My project is to add a digital signature to an existing PDF if Windows, in C++, with PAdES support. Studying base aspects of the PDF format and the output of jSignPdf, I 've managed to be able to sign successfully any existing PDF file and Adobe Acrobat finds the PAdES B-T level signature successfully.
My problem is now to preserve existing PDF information. As far as I have understood it, adding extra information to an existing PDF requires:
- Creating a news ection %%EOF, startxref, trailer, xref table.
- Creating some new objects to describe the signature.
jSignPDF (and my tool) creates 9 new objects, 8 of them are self-describing (i.e. they reference parts to the same section:
12 0 obj
<</F 132/Type/Annot/Subtype/Widget/Rect[0 0 0 0]/FT/Sig/DR<<>>/T(Signature1)/V 10 0 R/P 9 0 R/AP<</N 11 0 R>>>>
endobj
10 0 obj
<</Contents<....>
/Type/Sig/SubFilter/ETSI.CAdES.detached/M(D:20181004191406+00'00')/ByteRange [0 829 60831 1193]/Filter/Adobe.PPKLite>>
endobj
13 0 obj
<</BaseFont/Helvetica/Type/Font/Subtype/Type1/Encoding/WinAnsiEncoding/Name/Helv>>
endobj
14 0 obj
<</BaseFont/ZapfDingbats/Type/Font/Subtype/Type1/Name/ZaDb>>
endobj
11 0 obj
<</Type/XObject/Resources<</ProcSet [/PDF /Text /ImageB /ImageC /ImageI]>>/Subtype/Form/BBox[0 0 0 0]/Matrix [1 0 0 1 0 0]/Length 8/FormType 1/Filter/FlateDecode>>stream
...
endstream
endobj
9 0 obj
<</Parent 8 0 R/Contents 5 0 R/Type/Page/Resources<</Font<</Helv 13 0 R>>>>/MediaBox[0 0 200 200]/Annots[12 0 R]>>
endobj
8 0 obj
<</Type/Pages/MediaBox[0 0 200 200]/Count 1/Kids[9 0 R]>>
endobj
7 0 obj
<</Type/Catalog/AcroForm<</Fields[12 0 R]/DR<</Font<</Helv 13 0 R/ZaDb 14 0 R>>>>/DA(/Helv 0 Tf 0 g )/SigFlags 3>>/Pages 8 0 R>>
endobj
15 0 obj
<</Producer(AdES Tools)/ModDate(D:20181002132630+03'00')>>
endobj
xref
7 9
....
trailer
<</Root 7 0 R/Prev 492/Info 15 0 R/Size 12>>
startxref
61760
%%EOF
The problem is the object 9 in this.
9 0 obj
<</Parent 8 0 R/Contents 5 0 R/Type/Page/Resources<</Font
<</Helv 13 0 R>>>>/MediaBox[0 0 200 200]/Annots[12 0 R]>>
This object refers to the original PDF section. My problem is, how to create a correct reference to it.
The original document is this:
%PDF-1.7
1 0 obj % entry point
<<
/Type /Catalog
/Pages 2 0 R
>>
endobj
2 0 obj
<<
/Type /Pages
/MediaBox [ 0 0 200 200 ]
/Count 1
/Kids [ 3 0 R ]
>>
endobj
3 0 obj
<<
/Type /Page
/Parent 2 0 R
/Resources <<
/Font <<
/F1 4 0 R
>>
>>
/Contents 5 0 R
>>
endobj
4 0 obj
<<
/Type /Font
/Subtype /Type1
/BaseFont /Times-Roman
>>
endobj
5 0 obj % page content
<<
/Length 44
>>
stream
BT
70 50 TD
/F1 12 Tf
(Hello, world!) Tj
ET
endstream
endobj
xref
0 6
0000000000 65535 f
0000000010 00000 n
0000000079 00000 n
0000000173 00000 n
0000000301 00000 n
0000000380 00000 n
trailer
<<
/Size 6
/Root 1 0 R
>>
startxref
492
%%EOF
Now I have created a simple PDF parser in C++, and as far as I understand I need to:
- Find the root object (from trailer)
- Find the pages descriptor from the root object (in this case, /Pages 2 0 R)
- Find the first (?) kid object from the pages descriptor (in this case, /Kids [ 3 0 R ]
- From the referenced object, get the contents (in this case, 5)
- Build the obj 9 object in my PDF section.
This is merely experimental though. What I need is some reference to PDF document which would explain the proper procedure to refer to the previous PDF section of the file, or some working C++ PDF-add-a-new-document updating.
If I have, I will look through the PDF specification and/or iText source, but I hope I can avoid that.
Thanks a lot.
