如何检查PDF文件已签名多少次？

Question

How can I check how many times a PDF file had been signed, using C# and preferably iTextSharp 5.5.11? I'm open to any other .NET PDF library as long as it does the job.

I would like to choose a PDF file from my hard drive and show on a form how many times the PDF file had been signed.

Showing the certificate source would be a bonus.

Thank you.

Answer 1

Since iText exists both in Java and C# and the Digital Signatures tutorial for iText was written in the former, I will give the answer in Java and there should be no problem to port it to c#:

One can use SignatureUtil#getTotalRevisions() to get the number. It has been designed to give you the number of signatures in the document, or that number plus one if changes were applied after the last time the document was signed.

Please check C5_01_SignatureIntegrity . Here three documents are processed: the first had been signed, but then modified, the second was signed by four persons and the third had been signed by 4 persons and then modified. SignatureUtil#getTotalRevisions() returns 2, 4 and 5 for them respectively.

The results in the first and the third cases are a bit misleading, but one can use SignatureUtil#signatureCoversWholeDocument(String) to check whether the last revision corresponds to the last signing or not.