Note: Descriptions are shown in the official language in which they were submitted.
CA 02557502 2006-08-24
METHOD FOR SECURING ENCRYPTED CONTENT BROADCAST BY A
BROADCASTER
This invention concerns the domain of security for security modules, these
modules being intended to contain personal and secret data allowing access
to services or benefits.
This invention applies more precisely to the Pay-TV domain, in which a
content is transmitted in an encrypted form, the decryption of these content
being authorized under determined conditions.
It is well known that in order to watch a Pay-TV event, such as a film, a
sports
event or a game in particular, several streams are diffused to a multimedia
unit, for example to a decoder. These streams are in particular, on one hand
the file of the event as an encrypted data stream and on the other hand, a
stream of control messages allowing the decryption of the data stream. The
content of the data stream is encrypted by "control words" (cw), which are
regularly renewed. The second stream is called ECM stream (Entitlement
Control Message) and can be formed in two different ways. According to a
first way, the control words are encrypted by a key, called transmission key
TK that generally pertains to the transmission system between the
management centre and a security module associated with the
receiver/decoder. The control word is obtained by decrypting the entitlement
control messages by means of the transmission key TK.
According to a second way, the ECM stream does not directly contain the
encrypted control words, but contains data allowing the determination of the
control words. Said determination of the control words can be carried out
through different operations, in particular by decryption, this decryption
being
able to lead directly to the control word, which corresponds to the first way
described above. But the decryption can also lead to data that contains the
control word, said control word still having to be extracted from the data. In
particular, the data can contain the control word as well as a value
associated
to the content to be diffused, and in particular the access conditions to
these
content. Another operation allowing the determination of the control word can
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for example use ' a one-way hash function of this piece of information in
particular.
The security operations are generally carried out in a security module
associated to the multimedia unit or to the decoder. This type of security
module can be produced in particular according to four different forms. One of
these consists in a microprocessor card, a smart card, or more generally an
electronic module (taking the form of a key, of a badge,...). This type of
module is generally removable and connectable to the decoder. The most
used form is the one with electric contacts, but does not exclude a connection
without contact, for example of the ISO 14443 type.
A second known form consists in an integrated circuit chip, generally placed
in
the decoder shell in a definitive and irremovable way. An alternative is made
up of a circuit wired on a base or connector such as a SIM module connector.
In a third form, the security module is integrated into an integrated circuit
chip
that also has another function, for example in a descrambling module of the
decoder or the microprocessor of the decoder.
In a fourth embodiment, the security module is not realised as a hardware, but
rather its function is implemented only as software. Given that in the four
cases the function is identical although the security level differs, it will
be
talked of security module regardless of the way in which its function is
realized
or the form that can be taken by this module.
At the time of the decryption of a entitlement control message (ECM), it is
verified if the right to access to the content in question is present in the
security module. This right can be managed by authorization messages (EMM
= Entitlement Management Message), which load this right into the security
module.
The diffusion of conditional access digital data is schematically divided into
three modules. The first module handles the encryption of the digital data by
the control words cw and the diffusion of this data.
The second module prepares the control messages ECM containing the
control words cw, as well as the access conditions and diffuses them for the
users.
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The third module' prepaies and transmits the authorization messages EMM
that handle the definition of the reception rights in the security modules
connected to the receivers.
While the first two units are generally independent from the addressees, the
third module manages the set of users and diffuses data for one user, for a
group of users or for all the users.
One of the methods used to bypass security, that is laborious but workable,
consists in analysing the content of an authorized security module (reverse
engineering) in order to imitate the security part (decryption of the
messages)
and at the same time bridge the verification part of the rights. It is thus
possible to produce a "clone" of a real security module. This clone will thus
have the transmission key that will allow it to decrypt the control words cw
contained in the control messages ECM. Since the rights are not verified in
this clone, it will operate as the original as far as the decryption means are
concerned, but without needing to have the rights to carry out this
decryption.
In a Pay-TV system, it is possible to change the transmission key. In
principle,
two methods can be used for this. The first consists in diffusing the new
destination transmission key to all the decoders. Said decoders can then be
updated so that when the new key is used, they can decrypt the events. This
type of updating does not allow the exclusion of a cloned decoder because it
can also receive the updating messages since it has corresponding
decryption keys.
Since each security module includes at least one single key, the second
approach consists in transmitting the new transmission key in an encrypted
message using this single key. In this case, the number of messages is at
least equal to the number of installed security modules in order to renew
individually this transmission key. It is known that if a module is released
(that
is to say if the host apparatus is not supplied), it will not receive this
message
and could not offer any further services to the user, to which he or she would
have by legitimate right. To compensate for this, when a message is sent to a
module, this message is repeated several times to be sure that the addressee
has received it.
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Given the available bandwidth and to ensure that each subscriber has
received the new key, it is necessary to transmit the message well before the
use of this new key, for example one month in advance.
Therefore, the possessor of a clone module will inform the technician that has
supplied him with this clone and has means to extract the new transmission
key from an authentic module. When the key is available, for example on the
Internet, all the clones can then be updated before the activation of the new
key. In this way, the clones are always operational.
As a result, the sending of transmission keys by global transmission as well
as by individual transmission has drawbacks and does not allow the
elimination of a cloned module.
Therefore, the aim of this invention is to propose a method to prevent the
abusive use of conditional access data, in particular by means of security
module clones whose security has been compromised.
This aim is reached by a method for protecting an encrypted content by
means of at least one encryption key and transmitted by a broadcaster to at
least one multimedia unit associated to a security module, a value allowing
the determination of the encryption key(s) of this content also being
transmitted to the multimedia unit by said broadcaster, said security module
comprising the means to determine the encryption key on the basis of said
value, this method comprising the following steps:
- generation of a temporary encryption key (MCW),
- encryption by the temporary key (MCW) of the value allowing the
determination of the encryption keys (cw) of the content;
- transmission of this encrypted value to said multimedia unit,
- encryption and transmission of at least two cryptograms comprising the
temporary key (MCW) encrypted by an authorization key (G), the first
cryptogram being encrypted by a first authorization key pertaining to a first
security module and the second cryptogram being encrypted by a second
authorization key, which pertains to a group of security modules whose
first security module is excluded.
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The aim of this invention is also reached by a method for protecting an
encrypted content by at least one encryption key and transmitted by a
broadcaster to at least one multimedia unit associated to a security module, a
value allowing the determination of the encryption key(s) of this content also
being transmitted to the multimedia unit by said broadcaster, said security
module comprising means to determine the encryption key on the basis of
said value, this method comprising the following steps:
- generation of said value allowing the determination of the encryption keys;
- transmission to the multimedia unit of said value allowing the deduction of
the encryption key (cw) of the content,
- generation of a temporary encryption key (MCW),
- transformation, by the temporary key (MCW) of the value allowing the
determination of the encryption keys of the content, said transformation
giving as a result said encryption key (cw) of the content;
- encryption and transmission of at least two cryptograms comprising the
temporary key (MCW) encrypted by an authorization key (G), the first
cryptogram being encrypted by a first authorization key pertaining to a first
security module and the second cryptogram being encrypted by a second
authorization key, which pertains to a group of security modules whose
first security module is excluded.
The method of the invention enables to carry out a pseudo-individual
encryption of messages and at the same time avoids the necessity of
encrypting the same message with each personal key of each security
module. This allows to permit the decryption of a decryption key only by the
non-cloned modules and to forbid the decryption of such a key by the clones,
so that they will not be able to decipher the future data.
One of the aims of the invention consists in combining the "individual"
encryption of a decryption key of the data, with the frequent change of this
key. These two notions are a priori incompatible because of the fact that it
is
necessary, for individual encryption, to transmit a number of messages equal
to the number of security modules, which as a consequence occupies a large
bandwidth at the time of the transmission. This characteristic is incompatible
with the frequent change of the key, which is a condition for optimal
security.
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The proposed solution consists in integrating into the authorization messages
ECM, not the control words that have been used to encrypt the data, but
modified control words, from which it is possible to determine the original
control words, on the condition that the security module has a valid key that
has not been revoked. In this invention, the determination of the original
control words cw can be carried out if the security module receives
"authorization data". The revocation of a security module is carried out by
simply not sending the authorization data in question.
A stream, that can be the entitlement management message stream EMM or
another specific stream, transmits this "authorization data", which will allow
the security modules to decrypt the modified control words and deduce the
control words cw, in order to be able to decrypt the content.
The authorization data used to encipher these control words cw is formed by
means of keys organized according to a tree structure in which the highest
level is made up of unique keys for each security module, the lower levels
being made up of keys common to a security module group, and so on. The
more the level descend, the more the number of security modules per group
increases. In this way, a collection of specific keys is associated to each
security module.
This plurality of keys associated to different groups of security modules
allows
a finer addressing of the "authorization data", and also thus allows the
reduction of the bandwidth needed for the transmission of this "authorization
data".
This has the advantage that it is possible to change the encryption key more
frequently than in the conventional systems, for example every 1 to 5 minutes,
so that possible pirates have no time to obtain the key and to diffuse it to
other
pirates. Furthermore, when a security module has been used for the
production of a clone, it is possible to identify this security module by
determining its key collection. It is then simple to revoke this security
module
and this clone.
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The invention will be better understood thanks to the following detailed
description, which refers to the enclosed drawings given as a non-limitative
example, in which:
- Figure 1 shows schematically the data encryption and decryption
according to a first embodiment of the process of the invention;
- Figure 2 shows the data encryption and decryption according to a second
embodiment of the process of the invention;
- Figure 3 shows the data encryption and decryption according to a third
embodiment of the process of the invention;
- Figure 4 describes the hierarchical structure of the keys used in the
invention;
- Figure 5 shows an example of the keys contained in certain security
modules; and
- Figures 6a to 6g represent schematically the messages transmitted by the
broadcaster to the security modules.
Figures 1 to 3 illustrate the implementation of the process according to the
invention, the encryption side as well as the decryption side. The encryption
is
carried out at the level of the management centre CG, which sends streams to
decoders STB associated to a security module SC that handles the decryption
of the data.
As shown in Figure 1, the management centre CG generates three streams,
which are detailed below.
This management centre first generates, in a control words generator cwg,
control words cw that are used for a first time in a conventional way for the
encryption of a content CT to be diffused. The content CT is encrypted during
a step referenced as Scramb in Figure 1. These content is transmitted as an
encrypted data stream CT' = cw(CT). As it is well known, the control words
are changed at regular intervals, for example every 2 to 10 seconds, although
other intervals can be considered.
When the control words are generated, they are also encrypted by a
temporary encryption key, generated in principle in a random way by a
generator MCWG, and called master control word MCW. Modified control
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words cw'= MCW(cw) are thus obtained. These master control words are also
changed at regular intervals, for example every 1 to 10 minutes. Other
durations can also be used.
In the embodiment of Figure 1, the modified control words cw' are formed,
particularly by adding the access conditions to the content and a header. They
are then encrypted by means of the transmission key TK before their diffusion
as conventional control messages ECM.
It can be noticed that it is also possible to add the access conditions CD to
the
control word and to encrypt the set with the master control word MCW. This
would allow the possibility of not using encryption by the transmission key
TK.
At the same time, the master control word MCW is encrypted by several
different keys to be sent to the different non-revoked groups of security
modules. These encryptions are carried out with keys G, called authorization
keys and are described in more detail hereafter. Each security module in fact
includes several authorization keys, some of them being unique and different
for each module, others being common to several modules. The authorization
keys are introduced into the security modules at the time of their
personalization.
When the master control word MCW is encrypted by an authorization key G,
an authorization block G(MCW) is obtained. These authorization blocks are
diffused either in a specific stream, or in an entitlement management
message stream EMM.
Thus, the decoders receive three streams: the encrypted data stream CT', the
entitlement control messages ECM and the authorization blocks G(MCW).
The stream of entitlement control messages ECM is filtered in a conventional
way and processed by the security module so that the modified control words
cw' are extracted. To that effect, it is first necessary to decrypt the
message
by means of the transmission key TK.
At the same time, the EMM type stream or another stream containing the
authorization blocks received by a decoder is filtered in a filter FT and
processed so that the authorization block pertaining to this decoder is
extracted. The latter is then transmitted to its security module SC. The
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authorization block G(MCW) is decrypted by means of one of the
authorization keys G, which allows the deduction of the master control word
MCW. The latter is then used to decrypt the modified control word cw', which
allows the determination of the control word cw used for the encryption of the
content. Thus, the content can be decrypted in a descrambling module Desc.
Therefore, the content CT is obtained in clear.
In the embodiment showed in Figure 2, first of all a variable element RN is
generated, which can be advantageously a random or a pseudo-random
value. According to an alternative, another element depending on the content
can also be joined to the variable element, this other element being able for
example to be linked to the access conditions CD of these content. In a first
instance, an operation is carried out on the variable element RN with or
without the element CD depending on the content. This operation can be a
one-way hash function or another cryptographic operation. The result of this
operation is the control word cw that will be used for the encryption of the
content.
The variable element RN with or without the access conditions CD is
encrypted by the master control word MCW in order to give the modified
control words cw*. These are then processed as in the embodiment in Figure
1, that is to say they are encrypted by the transmission key, formed and
diffused as control messages.
The master control words MCW are also encrypted by authorization keys G
and diffused in an authorization messages stream EMM or in another specific
stream, to arrive at the decoders.
For the decryption of the encrypted content CT', the security module
processes the control messages ECM in order to extract the modified control
word cw*. It also processes the authorization messages G(MCW) in order to
extract the master control word MCW. The latter is then used to extract the
variable element RN, possibly with the element CD depending on the content,
from the modified control word cw*. The same operation as the one used at
the management centre to generate the control words cw from the variable
element RN is applied to the extracted elements, the variable element RN and
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possibly the element depending on the content CD. The control words cw thus
obtained could be used to decrypt the encrypted content CT' in order to obtain
the content CT in clear.
In the embodiment of Figure 3, the control words cw are generated from a
variable element RN, for example a random element with a possible element
CD depending on the data access conditions. In the following description, in
order to simplify, it is supposed that an element depending on the content is
used and that this element is linked to these content's access conditions. In
practice, an element, which does not depend on the access conditions or only
the variable element can be used.
As previously, master control words MCW are also generated. Then the
variable element RN and the access conditions CD are subjected to an
operation depending on the master control words MCW. This type of
operation is typically a hash operation with key, the key being the master
control word. The result of this operation is the control word cw used for the
encryption of the content CT.
The variable element RN and the access conditions CD are encrypted by the
transmission key TK, formed and sent as a control message ECM to the
decoder. The authorization blocks containing the master control words
encrypted by means of the authorization keys G are also transmitted to the
decoder.
The security module associated to the decoder extracts the variable element
RN and the access conditions. It also decrypts the master control words
MCW. From these elements, it applies the operation used to create the
control words in the management centre, this operation being, in the
described example, a hash function with key, the key being the master control
word. The control words cw are thus obtained and then used to decrypt the
encrypted content CT'.
According to the process of the invention, in order to revoke a security
module, it is necessary to determine the set of authorization keys present in
the module to be revoked, then it is necessary to use none of the keys of the
module to be revoked to generate authorization blocks. In other words,
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revoking a security module and its clones corresponds to revoking all the
authorization keys that are present in the security module in question. On the
other hand, it is necessary to use the keys of the non-revoked modules to
generate the authorization blocks needed for the proper functioning of these
non-revoked modules. With reference to optimizing bandwidth use, it is not
possible or desirable to send encrypted authorization blocks with a single key
for each security module. In fact, as the number of security modules to be
managed becomes larger, the available bandwidth will probably be
insufficient. To solve this problem, each security module contains, as
previously indicated, a plurality of authorization keys G. These keys are
organized according to a tree structure described in detail with reference to
Figure 4.
Figure 4 shows an embodiment in which 27 different security modules are
managed. These modules are divided up into groups of three elements.
These nine groups of three elements are also grouped into three. The
principle of the invention is strictly the same, regardless of the size of the
groups. In Figure 4 all the authorizations keys distributed in the 27 security
modules are represented. The system in Figure 4 has four key levels, which
means that each security module contains four authorization keys.
One of these keys, referenced as GO in Figure 4 is common to 33 thus to 27
security modules, which represents the totality of the modules in the example
in question. This key is used to encrypt the master control words MCW as
long as no security module is to be revoked, thus forming an authorization
block G(MCW).
A second authorization key, called level 1 key and referenced as G1 in the
Figure is common to a group of 32 thus 9 security modules. Therefore, three
keys of level 1 are necessary to cover the set of 27 security modules. In the
Figure, these keys have G1 as reference followed by 1, 2 or 3, each
corresponding to a group of 9 security modules.
A third authorization key, called level 2 key, with the reference G2, followed
by
1, 2 or 3 of the 1st level group to which it belongs and followed by 1, 2 or 3
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corresponding to the second level group. A second level key is common to 31,
that is to say 3 security modules.
Furthermore, the security modules contain a 3rd level authorization key, with
the reference G3, this reference also containing the identifiers of the 1st
and
2nd level groups, followed by 1, 2 or 3. This key is common to 30, that is to
say
1 security module. In other words, these level 3 keys are unique for each
security module.
Therefore, according to the level of the key, a key is common for 30 = 1
security module, 31 = 3 modules, 32 = 9 modules and 33 = 27 modules. As
previously indicated, the explained embodiment example is limited to 27
decoders, for the clarity of the illustration. In practice, groups formed by a
power of 2 modules can be used, for example of 16 security modules, with for
example 7 levels keys, that is to say a collection of 7 keys loaded in each
security module, which would allow the management of more than 16 million
modules.
As showed in Figure 5, all the security modules contain, according to a
determined hierarchy, the authorization key GO of level 0, one level 1 key,
one
level 2 key and one level 3 key. It should be noted that, in order to ensure
the
security of the security modules, the keys that they contain are preferably
not
sent in a message, but are loaded in the factory during a module
personalization stage.
In normal operating conditions, that is to say when all the security modules
are active, the authorization data is systematically encrypted by the global
key
GO known by all the security modules, as previously explained.
When the use of the global key GO is no longer desired, for example when
one of the security modules is considered to be a clone and therefore must be
revoked, the authorization blocks are only encrypted with the authorization
keys of level G1, excluding of course the G1 level key which is present in the
module to be revoked. As the revoked key G1 is shared by several security
modules, the level of the used key is lower only for the modules which have
the same key G1 as the module to be revoked and the authorization blocks
are encrypted with the G2 level keys, except with that who is contained in the
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module to be revoked. This process is iterative until the used key is that of
the
final level which corresponds to a unique key for each security module.
To explain the process according to the invention referring to Figure 4, it is
supposed that the security module having the unique authorization key
G3.3.2.1 is considered as a clone and that the right for this module to
decrypt
the data must be denied. Since this module belongs to the group having the
authorization key GO, as all the modules, it is no longer possible to use this
key GO, as schematically represented in Figure 6a. Also the module to be
revoked belongs to the group having the first level G1.3 key. Thus, this
authorization key must no longer be used. The suppression of the key GO for
decryption prevents all security modules from decrypting data, which is
obviously not desired. In order to allow the correct functioning of the non-
revoked modules, it is necessary to use another authorization key. In
practice,
one will use the valid encryption keys for the group containing the largest
possible number of security modules, provided this group does not contain the
module to be deactivated. According to this rule, the key GO is no longer
used, but the first level keys G1.1 and G1.2 can be used, since the module to
be deactivated, having the G3.3.2.1 unique key, does not belong to the group
having these 1st level keys. The encryption and the sending of the master
control word MCW encrypted with these 18t level keys are schematically
showed in Figures 6b and 6c respectively.
Thus, by sending encrypted messages using two different authorization keys,
namely G1.1 and G1.2, all the modules belonging to these groups, which
correspond to 18 modules, function correctly. The G1.3 key has not been
used because the module to be deactivated belongs to the group using this
G1.3 key. The effect of this is to render unusable the 9 modules belonging to
this group. The module to be blocked belongs to the group having the 2nd
level G2.3.2 key. Therefore, the other keys of the 2nd level are used, namely
G2.3.1 and G2.3.3, as shown in Figures 6d and 6e respectively, without using
G2.3.2. This has the effect of blocking three modules namely those having the
G3.3.2.1, G3.3.2.2 and G3.3.2.3 keys. Only one of these modules must be
blocked. In order to allow the functioning of the other two modules, the
G3.3.2.2 and G3.3.2.3 keys are used, as shown in Figures 6f and 6g, which
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only prevents the module to be eliminated from decrypting any further data.
Therefore, in the described embodiment example, in order to deactivate a
security module, it is necessary to use two keys from level 1, two keys from
level 2 and two keys from level 3, that is to say 6 keys, for a total of 27
security modules. It should be noted that in a conventional system, it would
be
necessary to encrypt the messages with 26 different keys to allow the
functioning of all the modules except one.
By extension, it can be demonstrated that if the number of levels is called
lc,
that is to say the number of authorization keys stored in each security
module,
and n is the number of security modules per group, the number of
manageable modules is equal to nk-1) and the number of keys to be used for
the elimination of a module among these nk-1) modules is equal to (n-1)*(K-1).
In an embodiment where the number of modules per group is 16 (n=16) and
has 7 keys levels (K=7), 90 authorization data must be sent, using 90
different
authorization keys to isolate a module among more than 16 million modules
(16777'216 precisely).
The number of keys to be used to isolate a second module depends on the
relation between the two modules to be eliminated or, in other words, on their
relative position in the tree structure shown in Figure 4. The most favourable
case corresponds to two modules to be eliminated belonging to the same
group of the penultimate level, that is to say, with reference to Figure 4,
two
modules having the same second level key G2. In this case, the number of
keys to be used corresponds to one less than that needed for the elimination
of a single module, namely [(n-1)*(K-1)]-1 in the general case, 89 keys in the
context of the groups of 16 modules distributed in 7 levels and 5 keys in the
embodiment example in Figure 4.
The most unfavourable case is when the two modules have as unique
common key, the level 0 key GO. In this case, the number of different keys to
be used is equal to (n-2)+2(n-1)(K-2) in general that is to say 164 keys in
the
context of the groups of 16 modules distributed in 7 levels and 9 keys in the
embodiment example in Figure 4.
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This invention is Particularly interesting because the number of messages to
be encrypted with different keys can be extremely reduced. Therefore, it is
possible to change frequently the master control word, for example once a
minute, so that possible acts of piracy can be deterred.
In order to use as less keys as possible and accordingly to generate as less
messages as possible, the keys common to the largest possible number of
security modules are used, excluding at the same time the module to be
deactivated. Therefore, referring to Figure 4, the G1.1 key will be used
rather
than the 3 keys G2.1.1, G2.1.2 and G2.1.3. Nevertheless, this process also
operates by using these three keys, however without minimising the number
of messages to be transmitted.
As previously indicated, the security modules contain several authorization
keys. When a security module receives a message, it can be decided that the
lowest possible level key will have to be used. For example, if a module
receives an encrypted message by means of a level 1 key and a level 3 key,
the level 1 key will have to be used to decrypt the message. It is possible to
provide other means to determine the key to be used, these means should
allow the knowledge of the level of the key to be used.
It should be noted that the master control words MCW are generally the same
for several channels. This allows fast decryption when the user changes
channel. However, it is also possible to use different master control words
MCW, this is generally the case when the encryption is carried out by various
providers.
