The case of the 9 bit AES key
Summary
The article analyzes weaknesses in PDF encryption key generation where AES-256 keys are derived from insecure randomness. It reviews multiple implementations (MuPDF, Foxit, OpenPDF, iText) and demonstrates how a small state space enables practical brute-forcing, undermining the advertised 256-bit security. It also notes vendor responses and provides decryptors released by the researchers.