Cannot open secure PDF's created by Adobe Acrobat

[jmanm]

Tested using Acrobat Pro DC version 2019.010.20069.

Steps to reproduce:

• Create secure PDF in Acrobat (restrict either open or edit)
• Save using any compatibility level (6.0, 7.0, or X)

var doc = PdfReader.Open("<path to secure pdf>", "<open password>");
//throws PdfSharp.SharpZipLib.SharpZipBaseException:  Header checksum illegal

[itsunbelievable]

Hi, by any chance have you found some solution for this issue? Thanks

[jmanm]

No I have not. I haven't looked at it in a few months, but so far I've tried:

• swapping out SharpZipLib for System.IO.Compression.GZipStream
• comparing the deflate code with the equivalent code in pdf.js

But no luck.

[sisve]

I have managed to track this down. This comes from PdfDictionary.TryUnfilter() which tries to call Filtering.Decode(...) where FlateDecode tries to use the embedded SharpZipLib to decompress the stream. However, the value is still encrypted, and needs to be decrypted. Catching the exception and decrypting and retrying works for me.

Old code: https://github.com/jmanm/PDFsharp-netstandard2.0/blob/51e048bef7e8e0489cbd6a172ae9b59c2412c51b/src/PdfSharp/Pdf/PdfDictionary.cs#L1684

New code:

byte[] bytes;

try {
    bytes = Filtering.Decode(_value, filter);
} catch (SharpZipBaseException) {
    _ownerDictionary._document.SecurityHandler.DecryptObject(_ownerDictionary);
    bytes = Filtering.Decode(_value, filter);
}

I have not submitted a pull request because this fix is a bit weird.

• It's weird that PdfDictionary should know about SharpZipBaseException (and SharpZibLib), which is an implementation details of FlateDecode. However, the FlateDecode filter does not have access to the security handler, and cannot decrypt the bytes.
• We're being called from PdfReader.Open, via ReadIRefsFromCompressedObject, which is done before the call to DecryptDocument. Perhaps the ordering here is wrong?
• I am unable to run the unit tests locally, and cannot be sure if I break something else.

My changes can be seen at https://github.com/sisve/PDFsharp-netstandard2.0/commit/6227b9f163f1c53add12faaf95fb55ed39c88b15