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https://issues.apache.org/jira/browse/PDFBOX-4439?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16753202#comment-16753202
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Michael Klink commented on PDFBOX-4439:
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This indeed is a left-over of old signature validation result visualization
code in Adobe software (mirrored in Foxit Reader).
*n1* used to be the "layer" activated by Adobe Reader whenever the validity of
the signature was UNKNOWN (yet). In the very first implementations Adobe Reader
indeed used the *n1* XObject contained in the PDF file. Then Adobe discovered
that people (surprise, surprise!) could provide layers for UNKNOWN and INVALID
that look like a successful validation and so fake successful validation; thus,
Adobe started using a different XObject (hardcoded in the Reader) instead of
the XObject provided by the PDF.
What you see in Adobe Reader and Foxit Reader now is that hardcoded replacement
for the *n1* layer, not the *n1* XObject you identified in the PDF using the
PDFDebugger. It differs not only in the face but also in an additional shadow
of the question mark.
Officially Adobe stopped support for the *n1* layer in Adobe Reader 6 but this
only meant they don't create new signatures with those layers anymore while
they still use layers to display the validation state of a signature if
XObjects with those names are used in the underlying PDF document.
Switching these XObjects off and on and replacing them under the hood is
covered by the PDF specification, it is an historic Adobe gimmick which
contradicts a number of current standards.
> Digital signature image is not rendered
> ---------------------------------------
>
> Key: PDFBOX-4439
> URL: https://issues.apache.org/jira/browse/PDFBOX-4439
> Project: PDFBox
> Issue Type: Bug
> Components: Rendering
> Affects Versions: 2.0.13
> Reporter: simon steiner
> Priority: Major
> Attachments: notembedded_pm65.pdf
>
>
> java -jar ~/pdf-box-svn/app/target/pdfbox-app-3.0.0-SNAPSHOT.jar PDFToImage
> notembedded_pm65.pdf
>
> Picture of a face is not rendered
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