HDCP (High Bandwidth Digital Protection) is an encryption system for digital video, which was developed in 2003 by Intel. It applies to all content on Blu-ray Discs and HD DVDs, and allows the best possible resolution for display on the screen. This encryption is designed especially for copy protection, to avoid the connection of a recording device in-between the link to copy and illegally distribute the contents.

Since HDCP is designed for digital video, it is supported only from digital devices with DVI, HDMI or DisplayPort interface. If you try to be smart and connect a Blue-ray player together with an analog devices (eg VGA or component video), the HDCP protection prevents an image display totally or shows only a very poor resolution.

The encryption of the content is based on the very secure AES-128 standard and uses a 128-bit encryption. The key for decoding is partly stored within the player and the second part on the medium (disc) itself. Upfront the transfer, the concerned devices agree on a HDCP key, all  can handle. Each HDCP-enabled device supports around 40 keys that are changed with each transmission, sometimes are even altered during the transmission. The HDCP encryption actually doesn’t alter the video signals itselves, but opens or closes the video channel.

The transmission of HDCP protected content has been originally envisaged only for the direct connection between two devices. Both devices need to support the HDCP standard to ensure the exchange of the key information.

This works quite well in private households. In a professional environment, it is often necessary to transmit the audio/video signals over long distances or through switching systems. In this case the corresponding video splitters, video extenders, and video switches all must support HDCP to take part of the key information exchange. Every active device in the transmission chain must participate in the vote on the HDCP key to ensure an open video channel.

Even if all devices offer HDCP support, it can happen that the devices involved cannot find a shared key and the video is not displayed on the output device. This problem often occurs when connecting a HDMI device with a DVI device. In this case, you can try to force the source to the output HDMI video with RGB color encoding or replace the DVI device with a HDMI device.
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