For Mac OS X, what is required for HDCP (video copy-protection) on an external monitor with HDMI? I have a (non-Retina) MacBook Pro (mid-2012) and I have just purchased an Asus VN248H-P external monitor with HDMI input, connected to the MacBook Pro with a Monoprice Mini-DisplayPort to HDMI cable. This is not a television set; it is a peripheral computer monitor.
For the first time, I tried to play a movie purchased and downloaded from iTunes on the external monitor, and I got an error message in iTunes saying that this is not supported because the Mac cannot verify HDCP support for my configuration. (This refers to the High-bandwidth Digital Content Protection encryption scheme).


The selected movie won't play on one of your connected displays.
This movie can be played only on displays that support HDCP (High-bandwidth Digital Content Protection).
Try moving the iTunes window to a different display. Make sure the entire window is on the supported display.

iTunes will only let me play the movie on the built-in screen on my (non-Retina) MacBook Pro, which is smaller and lower-resolution.
Is my Monoprice cable incompatible, and do I need to replace it with a different cable or adapter? Is there such a thing as a cable or adapter that works with HDCP and will enable this? Or is playing HDCP-encrypted content simply not possible on my configuration?
Please don't give me an answer that involves using software to break the HDCP encryption or to transcode to a different video format; I'd like to know if there is a hardware solution for full compatibility so that I don't lose any video resolution by transcoding.
The answer is that I seem to have encountered a bug in the implementation of dual displays in Mac OS X 10.9.1 Mavericks.

If I am using both the internal display and the external display at the same time, without using "Mirror" mode, then iTunes will not permit playback of the iTunes HDCP-encrypted movie on the external HDMI screen.

However, there is a workaround: if I close the MacBook Pro's lid, disconnecting the internal display, so that the MacBook Pro is only driving the external display, then iTunes will play the HDCP-encrypted movie on the external display over HDMI, in full-screen mode -- although iTunes still displays the same error message saying that it can't play back! 

This is quite inconvenient, because I want to use both displays at once, without Mirror mode, at all times, so, for example, I can have Apple Mail and Microsoft Word up and running on the internal display while I'm watching the iTunes HDCP movie on the external display. 

It looks as if I can further work around this bug, by first starting iTunes, then closing the lid, then starting the movie in iTunes on the external monitor, then raising the lid on the laptop again to activate the internal screen, then continuing to use other applications on the internal screen while the movie continues to play in iTunes on the external screen.