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Clément K.
Clément K.
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PDF cross-reference streams

Jan 5, 2012 10:40 AM

Tags: #signature #cross-reference_streams

Hello,

 

I noticed a behaviour I don't understand with some PDFs. They are hybrid-reference file where both cross-reference sections carry the same informations (there are 9 meaningful objects and the 9 are referenced by both sections).

When I sign the document twice, the second signature invalidates the first one. This seems to be due to the copy in the update trailers of the reference to the original cross-refernce stream, i.e. :

 

 

trailer

<</XRefStm 78305/ [...]>>

Is this a bad practice?

832 Views   2 Replies   Latest reply: Clément K., Jan 6, 2012 1:36 AM
 
Replies
  • lrosenth
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    1. lrosenth,
    Jan 5, 2012 10:46 AM   in reply to Clément K.
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    It’s hard to understand what you mean.

     

    Are you saying that you have a file which started out as compressed object/xref streams and was then updated using classic xrefs?  If so, that’s going to be a problem as it’s not fully supported according to the standard.  (sort of like Ghostbusters – don’t mix the streams!)

     

    If you are signing such a file (or any non-signed PDF for that matter), it’s always a good idea to a “full, garbage collected” save first.  THEN sign away.

     
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  • Clément K.
    Currently Being Moderated
    2. Clément K.,
    Jan 6, 2012 1:36 AM   in reply to lrosenth
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    As a picture is often better than a long speech, here is the mess:

     

     

    10 0 obj

    <</Type/XRef/Size 10/W[ 1 4 2] /Root 1 0 R/Info 7 0 R/ID[<FAC18A20B074DF4FA664118D8354A667><FAC18A20B074DF4FA664118D8354A 667>] /Filter/FlateDecode/Length 46>>

    stream

    % stream data

    endstream

    endobj

     

    xref

    0 11

    0000000000 65535 f

    0000000017 00000 n

    0000000078 00000 n

    0000000134 00000 n

    0000000354 00000 n

    0000000476 00000 n

    0000000642 00000 n

    0000000880 00000 n

    0000001111 00000 n

    0000001137 00000 n

    0000078305 00000 n

    trailer

    <</Size 11/Root 1 0 R/Info 7 0 R/ID[<FAC18A20B074DF4FA664118D8354A667><FAC18A20B074DF4FA664118D8354A 667>] >>

    startxref

    78550

    %%EOF

     

    xref

    0 0

    trailer

    <</Size 11/Root 1 0 R/Info 7 0 R/ID[<FAC18A20B074DF4FA664118D8354A667><FAC18A20B074DF4FA664118D8354A 667>] /Prev 78550/XRefStm 78305>>

    startxref

    78926

    %%EOF

     

    Well, this seems to be what you suggested: mixed streams. Unfortunatly I can't rewrite the file entirely as my tool cannot produce PDF from scratch.

    I found as a workaround that updating the file with standard cross-reference table does not invalidate signatures.

     

    Thank you Leonard for your help!

     
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