A researcher found a PDF key-generation path in MuPDF versions before 1.28.5 with only nine bits of effective randomness: 512 possible keys, with recovery demonstrated in roughly 11 milliseconds. The weakness was not in AES. MuPDF’s release history confirms a cryptographic random-number-generator fix in 1.28.5. The research also identified time-based key-generation problems in other PDF libraries. Updating software does not change keys in files already distributed, so sensitive documents may require separate review and re-encryption.
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Why some AES-256 PDFs had only nine bits of randomness
MuPDF fixed key generation, but previously distributed PDFs need separate review.