Can someone else plz explain to me how the DRM in HD video content works?

[Sureshot324]

The way I understand it, the content is encrypted, and the decryption takes place in the monitor/TV. So if you were to play an HD-DVD in a computer, the computer would read the encrypted content from the disk, send it to the monitor still encrypted, and the monitor would decrypt it and display it.

However, as far as I can tell, this would mean there would have to be only ONE encryption key for all HD content. There could not be a different encryption key for every HD-DVD, because then the computer would have to either read the key from the disk or get it from the internet, which would mean a software program could get the key and decrypt the content in software, rendering the DRM useless.

However, if there's only ONE encryption key and that key is stored in every computer monitor and TV in the world, it's only a matter of time before that key is public knowledge, again rendering the DRM useless.

Can someone please explain to me what I'm missing?

 

[Aoleleb]

That is HDCP here is the wikipedia article on it. In addition to the moniter supporting it, the video card also has to support it.

Here is an article about the recently released HD DVD backup software.

 

[MrChad]

HDCP is only part of the story. AACS is the actual DRM system, and it is explained well on Wikipedia.

There is a unique encryption title key for each DVD, which is encrypted and stored on the disc. Each licensed player has its own key for decryption, and player keys can be revoked (a blacklist exists on each title) if the player is compromised.

 

[Sureshot324]

Ok I think I have a better understanding now.

-Each disc has a unique key
-That key is stored on the disk, but the key itself is encrypted
-Each model of player has it's own key to decrypt the key on the disk
-If a model becomes compromised (it's key becomes publicly known) that model will be blacklisted, so the keys on all future disks will be encrypted in a way that they cannot be decrypted by that model's key, but can still be decrypted by any other model
-Once the key on the disk is decrypted, it is sent directly to the monitor through hardware, either through a cable from the drive to the video card, or through the monitor. The key never goes into system ram, so there's no way the software can read it.
-The video is sent to the monitor still encrypted, and is then decrypted in the monitor.


Seems like a pretty solid system, and DVD rippers as we know them will probably be impossible. However, I think what will happen is a determined hacker will modify the hardware of a computer to intercept that key. After all, it has to be sent from the drive to the monitor unencrypted.

He can then rent a bunch of HD-DVD/Blu-Ray disks and publish the keys on the internet. Someone can then create a ripper that simply requires you to enter the key of the movie you are ripping.

 

[shortylickens]

I know this has been dicussed ad nauseum, but I'm pretty sure there will be generic or "open" players very soon and I am also sure that the so-called "encryption" will be cracked very soon.

 

[MrChad]

Originally posted by: Sureshot324
Ok I think I have a better understanding now.

-Each disc has a unique key
-That key is stored on the disk, but the key itself is encrypted
-Each model of player has it's own key to decrypt the key on the disk
-If a model becomes compromised (it's key becomes publicly known) that model will be blacklisted, so the keys on all future disks will be encrypted in a way that they cannot be decrypted by that model's key, but can still be decrypted by any other model
-Once the key on the disk is decrypted, it is sent directly to the monitor through hardware, either through a cable from the drive to the video card, or through the monitor. The key never goes into system ram, so there's no way the software can read it.
-The video is sent to the monitor still encrypted, and is then decrypted in the monitor.


Seems like a pretty solid system, and DVD rippers as we know them will probably be impossible. However, I think what will happen is a determined hacker will modify the hardware of a computer to intercept that key. After all, it has to be sent from the drive to the monitor unencrypted.

He can then rent a bunch of HD-DVD/Blu-Ray disks and publish the keys on the internet. Someone can then create a ripper that simply requires you to enter the key of the movie you are ripping.

There is a program called BackupHDDVD that was released a week or so ago that claims to decrypt HD-DVDs as long as you have the disc's title key available. The problem is that no one knows how the author got the title key(s), and he refuses to tell (except for a few vague hints).

Remember, the purpose of HDMI/HDCP is to keep the signal encrypted all the way to the monitor. If content providers elect to enable the Image Constraint Token (ICT) on their DVDs, the signal will be downsampled to a lower resolution if it is intercepted or output over a non-HDCP signal.

 

[Sureshot324]

Originally posted by: MrChad

Originally posted by: Sureshot324
Ok I think I have a better understanding now.

-Each disc has a unique key
-That key is stored on the disk, but the key itself is encrypted
-Each model of player has it's own key to decrypt the key on the disk
-If a model becomes compromised (it's key becomes publicly known) that model will be blacklisted, so the keys on all future disks will be encrypted in a way that they cannot be decrypted by that model's key, but can still be decrypted by any other model
-Once the key on the disk is decrypted, it is sent directly to the monitor through hardware, either through a cable from the drive to the video card, or through the monitor. The key never goes into system ram, so there's no way the software can read it.
-The video is sent to the monitor still encrypted, and is then decrypted in the monitor.


Seems like a pretty solid system, and DVD rippers as we know them will probably be impossible. However, I think what will happen is a determined hacker will modify the hardware of a computer to intercept that key. After all, it has to be sent from the drive to the monitor unencrypted.

He can then rent a bunch of HD-DVD/Blu-Ray disks and publish the keys on the internet. Someone can then create a ripper that simply requires you to enter the key of the movie you are ripping.

There is a program called BackupHDDVD that was released a week or so ago that claims to decrypt HD-DVDs as long as you have the disc's title key available. The problem is that no one knows how the author got the title key(s), and he refuses to tell (except for a few vague hints).

Remember, the purpose of HDMI/HDCP is to keep the signal encrypted all the way to the monitor. If content providers elect to enable the Image Constraint Token (ICT) on their DVDs, the signal will be downsampled to a lower resolution if it is intercepted or output over a non-HDCP signal.

The movie is encrypted all the way to the monitor, but the title key is not. It is decrypted in the drive/player, and then sent to the monitor unencrypted. Otherwise how would the monitor be able to decrypt the movie?

I'm not sure how they send the key to the monitor, but I'm sure it's in a way that software can't read it. For example, if there was a cable from the drive to the video card just to transfer the key, then there's no way someone could write a software hack to get that key. It would require modification of the hardware, but I'm sure that could be done.

 

[Mark R]

There are several stages.

1. Disc -> Drive
The disc is encypted with a 'title key'. On the disc are a long list of encrypted 'title keys'. Each model of drive will have it's own drive key. It's drive key should then be able to decrypt one of the title keys. (The other encrypted keys are decryptable by other models of drive).

The disc also contains data on whether particular drives have been blocked, particular software has been blocked, or whether there are new keys. The drive will read this whenever a disc is inserted. If the data on the disc is newer than the data in the drive's firmware, it will automatically flash the new data into its firmware.

The encrption key system is designed so there are there are many levels of 'master keys' - a so called 'broadcast encryption' system. This means each individual drive could have its own set of keys, yet the key list on the disc wouldn't need to be very long. It is possible to choose the list of keys included on the disc in such a way that they can deactivite either a single drive key, a set of keys belonging to a model, or a set of keys belonging to a whole manufacturer.

2. Drive -> PC

The drive verifies that the PC OS (or DVD player device) is authorised to access the drive. The drive has a list of authorised OSs/software/Player hardware (the host), and will verify that the device posseses the correct encryption certificate to access the data on the disc.

As in step 1, the encryption system allows individual player softwares, etc. to be blocked. The drives also hold a list of blocked hosts in firmware (autoflashed when a disc is inserted) and will shutdown if the host is on the blocked list.

Once the host has been authorised, the decrypted 'title key' is given to the host. The host can then read the encrypted data off the drive.

3. Player

Using the decryption key, the player decrypts the data it reads from the main files on the disc. It then passes the data through the appropraite CODEC to produce a series of uncompressed frames which need to be sent to the screen.

(The AACS 'hack' attacked this stage - by using debugger software it was possible to extract the 'title key' from RAM being used by a software HDDVD player)

4. Graphics card/hardware

The uncompressed images are sent to the graphics card for display. In the case of PCs, the software should check if the graphics card supports HDCP. If it doesn't the software should refuse to decode the video. In the case of Windows Vista, the OS will somehow detect that a premium disc is being played, and intercept the data before it gets to the graphics card, degrading it.

The data at this stage may not be encrypted. However, in Vista - MS have added a facility that would allow the data to be re-encrypted on its way to the graphics card.

5. Graphics hardware -> Display

The HDCP encrypter (grpahics card) and decrypter (monitor) have their own set of keys (each model of device has a unique set of keys). The two systems negotiate encrypted communication between each other.

The encrypter has in its firmware a list of blacklisted decrypters. (So if someone buys a set of grey-market HDCP decrypter chips, and makes them into decryption boxes instead of plasma TV as they promised, those chips can be blocked). The firmware can be updated with new lists as newly discovered compromised decryption chips are discovered.

I'm not sure how this firmware update would take place (possibly new graphics card drivers may include new lists), or possibly there is someway for the data on protected discs to automatically flash the HDCP chip (much as how the drive self-flashes).

Again, the driver issue may be a possibility, as Vista 64 bit (which is the only version to support HDCP fully) has the ability to blacklist driver versions. So if a certain type of decrypter box was released to market, updated drivers could be issued that could not communicate with that box. Subsequently the old drivers could be blacklisted, so could no longer be installed.

 

[Sureshot324]

Thanks for the explanation, but if that is true, the whole system seems useless to me. If the unencrypted key is going to be in system ram, accessible by any software program, why bother encrypting the movie at all? Why re-encrypt the video in the video card as it's transfered to the monitor when software is the critical step where everyone will be copying their movies?

 

[Mark R]

Originally posted by: Sureshot324
Thanks for the explanation, but if that is true, the whole system seems useless to me. If the unencrypted key is going to be in system ram, accessible by any software program, why bother encrypting the movie at all? Why re-encrypt the video in the video card as it's transfered to the monitor when software is the critical step where everyone will be copying their movies?

Well, exactly.

The key has to be in RAM in order for the decryption to take place, so that the data can be decompressed. However, in a well designed OS there should be no way for 1 program to access RAM allocated to another. (The exception is debugger software).

However, even despite this, there are things that can be done to try and defeat debugger software. There are a number of obfuscation techniques that can be used (e.g. complex code with multiple loops, redundant code, fake code, etc.).

There are also active techniques such that the decryption key is itself kept encrypted, and every second or so, the concealment key is changed, and the key moved to a different area in memory. These types of technique are established in security applications.

Nevertheless, these are only band-aids on an open sore. The studios are aware of this, and this is the reason they have not allowed MS to support 'premium content' in 32 bit versions of Vista.

The new 64 bit Vista has a number of security enhancements, including the ability for encryption keys to be concealed in secure kernel RAM. This is beyond the prying eyes of any program or debugger. This RAM is only available to drivers and the Windows core. However, hackers will have difficulty writing hacked drivers to access this RAM - Vista 64 will not allow privilieged drivers to run unless digitally signed by MS as genuine (i.e. there will be no way to install 'modded' graphics card drivers).

 

[MrChad]

Originally posted by: Mark R
The studios are aware of this, and this is the reason they have not allowed MS to support 'premium content' in 32 bit versions of Vista.

This is not true, unless something new has come to light. 32-bit versions of Vista should support HD content.

http://www.engadget.com/2006/08/25/micr...-and-blu-ray-playback-in-32-bit-vista/