When trying to optimize or reduce the file size of a PDF document that has a DocuSign document within it, I get the message the Signature is Invalid and Signed by Unknown. I've tried several different things found on this forum but no luck (see attached.).
We are on version Continuous Release 2021.007.20102 of Acrobat Pro DC.
The signature in a digitally signed PDF in particular contains a checksum over the bytes of the whole signed document revision (except the bytes of the embedded cryptographic signature container, of course). When you optimize the PDF, you change those signed bytes; thus, if Adobe Reader now calculates the checksum over the signed bytes, it gets a different result indicating a manipulated document.
Even worse, the signing process writes the offsets and lengths of the signed byte ranges to the PDF. These data imply the position of the embedded cryptographic signature container - the gap between the two ranges. This is where Adobe Reader looks for the signature container. Most likely, though, optimization does not leave the signature container in place. Thus, when Adobe Reader tries to retrieve the signature container, it gets something entirely different. Consequentially you get the error message "There are errors in the formatting or information contained in this signature (support information: SigDict /Contents illegal data)".
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