U.S. patent application number 09/022473 was filed with the patent office on 2001-11-15 for electronic watermark system.
Invention is credited to NAKANO, HIROTAKA.
Application Number | 20010040977 09/022473 |
Document ID | / |
Family ID | 12231473 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010040977 |
Kind Code |
A1 |
NAKANO, HIROTAKA |
November 15, 2001 |
ELECTRONIC WATERMARK SYSTEM
Abstract
An electronic watermark system invisibly embeds watermark
information into original image data and the watermarked image data
is transferred to a first medium. At the same time, the embedded
watermark information is transferred to a second medium. When a
medium questioned is found, its watermark information can be easily
identified by searching the second medium. Alternatively, the
watermarked image data and the embedded watermark information are
transferred to different storage areas of the same medium,
respectively.
Inventors: |
NAKANO, HIROTAKA; (TOKYO,
JP) |
Correspondence
Address: |
FOLEY & LARDNER
3000 K STREET NW SUITE 500
PO BOX 25696
WASHINGTON
DC
200078696
|
Family ID: |
12231473 |
Appl. No.: |
09/022473 |
Filed: |
February 12, 1998 |
Current U.S.
Class: |
382/100 ;
375/E7.089; 375/E7.211; 386/E9.013; G9B/20.002 |
Current CPC
Class: |
G06T 2201/0065 20130101;
G11B 20/00557 20130101; G11B 20/00884 20130101; G11B 20/00094
20130101; G11B 20/00086 20130101; G11B 20/0021 20130101; H04N
1/32154 20130101; H04N 2201/3233 20130101; H04N 2201/327 20130101;
H04N 21/8358 20130101; G11B 20/00855 20130101; G06T 1/0021
20130101; G06T 2201/0052 20130101; H04N 9/8063 20130101; H04N
1/32277 20130101; H04N 5/781 20130101; H04N 1/32165 20130101; H04N
19/61 20141101; H04N 21/23892 20130101; H04N 2201/3235 20130101;
H04N 9/8042 20130101; H04N 2201/3274 20130101; H04N 19/467
20141101 |
Class at
Publication: |
382/100 |
International
Class: |
G06K 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 12, 1997 |
JP |
9-027815 |
Claims
What is claimed is:
1. A system comprising: an information generator for generating
watermark information; a combiner for invisibly embedding the
watermark information into original image data to produce
watermarked image data; and a transfer controller for transferring
the watermarked image data to a first medium and the watermark
information to a second medium.
2. The system according to claim 1, wherein the transfer controller
further transfers medium identification information identifying the
first medium to the second medium.
3. The system according to claim 2, wherein the second medium
stores the watermark information and the medium identification
information to allow retrieval.
4. The system according to claim 1, wherein the first medium is a
first storage medium for storing the watermarked image data; and
the second medium is a second storage medium for storing the
watermark information and medium identification information
identifying the first storage medium to allow retrieval.
5. The system according to claim 4, wherein the medium
identification information is a manufacture serial number of the
first storage medium.
6. The system according to claim 1, wherein the first medium is a
communication medium through which the watermarked image data is
transmitted to a destination in a communication network; and the
second medium is a storage medium for storing the watermark
information and medium identification information identifying the
destination to allow retrieval.
7. The system according to claim 6, wherein the medium
identification information is an address of the destination in the
communication network.
8. A system comprising: an information generator for generating
information; a combiner for invisibly embedding the watermark
information into original image data to produce watermarked image
data; and a transfer controller for transferring both the
watermarked image data and the watermark information to a single
medium.
9. The system according to claim 8, wherein the single medium is a
storage medium having a first storage area for storing the
watermarked image data and a second storage area for storing the
watermark information, wherein the second storage area is dedicated
to the watermark information so that only the watermark information
is allowed to be read.
10. The system according to claim 8, wherein the transfer
controller encrypts the watermark information and transfers
encrypted watermark information to the single medium.
11. The system according to claim 10, wherein the single medium is
a storage medium having a first storage area for storing the
watermarked image data and a second storage area for storing the
encrypted watermark information, wherein the second storage area is
dedicated to the encrypted watermark information so that only the
encrypted watermark information is allowed to be read.
12. An authorization check system for checking whether a medium
questioned is authorized, comprising: a storage for storing
watermark information and medium identification information
identifying a medium, the medium storing watermarked image data
which is produced by invisibly embedding the watermark information
into original image data; a retrieval controller for retrieving
watermark information corresponding to medium identification
information of the medium questioned from the storage; watermark
extractor for extracting watermark information questioned from
image data stored in the medium questioned; and a checker for
checking whether the medium questioned is authorized by comparing
the extracted watermark information with the retrieved watermark
information.
13. The authorization check system according to claim 12, wherein
the medium identification information is a manufacture serial
number of the medium.
14. The authorization check system according to claim 12, wherein
the medium is a communication medium through which the watermarked
image data is transmitted to a destination in a communication
network, wherein the medium identification information is an
address of the destination in the communication network.
15. An authorization check system for checking whether a medium
questioned is authorized, the medium questioned having a first
storage area for storing watermarked image data and a second
storage area for storing watermark information, the system
comprising: a watermark extractor for extracting watermark
information from image data stored in the medium questioned; and a
checker for checking whether the medium questioned is authorized by
comparing the extracted watermark information with the watermark
information stored in the second storage area of the medium
questioned.
16. The authorization check system according to claim 15, wherein
the watermark information stored in the second storage area is
encrypted.
17. A method for checking whether a medium questioned is
authorized, comprising the steps of: generating watermark
information; invisibly embedding the watermark information into
original image data to produce watermarked image data; transferring
the watermarked image data to an authorized medium; transferring
the watermark information and medium identification information
identifying the authorized medium to a retrieval medium; retrieving
watermark information corresponding to medium identification
information of the medium questioned from the retrieval medium;
extracting watermark information questioned from image data stored
in the medium questioned; and checking whether the medium
questioned is authorized by comparing the extracted watermark
information with the retrieved watermark information.
18. The method according to claim 17, wherein the medium
identification information is a manufacture serial number.
19. A method for checking whether image data questioned is
authorized, comprising the steps of: generating watermark
information; invisibly embedding the watermark information into
original image data to produce watermarked image data; transmitting
the watermarked image data to an authorized destination through a
network; transferring the watermark information and medium
identification information identifying the authorized destination
to a retrieval medium; retrieving watermark information
corresponding to identification information of a data provider
providing the image data questioned from the retrieval medium;
extracting watermark information from the image data questioned
received from the data provider; and checking whether the image
data questioned is authorized by comparing the extracted watermark
information with the retrieved watermark information.
20. A method for checking whether a medium questioned is
authorized, comprising the steps of: generating watermark
information; invisibly embedding the watermark information into
original image data to produce watermarked image data; transferring
both the watermarked image data and the watermark information to an
authorized medium; extracting watermark information from image data
stored in the medium questioned; and checking whether the medium
questioned is authorized by comparing the extracted watermark
information with watermark information stored in the medium
questioned.
21. The method according to claim 20, wherein the authorized medium
is a storage medium having a first storage area for storing the
watermarked image data and a second storage area for storing the
watermark information, wherein the second storage area is dedicated
to the watermark information so that only the watermark information
is allowed to be read.
22. The method according to claim 21, wherein the watermark
information is encrypted and encrypted watermark information is
stored to the authorized medium.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to an authorization
system and in particular to an authorization system using
identification information embedded into image data to prevent
unauthorized duplication of the image data.
[0003] 2. Description of the Related Art
[0004] Recently, unauthorized copying of digital image data has
become a serious problem because duplication of digital data can be
extremely easy. To prevent unauthorized copying, several
authorization systems using identification information (an
electronic watermark) embedded into image data have been
proposed.
[0005] A visible-watermarking system is disclosed in Japanese
Patent Unexamined Publication No. 8-241403 which corresponds to
U.S. Pat. No. 5,530,759. In this system, a visible watermark is
placed on a digital image such that the corresponding pixel of
original image changes its brightness but its chromaticities.
[0006] An invisible-watermarking system is disclosed in NIKKEI
ELECTRONICS 1996.4.22 (no. 660). In this system, original image
data is converted into frequency spectrum and then ID information
is embedded into the frequency spectrum which is in turn converted
into image data invisibly including the ID information. In
authorization check, image data questioned is converted into
frequency spectrum. ID information embedded in the image data
questioned is obtained from a difference between the frequency
spectrum questioned and the original frequency spectrum. Since the
ID information is embedded into frequency spectrum of the original
image data, it has little effect on the image quality and further
it has become very difficult to delete the ID information from the
image data.
[0007] In the above invisible-watermarking system, since the ID
information cannot be seen, it is difficult to check whether image
data is authorized in the case where no one knows the ID
information. Especially, in a distribution system of data storing
medium including CD-ROMS, there are cases where a different
watermark is used for each distribution channel. In this case, the
authorization check and manufacture management become very
complicated and time-consuming jobs.
SUMMARY OF THE INVENTION
[0008] An object of the present invention is to provide a system
which can rapidly check whether image data is authorized to
effectively prevent unauthorized copying of the image data.
[0009] Another object of the present invention is to provide a
system which can easily identity an electronic watermark invisibly
embedded in image data to check whether the image data is
authorized.
[0010] According to an aspect of the present invention, watermarked
image data having watermark information invisibly embedded is
transferred to a first medium and the embedded watermark
information is transferred to a second medium. Since the embedded
watermark information is stored in the second medium, watermark
identification can be easily made.
[0011] Further, in an authorization check system for checking
whether a medium questioned is authorized, a storage stores
watermark information and medium identification information
identifying a medium, the medium storing watermarked image data
which is produced by invisibly embedding the watermark information
into original image data. A retrieval controller retrieves
watermark information corresponding to medium identification
information of the medium questioned from the storage. A watermark
extractor extracts watermark information questioned from image data
stored in the medium questioned. A checker checks whether the
medium questioned is authorized by comparing the extracted
watermark information with the retrieved watermark information.
[0012] According to another aspect of the present invention,
watermarked image data having watermark information invisibly
embedded and the embedded watermark information are both
transferred to a single medium. Preferably, the embedded watermark
information may be stored in a dedicated storage area of the single
medium. Further, the embedded watermark information may be
encrypted and stored. Since the embedded watermark information is
stored in the same medium, watermark identification can be easily
made.
[0013] Further, in an authorization check system for checking
whether a medium questioned is authorized, the medium questioned
having a first storage area for storing watermarked image data and
a second storage area for storing watermark information. After
extracting watermark information from image data stored in the
medium questioned, a checker checks whether the medium questioned
is authorized by comparing the extracted watermark information with
the watermark information stored in the second storage area of the
medium questioned.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a schematic block diagram showing an encoding side
of an electronic watermark system according to the present
invention;
[0015] FIG. 2 is a schematic block diagram showing a watermark
check section of the electronic watermark system according to the
present invention;
[0016] FIG. 3 is a schematic diagram showing an example of
watermarking section of the encoding side of FIG. 1;
[0017] FIG. 4 is a schematic diagram showing an example of a
watermark extractor of the watermark check section of FIG. 2;
[0018] FIG. 5 is a diagram showing a first embodiment of an
electronic watermark system according to the present invention;
[0019] FIG. 6 is a diagram showing an electronic watermark system
according to a second embodiment of the present invention;
[0020] FIG. 7A is a schematic diagram showing an encoding side of
an electronic watermark system according to a third embodiment of
the present invention; and
[0021] FIG. 7B is a schematic diagram showing a decoding side of
the electronic watermark system according to the third
embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0022] Referring to FIG. 1, an encoding side of an electronic
watermark system inputs a stream of original image data D.sub.0 and
produces both MPEG (Motion Picture Experts Group) data stream
D.sub.4 and watermark data which is invisibly embedded into the
MPEG data D.sub.4. More specifically, a stream input section 101
inputs a stream of the original image data which is converted to
frequency spectrum data D.sub.1 by a DCT (Discrete Cosine
Transform) section 102. A watermarking section 103 reads a
watermark D.sub.WM selected from a plurality of watermarks stored
in a watermark data table 104, and then embeds the selected
watermark D.sub.WM into the frequency spectrum data D.sub.1 to
produce watermark-embedded frequency spectrum data D.sub.2. The
watermark-embedded frequency spectrum data D.sub.2 is quantized by
a quantizing section 105 and the quantized data D.sub.3 is encoded
to produce MPEG data by an encoding section 106. A transfer
controller 107 inputs the MPEG data from the encoding section 106
to produce a stream of the MPEG data D.sub.4 and a transfer
controller 108 inputs the selected watermark D.sub.WM from the
watermark data table 104 to produce selected watermark data. The
stream of the MPEG data D.sub.4 is transferred to a storage medium
or a client through a communication channel and the selected
watermark data is transferred to another storage medium or the same
storage medium, as will be described later.
[0023] It should be noted that the above sections 101-106 may be
implemented with a program-controlled processor such as CPU or DSP.
In other words, the processor runs programs including the functions
of the above section 101-106 to perform the watermarking and MPEG
data compression as described above.
WATERMARK CHECKING SECTION
[0024] Referring to FIG. 2, in a watermark checking section of the
electronic watermark system, a stream data input section 201 inputs
a stream of MPEG data D.sub.4' and a watermark input section 202
inputs watermark data associated with the input MPEG data D.sub.4'.
The input MPEG data D.sub.4' is decoded to produce date D.sub.3' by
a decoding section 203 and then the date D.sub.3' is
inverse-quantized to produce data D.sub.2' by an inverse-quantizing
section 204. On the other hand, the watermark checking section is
provided with an original image memory 205 storing the original
image data D.sub.0. The original image date D.sub.0 is converted to
frequency spectrum data D.sub.1 by a DCT section 206.
[0025] When receiving the input frequency spectrum data D.sub.2'
from the DCT section 204 and the original frequency spectrum data
D.sub.1 from the DCT section 206, a watermark extractor 207
extracts a watermark D.sub.WM1 from the input frequency spectrum
data D.sub.2' by calculating a difference between the input
frequency spectrum data D.sub.2' and the original frequency
spectrum data D.sub.1. An inner product calculating section 208
inputs the extracted watermark D.sub.WM1 from the watermark
extractor 207 and the received water ark D.sub.WM2 from the
watermark input section 202, and performs the inner product thereof
to produce a degree of statistical similarity between them. An
authorization check section 209 checks whether the extracted
watermark D.sub.WM1 is identical to the received watermark
D.sub.WM2 by comparing the degree of statistical similarity with a
reference value.
[0026] The input MPEG data D.sub.4' may be received from a storage
medium or a communication network. The watermark data associated
with the input MPEG data D.sub.4' is received from a management
database storing manufacture management data or the same storage
medium as the MPEG data D.sub.4', as will be described later. In
the case where the input MPEG data D.sub.4' and/or the watermark
data are received from a communication network, the watermark
checking section is provided with a communication means such as a
network interface or a radio transceiver.
[0027] It should be noted that the above sections 201-209 may be
implemented with a program-controlled processor such as CPU or DSP.
In other words, the processor runs programs including the functions
of the above section 201-209 to perform the watermark extracting,
MPEG data decompression and authorization check as described
above.
WATERMARKING
[0028] Referring to FIG. 3, the watermarking section 103 receives
the frequency spectrum data D.sub.1 from DCT section 102 and
selects a set of N data samples: f(1)-f(n) which are greater than a
predetermined threshold level from the frequency spectrum data
D.sub.1. Further, the watermarking section 103 selects a set of
watermark data: w(1)-w(n) from random numbers depending on a normal
distribution with a mean of 0 and a variance of 1. The watermarking
section 103 calculates
F(i)=f(i)+.alpha..vertline.f(i).vertline.*w(i) for each variable i
(1.ltoreq.i.ltoreq.n) using multipliers 301 and 302, a constant
.alpha. and an adder 303, where .alpha. is a scaling element
(hereinafter, assuming .alpha.=1). The calculated data samples
F(1)-F(n) are substituted for the selected N data samples f(1)-f(n)
of the frequency spectrum data D.sub.1 to produce watermarked DCT
frequency spectrum data which will be subject to inverse DCT in the
following stage 105. In this manner, the selected watermark
D.sub.WM is invisibly embedded into the original image data
D.sub.0.
WATERMARK CHECK
[0029] Referring to FIG. 4, the watermark extractor 207 inputs the
received data samples F(1)-F(n) of the received frequency spectrum
data D.sub.2' and the original data samples f(1)-f(n) of the
original frequency spectrum data D.sub.1. The watermark extractor
207 calculates w.sub.1(i)=(F(i)-f(i))/f(i) for each variable i
(1.ltoreq.i.ltoreq.n) using subtracter 401 and a divider 402 to
extract the watermark D.sub.WM1=(w.sub.1(1), w.sub.1(2), . . . ,
w.sub.1(n)).
[0030] Subsequently, the inner product calculating section 208
calculates a degree of statistical similarity C between the
extracted watermark D.sub.WM1=(w.sub.1(1), w.sub.1(2), . . . ,
w.sub.1(n)) and the received watermark D.sub.WM2=(w.sub.2(1),
w.sub.2(2), . . . , w.sub.2(n)) using the following equation:
C=D.sub.WM1*D.sub.WM2/.vertline.D.sub.WM1.vertlin-
e.*.vertline.D.sub.WM2.vertline.. If C is equal to or greater than
the predetermined level, it is determined that the received
watermark is embedded into the received MPEG data D.sub.4' and
therefore the received MPEG data D.sub.4' is the authorized data.
If C is smaller than the predetermined level, it is determined that
the received MPEG data D.sub.4' is an unauthorized duplication.
FIRST EMBODIMENT
[0031] Referring to FIG. 5, the encoding side of the electronic
watermark system is comprised of a processor 501, the watermark
data table 104, a data storage controller 502 and a data transfer
controller 503. As described before, the processor 501 runs
programs including the functions of the sections 101-106 as shown
in FIG. 1 to perform the watermarking and MPEG data
compression.
[0032] The processor 501 inputs a stream of original image data
D.sub.0 and produces the water-marked MPEG data stream D.sub.4
which is stored onto a storage medium 504 such as CD-ROM or
magneto-optic disc by the data storage controller 502. At the same
time, the data storage controller 502 outputs a serial number of
the storage medium 504 to the data transfer controller 503. In this
manner, an authorized medium 504 storing MPEG data stream D.sub.4
into which the selected watermark data D.sub.WM is embedded is
manufactured. If the processor 501 selects another watermark data,
an authorized medium 504 storing the same MPEG data stream D.sub.4
into which a different watermark data is embedded is easily
manufactured. Needless to say, the data storage controller 502
outputs the serial number of the storage medium 504 to the data
transfer controller 503.
[0033] The data transfer controller 503 inputs the embedded
watermark data D.sub.WM from the watermark data table 104 and then
transfers a pair of the embedded watermark data D.sub.WM and the
serial number of the storage medium 504 to a manufacture management
database 505 provided in a management center. Therefore, the
watermark for each authorized medium 504 can be easily identified
by searching the manufacture management database 505. In the case
where the management center is located at a distance from the
encoding side, the data transfer controller 503 may transfer them
through a local-area network or a radio communication channel.
[0034] When a suspect medium 506 is found, the authorization check
is performed by an authorization check section 507 running the
programs including the functions of the above sections 201-209 as
shown in FIG. 2 to perform the watermark extracting, MPEG data
decompression and authorization check. In this case, the
authorization check section 507 includes data communication
means.
[0035] First of all, the authorization check section 507 reads the
serial number from the suspect medium 506 and transmits a retrieval
request using the read serial number to the manufacture management
database 505 through a network. Upon receipt of the retrieval
request, the manufacture management database 505 is searched for
the corresponding watermark to the serial number. If the
corresponding watermark is found, the retrieved watermark data is
sent back to the authorization check section 507.
[0036] Using the retrieved watermark data, the authorization check
section 507 performs the watermark extracting and watermark
checking operations as described before. That is, if the degree of
statistical similarity C between the extracted watermark and the
retrieved watermark is equal to or greater than the predetermined
level, it is determined that the medium 506 is one of authorized
media. If C is smaller than the predetermined level, it is
determined that the medium 506 is an unauthorized duplication.
SECOND EMBODIMENT
[0037] Referring to FIG. 6, the encoding side of the electronic
watermark system is comprised of a processor 601, the watermark
data table 104, a data communication controller 602 and a data
transfer controller 603. As described before, the processor 601
runs programs including the functions of the sections 101-106 as
shown in FIG. 1 to perform the watermarking and MPEG data
compression.
[0038] The processor 601 inputs a stream of original image data
D.sub.0 and produces the water-marked MPEG data stream D.sub.4
which is transmitted to a client 604 by the data communication
controller 602. Here, it is assumed that the water-marked MPEG data
is distributed depending on a data transmission request received
from the client 604. At the same time, the data communication
controller 602 outputs the address number of the client 604 to the
data transfer controller 603. In this manner, the MPEG data stream
D.sub.4 into which the selected watermark data D.sub.WM is embedded
is transmitted to the authorized client 604. If the data
transmission request is received from another client, the processor
601 selects another watermark data which is embedded into the MPEG
data. The MPEG data stream D.sub.4 into which a different watermark
data is embedded is transmitted to the new client. Needless to say,
the data storage controller 602 outputs the address number of that
new client to the data transfer controller 603.
[0039] The data transfer controller 603 inputs the embedded
watermark data D.sub.WM from the watermark data table 104 and then
transfers a pair of the embedded watermark data D.sub.WM and the
client address number to a manufacture management database 605
provided in a management center. Therefore, the watermark for each
authorized client 604 can be easily identified by searching the
manufacture management database 605. In the case where the
management center is located at a distance from the encoding side,
the data transfer controller 603 may transfer them through a
local-area network or a radio communication channel. Further the
management database 605 is provided with data communication
controller 606.
[0040] When a suspect data provider 607 is found, the authorization
check is performed by an authorization check section 608 running
the programs including the functions of the above sections 201-209
as shown in FIG. 2 to perform the watermark extracting, MPEG data
decompression and authorization check. In this case, the
authorization check section 508 includes data communication
means.
[0041] First of all, the authorization check section 608 receives
the address number from the suspect data provider 607 and transmits
a retrieval request using the address number to the manufacture
management database 605 through the data communication controller
606. Upon receipt of the retrieval request, the manufacture
management database 605 is searched for the corresponding watermark
to the address number. If the corresponding watermark is found, the
retrieved watermark data is sent back to the authorization check
section 608.
[0042] Using the retrieved watermark data, the authorization check
section 608 performs the watermark extracting and watermark
checking operations as described before. That is, if the degree of
statistical similarity C between the extracted watermark and the
retrieved watermark is equal to or greater than the predetermined
level, it is determined that the data provider 607 is one of
authorized clients. If C is smaller than the predetermined level,
it is determined that the data provider 607 distributes an
unauthorized duplication.
THIRD EMBODIMENT
[0043] Referring to FIG. 7A, the encoding side of the electronic
watermark system is comprised of a processor 701, the watermark
data table 104, an encryption section 702 and a data storage
controller 703. As described before, the processor 701 runs
programs including the functions of the sections 101-106 as shown
in FIG. 1 to perform the watermarking and MPEG data compression.
Further, the processor 701 may include the encryption section 702
and the data storage controller 703.
[0044] The processor 701 inputs a stream of original image data
D.sub.0 and produces the water-marked MPEG data stream D.sub.4
which is stored onto a storage medium 704 such as CD-ROM or
magneto-optic disc by the data storage controller 703. At the same
time, the encryption section 702 encrypts the selected watermark
data D.sub.WM and the encrypted watermark data is stored onto the
same storage medium 704 by the data storage controller 703. It is
preferably that the storage medium 704 has a first area 705 for
storing the water-marked MPEG data and a second area 706 dedicated
to the encrypted watermark data. In the case of MPEG data reading
mode, no data can read from the second area 706.
[0045] In this manner, an authorized medium 704 storing the
encrypted watermark data D.sub.WM and the watermarked MPEG data
stream D.sub.4 is manufactured. If the processor 701 selects
another watermark data, an authorized medium 704 storing the new
watermark data and the same MPEG data stream D, into which the new
watermark data is embedded is easily manufactured.
[0046] Referring to FIG. 7B, when a suspect medium 707 is found,
the authorization check is performed by an authorization check
section 708 running the programs including the functions of the
above sections 201-209 as shown in FIG. 2 to perform the watermark
extracting, MPEG data decompression and authorization check. In
this case, the authorization check section 708 includes decryption
section 709.
[0047] First of all, the decryption section 709 reads the watermark
data from the second area dedicated to watermark of the suspect
medium 707 and decrypts it to output watermark data D.sub.MW' to
the authorization check section 708. Using the read watermark data
D.sub.WM', the authorization check section 708 performs the
watermark extracting and watermark checking operations as described
before. That is, if the degree of statistical similarity C between
the extracted watermark and the read watermark D.sub.WM' is equal
to or greater than the predetermined level, it is determined that
the medium 707 is one of authorized media. If C is smaller than the
predetermined level, it is determined that the medium 707 is an
unauthorized duplication.
[0048] As described above, since the authorized watermark can be
easily obtained from a management center or the storage medium
storing the watermarked MPEG data, the authorization check is
rapidly performed with reliability.
* * * * *