December 14, 2017

Why Linux HDCP Isn't the End of the World

Recently, Sean Paul from Google's ChromeOS team, submitted a patch series to enable HDCP support for the Intel display driver. HDCP - or High-bandwidth Digital Content Protection to its parents - is used to encrypt content over HDMI and DisplayPort links, which can only be decoded by trusted devices.

By Daniel Stone, Graphics Lead at Collabora.

HDCP is typically used to protect high-quality content. A source device will try to negotiate a HDCP link with its downstream receiver such as your TV or a frame-capture device. If a HDCP link can be negotiated, the pixel content will be encrypted over the wire and decrypted by the trusted downstream device. If a HDCP link cannot be successfully negotiated and pixel data remains unencrypted, the typical behaviour is to fall back to a lower resolution, or quality that is in some way less desirable to capture.

This is a form of copy protection usually lumped in with Digital Rights Management, something the open source community is often jumpy about. Most of the sound and fury typically comes from people mixing up the acronym with the kernel's display management framework called the Direct Rendering Manager; this is thus the first known upstream submission of DRM for DRM.

Regardless, there is no reason for the open-source community to worry at all.

HDCP support is implemented almost entirely in the hardware. Rather than adding a mandatory encryption layer for content, the HDCP kernel support is dormant unless userspace explicitly requests an encrypted link. It then attempts to enable encryption in the hardware and informs userspace of the result. So there's the first out: if you don't want to use HDCP, then don't enable it! The kernel doesn't force anything on an unwilling userspace. Sinks (such as TVs) cannot demand an upstream link provide HDCP, either.

HDCP support is also only over the wire, not on your device. A common misconception is that DRM means that the pixel frames coming from your video decoder are encrypted. Not so: all content is completely unencrypted locally, with encryption only occurring at the very last step before the stream of pixels becomes a stream of physical electrons on a wire.

Continue reading on Collabora's blog.

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