CD copy protection

CD/DVD copy protection is a blanket term for various methods of copy protection for CDs and DVDs. Such methods include DRM, CD-checks, Dummy Files, illegal tables of contents, over-sizing or over-burning the CD, physical errors and bad sectors. Many protection schemes rely on breaking compliance with CD and DVD standards, leading to playback problems on some devices.

Protection schemes rely on distinctive features that:

Most CD-ROMs use the ISO9660 file system to organize the available storage space for use by a computer or player. This has the effect of establishing directories (i.e., folders) and files within those directories. Usually, the filesystem is modified to use extensions intended to overcome limitations in the ISO9660 filesystem design. These include Joliet, RockRidge and El Torito extensions. These are, however, compatible additions to the underlying ISO9660 structure, not complete replacements or modifications. The most basic approach for a distinctive feature is to purposely fake some information within the filesystem. Early generations of software copied every single file one by one from the original medium and re-created a new filesystem on the target medium.

A sector is the primary data structure on a CD-ROM accessible to external software (including the OS). On a Mode-1 CD-ROM, each sector contains 2048 bytes of user-data (content) and 304 bytes of structural information. Among other things, the structural information consists of

Using the EDC and ECC information, the drive can detect and repair many (but not all) types of read-error.

Copy protections can use these fields as a distinctive feature by purposely crafting sectors with improper EDC/ECC fields during manufacture. The protection software tries to read those sectors, awaiting read-errors. As early generations of end-user soft/hardware were not able to generate sectors with illegal structural information, this feature could not be re-generated with such soft/hardware. If the sectors forming the distinctive feature have become readable, the medium is presumed to be a copy.

Modern soft/hardware is able to reproduce raw sectors, therefore this approach cannot be used to give protection any more.

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Wikipedia