HDCP… how is this still a thing?
The movie industry has foisted yet another version of the High-bandwidth Digital Content Protection system onto consumers in the likely futile hope of ‘stopping piracy’, writes Stephen Dawson.
We can all sympathise with anyone who wants to protect the fruits of their hard – and extremely expensive – labour from unauthorised use. But the movie industry seems to be quite prepared to sacrifice consumer satisfaction for a protection that has so far proved pointless, and will likely continue to be pointless.
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What it’s supposed to do
High-bandwidth Digital Content Protection (HDCP) is a system designed to protect against one form of digital copying.
There are essentially two ways that digital content can be copied. One way is by ‘ripping’. That is, by cracking whatever protections limit the use of a source file so that it can be transferred to a new, unprotected disc or placed on a media server.
The other way is to copy the material from the normal output of a playback device. HCDP offers a protection against this type of copying, and only this type.
Output protection has a long history. Both VHS and DVD implemented ‘Macrovision’. In the case of VHS, this imposed an excessive voltage in the vertical blanking interval of the analogue video signal, which essentially confused the input circuitry of a video recorder attempting to copy the signal, leading to terrible picture quality. DVD recorders had a more robust input system, but they respected Macrovision by detecting the marker on the analogue signal and simply refusing to record.
The picture signal on a DVD could not carry the traditional Macrovision marker since there are no blanking intervals in digital images. Instead the presence of Macrovision protection was indicated by a flag in the signal, and the DVD player would insert the necessary marker into its analogue output.
Of course, Macrovision was only applied to commercial releases – movies and TV shows and such – and even then not always. To use Macrovision a licence fee had to be paid. Indeed, the original 4:3 aspect ratio version of Harry Potter and the Philosopher’s Stone on DVD lacked Macrovision protection.
With the move to Blu-ray concern about copying continued. The rules governing Blu-ray insisted that no Blu-ray player launched after 1 January 2011 could output high definition video via its analogue video outputs with disc content which was protected by the AACS encryption used on Blu-ray (ie. just about all of it). As if there were high definition component video recorders readily available. And from the end of 2013 even standard definition analogue video output was prohibited. If you’ve been wondering about the paucity of outputs on modern Blu-ray players, now you know why.
Going Digital
But of course HDCP has nothing to do with analogue video. It is digital only, and is mostly seen with HDMI connections, although it may also be implemented on DVI and DisplayPort connections. Its main purpose is to allow only legitimate devices to receive and decode a HDMI signal.
It does this by requiring the source device to encrypt the digital data, and the receiving device, such as a TV (called a ‘sink’), to decrypt it again. Just in case, you know, someone has put some kind of intercept on the HDMI cable between source and sink.
Another type of device is also permitted: a repeater. This would typically be a home theatre receiver. The repeater doesn’t actually just ‘repeat’ the signal. It decrypts it and then re-encrypts it. It will definitely need to pull the audio out for playback, and may also apply processing to the video signal, and this can only be done to the original signal, not its encrypted version.
Encryption is of course what spies use, and also what your computer uses while you’re doing on-line banking. In these days of digital technology it is fairly efficient, but the weak point of encryption and decryption has always been key management. You need a key to encrypt and a key to decrypt and they must be related in some way.
All devices licensed to use HDCP include a set of component numbers which when combined with additional components provided by connected devices can generate unique keys for a particular session. HDCP provides for the revocation of the component numbers in a particular device should it be breached (this revocation list is carried by new DVDs and Blu-ray discs), giving equipment manufacturers a strong incentive to keep those component numbers secret. Unlicensed HDMI sinks cannot work with HDCP protected video because they don’t have the necessary component numbers built in.
Broken
When HDMI outputs first appeared on DVD players, and HDMI inputs on home theatre projectors, I always experienced a moment of breath-holding whenever switching things on. Would it all work? Or would the projector display a largely white screen sprinkled with small coloured dots? The latter indicated a HDCP failure, typically at the ‘handshaking’ key-exchange stage. Then there’d be a ritual of switching things off and on in different orders in order to invoke a new handshake. This became even more complicated once a HDMI-equipped receiver sat between source and sink.
Not that it happened a lot, but even one time in 20 is unsettling.
Over the years the bugs were worked out, and it is very rare that HDCP is anything other than totally invisible these days.
Meanwhile, the key system for HDCP 1.x – the system applying to Blu-ray and DVD – was alleged to have been broken back in the first half of the 2000s, and more definitively by 2013. Not, it must be noted, by copyright pirates, but by academics and enthusiasts who like challenges of this kind. Aside from upsetting copyright owners, this has had little practical import because no-one copies by means of the HDMI output stream.
The fact is, since soon after the introduction of the DVD, and then again soon after the introduction of Blu-ray, the encryption systems applied to the actual data on the actual discs was broken. Anyone wanting to copy high quality versions of the video for their own purposes, or for nefarious ones, has been able to rip the contents of discs at very little cost. The only disc format to avoid such a fate so far has been the SACD, and that’s only because SACD-compatible computer drives were never released (software running on the first couple of PS3 models – now fairly rare – can be and is used to rip SACDs).
So no-one is particularly interested in intercepting HDMI streams.
Still, Hollywood persists, with a new version of HDCP introduced. After a couple of abortive (ie. quickly broken) HDCP 2.x versions, HDCP 2.2 was settled upon for the protection of 4K content.
HDCP 2.2 is not backwards compatible with HDCP 1.x. It features beefed up encryption standards and a more robust key-establishment function, but shares the key revocation mechanism.
And as with HDCP 1.x when it was first introduced, it is causing problems.
Chipsets
The main problem with HDCP 2.2 is a hardware one. As devices were rolled out throughout 2014 with HDCP 2.2 support, it became apparent that HDCP 2.2-capable inputs had limitations. It turned out that while the chipsets required for HDCP 2.2 supported UHD and 4K video, they would not support the enhanced colour depths provided by many source devices. So they were limited to 8 bit colour.
In theory this should not make much difference at the moment because most sources of HD video – particularly Blu-ray and HDTV – are inherently 8 bit colour sources. While many quality Blu-ray players upconvert to 10 or more bits, there’s no reason why this cannot be done by the display from the same 8 bits.
However, future 4K/UHD source devices may well offer higher colour resolution – that alone would deliver a significant improvement in picture quality – and most existing displays won’t support this.
Indeed, most brands of home theatre receiver completely eschewed HDCP 2.2 for their 2014 models, citing inadequate performance. At least one brand offers HDCP 2.2 for one or two inputs, while the others support greater colour depths.
Likewise with 2014 model TVs: they typically omit HDCP 2.2, have it switchable, or have it fitted to just one HDMI input.
Conclusion
No doubt improved chipsets will appear this year and in future years. No doubt HDCP 2.2 will eventually, if not so immediately, become as transparent as HDCP 1.3 presently is.
But the question remains: why do they bother?
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