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DVD Content Scramble System (CSS)

What is DVD Content Scramble System (CSS)? Here is a simple explanation. If you want a more detailed explanation of it, click this link: dvdcss

The Content Scramble System (CSS) is a Digital Rights Management (DRM) scheme introduced in 1996.

Almost all commercial dvd movies are encrypted with CSS copy protection. You'll usually get an error if you try to copy a DVD to a hard drive.

There are two important steps to copy dvd with copy-protection - Authentication and Descrambling.

Step 1. Authentication between the drive and the computer.

When a dvd-video disc is inserted into a dvd drive, the computer and the drive must authenticate each other before reading data from the disc.
If the authentication fails, an crc error (cyclic redundancy check) can occur when you copy the dvd.

crc error

You can use a software player (e.g. PowerDVD) to play the dvd movie first. As soon as the player starts playing the movie, the authentication process should be successful; then you can copy the dvd without error. But the copied video is crambled and unusable. We have to descramble it when we copy it. (An Authentication Key is used for the authentication process)

Step 2. Descrambling DVD-Video data.

The CSS uses a cipher (stream cipher algorithm) to encrypt all the VOB files on a dvd disc. The encryption key is called Title key - a 40-bit stream cipher key. One or more title keys are stored on a copy protected dvd. But the Title keys are also encrypted by a key which is called Disc key.

There is one Disc key per protected disc. The Disc key is also encrypted by a key called Player key. The Disc key is stored on a special area of the disc - lead-in area. The lead-in area is unable to accessed, unless the authentication process is successful. For this reason, when you copy a dvd, you get an CRC error and after successful authentication, the error disapears.

Each DVD Player (e.g. PowerDVD) has a Player key. Each Player key is 5 bytes (40 bits) length. One DVD player manufacturer is allocated one Player key. There are approximately 400 player keys. A list of Player keys: click here.

dvd css image

Now we can retriev a encrypted Disc key and one or more encrypted Title keys from dvd disc; then we use the Player key to recovery the Disc key and Title keys. Finally we can decrypt dvd video with the Title keys.

Brute-force attack

Brute-force attack is another way to get Title keys. In this way, we can decrypt a copy protected dvd without the Player key and Disc key. So using the brute-force attack, we can decrypt a dvd on a hard drive. The brute-force attack code is in the libdvdcss project.

Other copy protection on dvd

ARccOS (Advanced Regional Copy Control Operating Solution) is a copy-protection system made by Sony that is used on some DVDs.

Content Protection for Recordable Media and Pre-Recorded Media (CPRM/CPPM)

DVD Decrypter Programs

DeCSS - the earliest decrypting CSS code which was released in October 1999.

libdvdcss - released after DeCSS, it implements a brute force attack on CSS. Libdvdcss is part of VLC Project.

Free DVD Decrypter - an easy-to-use freeware designed to decrypt and copy a dvd with protection.