Thursday, April 12, 2007

CD Copy Protection

by Allan Hall

There are a number of ways to protect your CD from being copied. Most of them depend on breaking conformity with the CD and DVD standards. In this article we will discuss the use of dummy files and illegal sectors, although there are other methods of protecting CDs.

Dummy files
For the most part CD-ROMs use an ISO9660-filesystem to arrange the existing space into files and folders. The majority of times it is used underneath a further sophisticated filesystem like Joliet to get round a few limitations, but the ISO9660-filesystem is at all times there. The most fundamental approach for a distinguishing characteristic is to deliberately fake a little information inside the filesystem. Early production of software copied every file one at a time from the original medium and re-created a new filesystem on the target medium, losing the faked information.

Illegal sectors
The top-level data structure of a CD-Rom is called a sector and it is the only one that is available to software (counting the OS). Every sector contains 2048 byte of user-data and 304 bytes of structural details (for a MODE1 CD-ROM). Along with other things the structural details consist of

1. the sector number, signifying the sector's absolute and relative logical location
2. an error detection code (EDC), a checksum to detect read-errors
3. an error correction code (ECC), specially crafted entropy encoding of the user-data Using the EDC and ECC-field, the drive can spot and fix read-errors.

Deliberately crafting sectors with improper EDC/ECC fields at some stage in manufacturing provides a distinguishing feature used by copy protections. The protection's software tries to read those sectors, awaiting read-errors. Since early versions of end-user software and hardware were not capable of generating sectors with illegal structural details, this characteristic could not be re-generated with such software and hardware. The disc is a copy if the sectors forming the distinguishing attribute have become readable.

This approach does not give protection anymore as current software and hardware are capable of reproducing raw sectors. Another interesting new technology on cd copy protection can be found here.

An adaptation of this approach uses large areas of unreadable sectors with tiny islands of readable ones in between. For the most part software trying to reproduce protected media will miss intervals of sectors once confronted with unreadable ones, expecting them all to be bad.

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