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These mechanisms vary widely and include DRM, CD-checks, Dummy Files, illegal table of contents, over-sizing or over-burning the CD, physical errors, bad sectors and more. Many of these protection schemes rely on breaking compliance with the CD and DVD standards, leading to playback problems on some devices. All CD/DVD copy protection schemes rely on some kind of distinctive feature that
can be applied to a medium during the manufacturing process, making a protected medium distinguishable from an unprotected one,
The medium has to comply with industry-standards. Application of the protection must not harm the medium's ability of standard compliance.;
Hard- and software found in the end-user environment must be able to gather some kind of information from the medium that makes it possible to distinguish between an original/protected medium and a copied/unprotected one without impairing the medium's original purpose.;
Technology
Illegal filesystems / Dummy filesMost CD-ROMs use an ISO9660-filesystem to organize the available space into folders and files. Most times it is superposed by a more advanced filesystem like Joliet to circumvent some restrictions, but the ISO9660-filesystem is always present. The most basic approach for a distinctive feature is to purposely fake some information within the filesystem. Early generation of software copied every single file one by one from the original medium and re-created a new filesystem on the target medium.
Illegal sectorsA sector is the top-level data structure of a CD-ROM and the only one that is accessible by software (including the OS). Each sector contains 2048 byte of user-data (content) and 304 bytes of structural information (for a MODE1 CD-ROM). Among other things the structural information consists of the sector number, representing the sector's relative and absolute logical position; an error detection code (EDC), a checksum to detect read-errors; an error correction code (ECC), specially crafted entropy encoding of the user-data; Using the EDC and ECC-field, the drive can detect and repair read-errors. Copy protections can use these fields as a distinctive feature by purposely crafting sectors with improper EDC/ECC fields during manufacturing. The protection's 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 supposed to be a copy.
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