Fix issues with object importing and tolerate unpadded empty strings in encrypted documents

MatthiasValvekens commented 3 months ago

Description of the changes

Make object importer more resilient
- Correctly copy references to the root object
- Deal with deep object graphs in a more stack-safe manner (we now only recurse within top-level objects, those usually are not that deep)
When decrypting strings and streams using one of PDF's AES-CBC-based crypt filters, if the plaintext is empty after decryption, do not reject it for lack of padding.

See #412.

Checklist

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MatthiasValvekens commented 3 months ago

@redfast00, you might want to throw your files at this one.

codecov-commenter commented 3 months ago

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.83%. Comparing base (6177c1f) to head (a605c21). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #414 +/- ## ======================================= Coverage 98.83% 98.83% ======================================= Files 113 113 Lines 16265 16298 +33 ======================================= + Hits 16075 16108 +33 Misses 190 190 ``` | [Flag](https://app.codecov.io/gh/MatthiasValvekens/pyHanko/pull/414/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthias+Valvekens) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/MatthiasValvekens/pyHanko/pull/414/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthias+Valvekens) | `98.83% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthias+Valvekens#carryforward-flags-in-the-pull-request-comment) to find out more.

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