There's no method to find out what changes were made, only that changes
were made, or rather, that the signature doesn't go over the whole PDF:
https://stackoverflow.com/questions/44001185/check-with-pdfbox-v2-x-x-if-a-pdf-form-was-changed-after-signing/44007920#44007920
Maybe it's possible to find out what objects were changed after the last
signature, but for that one would have to look very deep into the parser
/ xref tables sources :-(
Tilman
Am 05.06.2017 um 08:26 schrieb NiKiFoR:
Hi,
Does PDFBOX support detection of changes that have been applied to pdf
document after it was signed? Especially changes that was done in active
form fields (which field and from what to what). I cannot find any
information in API and Stack Overflow.
I have a pdf document with some active forms (checkboxes, text fields
etc.). I fill in those fields and then I sign the document (CAdES/PAdES).
After that in Acrobat Reader a see that everything is ok (Signatures
bookmark). Then I change one of active fields and save the document. I see
now that verification is ok, but some changes was made in document. I don’t
want to block possibility to make changes after sign, but only detect that
some fields was changed. Now I take all current field:
document.getDocumentCatalog().getAcroForm().acroForm.getFields();
and compare them to those which I take from signed content:
PDSignature sig = document.getSignatureDictionaries()[0];
byte[] buf = null;
try (InputStream fis = new FileInputStream( pdf )) {
buf = sig.getSignedContent( fis );
}
catch ( IOException e ) {
//
}
String newSignedSequence = new String( buf, charset );
newSignedSequence = newSignedSequence.replaceAll( "/Contents >>",
"/Contents <>>>" );
buf = newSignedSequence.getBytes( charset );
try (PDDocument subDocument = PDDocument.load( buf )) {
// comparing current fields and that from signed content
} catch ( IOException ex ) {
//
}
Is there any other way to do this better (a workaround with replace is
necessary, because in other case I cannon load extracted signed sequence)?
Any build in method? Maybe exists method that detects that something was
changed, but doesn't tell what exactly?
Thank you in advance,
Lukas
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