"Unknown encryption type R = 5" about openpdf HOT 6 OPEN

@Lonzak already correctly explained that R=5 encryption is part of an Adobe extension to PDF that never became standard and only is mentioned as deprecated and forbidden to use in ISO 32000-2.
In contrast to his assumption "Maybe it is only extending R=4 with AES256", though, the R=5 algorithm is more related to R=6 than R=4. The differences between R=5 and R=6 mostly are additional variable numbers of hashing rounds in the latter to make breaking the encryption harder for brute force and quantum approaches.

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MKL implemented support for R=6 some time ago (one of the best additions to openPDF imho).
However R=5 is a proprietary adobe extension which has been deprecated and should not be used. (Thats why it wasn't implemented)
You are of course free to implement it - perhaps mkl can give you some pointers. Maybe it is only extending R=4 with AES256?

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In contrast to his assumption "Maybe it is only extending R=4 with AES256", though, the R=5 algorithm is more related to R=6 than R=4.

...was just a (wild) guess ;-)

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Let's pretend that I write a perfect PR adding R = 5 support and it's merged. How long would it take to get a new version out in Maven Central?

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You are of course free to implement it

I'm going to keep getting PDFs with R = 5. So, either I'll have to:

1. implement it as you suggest, or
2. switch to PDFBox, which would be a pretty significant change to our code base.

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@brettrowberry please submit that perfect PR adding R = 5 support. I am sure the new version would be available within reasonable time in Maven Central. I am also seeing this Unknown encryption type R = 5 error in production, however the percentage is not very large.

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