A device and method for recovering pdf files

Abstract

The invention discloses a PDF document recovery device and method; the device comprises the following units: an overall key obtain unit using a rainbow table to obtain the overall key of a to-be-decrypted document; a PDF title and linearization parameter dictionary object generation unit used for generating a title and linearization parameter dictionary object content; a cross-reference table generation unit used for generating a main cross-reference table and a second cross-reference table; a tail dictionary generation unit used for generating a tail dictionary corresponding to the main cross-reference table and second cross-reference table; an object number processing unit used for processing a decrypted object number; an indirect reference object processing unit used for processing object numbers of all reference objects in the present object; an object content processing unit used for determining the type of the object content, and decrypting a flow object. The novel device and method can neglect the initial deviant calculating difficulty caused by content length changes before and after decryption; modularization design is convenient, thus fast recovering PDF documents.

Description

technical field [0001] The invention relates to the field of PDF (Portable Document Fotmat) file processing, in particular to a device and method for restoring a PDF file. Background technique [0002] As we all know, the full name of PDF is Protable Document Format, which can be translated into Portable Document Format and is an electronic file format. This file format has nothing to do with the operating system, that is to say, PDF files are universal no matter whether they are Windows, Uinx or Apple's Mac OS operating systems. Ideal document format for information dissemination. More and more e-books, product descriptions, e-mails, and network materials start to use PDF format files. [0003] The PDF file structure mainly consists of header, linearization parameter dictionary, second cross-reference table and corresponding tail dictionary, object content, main cross-reference table and corresponding tail dictionary. [0004] From 1993 to now, the PDF specification has ...

AI Technical Summary

Problems solved by technology

After decryption, the encrypted dictionary object is removed, which causes the content length of some objects to change. For example, the object number of object A is 100 before decryption, but becomes 99 after decryption. The length of the object number changes from 3 to 2, and the total length of object A The length is also correspondingly reduced by 1, which in turn causes the starting offset value of all objects behind object A to be reduced by 1; in addition, object A, as an indirect reference object, may also be referenced by object B, so the above problems will also occur

Since the positions of objects A and B in the PDF file structure are not known, the starting offset value of each object cannot be pre-calculated, but the starting offset value of each object is calculated in real time, thus bringing PDF document recovery belt Difficult to recover PDF documents quickly

In addition, assuming that the length of each object remains unchanged before and after decryption, although the initial offset value of each object after decryption can be quickly calculated, it is still necessary to process the object whose length has changed. If the processing is not correct, the PDF document cannot be restored correctly.

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