MAINT: Simplify file identifiers generation

[exiledkingcc]

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (ec85a27) 94.54% compared to head (40bb17f) 94.52%. Report is 1 commits behind head on main.

Files	Patch %	Lines
pypdf/_writer.py	90.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

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- Coverage   94.54%   94.52%   -0.02%     
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  Files          43       43              
  Lines        7549     7549              
  Branches     1490     1491       +1     
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- Hits         7137     7136       -1     
  Misses        253      253              
- Partials      159      160       +1     

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[MartinThoma]

What impact do the file identifiers have? Who/what makes use of them?

[exiledkingcc]

the PDF standard says:

The calculation of the file identifier need not be reproducible; all that matters is that the identifier is likely to be unique. For example, two implementations of the preceding algorithm might use different formats for the current time, causing them to produce different file identifiers for the same file created at the same time, but the uniqueness of the identifier is not affected.

the identifiers are also be used for encryption.

@MartinThoma so i think it's ok to make it simple.

[MartinThoma]

Having a deterministic way to generate PDFs is valuable to several developers. Does the current deterministic identifier generation cause any issues?

[exiledkingcc]

first of all, it cost too much for big pdf files. and for aes encrypted pdf, it's not deterministic. when PdfWriter.encrypt called, the identifiers are genearated by uncrypted pdf stream, then PdfWriter.write called, the content of pdf file is encrypted, so the hash changed. for encrypted pdf, identifiers must be generated before write to stream, since the identifier will be used to calculate the key, so the identifiers cannot be the hash of pdf stream content.