Excel displays "Valid partial signature" when using C# to sign Excel 2016 xlsx doc

Question

Excel displays "Valid partial signature" when using C# to sign Excel 2016 xlsx doc

Brian Tabone 46

I am using .net 5 to digitally sign Excel documents with my digital signature certificate. I followed the example from

https://learn.microsoft.com/en-us/dotnet/api/system.io.packaging.packagedigitalsignaturemanager.sign?view=net-5.0#System_IO_Packaging_PackageDigitalSignatureManager_Sign_System_Collections_Generic_IEnumerable_System_Uri__System_Security_Cryptography_X509Certificates_X509Certificate_

and all seems to work without issue except that Excel shows that the while the signature is valid, it is only partial. The verbiage is

"Valid partial signature - This signature only signs a portion of the content"

When I extract the signature xml, it appears to sign all parts of the package but does not contain an idOfficeObject nor an idSignedProperties section, just the idPackageSignature . When I sign with Excel I get the idOfficeObject and idSignedProperties section.

Is there a way to sign an Excel document from C# without it showing the valid partial signature notification?

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Answer 1

Try this code if it works for you：

            string certFileName = "AsposeDemo.pfx";  
            string password = "aspose";  
  
            Aspose.Cells.Workbook workbook = new Aspose.Cells.Workbook("sampleDigitallySignedByCells.xlsx");  
  
            Aspose.Cells.DigitalSignatures.DigitalSignatureCollection dsCollection = new Aspose.Cells.DigitalSignatures.DigitalSignatureCollection();  
  
            System.Security.Cryptography.X509Certificates.X509Certificate2 certificate = new System.Security.Cryptography.X509Certificates.X509Certificate2(certFileName, password);  
  
            Aspose.Cells.DigitalSignatures.DigitalSignature signature = new Aspose.Cells.DigitalSignatures.DigitalSignature(certificate, "Aspose.Cells added new digital signature in existing digitally signed workbook.", DateTime.Now);  
            dsCollection.Add(signature);  
  
            workbook.AddDigitalSignature(dsCollection);  
  
            workbook.Save("outputDigitallySignedByCells.xlsx");  
            workbook.Dispose();

I used this nuget package.

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Brian Tabone 46 Reputation points

2021-02-09T22:26:06.843+00:00

This works but licensing for Aspose is costly, is there a way to sign using the Microsoft APIs which will give a fully signed office document?

Answer 2

I found how to fix the issue directly using the PackageDigitalSignatureManager

The example code I used is from:

https://learn.microsoft.com/en-us/previous-versions/dotnet/articles/aa905326(v=msdn.10)?redirectedfrom=MSDN

And I added

// Package part relationships must also be added otherwise you get only a partial signature  
foreach (PackageRelationship relationship in packagePart.GetRelationships())  
{  
    relationshipSelectors.Add(new  
    PackageRelationshipSelector(relationship.SourceUri,  
    PackageRelationshipSelectorType.Id, relationship.Id));  
}

Here's the digital signature method in its entirety:

        private static bool SignOfficeDoc(string officeDocPackagePath, X509Certificate2 certificate)  
        {  
            using (var package = Package.Open(officeDocPackagePath))  
            {  
                var signatureManager = new PackageDigitalSignatureManager(package);  
                signatureManager.CertificateOption = CertificateEmbeddingOption.InSignaturePart;  
                //Initialize a list to hold the part URIs to sign.  
  
                List<Uri> partsToSign = new List<Uri>();  
  
                //Create list of selectors for the list of relationships  
  
                List<PackageRelationshipSelector> relationshipSelectors =  
                     new List<PackageRelationshipSelector>();  
  
                //Add each part to the list, except relationships parts.  
                foreach (PackagePart packagePart in package.GetParts())  
                {  
                    if (!PackUriHelper.IsRelationshipPartUri(packagePart.Uri))  
                    {  
                        partsToSign.Add(packagePart.Uri);  
  
                        // Package part relationships must also be added otherwise you get only a partial signature  
                        foreach (PackageRelationship relationship in packagePart.GetRelationships())  
                        {  
                            relationshipSelectors.Add(new  
                                PackageRelationshipSelector(relationship.SourceUri,  
                                PackageRelationshipSelectorType.Id, relationship.Id));  
                        }  
                    }  
                }  
  
                //Create one selector for each package-level relationship, based on id  
                foreach (PackageRelationship relationship in package.GetRelationships())  
                {  
                    relationshipSelectors.Add(new  
                        PackageRelationshipSelector(relationship.SourceUri,  
                        PackageRelationshipSelectorType.Id, relationship.Id));  
                }  
  
                PackageDigitalSignature signature = signatureManager.Sign(partsToSign, certificate, relationshipSelectors);  
  
                //After signing, close the package.  
                //The signature will be persisted in the package.  
                package.Close();  
            }  
  
            return true;  
        }

Premnath Jothi 1 Reputation point

2021-11-13T19:44:17.13+00:00

Hi @Brian Tabone ,

I have tried the same piece of code (.net framework version 4.6). I got the error that I'm unable to access the sign method
from PackageDigitalSignatreManger (Method does not exist). Please let me know if any thing I'm missing.

Thanks

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Excel displays "Valid partial signature" when using C# to sign Excel 2016 xlsx doc

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