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https://issues.apache.org/jira/browse/PDFBOX-3853?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16072930#comment-16072930
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Tilman Hausherr commented on PDFBOX-3853:
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https://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/PPKAppearances.pdf
Signature annotation with no appearance
PDF signatures can be visible or invisible. In both situations there is an
associated annotation. For invisible
signatures, the annotation has a rectangle array with values [ 0 0 0 0 ]. This
annotation is usually attached
to the page that is being viewed when the signature is created. Despite not
having an appearance, the
annotation AP and N dictionaries may be present in some versions of Acrobat. If
present, N references the
DSBlankXObj (blank) XObject.
> Invisible sigaure's catalog tree is not updated
> -----------------------------------------------
>
> Key: PDFBOX-3853
> URL: https://issues.apache.org/jira/browse/PDFBOX-3853
> Project: PDFBox
> Issue Type: Bug
> Components: Signing
> Affects Versions: 2.0.6
> Reporter: ryuukei
> Priority: Minor
> Labels: signature
> Attachments: PDDocument.patch
>
>
> According latest Acrobat PDF 1.7 speficication:
> http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/pdf_reference_1-7.pdf
> Page 695, section [Signature Fields]:
> * quote 1:
> The field type ( FT ) is Sig , and the field value
> ( V ) is a signature dictionary containing the signature and specifying
> various at-
> tributes of the signature field.
> * quote 2:
> Like any other field, a signature field may actually be described by a widget
> anno-
> tation dictionary containing entries pertaining to an annotation as well as a
> field
> (see “Widget Annotations” on page 640). The annotation rectangle ( Rect ) in
> such
> a dictionary gives the position of the field on its page. Signature fields
> that are not
> intended to be visible should have an annotation rectangle that has zero
> height
> and width.
> All in all, invisible signature also should be as a Signature Field, which is
> one type of Annotation, thus the page catalog tree should be updated as well.
> For fixing this improvement issue, in the class PDDocument.java L351~L353,
> instead of terminating the process after [prepareVisibleSignature] by return,
> continue with later on annotation update process would be better.
> Personally I'm using itext-rups to analyse the pdf catalog structure, after
> applying the patch, the signature present in the catalog tree
> pages/page/annot/sig correctly.
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