PDF Signature: "Expected a dict object" - pdf

Question

I'm creating a library for digitally signing a PDF document. During my quest I stumbled upon an other problem.
In Acrobat I'm getting the error:
Error during signature verification.
Adobe Acrobat error.
Expected a dict object.
I know it expects a dictionary object somewhere. But I have no idea where.
This problem shows up when I add the image to the AP of the signature.
For this I'm basing my implementation on the spec, and " Insert multiple digital approval signatures without invalidating the previous one "
Most of this seems to work correctly, but when the image is present it results in the error. The image is correctly visible.
Current working:
(This is a very short overview of the part where the error is, it might be slightly different, but hope this helps)
I update the signature annotation. Add link to object that contains normal appearance.
16 0 obj
<<
/Type/Annot
/Subtype/Widget
...snip...
/AP<<
/N 21 0 R
>>
>>
Add image as XObject
20 0 obj
<<
/Type/XObject
/Subtype/Image
...snip...
/Length 29569
>>
stream
...snip...
endstream
endobj
Add XObject (Normal appearance)
21 0 obj
<<
/Type/XObject
/Subtype/Form
/Resources<<
/XObject<<
/UserSignature272 20 0 R
>>
>>
/BBox[0 0 135 37.5]
/Length 44
>>stream
q
135 0 0 37.5 0 0 cm
/UserSignature272 Do
Q
endstream
endobj
I think the problem happens somewhere in obj (21 0), but I'm not sure.
Here is a minimal file that can be used for testing.
https://drive.google.com/file/d/17sdz2xJy3VhN6i9YiuPrJ6x2s5kU2sra/view?usp=sharing
Any help, or hints would be welcome.
(This post is a continuation of PDF Digital Signature has "Bad parameter" in Acrobat, but is about a different problem, same subject area.)

Answer 1

You're running into a bug of Adobe Acrobat here: If you display a XObject from inside your signature appearance stream, it expects that XObject to have a Resources entry. This may make sense in case of form XObjects but it doesn't for image XObjects like in your case.
A work around is to add an empty Resources dictionary to your image XObject.
I checked this by replacing the /BBox[1 0 0 1 0 0] in your image XObject (which is not needed there anyways) by /Resources<< >>.
When Adobe Acrobat creates its own signature appearances, it creates a hierarchy of form XObjects here with Resource dictionaries all over including those for the "layers". I assume Adobe Reader, seeing the Do operator attempts to collect information on such "layers", not expecting to immediately be confronted with an image XObject.