7 ms·

So, the only thing stopping a user from obtaining a clean copy of the content is the fact that the CDM is closed source? What if a CDM vendor leaked the source

So, the only thing stopping a user from obtaining a clean copy of the content is the fact that the CDM is closed source? What if a CDM vendor leaked the source code? I guess the DRM server would block them after a while, but before they notice, the user would have free access to the content, right?

deleted 6y ago
[deleted]
drudu 6y ago
You're completely right but the source code doesn't even need to be leaked. Just reverse engineered. How else would we pirate movies from Netflix/Amazon/etc :)
grishka 6y ago
DRM is the epitome of security through obscurity.
ponderingfish 6y ago
Exactly - which is why the CDM is a contentious topic because you have no way to see what's inside it. Also, technologies such as HDCP, Trusted Execution Environment prevent the decrypted and decoded frames from being leaked. At least, that's the intention. And the CDM vendors are Google, MSFT, and Apple. There's no way they are going to leak their code :) What's really strong is the rotation of decryption keys. Content providers rotate the keys often (at times, in the middle of a movie, and very often during a live stream) to deter people from cracking the key. Cracking DRM can be done and the goal from the other side is to make it hard and expensive. Nothing is impossible :)
- grishka 6y ago
  IMO it isn't that hard to crack a DRM system. My understanding is that most of the protections are legal, not technical, as in, if you do it, they'll sue the hell out of you and win.
  - svenpeter 6y ago
    Yeah, there are many trivial systems out there that mostly provide legal protection. But there are also some pretty sophisticated DRM systems that are technologically quite well implemented and take a large effort to break. BD+ is one of these for example. And maybe going to platforms where the whole hardware is specialized is not what most people have in mind when they think DRM but the basic building blocks are similar. Microsoft's "Guarding Against Physical Attacks: The Xbox One Story" talk is also one of the few examples I know of where a manufacturer explains how they implemented such a system. [1] [1] https://www.platformsecuritysummit.com/2019/speaker/chen/ https://www.platformsecuritysummit.com/2019/speaker/chen/
    - grishka 6y ago
      Thanks for linking this talk, it was indeed interesting. Especially the part where he said that "if the hack costs more than 10 games, we're fine". Indeed, you probably aren't breaking this thing unless you have millions of dollars worth of equipment and the expertise to use it. But then video game consoles are special. They were always these closed, tightly-controlled ecosystems. General-purpose devices, like phones and computers, typically lack this kind of integration between their components. Yes, there's ARM TrustZone, and that's about it. Thankfully. And I'm not aware of anything similar on x86 PCs (besides the Apple T2, which was recently compromised).
      - vengefulduck 6y ago
        Intel SGX?
        grishka 6y ago
        I've seen this name but still don't understand where's its root of trust. In other words: - How does the program making use of it make sure it's not been patched so the code that's supposed to run in the enclave runs without one? - How does it know it's not being run on an emulated CPU or in a VM? And that those checks haven't been patched out, too? - How does it make sure the OS is cooperating? I assume there is some cooperation required from the kernel at least.
- MaxBarraclough 6y ago
  > the CDM vendors are Google, MSFT, and Apple Also Adobe, although I'm not sure if their CDM (Adobe Primetime) is still relevant. > which is why the CDM is a contentious topic because you have no way to see what's inside it Related to this: Firefox takes steps to try to contain the CDM blob within a sandbox. [0] See also this old blog post from before Firefox switched from Adobe's CDM to Google's WideVine CDM. [1] [0] https://wiki.mozilla.org/Security/Sandbox/Architecture#GMP_process_.28Widevine.2C_Primetime.2C_OpenH264.29 https://wiki.mozilla.org/Security/Sandbox/Architecture#GMP_p... [1] https://hacks.mozilla.org/2014/05/reconciling-mozillas-mission-and-w3c-eme/ https://hacks.mozilla.org/2014/05/reconciling-mozillas-missi...
marcan_42 6y ago
It's security through obscurity. For pure software stuff, like Widevine L3, you can just patch the browser or insert a shim to spit out the decrypted video somewhere. https://github.com/artemist/widevineproxy https://github.com/artemist/widevineproxy
- Thorrez 6y ago
  Is that the decrypted and decompressed video, or decrypted and still compressed? If the former, that's going to be a gigantic file, and you're going to lose quality when recompressing (although if you were planning to recompress anyway the quality loss is no worse).
- bitwize 6y ago
  Illegal to distribute under the DMCA.
kmeisthax 6y ago
The primary purpose of content encryption tools is to trigger DMCA 1201 anti-circumention provisions to prohibit people from writing decryption tools. Nothing more and nothing less. All DRM schemes are a form of obfuscation; some particularly secure ones use hardware decoding enclaves not present on desktops that are harder to crack, but that's about it. The only practically useful DRM schemes are ones where any tool that could circumvent them would be so special-purpose as to be illegal. Here's how that works legally. DMCA 1201 actually comes in two parts: the first prohibits you from circumventing ANY technological measure (yes, even something as simple as a right-click blocker, domain lock, Youtube's rolling URL cipher, or what have you) and then lays out exceptions shaped similarly enough to fair use that the first part really doesn't alter the law beyond adding another tort to the pile. What's already infringement becomes infringement with a circumvention tort; and what's already fair use stays fair use. However, the second part is rather broad. It prohibits the marketing of any sort of tool whose sole purpose is circumvention, lacks non-circumvention uses, or is marketed for circumvention. In practice, this means that there's a dividing line between DRM that can be circumvented with standard tools and those that require domain specialists to write you a tool. You can circumvent YouTube's "rolling cipher" by just launching OBS. You can't circumvent DVD CSS with anything but a specialized tool that falls under the second part of DMCA 1201 and is thus illegal to provide - even for otherwise legal circumvention. (Yes, I know DVD CSS is laughably bad encryption; but the law doesn't care. You still have to write some code to brute force the laughably short ITAR-compliant key lengths.) If the CDM vendor themselves released their source code, I doubt that would be considered a circumvention tool per se. If they had also started telling people to pirate movies while they still can, then they are violating the law. However, the CDM vendor likely promises to their customers not to do so, possibly in contracts and definitely implied in their marketing material. Releasing source moves the DRM scheme from the "domain specialists only" side to the "10 minutes with developer tools" side.
- ponderingfish 6y ago
  Super answer!
- shmerl 6y ago
  So all this will become a pile of wasted effort once DMCA 1201 is repealed.
brigade 6y ago
For HD, you'd also need a valid vendor-signed key to do the key exchange (which would get swiftly revoked once public), and/or to crack the trusted execution environment.
- grishka 6y ago
  > which would get swiftly revoked once public And then what happens to all those devices people bought and expect to be able to watch Netflix on?
  - Mindwipe 6y ago
    They get downgraded to SD.
    - grishka 6y ago
      And then they keep wondering why people pirate...
- kevingadd 6y ago
  This isn't true. Multiple Google devices have had their Widevine keys leaked/recovered and Google has waited around before revoking them since it would break hardware playback on the device or cause other issues. The Nexus 6, for one, has been downgraded from Widevine L1 to L3 as a result.
  - brigade 6y ago
    I guess I'm sort of surprised that a unique key per device isn't common yet...

deleted 6y ago

[deleted]

drudu 6y ago

You're completely right but the source code doesn't even need to be leaked. Just reverse engineered. How else would we pirate movies from Netflix/Amazon/etc :)

grishka 6y ago

DRM is the epitome of security through obscurity.

ponderingfish 6y ago

Exactly - which is why the CDM is a contentious topic because you have no way to see what's inside it. Also, technologies such as HDCP, Trusted Execution Environment prevent the decrypted and decoded frames from being leaked. At least, that's the intention. And the CDM vendors are Google, MSFT, and Apple. There's no way they are going to leak their code :) What's really strong is the rotation of decryption keys. Content providers rotate the keys often (at times, in the middle of a movie, and very often during a live stream) to deter people from cracking the key. Cracking DRM can be done and the goal from the other side is to make it hard and expensive. Nothing is impossible :)

grishka 6y ago
IMO it isn't that hard to crack a DRM system. My understanding is that most of the protections are legal, not technical, as in, if you do it, they'll sue the hell out of you and win.
- svenpeter 6y ago
  Yeah, there are many trivial systems out there that mostly provide legal protection. But there are also some pretty sophisticated DRM systems that are technologically quite well implemented and take a large effort to break. BD+ is one of these for example. And maybe going to platforms where the whole hardware is specialized is not what most people have in mind when they think DRM but the basic building blocks are similar. Microsoft's "Guarding Against Physical Attacks: The Xbox One Story" talk is also one of the few examples I know of where a manufacturer explains how they implemented such a system. [1] [1] https://www.platformsecuritysummit.com/2019/speaker/chen/ https://www.platformsecuritysummit.com/2019/speaker/chen/
  - grishka 6y ago
    Thanks for linking this talk, it was indeed interesting. Especially the part where he said that "if the hack costs more than 10 games, we're fine". Indeed, you probably aren't breaking this thing unless you have millions of dollars worth of equipment and the expertise to use it. But then video game consoles are special. They were always these closed, tightly-controlled ecosystems. General-purpose devices, like phones and computers, typically lack this kind of integration between their components. Yes, there's ARM TrustZone, and that's about it. Thankfully. And I'm not aware of anything similar on x86 PCs (besides the Apple T2, which was recently compromised).
    - vengefulduck 6y ago
      Intel SGX?
      - grishka 6y ago
        I've seen this name but still don't understand where's its root of trust. In other words: - How does the program making use of it make sure it's not been patched so the code that's supposed to run in the enclave runs without one? - How does it know it's not being run on an emulated CPU or in a VM? And that those checks haven't been patched out, too? - How does it make sure the OS is cooperating? I assume there is some cooperation required from the kernel at least.
MaxBarraclough 6y ago
> the CDM vendors are Google, MSFT, and Apple Also Adobe, although I'm not sure if their CDM (Adobe Primetime) is still relevant. > which is why the CDM is a contentious topic because you have no way to see what's inside it Related to this: Firefox takes steps to try to contain the CDM blob within a sandbox. [0] See also this old blog post from before Firefox switched from Adobe's CDM to Google's WideVine CDM. [1] [0] https://wiki.mozilla.org/Security/Sandbox/Architecture#GMP_process_.28Widevine.2C_Primetime.2C_OpenH264.29 https://wiki.mozilla.org/Security/Sandbox/Architecture#GMP_p... [1] https://hacks.mozilla.org/2014/05/reconciling-mozillas-mission-and-w3c-eme/ https://hacks.mozilla.org/2014/05/reconciling-mozillas-missi...

marcan_42 6y ago

It's security through obscurity. For pure software stuff, like Widevine L3, you can just patch the browser or insert a shim to spit out the decrypted video somewhere. https://github.com/artemist/widevineproxy https://github.com/artemist/widevineproxy

Thorrez 6y ago
Is that the decrypted and decompressed video, or decrypted and still compressed? If the former, that's going to be a gigantic file, and you're going to lose quality when recompressing (although if you were planning to recompress anyway the quality loss is no worse).
bitwize 6y ago
Illegal to distribute under the DMCA.

kmeisthax 6y ago

The primary purpose of content encryption tools is to trigger DMCA 1201 anti-circumention provisions to prohibit people from writing decryption tools. Nothing more and nothing less. All DRM schemes are a form of obfuscation; some particularly secure ones use hardware decoding enclaves not present on desktops that are harder to crack, but that's about it. The only practically useful DRM schemes are ones where any tool that could circumvent them would be so special-purpose as to be illegal. Here's how that works legally. DMCA 1201 actually comes in two parts: the first prohibits you from circumventing ANY technological measure (yes, even something as simple as a right-click blocker, domain lock, Youtube's rolling URL cipher, or what have you) and then lays out exceptions shaped similarly enough to fair use that the first part really doesn't alter the law beyond adding another tort to the pile. What's already infringement becomes infringement with a circumvention tort; and what's already fair use stays fair use. However, the second part is rather broad. It prohibits the marketing of any sort of tool whose sole purpose is circumvention, lacks non-circumvention uses, or is marketed for circumvention. In practice, this means that there's a dividing line between DRM that can be circumvented with standard tools and those that require domain specialists to write you a tool. You can circumvent YouTube's "rolling cipher" by just launching OBS. You can't circumvent DVD CSS with anything but a specialized tool that falls under the second part of DMCA 1201 and is thus illegal to provide - even for otherwise legal circumvention. (Yes, I know DVD CSS is laughably bad encryption; but the law doesn't care. You still have to write some code to brute force the laughably short ITAR-compliant key lengths.) If the CDM vendor themselves released their source code, I doubt that would be considered a circumvention tool per se. If they had also started telling people to pirate movies while they still can, then they are violating the law. However, the CDM vendor likely promises to their customers not to do so, possibly in contracts and definitely implied in their marketing material. Releasing source moves the DRM scheme from the "domain specialists only" side to the "10 minutes with developer tools" side.

ponderingfish 6y ago
Super answer!
shmerl 6y ago
So all this will become a pile of wasted effort once DMCA 1201 is repealed.

brigade 6y ago

For HD, you'd also need a valid vendor-signed key to do the key exchange (which would get swiftly revoked once public), and/or to crack the trusted execution environment.

grishka 6y ago
> which would get swiftly revoked once public And then what happens to all those devices people bought and expect to be able to watch Netflix on?
- Mindwipe 6y ago
  They get downgraded to SD.
  - grishka 6y ago
    And then they keep wondering why people pirate...
kevingadd 6y ago
This isn't true. Multiple Google devices have had their Widevine keys leaked/recovered and Google has waited around before revoking them since it would break hardware playback on the device or cause other issues. The Nexus 6, for one, has been downgraded from Widevine L1 to L3 as a result.
- brigade 6y ago
  I guess I'm sort of surprised that a unique key per device isn't common yet...