Forensic watermarking (forensic marking) in cinemas

The DCI Digital Cinema System Specification makes forensic marking part of every compliant playback system. It defines a Forensic Marker as the security component that inserts markings, data indicating the time, date and location of playback, into both image and audio in real time during playback. It also requires image, audio and auxiliary data forensic marking capability in each media block.

The mark has to go in early. The specification requires it to be inserted at the earliest point after decryption, before the content appears on any data bus outside the media block. In practice that means inside the integrated media block, so there is no unmarked picture anywhere in the booth to tap.

The point of the mark is tracing. If someone camcords a screening, the copy carries a code that identifies the auditorium and the time. The mark does not identify an audience member. It identifies your screen, which is why distributors and trade bodies ask cinemas to take anti-camcording measures seriously.

The operational consequence is the one that bites. According to Cinepedia, the DCI specification requires playout to fail if the server's forensic marking is not working. A media block that reports a marking fault will refuse encrypted content, even with a valid KDM. A studio can set a flag in the key that disables marking for that title, which also lets it play on a failed marker, but that is the distributor's decision, not the cinema's.