Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02332008 2000-11-14
WO 00/55736 PCT/EP00/02276
COPY-PROTECTION ON A STORAGE MEDIUM BY RANDOMIZING LOCATIONS AND KEYS UPON
WRITE ACCESS
The invention Relates to a method for providing copy-protection to a data
storage medium, in particular to solid state memory modules. With advancing
teclhnology,
next generations of portable audio playback and recording; devices will be
based on solid
state technology. Arguments in favor are based on weight, power and
shockproofness
considerations.
Software providers, e.g. music publishers, :require measures against
unauthorized copying of the digitally stored information, with little or
preferably no
inconvenience to an authorized user. In addition, the method and system should
support such
business models as rental, try-before-you-buy, and controEled copying (e.g.
super
distribution). A particular problem is posed by devices that can potentially
access all
information on the storage medium, without complying with protection
standards.
Known anti-copying solutions use a unique; identification code (ID;n that is
'engraved' in the storage medium. At some point in time, this may be
disadvantageous
because of privacy considerations. Furthermore, as will be; explained below,
methods which
mainly rely on such an ll~ do not provide adequate protection against a
copying scheme
known as a 'replay attack'.
It is therefore an object of the invention to provide a method and system that
provides protection against replay attacks, without necessarily employing a
unique; >D, in a
relatively inexpensive manner that requires only moderate processing
facilities.
The basic idea for this copy protection method and system is that the data is
encrypted using a key that critically depends on the locations) in which the
data i.; stored,
and which is combined with a method that renders it impossible to predict
where the data will
be actually stored on the medium. Accordingly, copying of the data will result
in an
unpredictable change of the storage location, thus breaking; the critical
relation between the
latter and the encryption key. 7~herefore, once the data has been moved, it
can never be
recovered, provided that the cryptography is sufficiently sl:rong, the random
number
generator is cryptographically strong, and any secrets are l;ept well hidden.
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In consequence, amongst other things, it is an object of the present invention
to provide an inexpensive method for storing data on storage media, where the
relation
between encryption key and ;storage location will be disrupted upon copying
operations.
The present invention is particularly suited for solid state memory modules
~ which pro-vide easy random access to any location in the memory, be it on
the basis of a bit, a
byte or on some other entity such as a uniform sized sector that relates to
the access width of
the memory in question.
Now, therefore according to one of its aspects the invention is characterized
in
that the data on the storage medium are encrypted with a key K which depends
on the
position (L~, Lz, L3) of the data on the storage medium, and that in each
write operation the
data is stored in Iocations on the storage medium that are chosen at random.
The invention also relates to a system arranged for implementing a method as
claimed in claim 1, a player for playing a recording prep~~red according to a
method as
claimed in claim 1, and a record earner prepared according to a method as
claimed in claim
1. Further advantageous aspects of the invention are recited in dependent
Claims.
These and other objects of the invention will be apparent from and elucidated
with reference
to the embodiments describeCd hereinafter.
In the drawings:
Figure 1 shows a conceptual tvo-player arrangement
Figure 2 illustrates the mechanism of 'replay attacks' in the prior art.
Figure 3 shows a schematic diagram of a storage medium embodiment;
Figure 4 shows an example of a file structure
Figures SA and SB illustrate an example oaf a method in accordancf: with the
invention and how this method prevents 'replay attacks'.
Figures 6A and 6B illustrate a further example for a method in accordance
with the invention.
Figure 1 illustrates a conceptual two-player arrangement, with two players A
and B, and a
removable module C that may be transposed between the players. As shown, both
players
have appropriate means for inserting the module. In the rest of the discussion
it is assumed
that this removable module may be accessed by other means as well (e.g. PC
based readers).
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This poses the risk of unauthorized copying of the data o~n the module,
assuming that the
players A and B do not allow unauthorized copying. The preferred embodiments
are
described in relation to a Solid State Audio player and module, although the
invention may
be used in a broader context.
Within a few years, Solid State Audio (SSA) players are expected to become a
new standard for portable audio playback devices. This is mainly due to many
advantages on
weight, size, power use, and :>hock resistance, with respect to current
solutions using disc or
tape. Currently available SSA, players combine 32-64 MlB of flash memory and
audio
compression techniques such as MPEG 1 layer III {MP3) or AAC to achieve up t~o
are hour
of (near) CD quality music playing time. Due to the digital nature of these
devices and the
associated ease of copying, however, the music industry unsists on proper
copyright
protection features.
One of the tools for copy protection of digital content is encryption. While
encryption by itself does not prevent illegal copying, it does render such
copies useless, as
the original content can be retrieved only by decrypting ii: using the proper
key. As a result,
playback of the content is limited to those devices that have access to that
key. It its an
objective of the copy protection system to manage the keys in such a way that
illegal copying
is prevented, while at the same time not inconveniencing legal and intended
use of the
content.
Most of the memory modules for solid state multimedia storage applications
comprise a large
flash memory and an on-board controller. The controller may or may not be
integrated, and
multiple separate memory chips may be employed on the module. Examples of such
multimedia memory modules are: Memory Stick {Sony), ~SmartMedia (SSFDC Forum),
Miniature Card (MC Forum), Compact Flash (PCMCIA Forum), Multimedia Card (MMC
Association). In addition, these devices can be thought of as block devices,
similar to hard
disk drives, where memory accesses occur by addressing ;sectors (typically 512
bytes) on the
module. Indeed, some of the modules listed above emplo~r the ATA interface
standard, which
is used to connect hard disks and other peripherals to a PC'. This enables
easy duplication (bit
by bit) of the content of such rnemory modules using a PC,'. Other modules use
a proprietary
interface and command set, but still are block based, i.e. individual sectors
on the ;module can
be addressed and modified.
Tn the following, it is assumed (see Figure :l) that a SSA player employs
detachable memory modules, which can be accessed by other means as well (e.g.
)?C based
readers).
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Basically, two approaches exist for copy protection. The first is to bind the
audio to a specific player by providing each individual player with a unique,
secret, number
that is used as the key to encr5rpt the audio. Therefore, the audio stored on
memory modules
by one player will play on that player only. Of course, this is very annoying
if one has
S ~ multiple SSA players. it is required that one is able to pla~~ music
stored on a memory
rnoduIe, regardless of the SSA device used to download it onto the module.
What should be
prevented, however, is that a user can copy the audio content to another
module and be able
to play from both.
One known solution is to embed a unique identification code {ID) in the
memory module, which can lbe; read by the application, but which can rat be
changed. This
identification code can then be used to generate an encryption key, which is
specific for the
module.
Another known solution is to make use of dlefects in the memory modules,
which naturally occur as a result of the manufacturing processes used to
fabricate cheap but
high storage capacity flash memories. The locations of these natural defects
probably will be
unique for each module, and as such can act as a 'fingerpr7,nt' of that
device. Again, a unique
key can be generated, which is specific far the module.
These known solutions, however, necessitate a unique identification code, and
the~~ do not
provide protection against replay attacks. A 'replay attack' is a form of
copying inywhich an
unauthorized copy is made from one system (system 1) to another (system 2),
where the
unauthorized (but unplayable copy) on system 2 can be used fo restore a
playable copy on
system over and over again, even after expiration of the original copy. Figure
2 illustrates this
in more detail. Each system comprises a unique identificat;ian code,
represented by )D 1 far
system 1 and ID2 for system 2, and contains files in which the content is
stored as a sequence
of separate blocks. In this example the data in respect of ria;hts and usage
on the or7iginal copy
are encrypted with a key that is derived from I'Dl and a secret S. In a 'try-
before-you-buy' or
a rental business model, further access to the data is deniedl after a certain
period of time, or
after a number of uses. Copying the data to a system having a unique
identification code ID2
(second step in Figure 2) will not render a usable copy, since the
identification code does not
match the code >D1. However, this copy is exactly (bit-by-bit) the same as the
origiinal. It can
at any time be recopied back Pram system 2 to system 1 and that copy of a copy
can be used
again. This enables a fraudulent customer to retain on system 2 a copy that
can be recopied
again and again on system 1 where it wilt be usable. So, after obtaining
content on a 'try-
befare-you-buy' basis, the fraudulent customer copies the data from system 1
to system 2,
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and recopies it again and again from system 2 to system 1. in order to-keep
'trying'. 'Try-
before-you-buy' thus has become 'try-indefinitely.' Likewise this scheme can
be used to pay
once for a rental and have a copy for ever.
To effectively-use a storage. device, it is necessary to implement a file
system
5 by means of which the user data is organized and accessed. By treating the
memory module
as a block device, the creation and management of a file system is Left to the
application. In a
PC environment, Where the operating system already has 'built-in file system
support, this is a
logical choice: by supporting the ATA standard this support can be reused for
the memory
module without any modification. However, in stand-alone devices, such as a
SSA player,
the application is burdened with file system details, if the memory module
employs the block
device approach. Therefore, stand-alone (portable} applications which require
storage of
multimedia content, may be built more efficiently if a controller unit on the
memory module
takes care of the file system dE;tails.
Figure 3 represents a schematic diagram of a memory module embodiment 20.
For simplicity, electromechanical interfacing to the player has not been
detailed in the Figure.
The storage area 30 has an access time that is substantially independent of
the phy;;ical
storage location. The controller 22 controls the access to the storage proper.
Various sub-
systems have been shown ther~:in, the host interface 24, the memory interface
26, and the file
system 28. External write and internal selection to the memory are shown as
well. Within the
Application Programming Interface API the following functionality should be
present. For
memory formatting, an optiomal volume number is outputted that is either
uniquely fixed and
hard-wired, or a random number that is generated each time the command is
executed. This
number may only be changeal when executing the formatting command, thereby
destroying
all data on the device. The copy protection proper does not expressly need
this number. To
create a file, a reusable file 1D is produced for later referencing the file
in question. When
writing a block, a sector number is produced that is a random choice from the
free lblock list.
Depending on the implementation, the sector number that is produced can be the
actual sector
number in which the data propE;r were stored during the write operation, or it
can bN the
sector number will be stored during the next write operation. This amongst
others is possible
in solid state audio devices without appreciable loss of timt: because the
flash memory is not
hampered by a seek time as is common in disk based systems. Such random choice
in
addition helps to level wear over the entire device. The application may use
or discard the
sector number returned by the block write command as required. When reading a
biLock, the
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file 1D controls outputting the data proper and the sector number of the
current or next block
to be read.
Figure 4 illustrates an example of a file structure, that is distributed into
blocks, each having the size of a single sector of 512 bytes. The first block
carnes
S information regarding the file, while the others have the file data proper.
The above
organization will block the making of a bit wise copy of l:he module, inasmuch
as no
modification facility for individual sectors has been provided. Copying to an
intey-mediate
storage location and subsequently recopying the data on the module (which
constitutes the
'replay attack' as explained above) will copy the data to completely different
locations. This
in itself provides some protection against copying. Copy :protection is
further provided by
encrypting a data block through a key that is derived from a secret and also
from the location
(for instance and preferably the sector number) where the data in question is
stored. The latter
information may be derived from the block write function that returns the
sector number of
the next file sector. As this information is not available for the first
block, the latter may be
used for less sensitive data. This limitation is overcome by letting the file
create function
return the sector number of the first sector in the file where the data proper
(e.g. the file info)
will be written. For reading, the present or next sector number is available
before reading of
the actual data, allowing the application to calculate the proper decryption
key in vtirne. The
encryption key thus combines the location of the storage, and a method that
renders it
impossible to predict this location. Copying will change the storage location,
and :in
consequence will break the relation between location and decryption key. Note
th;~t the secret
used in the derivation of the key may be a globally shared secret between all
players, or may
be derived by other methods well known to those skilled in the art.
Figures 5A and 5B illustrate a method in accordance with the invention. Each
time data blocks are written, the controller 22 writes the data in randomly
chosen locations.
In Figures 5A and 5B the locations are indicated by L,, L;. etc. The data axe
encrypted with a
key which is dependent on a secret S and a location L; or ;~ combination of
locations L; (for
instance the location of the block that is written, or of the :previous block,
or of the: block that
is written and the previous block etc).
- Making a copy of the data of the memory nnodule will (see Figure 'iB) change
in a irreproducible manner the locations of the data. In fact such will happen
twice..
Therefore, a recopy of a copy will have data for which the: positions {L3",
L2" etc.) does not
correspond to the arguments needed for a proper decryption of the data.
Subsequently the
copy of the copy cannot be decrypted and is useless. The 'replay attack' is
prevented.
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Figures 6A and 6B show an embodiment of the invention in which all data are
encrypted with a key K (which may consist of a single kf:y or a block of
keys), that itself is
stored encrypted with a key K', whicl is the output of a hash function having
as arguments
the locations Ll, LZ, L3 etc and a secret S. K' thus depends on the positions
of the: data blocks,
in this case on the total sequence in which the data blocks are written. Since
at each write
access the locations LI, L2, L3 are changed in an unpredi<;table manner, the
result of the hash
function H and thereby the key K' is changed. If the content is copied and
recopied the player
will fail (as in the method illustrated in Figures SA and S:B) to recover the
keys because K' is
changed in an intractable manner. Accordingly any replay attack fails. Thus
copying is
i0 prevented in an inexpensive rnanner requiring only moderate processing
facilities and
without the need of a unique ftdentification code. It is noted that the
invention provides the
possibility of copy protection without the need for a unique identification
code. This does
not exclude use of such a code for other reasons or for extra protection.
It is also possible to arrange the data in groups of blocks, and groups of
blocks arc; written in
random locations. The same schemes as above may be used for groups of blocks,
instead of
single blocks. 'Random locations' within the concept of the invention in its
broadest sense
means locations that for all due purposes cannot be predicted in advance. 'For
all due
. purposes' is stated since to get random numbers or locations use is usually
made of some
kind of algorithm. Substantially truly random, i.e. substanrtially evenly
distributed throughout
the memory module is preferred to even out wear on the device. Although
preferably the
method is applied to all or substantially all data in the memory module, the
invention
encompasses embodiments in which the method is applied to only a part of the
data in the
. memory module. This could for instance be advantageous from the point of
view of speed of
operation. The invention is ~zot restricted to using one and only one
encryption method. When
the data are divided in groups, embodiments using different encryption methods
and different
ways of dependency of said encryption methods on the locations may be used for
different
groups. This reduces the risk of unauthorized decryption. Although the
controller may be
provided in the system apart from the memory module, preferably the controller
unit by
which the random locations are chosen is integrated in the memory module. This
makes it
difficult to circumvent the method or influence the choice of locations of
data.
In a method for providing copy-protection ;>ervices on storage media, the
locations where the data, preferably arranged in blocks, arse stored, are
chosen by a.
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(preferably built-in) controller on a random basis. Using an encryption key
which depends
critically on the position of the data the storage medium, decrypting copied
data is made
virtually impossible.
