CSS

The Content Scramble System (CSS) is a digital rights management (DRM) and encryption system employed on many commercially produced DVD-Video discs.

However, it’s heavily flawed. It was originally defeated back in 1999 by DeCSS. The 40-bit key size can be subjected to a brute-force attack as the effective key size is only about 16 bits. That’s a mere two bytes. It can be brute-forced in about a minute on even an old Pentium II and effectively instantly on modern hardware.

Where CSS is Applied

CSS is applied on ISO 9660 Sectors containing VOB data in it’s entirety, not on each file. Header and IFO sectors are unprotected. A sector can contain one or more files’ data, and one file’s data may reside on multiple sectors.

If you backup a protected disc to a VIDEO_TS folder without first decrypting CSS, it will be a lot harder to re-align each VOB file, get each VTS’s key for the corresponding VOBs, and decrypt, but not impossible.

Decrypting CSS

Since CSS is applied on each 2048-byte sector, decryption occurs while reading from the disc, as the disc would be read one sector at a time, in order from first VTS to last VTS. As the disc is read, header information for each VTS is also read before the VOB data, allowing the CSS key to be obtained for that VTS’s. It then decrypts the following VOB data sectors with that key. Once a new VTS sector is reached a new key for that VTS is obtained and the process repeats until all sectors have been decrypted.

Saving Decrypted Sectors

Once a sector is read and decrypted, it’s generally saved to the disk by appending each sectors data (2048 bytes) to a .ISO file.

Alternatively, it is possible to read the sector data as it’s being read and parse as an ISO 9660 file-system to read the filenames and data. This allows you to save as normal separate files into a folder and is what happens when you backup directly to a VIDEO_TS folder.

You may also get a VIDEO_TS folder with VOB/IFO files by extracting from the .ISO afterwards with a supported program like 7-zip. However, this method is slower and would use more disk space at least temporarily while you extract from the ISO.

CSS Keys and Region Codes

While I don’t fully understand it, there is Key negotiation between the Player Hardware and the DVD-Video disc that may fail if the region code of the Player and the Disc doesn’t match. Modern Backup Software has ways around this via brute-force and region emulation.

However, the workarounds don’t always work or result in the correct key. For example, I was backing of Region 1 (USA) The Fresh Prince of Bel-Air DVD using my Player which was set to Region 2. It failed to negotiate keys and used some kind of workaround on one of the VTS. The resulting key for that VTS specifically was incorrect. The resulting stream caused corrupted key frames at random parts of the video, and weird mushed pixels around edges of some frames.

While changing the Player region to match the disc did work, you can only do that a limited number of times before the player refuses to change region. So, I had to get a second Player as I often backup discs from two regions.

Can you just give me a way to do all this automatically…

Sure can. Give a look at AnyDVD HD it’s a fantastic piece of software that decrypts DVD-Video at a driver-level, essentially spoofing it as a disc with no CSS.

This allows you to use your DVD-Video disc on any software, including legal backup solutions like ImgBurn which is a much newer and still supported version of DVD Decrypter, which had to drop CSS decryption functionality after legal pressure.

You can also have AnyDVD HD back up the disc for you with a very minimal UI, no bloat, no adware. Simply back it up!

More Information

CSS has a pretty large history with an interesting legal battle. Check out the Content Scramble System Wikipedia page for more information.