Signature is invalid
I'm trying to validate a file in Adobe Acrobat Reader, but the result is that 1st signature is invalid ("There have been changes made to this document that invalidate the signature").
I tried many other 3rd party applications, and they all say that all of its signatures are valid (including DSS from ec.europa.eu, and many more "smaller" apps).
Is the verification process in Adobe Acrobat Reader incorrect? If not, could you tell me what is wrong with this file?
Michał Jakubowski
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mkl
commented
The PDF revision added for the second signature not only adds that second signature, it also adds an /OutputIntents entry to the document catalog.
Setting (or changing) that entry can considerably change the appearance of documents. Thus, Adobe Acrobat considers this change a violation of the first signature (the certification signature). (I don't know whether it would also have considered the first signature broken if it was only a normal approval signature.)
Thus, most other PDF validators will ignore this change because many of them don't do any MDP (Modification Detection and Prevention) checks at all or do different ones than Acrobat does.
The MDP checks are very poorly specified in the PDF spec. Both the validation results of Acrobat and those other applications, therefore, can be considered correct, they merely validate different properties.