Why does adding a second digital signature in the same session with iText C# cause an error?

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I'm generating a PDF with multiple signature fields using iText in C#. When the PDF is opened initially, signing one field works fine. However, if the user attempts to sign another field within the same session, an error occurs. Once the PDF is closed and reopened, signing all the fields works without issue.

Below is the simplified code that creates the document and adds the signature fields:

CSHARP
 public static void Main(string[] args)
 {
     string DEST = "path";
     PdfWriter writer = new PdfWriter(DEST);
     PdfDocument pdfDoc = new PdfDocument(writer);
     Document document = new Document(pdfDoc);

     PdfAcroForm form = PdfAcroForm.GetAcroForm(pdfDoc, true);

     AddSignatureField(form, "signature1", 36, 700, 200, 50);
     AddSignatureField(form, "signature2", 36, 600, 200, 50);
     AddSignatureField(form, "signature3", 36, 500, 200, 50);

     document.Close();
 }

 public static void AddSignatureField(PdfAcroForm form, string fieldName, float x, float y, float width, float height)
 {
     Rectangle rect = new Rectangle(x, y, width, height);
     PdfWidgetAnnotation pdfWidgetAnnotation = new PdfWidgetAnnotation(rect);
     PdfSignatureFormField signatureField = PdfFormCreator.CreateSignatureFormField(pdfWidgetAnnotation, form.GetPdfDocument());
     signatureField.SetFieldName(fieldName);
     form.AddField(signatureField);
 }

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Answers 2

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

The behavior you're observing is due to the way PDF digital signatures work. When you sign a PDF, the signing operation creates an incremental update that “freezes” part of the document. In other words, parts of the document (including the empty signature fields) become locked to preserve the integrity of the signed byte ranges. Trying to add or modify a signature field in the same document revision (session) will cause an error because it violates the integrity that the first signature established.

Once the PDF is closed and reopened, a new revision is started. This allows the incremental update mechanism to add additional signature updates (or sign other empty fields) without affecting the previously signed data.

In your code sample, after the first signature is applied, the document’s incremental update is in place. Any attempt to add a second signature field (or sign it) during the same session results in an error. Closing and reopening the PDF essentially “releases” that revision and prepares the document for a new incremental update, which is why multiple signatures in subsequent sessions work as expected.

A few things to consider when working with multiple signatures in a PDF:

Predefine All Signature Fields Before Signing: One common approach is to add all the necessary signature fields (and leave them empty) before any signing occurs. Each field can then be signed in separate sessions, ensuring that every signature is applied as part of a valid incremental update.
Incremental Updates: Understand that each digital signature creates an incremental update to the document. Any modifications after a signature (including the signing process itself) must not change the parts of the document that were included in the signed byte range.
Document Locking: After a signature is applied, certain parts of the document become locked. If you need to add additional elements, you’ll have to plan where incremental updates can occur.

By designing your signing workflow to either sign one field per document update or predefine all required fields, you can avoid this error.

In summary, the error occurs because the second signature attempt during the same session conflicts with the locked state of the document created by the first signature’s incremental update. Closing and reopening the PDF starts a new revision, allowing further signatures to be added.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by AuroraStargazer665 • 1 month ago

The error

Unfortunately you didn't tell which error occurred while signing with which software. Out of interest I tried it nonetheless, starting with Adobe Acrobat, and indeed, there was an error! After successfully signing the first signature field, while trying to sign the second one, I got the following messages, one after the other:

The cause

I created a similar file using Adobe Acrobat, i.e. a document with one page with three empty signature fields. With this file the error did not occur.

Thus, I compared the file generated by iText and the file generated by Acrobat with each other. The latter contained a number of additional entries but according to the PDF specification all of them are optional.

Nonetheless, removing those entries one after the other from the Acrobat generated file I eventually could reproduce the issue with that file, too. As it turned out, Acrobat needs the Type entry of the merged field/widget objects to be present!

As this entry is optional and it also is clear by structure that those objects are merged field/widget objects, this clearly is a bug of Adobe Acrobat. Thus, you should file a bug with Adobe.

A work-around

To be able to create signature field files for the current Acrobat, though, you can extend your code to set the widget annotation Type field accordingly, change your AddSignatureField method like this:

JAVA
Rectangle rect = new Rectangle(x, y, width, height);  
PdfWidgetAnnotation pdfWidgetAnnotation = new PdfWidgetAnnotation(rect);  
pdfWidgetAnnotation.Put(PdfName.Type, PdfName.Annot);  
PdfSignatureFormField signatureField = PdfFormCreator.CreateSignatureFormField(pdfWidgetAnnotation, form.GetPdfDocument());  
signatureField.SetFieldName(fieldName);  
form.AddField(signatureField);

(From AddSignatureField method AddSignatureFieldDharmendraSinsinwarImproved)

in particular, add the pdfWidgetAnnotation.Put(PdfName.Type, PdfName.Annot) line.

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Why does adding a second digital signature in the same session with iText C# cause an error?

2 Answers

The error

The cause

A work-around

Discussion

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Error Signing Multiple Signature Fields in One Session with iText C#