mar-07 9:19 
GeneralGreat article Pin
azam's22-Feb-07 18:07
memberazam's22-Feb-07 18:07 
GeneralRe: Great article Pin
Adam Tibi22-Feb-07 22:39
professionalAdam Tibi22-Feb-07 22:39 
GeneralWell done Adam! Pin
Grapes-R-Fun2-Jul-06 10:53
memberGrapes-R-Fun2-Jul-06 10:53 
GeneralGREAT! Pin
Kevin I13-Apr-06 18:04
memberKevin I13-Apr-06 18:04 
GeneralCool article Pin
Moustafa Banbouk11-Apr-06 17:58
memberMoustafa Banbouk11-Apr-06 17:58 
GeneralRe: Cool article Pin
Adam Tibi12-Apr-06 0:11
professionalAdam Tibi12-Apr-06 0:11 

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