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Comment by huytersd | Hacker News Reader

I don’t understand. If you’re watching it the bits have been decrypted. Can’t you just get them from local memory?

Realistically, no. You don't have access to the device that contains the decrypted content.

Of course, you could try to open up or bypass the DRM module (e.g., via the GPU driver during hardware accelerated decoding), but that won't work with a TPM module and signed kernel space.

If you look at the security technologies introduced into consumer decides over the last decades, it all serves the dual purpose of securing your hardware from ... you.

Couldn’t you just plug a capture card into the display out?

HDCP.

Wow, so the decryption happens in the monitor? Can’t the capture card replicate that?

No, decryption keys are tightly held and inaccessible to general populations and small manufacturers and those used by illicit capture cards are regularly revoked iirc.

In regards to that, i assume that manufacturers have to apply for a cert for their device and then embed that cert, correct? Then if the device is found to be stripping HDCP the consortium can revoke that cert, but how? Sure you can do it for PCs and consoles, but are blu-ray players connected to the internet and auto-updated nowadays? Otherwise it'd be pretty easy for Chinese manufactures out of reach of the DMCA to just release one every few years and have it work for all devices prior.

"are blu-ray players connected to the internet and auto-updated nowadays?"

Some blu-rays force you to update before you can watch them. Also, the key revocation lists can (I believe) be included in the blu-ray itself to make them work offline, too.

yes, keys can be revoked [in offline players] by inserting a newer disc with a newer version of the "media key block." wikipedia suggests they're up to version 78 of that data.

https://en.wikipedia.org/wiki/Media_Key_Block

Easier to just recover via camera at this point.

Kinda. You need access to a decryption key. The ones known to be leaked will be revoked, so you need one that cannot be revoked, because the financial fallout would be too painful. That's why WEB-DL rips are typically made using the DRM keys extracted out of very popular consumer TV models.

So the WEB-DL rips are saved using homemade capture cards with the decryption module from popular consumer TVs since those can’t be revoked as they would brick those TVs?

Alternatively you plug into the output of the decryption module in the monitor before it hits the screen?

Roughly yes.

I believe you can also in some instances skip the hardware capture card and instead capture the data stream out of the host's RAM.

Inside the monitor, the decryption module and the main SoC controlling the hardware are usually the same physical chip, so grabbing data there will be close to impossible.

Every time you say 'just' an engineer who's already spent hours trying out your simple idea pulls out another clump of hair.

Well I’m clearly coming at this from a naive perspective and I’m sure many people are learning this along with me.

Excise the word "just" from your engineering vocabulary and you'll be better able to come across as naive instead of condescending or dismissive. I'm not trying to be snarky here, this is a genuine recommendation that will help your ideas be better received, both in the workplace and on internet forums.

That’s a good point. The sentence does sound better without “just”. Though in this case I was trying to account for what I thought was a simple solution.

It's encrypted across the cable itself, the capture card wouldn't be able to understand the signal.

the monitor itself is doing the unencrypting. As the other person said, it's specifically to protect the video from ... you.

>Of course, you could try to open up or bypass the DRM module (e.g., via the GPU driver during hardware accelerated decoding), but that won't work with a TPM module and signed kernel space.

Nope. Thanks to HDCP the data never hits the kernel or the host memory unencrypted. It doesn't even require TPMs or signed kernels.

That's not true, AFAIK. The DRM module (e.g., widevine) has to negotiate keys with the host and will decrypt the stream for decoding. The HDCP keys are static and independent from the streaming service. That decryption can happen with hardware support but it can also happen in software. I think most streaming services fall back from 4k to FHD or lower if their "security requirements" aren't fully met.

Also, if the DRM module supports hardware decoding, the GPU will need to see a decrypted stream. The GPU memory is in principle accessible to the driver. So if you can manipulate the driver, or setup another kernel module, you might be able to grab that content.

HDMI splitters have been a thing for years.

Apparently they present as a compliant HDMI output to the source device, but they don't require HDCP or similar on the output they're passing it on to.

From there, you're free to do whatever you want with the output.

At least in theory, HDCP keys can be revoced.

Manufacturing non-compliant devices could become extremely risky.

Well, these things have already existed for years...

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