Before we delve into the intricacies of individual methods of anti-debugging let’s use this post to define the classes of anti-debugging that we will be discussing. While other classes may exist, the definition of these classes is an attempt to include the majority of anti-debugging methods in use today. There is some overlap between classifications and we may have left out some methods due to limited exposure or effectiveness.

API Based Anti-Debugging
API based anti-debugging is the most straightforward and possibly the easiest to understand for a typical developer. Using both documented and undocumented API calls, these methods query process and system information to determine the existence or operation of a debugger. From single line calls such as IsDebuggerPresent() and CheckRemoteDebugger() to slightly more complex methods including debugger detaching and CloseHandle() checks. These methods are generally trivial to add to an existing code base and many can even be implemented in as few as two or three lines.

Exception Based Anti-Debugging
Exception based anti-debugging is slightly different than your basic API based techniques. Many times when a debugger is attached to a process, exceptions are trapped and handled by the debugger without regard to passing the exception back to the application for continued execution. Occasionally these exceptions can even crash or terminate a process when run under a debugger and be handled gracefully when running clean. It is these discrepancies that makes exception based anti-debugging techniques possible.

Process and Thread Block Anti-Debugging
Some of the API based anti-debugging methods use published functions to query information from within the process and thread blocks for our running code. Many API based detections can be subverted within a debugger by hooking the API call and returning values that indicate a clean process. One way around this subversion is to directly query the process and thread blocks, bypassing the API calls. Direct analysis of the process and thread blocks, while more complex, can lead to a more accurate and high assurance result.

Modified Code Anti-Debugging
One of the methods that a debugger uses to signal a breakpoint is to insert a break byte into the running code at the location that it wishes to stop execution. The process execution breaks when this value is seen, giving control to the debugger. When the program is resumed, the breakpoint value is removed and replaced with the original byte, the execution backed up one byte, and the program is resumed. Detection of software based breakpoints can be achieved by analyzing the process for modifications from the expected norm.

Hardware and Register Based Anti-Debugging
A second way that a debugger can break the execution of a process is by using a hardware breakpoint. A hardware breakpoint relies upon CPU registers to store the pertinent information and to detect when the target break addresses are seen on the bus. A break interrupt is triggered at the appropriate time based on these register values. Reading or modifying the hardware can allow for the detection of a debugger.

Timing and Latency Anti-Debugging
Finally timing and latency can be used as an effective anti-debugging method. When executing a program within a debugger, specifically when single stepping, a much larger latency occurs between execution of instructions. This latency can be detected and compared against a reasonable threshold to detect the existence of a debugger attached to our process.

Each of the classes of anti-debugging outlined above has merit when used individually to protect a process. While none of them can be assured to ever protect a program from a determined reverse engineer or debugger, implementation of these techniques (or many of them if appropriate) can sufficiently slow down the debugging process and hopefully make the attacker spend his time on other, easier, ventures. In the remainder of this series on anti-debugging we will review in depth some of the more interesting methods of each of the above classes. So bring along your debugger and your development environment and let the games begin.

I'm not complaining about the fact that I can't see what I'm typing. I understand and laud that feature, because I don't want someone looking over my shoulder at the password I'm typing, and this even applies when I'm at home. I love my children, but I certainly don't want them knowing the password to my bank account!

No, what I'm bothered by is how a typical password text box behaves on a form that may incur multiple post-backs before it's finally submitted. If you use the built in ASP.NET TextBox control, it purposely does not repopulate the password text, which means if you press a button on the form that performs a post-back, or if you have a multi-page form that posts back on every step, that password disappears, and the user typically has to re-enter it. You could solve this with liberal use of ASP.NET Ajax UpdatePanels, but that adds its own complexities. I wanted a simpler solution.

So I did a little research to see what others had discovered about this problem, and I ended up deriving my own custom control from TextBox to make a much more user-friendly (and developer-friendly) TextBox control. I called it PasswordTextBox, and it acts just like a TextBox in password mode, but it retains the password while still giving the user the same level of protection the standard TextBox supplies.

My PasswordTextBox operates very simply: it stores the password in control state, and renders a series of fixed characters (with the same length as the actual password) into the text box so that it "looks" like the user's password has been rendered. Since control state is part of view state, and since view state is stored in a hidden field on the form, I encrypt the password before putting it into control state.

The result is quite nice - the user can post your form back as many times as she needs to, perhaps moving back and forth across wizard steps or tabs, and when she finally presses the "Finish" button (or whatever you call the last step of your input form), your code will be able to read the password by simply accessing the Text property on the PasswordTextBox. The user will believe that her password is sitting there on the form while she's working, as the same number of obfuscated characters will show up in the field as she typed in originally (what she doesn't know is that those characters aren't her real password anymore, but what she doesn't know won't hurt her!)

Note that to keep this simple, I used DPAPI to encrypt the password, which suited my purposes. But if you have a web farm, that won't work well at all if you don't know which machine the user's going to post back to, so you'll want to replace that with something more robust. I could see looking up the <machineKey> for entropy, as that tends to be sync'd already across the farm, but I've not yet spent the cycles to go down that road, since unfortunately all of the code for generating keys based on that config section are off limits in ASP.NET (most of the useful stuff is marked internal). I don't think it'd be that hard to do though.

Anyway, without further ado, here's the code, which you'll see is quite simple. I'd love feedback, especially if you see any glaring problems with the idea or the implementation!

public class PasswordTextBox : TextBox
{
    // unlikely that a string of these would be used for a password
    const char PasswordPlaceholderChar = '}';

    string password; // stored encrypted in control state

    protected override void OnInit(EventArgs e)
    {
        base.OnInit(e);
        Page.RegisterRequiresControlState(this);
    }

    protected override object SaveControlState()
    {
        byte[] encryptedPassword = ProtectPassword(password);

        object baseControlState = base.SaveControlState();
        if (null == baseControlState)
            return encryptedPassword;
        else return new Pair(baseControlState, encryptedPassword);
    }

    protected override void LoadControlState(object savedState)
    {
        byte[] encryptedPassword;

        Pair pair = savedState as Pair;
        if (null != pair)
        {
            base.LoadControlState(pair.First);
            encryptedPassword = pair.Second as byte[];
        }
        else encryptedPassword = savedState as byte[];

        password = UnprotectPassword(encryptedPassword);
    }

    /// <summary>
    /// This control always uses TextMode=Password
    /// </summary>
    public override TextBoxMode TextMode
    {
        get
        {
            return TextBoxMode.Password;
        }
        set { }
    }

    /// <summary>
    /// TextBox doesn't render value attribute for TextMode=Password
    /// So we add code that renders a placeholder text instead
    /// </summary>
    /// <param name="writer"></param>
    protected override void AddAttributesToRender(HtmlTextWriter writer)
    {
        base.AddAttributesToRender(writer);

        string text = Text;
        if (text.Length > 0)
            writer.AddAttribute(HtmlTextWriterAttribute.Value,
                GetPlaceholderPassword(text));
    }

    /// <summary>
    /// TextBox doesn't save the "Text" viewstate in
    /// TextMode=Password and we don't want our behavior to break
    /// if ViewState is turned off so we store the password in
    /// Control State, encrypted with MachineKey
    /// </summary>
    public override string Text
    {
        get
        {
            return password ?? string.Empty;
        }
        set
        {
            // this prevents us overwriting the actual
            // password with a placeholder
            if (!string.IsNullOrEmpty(password) &&
                value.Equals(GetPlaceholderPassword(password)))
                return;

            password = value;
        }
    }

    private string GetPlaceholderPassword(string realPassword)
    {
        int length = 12;
        if (!string.IsNullOrEmpty(realPassword))
            length = realPassword.Length;

        StringBuilder sb = new StringBuilder();
        sb.Append(PasswordPlaceholderChar, length);

        return sb.ToString();
    }

    public byte[] ProtectPassword(string password)
    {
        if (string.IsNullOrEmpty(password))
            return null;
        byte[] cleartext = Encoding.UTF8.GetBytes(password);
        return ProtectedData.Protect(cleartext, null,
            DataProtectionScope.LocalMachine);
    }

    public string UnprotectPassword(byte[] ciphertext)
    {
        if (null == ciphertext)
            return null;
        byte[] cleartext = ProtectedData.Unprotect(ciphertext, null,
            DataProtectionScope.LocalMachine);
        return Encoding.UTF8.GetString(cleartext);
    }
}