System/Posix/Ap4PosixRandom uses /dev/random. The read function to /dev/random can be blocking. In our case this is not acceptable. Using /dev/urandom is sufficient for encryption key generation and the read function is never blocking. For an extended explanation I recommend http://www.2uo.de/myths-about-urandom/.

Best Regards,
Armin Trattnig (bitmovin)