 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates generally to conditional access systems of
the type used to control cable subscriber set-top boxes and particularly
concerns a countdown technique for increasing the security of such
conditional access systems.
Conditional access systems for subscriber units such as cable subscriber
set-top box decoders are well-known in the art. Conditional access is
conventionally achieved by downloading one or more authorization levels
for storage in the decoder. The stored authorization levels may be
represented by a bit map or a list of individual multi-bit codes, or a
combination of both. Each received subscription program, which is normally
scrambled to prevent access thereto by an unauthorized subscriber,
includes an authorization code (sometimes referred to as a program tag)
identifying the associated program. If the program tag corresponds to an
authorized level in the stored bit map or to a stored listed authorization
level of the subscriber, a descrambling circuit within the subscriber's
decoder is enabled to descramble the signal for viewing by the subscriber.
On the other hand, if the received program tag does not match any stored
authorization level descrambling of the accompanying program is inhibited.
Depending on the desired resolution, recent advances in technology have
made possible the transmission and reception of one or more digitally
compressed television signals over a 6 MHz television channel. The
television signal is preferably compressed and arranged for transport in
accordance with international standards established by the Moving Pictures
Expert Group (MPEG). In accordance with the MPEG standard, the compressed
digital television information may be arranged for transmission in the
form of a multiplexed transport stream of fixed length MPEG packets
including, for example, video packets, audio packets and conditional
access packets (all packets other than conditional access packets being
referred to as product packets). Each packet in the transport stream
includes a 4-byte header comprising a 13-bit packet identification code
(PID) identifying the so-called payload (184 bytes) of the respective
packet. A PID having a value equal to one (i.e. 00 . . . 1) has been
reserved for use with conditional access packets.
In a general sense, conditional access for digitally compressed
subscription systems of the foregoing type may be achieved using
techniques quite similar to those employed in prior art analog
subscription systems. However, since it is anticipated that the digital
systems will make much more extensive use of services such as
video-on-demand, it is desirable to provide increased confidence that the
operation of the conditional access system is largely tamper-proof. For
example, the security of the conditional access system may be compromised
by interrupting the conditional access data stream to the decoder, the
feasibility of such interruption being facilitated by the fact that the
MPEG packet headers are not encrypted. Such interruption of the
conditional access data could lead to a situation where, for example, it
becomes impossible to retract a previously established authorization
level. This presents the subscription system operator with the dilemma of
not being able to reuse authorization levels in a timely manner or assume
the risk that a non-paying subscriber may have access to the service
associated with a reused authorization level. The problem is compounded in
connection with services such as pay-per-view television which require
frequent recycling of authorization levels.
OBJECTS OF THE INVENTION
It is therefore a basic object of the present invention to provide an
improved conditional access system for a subscription service such as a
pay cable television system.
It is a more specific object of the invention to provide a conditional
access system which is relatively resistant to unauthorized tampering even
though the PID's identifying the conditional access data stream are not
encrypted.
It is yet a more specific object of the invention to provide a conditional
access system in which interruption of the conditional access data stream
will automatically result in denial of further subscription services.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advances of the invention will be apparent upon
reading the following description in conjunction with the drawings, in
which:
FIG. 1 is a simplified block diagram of a subscription television decoder;
FIG. 2 is a block diagram of the DCAM shown in FIG. 1;
FIGS. 3A-3B are diagrammatic representations of different CA packets
according to the invention; and
FIG. 4 illustrates the PID and countdown registers of the DCAM of FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 sets forth a block diagram of a television subscriber terminal
constructed in accordance with the present invention and generally
referenced by numeral 10. Subscriber terminal 10 includes a tuner 12
coupled to a cable television distribution system (not shown) or other
suitable transmission medium by a cable 14. The output of tuner 12 is
coupled to an intermediate frequency (IF) filter 16, typically a SAW
filter, and therefrom to the input of an IF amplifier and demodulator
circuit 18. Demodulator 18 may comprise, for example, a multi-level VSB or
QAM demodulator.
The output of demodulator 18 comprises an MPEG transport bitstream
including a series of MPEG product and conditional access (CA) packets. As
previously mentioned, each such packet includes an unencrypted 4-byte
header comprising a 13-bit PID identifying the contents of the packet
followed by 184-bytes of encrypted payload, 20-bytes of which are used for
forward error correction. A product packet may comprise a compressed video
packet, a compressed audio packet or a packet containing auxiliary data.
Each such packet is identified by its own unique PID, with a PID having a
value of one (00 . . . 01) reserved for CA packets. Depending on the
degree of compression employed and on the maximum bit-rate provided by the
transmission system, the transport bitstream derived from a tuned 6 MHz
television channel may represent one or more television programs, the
components (e.g. video and audio) of each television program being
identified by their own respective PID's.
The MPEG transport bitstream developed at the output of demodulator 18 is
coupled to a digital condition access module (DCAM) 20 which will be
described in further detail hereinafter. For now it is sufficient to
understand that DCAM 20 is responsive to CA packets multiplexed in the
transport bitstream for selectively authorizing and deauthorizing
subscriber terminal 10 for various television programs and other services.
DCAM 20 is also operative for decrypting the payloads of product packets
having PID's corresponding to a program selected for viewing by the
subscriber and for which the subscriber has appropriate authorization. The
output of DCAM 20 is coupled to an MPEG transport demultiplexer 22 which
couples the decrypted video packets to a video decompression circuit 24
and the decrypted audio packets to an audio decompression circuit 26.
Video decompression circuit 24 may include a random access memory 28
coupled thereto. The decompressed video signal developed at the output of
video decompression circuit 24 is applied to a D/A converter 29 which is
coupled to a suitable video display. Correspondingly, the decompressed
audio signal developed at the output of audio decompression circuit 26 is
applied to a D/A converter 30 which is coupled to a suitable audio system.
Subscriber terminal 10 further includes a microprocessor 32 having a
channel selection output 34 for controlling tuner 12 (i.e. for tuning a
selected 6 MHz television channel) and an output 36 for controlling
demodulator 18. Microprocessor 32 also has an output 38 for controlling
transport demultiplexer 22 and receives data, e.g. network directories and
program map tables, from the demultiplexer over a line 39. Microprocessor
32 is further bi-directionally coupled to DCAM 20 by a bus 40 and is
supplied with an interrupt signal by DCAM 20 over a line 42. An upstream
transmitter 44 is supplied by microprocessor 32 and has an output coupled
to cable 14 for providing upstream transmissions over the cable
distribution system. Microprocessor 32 is responsive to signals from a
user control interface 46 operable by a subscriber for selecting a program
for viewing.
In operation, a plurality of broadcast 6 MHz RF channels are coupled by
cable 14 to the input of tuner 12 which in response to a channel selection
signal supplied by microprocessor 32 couples a selected channel to
intermediate frequency filter 16. Filter 16 may be constructed in
accordance with conventional fabrication techniques and may, for example,
include a conventional surface acoustic wave filter or its equivalent. The
output of filter 16 is demodulated by intermediate frequency amplifier and
demodulator circuit 18. Demodulator 18, which may comprise a synchronous
demodulator, recovers the digital MPEG transport bitstream comprising CA
packets and product packets representing one or more television programs.
While different transmission signal formats and methods may be utilized in
communicating data through the cable distribution system, the preferred
embodiment shown in FIG. 1 utilizes a digital vestigial sideband (VSB)
modulation system in which N-level (e.g. 16, 8, 4 or 2-level) symbols
having a symbol rate of approximately 10.76 megahertz are transmitted and
received over cable 14. The transport bitstream generated at the output of
demodulator 18 is further selectively processed by DCAM 20 and thereafter
demultiplexed in demultiplex circuit 22 to provide selected input video
and audio signals to decompression circuits 24 and 26 respectively.
Circuits 24 and 26 perform conventional video and audio decompression
operations upon the applied video and audio data to produce decompressed
video and audio signals which are converted to corresponding analog
signals within digital to analog converters 29 and 30. The analog signals
thus provided may be applied to the video display and audio system of the
subscriber's television receiver.
The structure of DCAM 20 is shown in greater detail in FIG. 2. DCAM 20
preferably comprises an application specific integrated circuit (ASIC)
which implements the conditional access and decryption functions of
subscriber terminal 10. As shown in FIG. 2, the transport bitstream from
demodulator 18 is supplied to a payload crypto device 50, whose output is
coupled to transport demultiplexer 22 and also supplies a CA packet
interceptor 52. The transport bitstream is also supplied through a PID
filter 53 to a payload countdown circuit 54 which includes an output
coupled to the Enable input of payload crypto device 50. The output of CA
packet interceptor 52 is supplied to an embedded CPU 56 over a bus 58. Bus
58 also couples CPU 56 to payload countdown circuit 54, a RAM 60, a ROM
62, a one-time-programmable memory (OTPM) 64, a key source generator 66
and a CA crypto device 68. Communications between CPU 56 and processor 32
(see FIG. 1) are effected over lines 40 and 42.
The operation of DCAM 20 is controlled by CA packets (PID =1) coupled by CA
packet interceptor 52 to CPU 56. There are a number of different types of
CA packets (identified by the first 3-bits of the packet payload)
including CA initialization packets, CA configuration load packets and CA
PID authorization packets. The CA initialization packets (see FIG. 3A) are
used to initialize various keys used in DCAM 20 and to initialize the
subscriber authorization levels of a bit map (e.g. 256 bits) and an
authorization list stored in RAM 60. In particular, each subscriber DCAM
20 includes a unique 4-byte public serial number (S/N), a common load and
a private (master) load all stored in OTPM 64. The common and private
(master) loads are combined with selected bytes of ROM 62, which includes
a stored program for controlling the operation of cPU 56, for providing
respective common and private (master) keys. Each subscriber DCAM further
includes active CA and payload key sources and received CA and payload key
sources provided by key source generator 66. Each CA initialization packet
includes the public S/N of one or more subscriber terminals together with
the associated active and received CA and payload key sources and
authorization levels. The received public S/N, which is encrypted with the
network common key, is decrypted by payload crypto device 50 in response
to the common key derived from OTPM 64. The decrypted public S/N is
supplied to CPU 56 which determines whether it matches the public S/N
stored in OTPM 64. If a match exists, CPU 56 fetches the received active
and received CA key sources for storage in key source generator 66. These
key sources are encrypted first with the private key corresponding to the
packet public S/N and then with the network common key. They are therefore
decrypted first by payload crypto device 50 using the common key from OTPM
64 and then by the CA crypto device 68 using the private key from OTPM 64.
The active CA key source now provided by key source generator 66 is used
to build a decryption table used by CA crypto device 68 to decrypt further
CA encrypted data bytes. Such further data bytes include the active and
received payload key sources which after decryption are stored in key
source generator 66 and authorization levels which after decryption are
stored in the bit map and authorization list of RAM 60.
The CA configuration load packet is illustrated in FIG. 3B. It is similar
in format to the CA initialization packet and is used to provide new CA
and payload key sources for key source generator 66. The packet is also
used to refresh the authorization bit map stored in RAM 60. As shown in
FIG. 3B, the packet type and public S/N are encrypted using the active
payload key source previously downloaded in a CA initialization packet and
are therefore decrypted by payload crypto device 50 in response to the
corresponding decryption table. If CPU 56 establishes that the decrypted
public S/N matches the public S/N stored in OTPM 64, the received payload
and CA key sources are decrypted by CA crypto device 68. These key sources
will become the active key sources if they differ from the current active
key sources and result in rebuilt decryption tables for payload and CA
crypto devices 50 and 68.
Processing of the CA initialization and configuration load packets
therefore results in DCAM 20 having been individually addressed to
download various decrypted critical operating parameters (i.e. decryption
keys and authorization levels). In particular, the CA key source is
downloaded after it is decrypted using the private key of DCAM 20, the
download CA key source being used in turn to provide for downloading of
the payload key source. Moreover, a would-be pirate can neither
artificially create nor selectively filter subsequent CA packets since the
packet payload, including CA packet type, of each such CA packet is
encrypted using the payload and CA key sources. As will be explained in
further detail, the inability to either create illegitimate CA packets or
selectively filter de-authorizing packets prevents a would-be pirate from
compromising system security.
The format of a CA PID authorization packet is illustrated in FIG. 3C. This
packet, which is globally addressed using a public S/N=1, is the most
frequently transmitted CA packet. The packet comprises a type code and a
public S/N=1, both encrypted using the active payload key source and both
decrypted by payload crypto device 50. The packet further comprises one or
more PID's, each with an associated authorization level and countdown
register level, all encrypted using the active CA key source and therefore
decrypted by CA crypto device 68. As will be explained in further detail
hereinafter, each active PID within a given 6 MHz channel must be
transmitted and received by DCAM 20 in a CA PID authorization packet at a
predetermined minimum rate in order to maintain payload crypto device 50
operable for decrypting payloads of product packets with corresponding
PID's. Therefore, if the CA bitstream to DCAM 20 is interrupted, payload
crypto device 50 will become inhibited and thereby cease decrypting
product packets.
More specifically, assume a subscriber elects to view a particular
television program contained within a tuned 6 MHz television channel. The
selected program has an authorization level "A" and is comprised of
packets having PID's 317, 318 and 319. PID 317 may, for example, identify
compressed video packets, PID 318 compressed audio packets and PID 319
auxiliary data packets. Upon selecting the program (using user control
interface 46), external processor 32 causes CPU 56 to determine whether
the subscriber is authorized to view the program. That is, CPU 56 checks
RAM 60 to determine if authorization level "A" is found in the stored
authorization bit map or authorization list. Assuming authorization level
"A" is found in RAM 60, the three PID's 317, 318 and 319 of the selected
program are transferred over bus 58 for storage in respective PID
registers 72a, 72b and 72c (see FIG. 4) of payload countdown circuit 54.
At the same time, respective associated countdown registers 74a, 74b, and
74c of countdown circuit 54 are set to selected values.
Payload crypto device 50 is operative for decrypting product packets only
if the PID of the respective packet is stored in one of the ten PID
registers 72a-72j of countdown circuit 54 (the packet PID's are supplied
to countdown circuit 54 by PID filter 53) and the contents of the
corresponding countdown register does not equal zero. Therefore, payload
crypto device 50 begins decrypting payloads of product packets having
PID's 317, 318 and 319 and couples the decrypted packet payloads (together
with all unencrypted packets) to transport demultiplexer 22 for further
processing. Moreover, each time a product packet having a PID stored in a
PID register 72a-72C is received by countdown circuit 54, the associated
PID countdown register 74a-74c of countdown circuit 54 is decremented by a
factor of unity. If any one of the countdown registers reaches a value of
zero, countdown circuit 54 inhibits payload crypto device from decrypting
any further packets having the corresponding PID. For example, if
countdown register 74a, which is associated with PID register 72a storing
PID 317, reaches a zero count, countdown circuit 54 will inhibit payload
crypto device 50 from decrypting any further product packets having PID
317.
However, under normal operating conditions CA PID authorizations packets
are transmitted and received at a rate sufficient to prevent anyone of the
countdown registers 74a-74c from reaching a zero count. In particular,
after verification by CPU 56 that the authorization level of a received CA
PID authorization packet matches a subscriber authorization level in RAM
60 and that its PID matches a PID stored in one of the PID registers
72a-72j, the associated countdown register 74a-74j is set to the countdown
register level value of the received CA PID authorization packet. The
countdown register level preferably comprises one byte allowing the
countdown register to be set to any one of 256 different values. It will
thus be seen that by operating the network such that appropriate CA PID
authorization packets (i.e. having selected PID/authorization level pairs)
are transmitted and received before a given countdown register reaches a
zero value, payload crypto device 50 will remain operative for decrypting
product packets having the corresponding PID's. Authorization level
matches may be established by searching the authorization list and bit map
of RAM 60 in response to each received CA PID authorization packet.
Alternatively, the authorization list and bit map may be searched only
once in response to the viewer's program request. The corresponding
authorization level, assuming that a match exists, is then stored in a
reserved portion of RAM 60 associated with the PID's stored in PID
registers 72a-72j and is checked for a match with each received CA PID
authorization packet. The latter approach is, of course, less CPU
intensive since a search of RAM 60 is not effected in response to each
received CA PID authorization packet.
Assume, now for example, that a subscriber interrupts the CA bitstream in
an attempt to prevent deauthorization by denying network access to the
authorization levels stored in RAM 60. While interruption of the CA
bitstream may achieve this goal, it will also interrupt receipt of the CA
PID authorization packets, thereby allowing the PID countdown registers
74a-74J to assume zero counts and inhibit decryption of the product
packets having PID's stored in PID registers 72a-72j. As a result, the
subscriber will be denied access to the program even though the network
cannot access RAM 60 for deauthorizing the corresponding service level.
As a further option, the operation of DCAM 20 can be enhanced such that
countdown registers 74a-74j are each also decremented in response to the
receipt of any CA packet (PID=1). This will in effect clear (i.e. set to
zero) all countdown registers 74a-74j having corresponding PID registers
72a-72j storing inactive PID's.
What has thus been described is a novel conditional access system having a
unique countdown system providing increased security against unauthorized
tampering. It is recognized that numerous changes in the described
embodiment of the invention will be apparent to those skilled in the art
without departing from its true spirit and scope. The invention is to be
limited only as defined in the claims.
* * * * *
|
|
 |
|
 |
Description |
|
