How Secure Digital Memory Cards Work

The Security in Secure Digital

What makes a Secure Digital memory card secure? All SD cards have copyright protection for SD standard data types. This enables commercial distributors to put music or other media on the device so that it's protected from illegal copying. The technology behind this SD card feature is called Content Protection for Recordable Media (CPRM).

CPRM was developed by the 4C Entity, LCC, the organization handling copyright protection licensing for IBM, Intel, and SD co-founders Panasonic and Toshiba. 4C Entity also developed the related technology Content Protection for Pre-recorded Media (CPPM), which relates to the DVD-Audio format for music. CPRM and CPPM are generally associated with flash and DVD media formats.

CPRM doesn't apply automatically just because you're using an SD card. In order for data to be protected with CPRM, the device that's writing the data must first create a Media Identifier and Media Key Block (MKB) on the SD card. The Media Identifier and MKB are written to a physical portion of the SD card known as the Protected Area. Data stored in the Protected Area is inaccessible through the SD card's file system, but any CPRM-enabled device reading the SD card can use it for content protection operations.

To protect data with CPRM, a device uses the Media Identifier and MKB to encode the data as it's written to the SD card. To read the data, a CPRM-enabled device accesses the Media Identify and MKB and uses them to decrypt the data as it's read from the SD card. Therefore, even if you copy the CPRM-protected files from the SD card's file system, you won't be able to read them outside of that SD card.

In addition to the CPRM, there is another security feature you'll notice on the 32 x 24-millimeter SD cards (SD, SDHC and SHXC): a write-protection lock. The lock is a tiny slider along one side of the SD card. In unlocked position, you can read and write to the SD card as you would with a USB drive or any other read-write storage device. In the locked position, the SD card will not allow any changes to its file system, making it a read-only device. You might use this feature if you're concerned about accidentally overwriting data from a camera, for example, while reading the SD card on your laptop or desktop computer.

Until now we've focused on the capacity, speed and security associated with SD cards. On the next page, we're popping off the plastic cover and taking a look at the technology inside.