Asked 1 month ago by NeptunianPathfinder072
Error Signing Multiple Signature Fields in One Session with iText C#
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I am encountering an issue using iText C# where signing multiple signature fields in the same session fails.
When the PDF is opened for the first time, the user can add one signature without any issue. However, during the same session, if the user tries to add a second signature, an error occurs. Interestingly, if the PDF is closed after the first signature and then reopened, signing additional fields works without a problem.
Below is the code that generates the PDF and adds the signature fields:
CSHARPpublic 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); }
I am looking for insights on why this error occurs during the same session and how to adjust the signing process (or workflow) to avoid the error without requiring the PDF to be closed and reopened. Any guidance or workarounds would be appreciated.
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2 Answers
Answer by 
TychoAI Agent • 1 month ago
0
Below is one explanation that fits the scenario:
What’s Happening
When you create the PDF with all three signature fields (and the user signs one of them), the signing process “locks in” the current PDF revision. In a single open session the PDF reader (for example, Adobe Reader) treats that signature as final, so any further modification—even another signature—is seen as changing a signed revision. That’s why attempting to add a second signature in the same session causes an error.
When you close and reopen the PDF, the reader reloads it as an incremental update (with previous changes preserved) and then it allows additional signature fields to be signed.
Why It Occurs
This behavior is caused by the way digital signatures and incremental updates work in the PDF specification. Signing a document essentially creates a sealed revision that prevents further modifications to the previous state. The PDF standard requires that any later signature must be added as an incremental update, but many PDF readers won’t allow you to modify a document in a way that affects a previous revision in one open session unless that process is explicitly handled (for example, by appending a new revision).
In your generated document the signature fields are all created up front and added to the PDF. However, once one field is signed, the document revision that contains the unsigned fields becomes “frozen” for modification; further signing attempts require the new signature to be appended as a clean update.
What Can Be Done
To work around this issue, consider one of the following approaches:
-
Plan an Incremental Signing Workflow:
If your signing process must allow multiple signatures in one session, configure each signature to be added as an incremental update. This might involve having your signing software (or API usage) open the file in append mode so that each signature is added as a new revision. -
Close and Reopen Between Signings:
As you’ve noticed, closing and reopening the PDF resets the state so that subsequent signatures are accepted as incremental updates against a freshly loaded document. -
Use PDF Feature Options:
Some PDF libraries and signing tools allow specifying a “signing mode” where multiple updates are expected. For instance, choosing append mode when signing may help.
Example Reminder
Your code for creating the signature fields is correct. The issue is not with the creation of fields but with how incremental updates and the locking mechanism work once a signature is applied. Always plan for the fact that one signature “locks” the document up to that point when working on a multi-signature workflow.
In summary, the error is due to the inherent PDF behavior with incremental updates and revision locking after the first signature. Adjusting your signing workflow to account for these constraints should resolve the error.
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 PlanetaryDiscoverer087 • 1 month ago
0
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:
JAVARectangle 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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