Content Protection for
Recordable Media (CPRM), the copyright protection technology used for the SD
Memory Card, was developed by 4C (The digital contents copyright protection
technology licensing organization of IBM, Intel, Matsushita, and Toshiba.)
Having a proven record in DVD, this protection is enhanced in the SD Memory Card through the use of "key revocation" technology that is built into the card.
The card's control circuitry allows data to be read and written (in its protection area) only when appropriate external devices are detected. A check-out (copying) from a PC to the SD Memory Card is restricted to 3 copies in compliance with the SDMI specification. All SD-Audio products comply with SDMI.
The SD Memory Card copyright protection function has the following features:
Access to the SD Memory Card must be
enabled by authentication between devices
A random number is generated each time there is mutual authentication and exchange of security information
Here is an example of how copyright
protection is used for Electronic Music Distribution (EMD):
(1) Content is
downloaded through the Internet to a personal computer. At this time the content
will have been encrypted (network encryption) by the electronic distribution
system.
(2) Before the content is stored in the SD Memory Card, the personal computer
will check whether the card is legitimate, and the card will check whether the
personal computer software is a legitimate application. When mutual validation
has been confirmed, the content will be encrypted in accordance with the key on
each card and the key associated with the content.
(3) Simultaneously, the encrypted content information will be stored on the
card.
(4) In the same way, before the semiconductor player reads the content from the
card, the player will check whether the card is legitimate and the card will
check whether the player is a legitimate device. When mutual validation has been
confirmed, the player will acquire the key for decryption.
(5) Simultaneously, the encrypted content information will be read from the
card, decrypted by the semiconductor device, and replayed legitimately.
Since this
procedure is not possible with anything other than a legitimate personal
computer, memory card and semiconductor player, the result is that illegal
behavior can be prevented.
Apart from the area in the SD Memory Card
where the key is stored, there is a protected area that cannot be accessed
without mutual authentication, and a data area that the user can access normally
as with a normal memory card.
If copyrighted content is to be stored, after successful authentication, the
copyrighted information in that content will be encrypted in accordance with a
key that is unique for each card and stored in the protected area. Also, the
copyrighted information together with the card's own unique key will be
encrypted and stored in the data area.
If copyrighted content is to be read out and reproduced, after successful
authentication, the copyrighted information in that content will be read from
the protected area and decrypted in accordance with the key that is unique to
each card. Also, the main body of the content that has been encrypted in the
data area is read, and the copyrighted information and the card's own unique key
area are decrypted and made into information that can be reproduced.
The main body of content that was encrypted and stored in the data area can be
copied freely to another card, but, since the copyrighted information of the
content stored in the protected area cannot be copied to another card, this
means it cannot be reproduced or copied dishonestly.
Moreover, even if a dishonest copy of the copyrighted information stored in the
protected area could be made in some way, this information is still protected
because it has been encrypted in accordance with the key that is unique to each
card so that it cannot be decrypted.
If a particular SD device were to be hacked, and this access procedure were to
be analyzed, there is still a key revocation system that will invalidate the key
of the SD compliant device that has been hacked by altering the keys of future
cards.