VBA Code Signing Solution

2024-04-20 00:23 UTC gpt-4 Open in ChatGPT ↗

You

You are C# and Excel expert and you provide a C# solution for code-signing the VBA project inside an Excel file, using an appropriate EV code-signing certificate

ChatGPT

Code-signing a VBA (Visual Basic for Applications) project inside an Excel file using C# involves several steps. This process ensures that the macros and scripts inside the Excel document are from a trusted source and have not been tampered with after signing. To sign a VBA project programmatically with an Extended Validation (EV) code-signing certificate, you can use the OfficePrimaryInterop assemblies in conjunction with a security tool like signtool.exe (part of the Windows SDK).

Here is a high-level overview of the steps involved and how to implement them in C#:

Prerequisites

1. EV Code-Signing Certificate: You must have an EV code-signing certificate installed on your machine. This certificate can be obtained from a certificate authority like DigiCert, Comodo, etc.
2. Visual Studio: Ensure you have Visual Studio installed with Office developer tools.
3. Windows SDK: Contains signtool.exe, required for the signing process.

Steps to Sign a VBA Project

1. Setting Up Your C# Project

• Create a new C# Console Application in Visual Studio.
• Add references to Microsoft.Vbe.Interop and Microsoft.Office.Interop.Excel to interact with Excel and VBA.

2. Accessing the Excel File and VBA Project

• Open the Excel file and access the VBA project using Interop services.

3. Preparing the VBA Project for Signing

• The VBA project needs to be unlocked and configured to accept a digital signature.

4. Signing the VBA Project

• Use signtool.exe to sign the VBA project. You will execute signtool.exe from your C# application, targeting the VBA project.

Here’s a basic example of how you might set this up in C#:

using System;
using System.Diagnostics;
using Microsoft.Vbe.Interop;
using Microsoft.Office.Interop.Excel;

class Program
{
    static void Main(string[] args)
    {
        string excelFilePath = @"C:\path\to\your\file.xlsx";
        string certThumbprint = "YOUR_CERT_THUMBPRINT";
        string signtoolPath = @"C:\path\to\signtool.exe";

        Application excelApp = new Application();
        Workbook workbook = excelApp.Workbooks.Open(excelFilePath);
        VBProject vbaProject = workbook.VBProject;

        try
        {
            // Ensure the VBA project is unlocked
            // This may require setting or removing passwords, depending on the project security settings

            // Command to sign the VBA project
            string args = $"sign /fd SHA256 /a /sha1 {certThumbprint} \"{excelFilePath}\"";

            ProcessStartInfo psi = new ProcessStartInfo(signtoolPath, args)
            {
                UseShellExecute = false,
                CreateNoWindow = true,
                RedirectStandardOutput = true,
                RedirectStandardError = true
            };

            using (Process process = Process.Start(psi))
            {
                process.WaitForExit();

                string output = process.StandardOutput.ReadToEnd();
                string error = process.StandardError.ReadToEnd();

                Console.WriteLine("Output: " + output);
                if (process.ExitCode != 0)
                {
                    Console.WriteLine("Error: " + error);
                }
            }
        }
        finally
        {
            workbook.Close(false);
            excelApp.Quit();
        }
    }
}

Important Notes

• This script assumes you have access to modify and sign the VBA project. You might need to handle permissions and security settings programmatically based on the VBA project’s configuration.
• signtool.exe commands and parameters may vary based on your specific requirements and the version of the tool.

Testing and Validation

• After signing, test the Excel file by opening it on a different machine to ensure the signature is recognized and valid.
• Ensure that the security settings in Excel trust signed macros.

This example provides a starting point, but be sure to adapt and expand it based on the specific security requirements and environment setup of your organization.

You

I want the solution to be entirely portable. I don’t want to rely on the end user having anything installed. the certificate will be included as a Resource in my C# app

ChatGPT

Creating a completely portable solution for signing a VBA project inside an Excel file using a C# application involves embedding the necessary components, including the certificate and signing tool, directly within the application. This allows you to avoid dependencies on external installations on the user’s system. Here’s a guide on how you can achieve this:

Step 1: Include Resources in Your C# Application

1. Embed the Certificate: You can include your EV code-signing certificate as an embedded resource in your C# project. Make sure it’s exported with the private key and secured with a password.
2. Embed signtool.exe: You need to embed signtool.exe from the Windows SDK as a resource in your project. This is necessary because signtool.exe is typically part of the Windows SDK or Visual Studio installations. Check the licensing terms to ensure compliance with Microsoft’s software usage rights.

Step 2: Setup Your C# Project

• Use Visual Studio to create a new Console Application.
• Add references to Microsoft.Vbe.Interop and Microsoft.Office.Interop.Excel for accessing and manipulating Excel files and their VBA projects.
• Add the certificate and signtool.exe as resources to your project.

Step 3: Write Code to Extract Resources

Before you can use the resources, you’ll need to extract them from the assembly to a temporary location on the user’s machine:

using System;
using System.IO;
using System.Reflection;
using System.Diagnostics;
using Microsoft.Office.Interop.Excel;
using Microsoft.Vbe.Interop;

class Program
{
    static void Main()
    {
        string tempPath = Path.GetTempPath();
        string signtoolPath = Path.Combine(tempPath, "signtool.exe");
        string certificatePath = Path.Combine(tempPath, "myCertificate.pfx");
        string certificatePassword = "YOUR_CERTIFICATE_PASSWORD";

        ExtractResource("YourNamespace.signtool.exe", signtoolPath);
        ExtractResource("YourNamespace.myCertificate.pfx", certificatePath);

        SignVbaProject(@"C:\path\to\your\file.xlsx", signtoolPath, certificatePath, certificatePassword);

        // Cleanup if necessary
        File.Delete(signtoolPath);
        File.Delete(certificatePath);
    }

    static void ExtractResource(string resourceName, string outputPath)
    {
        using (var resource = Assembly.GetExecutingAssembly().GetManifestResourceStream(resourceName))
        using (var file = new FileStream(outputPath, FileMode.Create, FileAccess.Write))
        {
            resource.CopyTo(file);
        }
    }

    static void SignVbaProject(string excelFilePath, string signtoolPath, string certPath, string certPassword)
    {
        Application excelApp = new Application();
        Workbook workbook = excelApp.Workbooks.Open(excelFilePath);
        VBProject vbaProject = workbook.VBProject;

        try
        {
            string args = $"sign /f \"{certPath}\" /p {certPassword} /fd SHA256 /a \"{excelFilePath}\"";
            ProcessStartInfo psi = new ProcessStartInfo(signtoolPath, args)
            {
                UseShellExecute = false,
                CreateNoWindow = true,
                RedirectStandardOutput = true,
                RedirectStandardError = true
            };

            using (Process process = Process.Start(psi))
            {
                process.WaitForExit();

                string output = process.StandardOutput.ReadToEnd();
                string error = process.StandardError.ReadToEnd();

                Console.WriteLine("Output: " + output);
                if (process.ExitCode != 0)
                {
                    Console.WriteLine("Error: " + error);
                }
            }
        }
        finally
        {
            workbook.Close(false);
            excelApp.Quit();
        }
    }
}

Key Considerations:

• Security: Embedding sensitive resources like certificates in your application can pose security risks, especially if the private key is included. Ensure that your application is secured and consider using obfuscation techniques to protect the contents.
• Legal and Compliance: Verify that you comply with all legal requirements for software distribution, especially concerning the redistribution of signtool.exe and use of the certificate.
• Testing: Thoroughly test the application in different environments to ensure that the signing process works as expected without requiring additional installations.

This approach provides a portable solution that does not rely on the end user having specific tools installed, ensuring that your application can run in a self-contained manner.

You

what if I use the OpenXML library ?

ChatGPT

Using the Open XML SDK for dealing with Office documents provides a more direct approach for interacting with the components of Excel files (like .xlsx), but it’s important to clarify that the Open XML SDK itself does not support direct manipulation or signing of VBA projects (macros) within those documents. For signing VBA macros, you generally have to interact with the VBA project part of the file, which is not accessible through the Open XML SDK as it primarily handles the XML parts of the file such as worksheets, styles, etc.

Alternative Approach for a Portable Solution

If you want to stick to a fully managed and potentially more portable solution that doesn’t involve signtool.exe or external dependencies, consider using a .NET library that can manipulate the digital signatures within the compound file binary format used by Excel’s VBA projects. However, such a library would need to:

1. Modify the VBA project binary within the Excel file.
2. Embed and use the digital signature directly within this binary format.

Libraries and Steps

You could explore libraries like DSOFile or other third-party libraries capable of manipulating OLE structured storage (which is the format used for VBA projects within Excel files). Unfortunately, these libraries often still require COM interop or similar, and complete solutions in pure .NET that handle this specific need might not be readily available or would require significant custom development.

Suggested Strategy

Given your requirements for portability and embedding all components within a single executable:

1. VBA API Access: Continue using the COM-based approach or similar to manipulate and prepare the VBA project for signing.
2. Custom Certificate Embedding and Signing:
   • Embed your certificate within your C# application.
   • Develop or use a library that can apply a PKCS #7 signature to the VBA project segment of the Excel file, directly manipulating the file’s binary structure.

Example Code Sketch

Here’s a conceptual outline of how this might be implemented in C#. This example is theoretical and focuses on the high-level steps you’d need to take:

using System;
using System.IO;
using System.Reflection;
using System.Security.Cryptography.X509Certificates;
using OfficeOpenXml;
using SomeVBASigningLibrary; // Hypothetical library

class Program
{
    static void Main()
    {
        string excelPath = @"C:\path\to\your\file.xlsx";
        string certificatePath = ExtractEmbeddedResource("YourNamespace.myCertificate.pfx");
        string certificatePassword = "YOUR_CERTIFICATE_PASSWORD";

        using (var package = new ExcelPackage(new FileInfo(excelPath)))
        {
            var certificate = new X509Certificate2(certificatePath, certificatePassword, X509KeyStorageFlags.Exportable);

            // Hypothetical method to sign a VBA project inside an Excel file
            SignVBAProject(package, certificate);

            package.Save();
        }

        File.Delete(certificatePath); // Clean up extracted certificate
    }

    static string ExtractEmbeddedResource(string resourceName)
    {
        string tempPath = Path.GetTempFileName();
        using (var resource = Assembly.GetExecutingAssembly().GetManifestResourceStream(resourceName))
        using (var file = new FileStream(tempPath, FileMode.Create, FileAccess.Write))
        {
            resource.CopyTo(file);
        }
        return tempPath;
    }

    static void SignVBAProject(ExcelPackage package, X509Certificate2 certificate)
    {
        // This function would need to be implemented in a library that supports VBA project signing
        VbaProjectSigner.Sign(package, certificate);
    }
}

Important Considerations:

• Availability of Libraries: As of now, there are no well-known .NET libraries that support direct manipulation and signing of VBA projects within Excel files. You might end up needing to develop custom solutions or extend existing libraries.
• Security and Compliance: Ensure your solution handles the certificate and signing process securely, especially when embedding sensitive components like private keys within an executable.
• Testing: Extensively test any solution in multiple environments to confirm that the VBA project remains intact and that the signatures are valid and recognized by Excel.

Conclusion

While this approach provides a conceptual framework for a solution, implementing it could involve significant technical challenges and development, particularly in handling binary file formats and cryptographic operations without external tools like signtool.exe.

You

What if I want the signing to happen on a mac version of my c# app as well? I’m using Avalonia

ChatGPT

Signing a VBA project in an Excel file on both Windows and macOS using a C# application developed with Avalonia (a cross-platform framework) introduces several unique challenges. The primary issue is that tools like signtool.exe are not available on macOS, and standard .NET libraries do not support direct manipulation of VBA projects within Excel files, as previously mentioned.

Cross-Platform Considerations

For a truly cross-platform solution in C# that operates within a macOS environment as well, you’d need to consider alternative approaches or tools that can handle digital signatures in a platform-agnostic manner.

Possible Approaches

1. Mono or .NET 5/6+ and a Portable Library: Leverage Mono (if still using older .NET standards) or the latest .NET versions which support cross-platform functionalities. Use libraries that can manipulate the underlying OLE structure of Excel files (if any exist) or handle PKCS#7 signatures directly.
2. Custom Command-Line Tool: Develop or utilize a custom tool written in a language that compiles to both Windows and macOS, such as C++ or Python. This tool would handle the extraction, signing, and reinsertion of the VBA project. Your C# application could then call this tool via command line.
3. Use a Java-Based Solution: Java has several libraries that can manipulate Excel files and potentially handle embedded VBA projects. You can interface your C# application with a Java application or use JNI (Java Native Interface) to call Java code directly. Java is inherently cross-platform and might provide more mature libraries for these tasks.

Example of a Cross-Platform Approach

Here is a conceptual outline using a cross-platform compatible method like a Java tool or a custom command-line tool. This example assumes the existence or creation of a hypothetical command-line tool (vba-sign-tool) that can sign VBA projects and is executable on both Windows and macOS.

C# Code Using Avalonia

You would set up your Avalonia project and include process execution logic similar to the following:

using System;
using System.Diagnostics;
using System.IO;
using Avalonia;
using Avalonia.Controls;
using Avalonia.Markup.Xaml;

public class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
#if DEBUG
        this.AttachDevTools();
#endif
    }

    private void InitializeComponent()
    {
        AvaloniaXamlLoader.Load(this);
    }

    private void SignExcelFile(string excelFilePath, string certificatePath, string certificatePassword)
    {
        string toolPath = "path/to/vba-sign-tool"; // Adjust path based on the actual location and executable
        string arguments = $"\"{excelFilePath}\" \"{certificatePath}\" \"{certificatePassword}\"";

        ProcessStartInfo startInfo = new ProcessStartInfo
        {
            FileName = toolPath,
            Arguments = arguments,
            UseShellExecute = false,
            RedirectStandardOutput = true,
            RedirectStandardError = true,
            CreateNoWindow = true
        };

        using (Process process = new Process { StartInfo = startInfo })
        {
            process.Start();
            string output = process.StandardOutput.ReadToEnd();
            string errors = process.StandardError.ReadToEnd();
            process.WaitForExit();

            // Log or display the output and errors
            Console.WriteLine("Output: " + output);
            if (!string.IsNullOrEmpty(errors))
            {
                Console.WriteLine("Errors: " + errors);
            }
        }
    }
}

Key Considerations

• Development of the Command-Line Tool: The feasibility of this approach heavily depends on whether you can create or find a suitable command-line tool that handles the specific requirements of signing VBA projects.
• Security: Ensure secure handling of certificates and passwords, especially when passing them via command lines.
• Cross-Platform Functionality: Make sure any paths or platform-specific functionalities are correctly handled depending on the OS.

This approach allows you to maintain a single codebase in C# using Avalonia for the UI and other logic while offloading the specific task of signing the VBA project to a specialized, platform-compatible tool.

You

In which part of the OpenXML.Packaging.SpreadsheetDocument object is the VBA stored?

ChatGPT

In an Open XML file format, such as an Excel .xlsx file, any embedded VBA (Visual Basic for Applications) code is stored within a part of the file known as the VBA project part. This part is essentially a binary blob that contains the VBA macros, and it’s stored separately from the XML parts that describe the spreadsheet’s cells, styles, and other content.

When you’re using the OpenXML SDK to work with an SpreadsheetDocument object in a .xlsx file, the VBA project, if present, is accessed through the VbaProjectPart of the document. Here’s how you can interact with it:

Accessing the VBA Project Part

If your Excel file (xlsx) contains macros, it’s saved with a .xlsm extension. The VbaProjectPart can be accessed and manipulated via the Open XML SDK as follows:

using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Spreadsheet;
using System.IO;

class Program
{
    static void Main(string[] args)
    {
        string filePath = @"path\to\your\file.xlsm";

        using (SpreadsheetDocument document = SpreadsheetDocument.Open(filePath, true))
        {
            VbaProjectPart vbaProjectPart = document.WorkbookPart.VbaProjectPart;

            if (vbaProjectPart != null)
            {
                // Example of accessing the VBA project part data
                using (Stream stream = vbaProjectPart.GetStream())
                {
                    using (StreamReader reader = new StreamReader(stream))
                    {
                        string vbaCode = reader.ReadToEnd();
                        // Now you have the binary data of the VBA project as a string.
                        // Note: This data is binary, so reading it as a string is just for illustration.
                    }
                }
            }
            else
            {
                Console.WriteLine("No VBA project part found in the document.");
            }
        }
    }
}

Important Notes

• Binary Data: The VBA project data is stored in a binary format (not plain text or XML). This means you cannot simply “read” or “edit” it like XML parts without proper parsing tools or libraries designed to handle this binary format.
• Modifying VBA Code: Modifying or signing the VBA code directly through the Open XML SDK is not straightforward because it doesn’t provide tools for parsing the binary format of the VBA project part. Typically, modifications to this part would require extracting it, modifying it with an appropriate tool or library that understands the binary format, and then replacing it in the document package.
• Security: Manipulating VBA code programmatically can expose security risks, especially if the code originates from or is shared with untrusted sources. Always ensure that macro-enabled documents are handled securely.

This approach through Open XML SDK provides a way to access and extract the VBA project part, but any advanced manipulation (like editing or signing the code) would require additional tools or custom implementations suited to dealing with the binary format of VBA projects.