Click here to Skip to main content
14,737,515 members
Articles » Web Development » Web Security » Security
Posted 3 Apr 2006


112 bookmarked

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

Rate me:
Please Sign up or sign in to vote.
4.90/5 (38 votes)
3 Apr 2006Ms-PL
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.


Comments and Discussions

QuestionHow do I check cookie for tampering? Pin
Nitin S1-Jun-18 23:29
professionalNitin S1-Jun-18 23:29 4.5 update Pin
Crazy Bill1-Jun-17 0:53
MemberCrazy Bill1-Jun-17 0:53 
GeneralSystem.Web.Security.Cryptography.Purpose Pin
Member 104772152-Feb-17 22:17
MemberMember 104772152-Feb-17 22:17 
GeneralRe: System.Web.Security.Cryptography.Purpose Pin
Adam Tibi3-Feb-17 4:14
professionalAdam Tibi3-Feb-17 4:14 
GeneralRe: System.Web.Security.Cryptography.Purpose Pin
Nitin S2-Jun-18 0:03
professionalNitin S2-Jun-18 0:03 
GeneralRe: System.Web.Security.Cryptography.Purpose Pin
Adam Tibi7-Jun-18 0:30
professionalAdam Tibi7-Jun-18 0:30 
QuestionBug on 4.5 framework ? Pin
diego mesa tabares5-Oct-12 7:29
Memberdiego mesa tabares5-Oct-12 7:29 
AnswerRe: Bug on 4.5 framework ? Pin
Adam Tibi5-Oct-12 17:19
professionalAdam Tibi5-Oct-12 17:19 
GeneralRe: Bug on 4.5 framework ? Pin
Jordi Corominas11-Jul-13 22:10
MemberJordi Corominas11-Jul-13 22:10 
BugRe: Bug on 4.5 framework ? Pin
heroes003-Dec-13 21:30
Memberheroes003-Dec-13 21:30 
GeneralRe: Bug on 4.5 framework ? Pin
Adam Tibi4-Dec-13 2:19
professionalAdam Tibi4-Dec-13 2:19 
SuggestionInternal class and reflection Pin
vasekz11-Feb-12 8:33
Membervasekz11-Feb-12 8:33 
GeneralRe: Internal class and reflection Pin
Adam Tibi13-Feb-12 11:41
professionalAdam Tibi13-Feb-12 11:41 
GeneralRe: Internal class and reflection Pin
JasonJoh3-Aug-12 1:44
MemberJasonJoh3-Aug-12 1:44 
GeneralNot update cookie instantly Pin
ashutosh kumar jha20-May-11 1:15
Memberashutosh kumar jha20-May-11 1:15 
GeneralNot working reliably in a web farm, even though machineKeys are synced Pin
frankosaurus3-Dec-10 17:03
Memberfrankosaurus3-Dec-10 17:03 
GeneralRe: Not working reliably in a web farm, even though machineKeys are synced Pin
frankosaurus5-Dec-10 15:10
Memberfrankosaurus5-Dec-10 15:10 
GeneralWas working great - now throwing errors Pin
Chuck Bevitt4-Nov-10 14:42
MemberChuck Bevitt4-Nov-10 14:42 
GeneralRe: Was working great - now throwing errors Pin
moabi K15-May-11 18:55
Membermoabi K15-May-11 18:55 
GeneralRe: Was working great - now throwing errors Pin
Chuck Bevitt16-May-11 7:18
MemberChuck Bevitt16-May-11 7:18 
GeneralRe: Was working great - now throwing errors Pin
diego mesa tabares29-Sep-11 12:58
Memberdiego mesa tabares29-Sep-11 12:58 
GeneralI'm pretty sure I'm missing something, but... Pin
Rei Miyasaka1-Nov-10 22:26
MemberRei Miyasaka1-Nov-10 22:26 
GeneralRe: I'm pretty sure I'm missing something, but... Pin
frankosaurus5-Dec-10 15:06
Memberfrankosaurus5-Dec-10 15:06 
GeneralRe: I'm pretty sure I'm missing something, but... Pin
Rei Miyasaka5-Dec-10 15:46
MemberRei Miyasaka5-Dec-10 15:46 
GeneralThanks for the props Pin
ericnewton769-Jul-10 8:12
Memberericnewton769-Jul-10 8:12 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Praise Praise    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.