Re: [libreoffice-users] Encrypted Calc file got corrupted

2016-02-29 12:20 GMT+01:00 Adam Bujdoso <adam.bujdoso@gmail.com>:

> Does anyone have an idea how the file could be repaired? I am happy to
> forward the file if someone has a tool to examine it, or perhaps to recover
> it.

​Unfortunately, I can't do it myself because I lack the time to do so, but
if someone's up to the task, it could be interesting to have an "extractor"
for secured files.

From my understanding of the specification, password-protected files are
"just" like regular files, meaning that they still are ZIP containing
multiple files. The password-protection is not at the ZIP level, but
instead relevant files are encrypted before they are stored in the ZIP
file.​ To remain efficient, these files are compressed using deflate (the
same algorithm used in zip files) *before* encryption, and then simply
stored in the ZIP file.

An extracting tool could extract the content of the ODT/ZIP file, and using
the password decrypt the relevant files. This would make the file
"components" available for further examination. Since sometimes file
corruption can merely mean a malformed closing tag in the XML that makes up
the core of a document, it could be helpful.

This could also make for a useful side-tool for LibreOffice, since these
corruptions, as rare as they are, still happens sometimes even on
unencrypted files.

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