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.
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:
- Modify the VBA project binary within the Excel file.
- Embed and use the digital signature directly within this binary format.
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.
Given your requirements for portability and embedding all components within a single executable:
- VBA API Access: Continue using the COM-based approach or similar to manipulate and prepare the VBA project for signing.
- 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.
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.Security.Cryptography.X509Certificates;
using SomeVBASigningLibrary; // Hypothetical library
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);
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))
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);
- 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.
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.