How HDCP works

High-Bandwidth Digital Content Protection (HDCP) is one of those buzzwords that’s currently shrouded in rumour, misinformation and a fair bit of needless scare-mongering.

As its name suggests, HDCP is designed to protect the integrity of digital content as it travels from one device to another. Originally it operated only over the DVI (Digital Visual Interface) connections, but has since been updated to work over the HDMI (High Definition Multimedia Interface) connections common on consumer electronics devices.

Developed by Intel and licensed by Digital Content Protection, a subsidiary of Intel, the idea of HDCP is to prevent unscrupulous individuals or devices from intercepting protected digital content as it travels over the DVI or HDMI cables. It is not a copy protection technology, however.

It also ensures that compliant devices will only talk to other compliant devices. Only ‘trustworthy’ devices will be licensed, which means that manufacturers must take adequate steps to prevent unauthorised access to the content or risk having their licences revoked.

HDCP uses data encryption algorithms with unique encryption/decryption keys assigned to each compliant device. Devices can take the form of transmitters, receivers or, less commonly, repeaters.

In a typical PC-based scenario, your graphics card functions as an HDCP transmitter and your display as an HDCP receiver. Repeaters come into play when a single device must both receive and transmit the content as it passes through it – an example would be a home audio/video amplifier with multiple inputs and outputs.

Each HDCP-compliant device contains a unique set of 40 encryption keys. These keys are stored within the device in a form that’s designed to be inaccessible to users – usually on a Rom (read-only memory) chip.

The secrecy and integrity of these keys is of the utmost importance and forms part of the HDCP licence agreement.

Incidentally, it is the Rom containing these keys that was absent from the first models of ‘HDCP-capable’ graphics cards that made it to market. Since HDCP is entirely reliant on these encryption keys for operation, no product supplied without them can function as an HDCP-compliant device.

A special key, called a Key Selection Vector (KSV) is exchanged during the initial setup phase of an HDCP connection. These KSVs combine with the device’s secret keys to form an encrypted link that can only be decrypted by the specific devices involved.

If a device is determined to have broken the terms of the licence agreement, then this key can be added to a ‘revocation list’. This list is then distributed with future content (discs or other digital media) and will cause HDCP transmitters to refuse to send protected content to the revoked device.

Any software sitting in the playback chain also needs to be HDCP capable, and again if the keys are compromised the software can be blacklisted by the revocation list.

For those with a strong stomach for technical details, the latest specification of the HDCP system currently stands at version 1.2 and can be downloaded directly from Digital Content Protection’s website.

This article is part of a group test of HDCP Vista-ready TFT monitors.
See also
Dell 3007WFP
Samsung SM205BW
Samsung SM215TW
Sony MFM-HT205
Viewsonic VP233wb
The HDCP chain
How we tested the monitors

Performance graphs and table of features can be read via our pdf downloads

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