| There are a number of ways to protect your CD | | | | 2. an error detection code (EDC), a checksum to |
| from being copied. Most of them depend on breaking | | | | detect read-errors |
| conformity with the CD and DVD standards. In this | | | | 3. an error correction code (ECC), specially crafted |
| article we will discuss the use of dummy files and | | | | entropy encoding of the user-data |
| illegal sectors, although there are other methods of | | | | Using the EDC and ECC-field, the drive can spot and |
| protecting CD's. | | | | fix read-errors. |
| Dummy files | | | | Deliberately crafting sectors with improper EDC/ECC |
| For the most part CD-ROMs use an | | | | fields at some stage in manufacturing provides a |
| ISO9660-filesystem to arrange the existing space | | | | distinguishing feature used by copy protections. The |
| into files and folders. The majority of times it is used | | | | protection's software tries to read those sectors, |
| underneath a further sophisticated filesystem like | | | | awaiting read-errors. Since early versions of end-user |
| Joliet to get round a few limitations, but the | | | | software and hardware were not capable of |
| ISO9660-filesystem is at all times there. The most | | | | generating sectors with illegal structural details, this |
| fundamental approach for a distinguishing | | | | characteristic could not be re-generated with such |
| characteristic is to deliberately fake a little information | | | | software and hardware. The disc is a copy if the |
| inside the filesystem. Early production of software | | | | sectors forming the distinguishing attribute have |
| copied every file one at a time from the original | | | | become readable. |
| medium and re-created a new filesystem on the | | | | This approach does not give protection anymore as |
| target medium, losing the faked information. | | | | current software and hardware are capable of |
| Illegal sectors | | | | reproducing raw sectors. Another interesting new |
| The top-level data structure of a CD-Rom is called a | | | | technology on cd copy protection can be found here. |
| sector and it is the only one that is available to | | | | |
| software (counting the OS). Every sector contains | | | | An adaptation of this approach uses large areas of |
| 2048 byte of user-data and 304 bytes of structural | | | | unreadable sectors with tiny islands of readable ones |
| details (for a MODE1 CD-ROM). Along with other | | | | in between. For the most part software trying to |
| things the structural details consist of | | | | reproduce protected media will miss intervals of |
| 1. the sector number, signifying the sector's absolute | | | | sectors once confronted with unreadable ones, |
| and relative logical location | | | | expecting them all to be bad. |