Ross Johnson created PDFBOX-5955:
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Summary: Support rare RC4 encryption where R=4, key length < 128
bits
Key: PDFBOX-5955
URL: https://issues.apache.org/jira/browse/PDFBOX-5955
Project: PDFBox
Issue Type: Improvement
Components: Crypto
Reporter: Ross Johnson
Attachments: R=4, V=4, 40-bit RC4.pdf, R=4, V=4, 48-bit RC4.pdf
I've come across some PDFs that open fine in Acrobat, but seemingly no other
viewer / parser. Upon further inspection, it seems these PDFs use a rare RC4
encryption scheme where R=4 & V=4, but the key length is given as 40 bits / 5
bytes instead of the normally expected 128 bits / 16 bytes.
I can't find this behavior explicitly described in the PDF specs, but it seems
that Acrobat extends the shorter key to 16 bytes (appending 0x00 bytes) in this
situation before the steps described in "Algorithm 1: Encryption of data using
the RC4 or AES algorithms".
I've been able to generate a sample file that demonstrates the issue and uses a
40-bit key. I also made a similar file with a 48-bit key. Note that these files
have an empty / blank user password, and Acrobat opens them without a password
prompt.
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