https://bugs.kde.org/show_bug.cgi?id=482682
Bug ID: 482682
Summary: Digital Signature not referenced in AcroForm
Classification: Applications
Product: okular
Version: 23.08.1
Platform: Debian unstable
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: PDF backend
Assignee: okular-de...@kde.org
Reporter: tobias.w...@electrologic.org
Target Milestone: ---
Created attachment 166560
--> https://bugs.kde.org/attachment.cgi?id=166560&action=edit
Test document and signed result
SUMMARY
When creating a digital signature with Okular, the signature dictionary is not
referenced in the AcroForm as required by the PDF/A Signature Tech Note [1].
This leads to problems in various signature validation software [2,3,4], namely
that the signature is not found.
[1]
https://pdfa.org/wp-content/uploads/2011/08/tn0006_digital_signatures_in_pdfa-1_2008-03-141.pdf
[2] https://ec.europa.eu/digital-building-blocks/DSS/webapp-demo/validation
[3] https://service.exceet.cloud/signature-check/web
[4] https://sws.firmacerta.it/SignEngineWeb/verifier.xhtml
STEPS TO REPRODUCE
1. Sign a PDF document with Okular
2. Verify with an external verification tool
OBSERVED RESULT
Signature is not found in external verification tools
EXPECTED RESULT
Signature is found and successfully validated
SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Debian unstable
(available in About System)
KDE Plasma Version:
KDE Frameworks Version: 5.103.0
Qt Version: 5.15.8
Poppler Version: 22.12.0
ADDITIONAL INFORMATION
The unsigned and signed document are attached for reference.
When inspecting the PDF specifications, the situation is more complicated.
The used test document is a PDF 1.4 document, so the signature field must be an
interactive form field, meaning it has to be referenced in the AcroForm.
The PDF 1.7 introduces non-interactive forms which, to my understanding, don't
need the AcroForm entry.
The signature field section (12.7.5.5 in PDF 2.0) does not state whether an
interactive or a non-interactive form field needs to be used.
While this explains while poppler and Adobe Acrobat Reader find the signature
and validate it successfully, thereby at least ignoring the undefined
non-interactive form type in PDF 1.4.
However there is still the PDF/A Signature Tech Report which should be taken
into account when creating signatures.
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