Compact Disc and DVD Copy Protection - Technology - Twin Sectors

Twin Sectors

This technique exploits the way the sectors on a CD-ROM are addressed and how the drive seeks from one sector to another. On every CD-ROM the sectors state their logical absolute and relative position in the corresponding sector-headers. The drive can use this information when it is told to retrieve or seek to a certain sector. Note that such information is not physically "hard-wired" into the CD-ROM itself but part of user-controlled data.

A part of an unprotected CD-ROM may look like this (simplified):

When the drive is told to read from or seek to sector 6553, it calculates the physical distance, moves the laser-diode and starts reading from the (spinning) disc, waiting for sector 6553 to come by.

A protected CD-ROM may look like this:

In this example, a sector was inserted ("Mary") with a sector-address identical to the one right before the insertion-point (6553). When the drive is told to read from or seek to sector 6553 on such a disc, the resulting sector-content depends on the position the drive starts seeking from.

• If the drive has to seek forwards, the sector's original content "Jill" is returned.
• If the drive has to seek backwards, the sector's twin "Mary" is returned.

A protected program can check whether the CD-ROM is original by positioning the drive behind sector 6553 and then reading from it — expecting the Mary version to appear. When a program tries to copy such a CD-ROM, it will miss the twin-sector as the drive skips the second 6553-sector, seeking for sector 6554.

There are more details about this technique (e.g. the twin-sectors need to be recorded in large extents, the SubQ-channel has to be modified etc.) that were omitted. If the twin sectors are right next to each other as shown, the reader would always read the first one, Jill; the twin sectors need to be farther apart on the disc.

Protection-vendors using this technique say it's "physically impossible" to copy this kind of protection. However, specialized software can produce perfect clones of such CD-ROMs, although the process is very time-consuming. What the vendors are thinking when they say "physically impossible" is probably that copying the protection requires technical features that the installed base of standard CD-ROM and CD-recorder drives do not have. Then, what they mean is "technically impossible with a typical drive," but that is still wrong, because with clever techniques it is technically possible, as the existence of programs that do it proves. No doubt, it would not even take excessive time if a specialized CD reader that made these workarounds unnecessary was available.