JP2006186871A - Broadcast type contents distribution system, and user key management method applied to the system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce costs required for user management and to effectively deal with a problem of illegal distribution of a decryption key in a broadcast type contents distribution system. <P>SOLUTION: Since an initial value (x+e<SB>0</SB>) of a user key results from adding a random number to a latest chaos sequence x at that time point, each time a prescribed time lapses (each time logistic mapping is repeated), an error between a latest chaos sequence value and a latest user key value is expanded. Therefore, since an encryption key generated based on high-order S bits of the latest chaos sequence and a decryption key generated based on high-order S bits of a latest user key match with each other before the lapse of the prescribed time, encrypted contents can be decrypted but after the lapse of the prescribed period of time (after the number (t) of times of mapping repetition increases), the encryption key and the decryption key do not match with each other, so that the encrypted contents can not be decrypted any more. It is substantially the same as providing validity for the user key. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a broadcast type content distribution system and a user key management method applied to the system, and more particularly to a technique for limiting users who can view content distributed from a provider side.

  2. Description of the Related Art Conventionally, in a content distribution system such as digital television broadcasting, only a user registered in a content provider computer (hereinafter referred to as a provider computer) is an encryption program distributed from the provider computer. In general, it is possible to restore and view the converted content. In order to realize such a mechanism, various broadcast encryption systems have been proposed. In general, in a broadcast encryption system, it is necessary to quickly encrypt and decrypt contents in order to realize a program that can be reproduced in real time. Therefore, the broadcast encryption system employs a common key (secret key) encryption method that has a higher processing speed for encryption / decryption than the public key encryption method.

  However, in general, the common key cryptosystem has a problem that it takes time to manage the common key. In other words, in the common key cryptosystem, both the sender and receiver of information must possess the same key, and the key must be kept secret from a third party, so a set of senders Each recipient will have a separate common key. Therefore, when the user performs encrypted communication with many other parties, the number of common keys owned by the user also increases, so it takes time to manage the common keys.

  As described above, since it is necessary to adopt a common key cryptosystem in the broadcast cryptosystem, the above problems related to the common key cryptosystem are also problems of the broadcast cryptosystem. That is, in the broadcast encryption system, since the common key is different for each user who is a viewer of the content, it is necessary for the provider side computer to create a ciphertext of different content with a different common key (encryption key) for each user. there were. In addition, when the user registration expires, it is necessary to perform processing such as communication with the information terminal of the corresponding user (hereinafter referred to as the user side information terminal) so that the common key of the corresponding user cannot be used. Therefore, the processing required for managing the common key of each user increases. For this reason, on the provider side, the amount of calculation processing and computer resources required for user management increases, and thus there is a problem that the cost required for user management increases.

  In addition, the broadcast encryption system has a problem of illegal distribution of a common key. That is, in the above system, there is a possibility that a registered user distributes a secret common key to non-registered users. In order to prevent this unauthorized distribution, Patent Document 1 below discloses a traitor tracing scheme. This mechanism enables a content provider to identify a registered user who has performed unauthorized distribution based on an illegally distributed key. In the unauthorized person tracking mechanism described above, data transmitted from the provider computer to the user information terminal is composed of two ciphertexts. One ciphertext is a ciphertext of a key (SEK (session-encrypting key)) for encrypting the content, and the other ciphertext is a ciphertext of the content encrypted using this SEK. is there. Only registered users having their own decryption key (KEK (key-encrypting key)) can decrypt the SEK ciphertext. Since the KEK of each user can be identified, the content provider can specify the owner of the KEK.

However, in order to track an unauthorized person who has illegally distributed a decryption key based on a user-specific decryption key in the mechanism for tracking an unauthorized person as described above, the content provider must use the decryption key itself on the network. Or, a transaction between a registered user and a non-registered user must be detected. However, in order to avoid detection by the provider, such an illegal transaction is often performed by an obscene method, and the number of times is not high. Therefore, the above-described illegal person tracking mechanism cannot effectively deal with the problem of illegal distribution of the decryption key because the probability that the illegal person can be detected is low.
Cho (B.Chor), 3 others, “Tracing traitors”, Transactions on Information Theory (USA), Institute of Electrical and Electronics Engineers, 2000 Year, Volume 46, Number 3, p893-910

  The present invention has been made to solve the above-described problem, and the content provider can reduce the cost required for user management as compared with a broadcast-type content distribution system that employs a conventional broadcast encryption system. Another object of the present invention is to provide a broadcast type content distribution system that can cope with the problem of illegal distribution of decryption keys.

  In order to achieve the above object, the invention of claim 1 is a computer on a provider side installed on a provider side of contents such as digital television broadcasting, and is connected to the computer on the provider side via a network. Only a user registered in the provider side computer restores and views the content distributed from the provider side computer. The user side information terminal is installed on the user side who is the content viewer. In the broadcast-type content distribution system, the provider-side computer is generated by the initial value generating unit that generates an initial value of a chaotic sequence that is a generation source of the content encryption key, and the initial value generating unit. For the initial value, a mapping of the chaotic map with the error propagation speed almost constant α times ( However, every time a predetermined time elapses after α is an integer equal to or greater than 1) and the latest chaotic sequence is obtained, the same chaotic mapping as the above chaotic mapping is applied to the latest chaotic sequence at that time. The chaos sequence update means for obtaining the latest chaos sequence, and the latest chaos sequence obtained by the chaos sequence update means are expressed in binary numbers, and the upper n bits constituting this binary number (where n is An encryption key generating unit that generates a cryptographic key for encrypting the content by extracting a predetermined number of bits or more, and using the cryptographic key generated by the cryptographic key generating unit Encryption means for encrypting content, content transmission means for transmitting the content encrypted by the encryption means to the user side information terminal, and user registration from the user side information terminal. Generated by the user key generating means by adding a random number to the latest chaotic sequence at that time obtained by the chaotic sequence updating means at the time of receiving the requested data, and generated by the user key generating means User key transmitting means for transmitting the user key to the user-side information terminal, wherein the user-side information terminal transmits registration request data to the provider-side computer, and the provider A user key receiving means for receiving a user key transmitted from the side computer, and the user key received by the user key receiving means as the latest user key in the initial state, every time a predetermined time elapses, User key update means for obtaining the latest user key by repeating the same chaotic map as the chaotic map α times for the user key, and the user The latest user key obtained by the updating unit is expressed in binary number, and the bit extraction process having the same content as the bit extraction process in the encryption key generating unit is performed on the binary number to decrypt the encrypted content. Using a decryption key generating means for generating a decryption key, a content receiving means for receiving encrypted content transmitted from the provider computer, and a decryption key generated by the decryption key generating means And a decrypting means for decrypting the encrypted content received by the content receiving means, wherein the encrypting means performs the encryption processing by a common key encryption method and generates the decryption generated by the decryption key generating means While the key matches the encryption key generated by the encryption key generation means, the decryption means enables decryption, and the decryption key generation means When the generated decryption key no longer matches the encryption key generated by the encryption key generation means, the user key is provided with an expiration date by disabling the decryption by the decryption means. is there.

  According to a second aspect of the present invention, in the broadcast-type content distribution system according to the first aspect, the chaotic map used by the chaotic sequence updating means is a logistic map.

  The invention of claim 3 is the broadcast type content distribution system according to claim 1 or claim 2, wherein the encryption key generating means represents the latest chaotic sequence obtained by the chaotic sequence updating means in binary, The upper n bits constituting this binary number are extracted to generate an encryption key for encrypting the broadcast type content.

  According to a fourth aspect of the present invention, in the broadcast-type content distribution system according to the second aspect, the initial value generated by the initial value generating means is represented by a binary number having a bit length of 256 or more.

  According to a fifth aspect of the present invention, in the broadcast type content distribution system according to the fourth aspect, α, which is the number of repetitions of the chaotic map by the chaotic sequence update means, is 16 or more.

  According to a sixth aspect of the present invention, in the broadcast-type content distribution system according to any one of the first to fifth aspects, the user key transmitting means of the provider side computer includes data representing an expiration date of the user key together with the user key. To the user side information terminal, the user key receiving means of the user side information terminal receives data representing the expiration date of the user key, and the user side information terminal determines the expiration date of the user key. An expiration determining means for determining whether or not the expiration date of the user key has expired based on the data to be represented, and when the expiration determination means determines that the expiration date of the user key has expired, before the expiration date The chaos mapping used by the user key update unit for the user key calculated last of the user keys calculated by the user key update unit Repeat the same chaotic mapping β times (where β is an integer equal to or greater than 1) to obtain a new chaotic sequence, and then extract the upper n bits of the chaotic sequence (where n is an integer equal to or greater than 1). The re-registration proof data generation means for generating proof data for re-registration user and the proof data transmission means for transmitting the proof data generated by the re-registration proof data generation means, and the provider The side computer further includes determination means for determining whether the user who transmitted the proof data is a user who has registered in the past based on the proof data transmitted from the user side information terminal. It is a thing.

  According to a seventh aspect of the present invention, in the broadcast type content distribution system according to any one of the first to sixth aspects, the user side information terminal is a digital television broadcast signal receiver, and the content transmission means is the encryption unit. The content encrypted by the means is transmitted to the digital television broadcast signal receiver in the form of a digital television broadcast signal by radio waves.

  The invention of claim 8 is a provider computer installed on a provider side of content such as digital television broadcasting, and a user who is connected to the provider computer via a network and who is a viewer of the content. In a user key management method applied to a broadcast-type content distribution system configured with a user-side information terminal installed on the side, the provider-side computer is a chaos sequence from which a content encryption key is generated A step of generating an initial value of the chaos, wherein the provider computer uses the generated initial value to generate a map in which a speed of error propagation in the chaotic map is substantially constant each time a predetermined time elapses. Performing α times (where α is an integer equal to or greater than 1) to obtain the latest chaotic sequence, In order to encrypt the content by expressing the latest chaotic sequence in a binary number and extracting a predetermined number or more of the upper n bits (where n is an integer of 1 or more) constituting the binary number Generating the encryption key of the provider, the provider computer encrypting the content using the encryption key, and the provider computer transmitting the encrypted content to the user side information terminal A step in which the user-side information terminal transmits user registration request data to the provider-side computer; and when the provider-side computer receives user registration request data from the user-side information terminal, Adding a random number to the latest chaotic sequence of generating a user key, wherein the provider computer uses the user key as the user key A step of transmitting to the information terminal, a step of receiving the user key transmitted from the provider-side computer by the user-side information terminal, and a step in which the user-side information terminal uses the received user key as an initial value. The user side information terminal is configured to obtain the latest user key by performing α times the same chaos map used for calculation of the latest chaos sequence in the provider side computer each time the time elapses. The latest user key is represented by a binary number, and this binary number is subjected to a bit extraction process having the same contents as the bit extraction process performed in the step of generating an encryption key in the provider computer. Generating a decryption key for decrypting the encrypted content, wherein the user side information terminal is transmitted from the provider side computer Receiving the encrypted content, and decrypting the encrypted content received from the provider computer by the user side information terminal using the decryption key. The encryption process in the step is performed by a common key cryptosystem, and while the decryption key generated in the step of generating the decryption key matches the encryption key generated in the step of generating the encryption key, Enables decryption of encrypted content received from a provider computer, and the decryption key generated in the step of generating the decryption key does not match the encryption key generated in the step of generating the encryption key The encrypted content received from the provider side computer cannot be decrypted. Ri, it is provided with a term of validity to the user key.

  The invention of claim 9 is the user key management method applied to the broadcast type content distribution system according to claim 8, wherein the chaotic map used in the step of obtaining the latest chaotic sequence is a logistic map. is there.

  The invention of claim 10 is the user key management method applied to the broadcast type content distribution system according to claim 8 or claim 9, wherein the step of obtaining the latest chaotic sequence in the step of generating the encryption key. The obtained latest chaotic sequence is represented by a binary number, and the upper n bits constituting the binary number are extracted to generate an encryption key for encrypting the broadcast type content.

  The invention of claim 11 is the user key management method applied to the broadcast type content distribution system according to claim 9, wherein the initial value generated in the step of generating the initial value is 2 with a bit length of 256 or more. It is expressed in decimal.

  According to a twelfth aspect of the present invention, in the user key management method applied to the broadcast content distribution system according to the eleventh aspect, the number of repetitions α of the chaotic map in the step of obtaining the latest chaotic sequence is 16 or more. There is something.

  According to a thirteenth aspect of the present invention, in the user key management method applied to the broadcast-type content distribution system according to the eighth to twelfth aspects, the provider computer transmits a user key in the step of transmitting the user key. Together with the data indicating the expiration date of the user key to the user side information terminal, the user side information terminal receiving the data indicating the expiration date of the user key in the step of receiving the user key The user-side information terminal determines whether or not the user key has expired based on the data representing the expiration date of the user key; and the user-side information terminal determines in the determining step When it is determined that the user key has expired, it is calculated at the end of the user key calculated before the expiration date. The same chaos mapping as that used in the step of obtaining the latest user key is repeated β times (where β is an integer of 1 or more) to obtain a new chaos sequence. Extracting the upper n bits (where n is an integer equal to or greater than 1) of the chaotic sequence to generate proof data indicating that the user is a re-registered user; and And whether the provider computer is a user who has registered in the past based on the proof data transmitted from the user side information terminal, based on the proof data transmitted from the user side information terminal. And a step of discriminating.

  The invention of claim 14 is the user key management method applied to the broadcast type content distribution system according to claim 8 to claim 13, wherein the user side information terminal is a digital television broadcast signal receiver, In the step of transmitting content, the encrypted content is transmitted to the digital television broadcast signal receiver in the form of a digital television broadcast signal by radio waves.

  According to the invention of claim 1 or claim 8, the chaos map used for calculation of the latest chaos sequence in the provider side computer and the chaos map used for calculation of the latest user key in the user side information terminal are: The mapping is the same, and the error propagation speed is almost constant. Therefore, since the initial value of the user key is a value obtained by adding a random number to the latest chaotic sequence at that time, the value of the latest chaotic sequence and the value of the latest user key each time a predetermined time elapses. The error increases. For this reason, until a predetermined period elapses, an encryption key generated based on the upper n bits of the latest chaotic sequence and a decryption key generated based on the upper n bits of the latest user key are Since they match, the encrypted content can be decrypted, but after a predetermined period of time, the upper n bits of the latest chaos sequence and the upper n bits of the latest user key do not match, The encryption key and the decryption key do not match, and the encrypted content cannot be decrypted. This is substantially the same as providing an expiration date for the user key.

  As described above, even when the user registration time limit (user key expiration date) expires, the provider computer performs processing such as communication with the user information terminal of the corresponding user, and the corresponding user. It is no longer necessary to make the user key unavailable. Further, the provider computer can encrypt the content to be transmitted to all the user side information terminals using the same encryption key generated based on the latest chaotic sequence at that time, and therefore differs for each user. Unlike the case where the content is encrypted with the encryption key, it is not necessary to transmit a ciphertext of a different content for each user side information terminal. In other words, the processing necessary for the provider computer to perform user management can be only the user key generation processing at the time of user registration. Accordingly, the cost required for user management can be reduced as compared with a broadcast type content distribution system employing a conventional broadcast encryption system.

  In addition, as described above, by providing an expiration date for the user key, the decryption key generated based on the user key cannot be used after a predetermined period of time has passed since the user key was issued. Even if it is distributed illegally, the impact can be minimized. In addition, by providing an expiration date for the user key as described above, the unauthorized distributor must frequently distribute the user key to unregistered users, so the provider detects unauthorized distribution of the user key. The possibility that it can be increased. That is, it is possible to effectively deal with the problem of illegal distribution of the decryption key.

  According to the invention of claim 2 or claim 9, the chaos map used for updating the chaos sequence and the user key is a logistic map in which the speed of error propagation is substantially constant, so that the above effect can be obtained accurately. be able to.

  According to the invention of claim 3 or claim 10, the latest chaotic sequence is represented by a binary number, and the upper n bits constituting the binary number are extracted to generate an encryption key for encrypting the content. I did it. Thereby, said effect can be acquired exactly.

  According to the invention of claim 4 or claim 11, the initial value of the chaotic sequence is represented by a binary number having a bit length of 256 or more, so that updating of the chaotic sequence by the chaotic sequence updating means and the user key are performed. Even when the user key is updated every day by the updating means, the expiration date of the user key can be set to a certain long period.

  According to the fifth or twelfth aspect of the present invention, by setting α, which is the number of repetitions of the above chaotic map, to 16 or more, a certain degree of resistance against brute force attacks can be obtained.

  According to the invention of claim 6 or claim 13, when the user side information terminal has expired, the user calculated at the end of the user keys calculated before the expiration date expires. For the key, the same chaotic mapping as that used to calculate the user key is repeated β times, a new chaotic sequence is obtained, and the upper n bits of this chaotic sequence are extracted, so that Proof data indicating that the user is a registered user is generated, and the generated proof data is transmitted to the provider computer. Then, based on the certification data transmitted from the user side information terminal, the provider side computer determines whether or not the user who transmitted the certification data is a user who has registered in the past. As a result, the provider's computer can confirm that the user who sent the certification data is a user who has registered in the past, so it is registered only for users who have registered in the past. It is possible to make a discount on the fee.

  According to the seventh or fourteenth aspect of the present invention, the encrypted content is transmitted to the digital television broadcast signal receiver in the form of a digital television broadcast signal by radio waves. The present invention can be applied to distribution systems such as John broadcasting and BS digital broadcasting.

  The best mode for carrying out the present invention will be described with reference to the drawings. The present invention relates to a broadcast-type content distribution system and a user key management method applied to the system, and more particularly to a technique for limiting users who can view content distributed from a provider side. . The embodiments described below do not cover the present invention, and the present invention is not limited to the following modes.

  FIG. 1 shows an outline of processing in a broadcast type content distribution system according to the present embodiment. A broadcast-type content distribution system 1 (hereinafter abbreviated as a content distribution system) includes a provider-side server 2 (provider-side computer) installed on the provider side of content 6 such as digital television broadcasting, and the provider side. It is connected to the server 2 via the network 5 and is configured from user-side information terminals 3 a and 3 b installed on the user side who is the viewer of the content 6. The user side information terminals 3a and 3b may be personal computers or digital television broadcast signal receivers. When the user-side information terminals 3a and 3b are digital television broadcast signal receivers, the provider-side server 2 converts the encrypted content into a digital television broadcast signal in the form of a digital television broadcast signal using radio waves. Send to receiver. Further, the network 5 may be a wired network such as the Internet or a wireless network.

The content distribution system 1 is of a type that allows only a user registered in the provider side server 2 to restore and view the ciphertext 7 of the content distributed from the provider side server 2. In the content distribution system 1, the process necessary for the provider side server 2 to perform user management can be only the issuance process of the user key U ₀ at the time of user registration. Therefore, the cost required for user management can be reduced as compared with the conventional content distribution system. The provider-side server 2 updates the chaotic sequence K _T that is the source of the encryption key every time a predetermined time (for example, 24 hours) elapses, and updates the upper n bits of the updated chaotic sequence K _T. And a new encryption key. In accordance with this, the user-side information terminal 3a, 3b also, at the same intervals as those of the updating of the chaotic sequence K _T, the predetermined time is a (e.g., 24 hours) every time elapses, the original decryption key user update the key U _T, the upper n bits of the user key U _T updated, and a new decryption key. Since the encryption key and the decryption key match within the expiration date of the user registration, the content 6 can be decrypted. However, when the expiration date of the user registration expires, the encryption key and the decryption key do not match, and the content Cannot be decrypted. Like the user-side information terminal 3b shown in the figure, a terminal whose user registration has expired can only be used after user registration with the provider-side server 2 again. Further, since the information terminal of the person who has not registered as the information terminal 4 does not have the decryption key, even if it can receive the ciphertext 7 of the content, it is natural that the ciphertext of the content 7 cannot be decrypted.

  Next, the hardware configuration of the content distribution system 1 will be described with reference to FIG. The provider-side server 2 includes a microprocessor 20 (discriminating means) that controls the entire apparatus, a hard disk 21 that stores various data and programs, and each program stored in the hard disk 21 during execution of each program. Data for the memory 22 into which the program is loaded, the operation unit 24 including a mouse and a keyboard for inputting various instructions, and the user side information terminal 3 (corresponding to the user side information terminals 3a and 3b in FIG. 1) A transmission / reception unit 23 (content transmission unit, user key transmission unit) and a display 25. The hard disk 21 generates a content DB 11 that is a database of various contents 6 for distribution, an encryption key generation PG 12 that is an encryption key generation program for encrypting the content 7, and a user key that is a source of the decryption key. User key generation PG13 which is a program for the above, encryption PG14 which is a program for encrypting the content 6 (packets thereof) with the encryption key generated by the encryption key generation PG12, various data including various work data, etc. 15 etc. are stored.

  The microprocessor 20 and the encryption key generation PG 12 correspond to the initial value generation means, the chaos sequence update means, and the encryption key generation means in the claims. Further, the microprocessor 20 and the user key generation PG 13 correspond to the user key generation means in the claims. Furthermore, the microprocessor 20 and the encryption PG 14 correspond to encryption means in the claims.

  Further, as shown in the figure, the user-side information terminal 3 has a configuration substantially similar to that of the provider-side server 2. The hard disk 31 of the user side information terminal 3 decrypts the decryption key generation PG 16, which is a program for generating a decryption key based on the latest user key, and the ciphertext 7 of the content transmitted from the provider side server 2. A decryption PG 17 that is a program for this purpose, various data 18 including other work data, and the like are stored. Since the other configuration of the user side information terminal 3 is basically the same as that of the provider side server 2, the description thereof is omitted here. The transmission / reception unit 33 of the user-side information terminal 3 corresponds to a registration request transmission unit, a user key reception unit, a content reception unit, and a certification data transmission unit in the claims. The microprocessor 30 and the decryption key generation PG 16 of the user side information terminal 3 correspond to a user key update unit and a decryption key generation unit in the claims. The microprocessor 30 and the decryption PG 17 of the user side information terminal 3 correspond to the decryption means in the claims. Further, the microprocessor 30 of the user side information terminal 3 corresponds to an expiration determination unit and a re-registration certification data generation unit in the claims.

  In the content distribution system 1 described above, an expiration date is provided for the user key (or the decryption key) using the characteristics of the chaotic map. The characteristics of the chaotic map used in the system 1 include initial value sensitivity (initial value dependency) and unpredictability. The initial value sensitivity is that a slight error in the initial value of the chaotic sequence increases exponentially by repeating the chaotic map. Unpredictability is a property that the behavior cannot be predicted unless the exact initial state of the chaotic sequence is known. Of these characteristics, it is mainly initial value sensitivity that is used to set an expiration date for a user key.

Specifically, when the initial value of the original chaotic sequence is x and the chaotic map is f (x), the chaotic map for the initial value x of the original chaotic sequence plus noise ε (x + ε) is f (x + ε). At this time, f (x), which is a result of performing chaotic mapping only once for the initial value x, and f (x + ε), which is a result of performing chaotic mapping only once for the initial value (x + ε), It becomes almost the same value. Then, the f ^t (x) is the value of the result of the chaotic map only t times the initial value x, the initial value (x + ε) for a value of the result of the chaotic map only t times f ^t (x + ε) is When the number of mappings is small, the values are almost the same, but when the number of mappings is large, the noise is increased and the values are not correlated at all.

FIG. 3 is a graph of f ^t (x) when an initial value x = 0.12345 and noise ε = 10 ⁻³⁰ using a logistic map (one of typical chaos maps) represented by the following equation (1). The graph 41 of the value of, and the graph 42 of the value of f ^t (x + ε) are shown.
x _{i + 1} = 4x _i (1−x _i ) = f (x _i ) (1)

  As shown in the figure, when the number of mapping iterations is small, the trajectories of these graphs 41 and 42 are extremely similar, but when they are increased, the trajectories are completely different.

The above logistic map is probably the most studied non-linear dynamic system in the study of chaos theory and has remarkable chaotic properties. The logistic map is generally a one-dimensional discrete-time system represented by the following nonlinear difference equation.
x _{i + 1} = μx _i (1−x _i ) (0 ≦ x _i ≦ 1, 0 ≦ μ ≦ 4)

In the content distribution system 1, the logistic map x _{i + 1} = 4x _i (1−x _i ) in the case of μ = 4 used for creating the graph of FIG. 3 is used. Since this chaotic map has a perfect chaotic characteristics, small differences in the initial value x ₀ is because bring large difference in future values x _n.

The logistic map generates a sequence of decimal numbers, but since the length of a decimal point number is infinite, it is converted into a binary sequence and calculated on a computer. When a conversion operation from decimal number to binary number is performed, the approximate expression becomes very important. If the chaotic sequence is represented with a finite accuracy of T bits, the above transformation can be realized by one of the following three functions.
floor _T (x) = [x · 2 ^T ] / 2 ^T (2)
round _T (x) = round (x · 2 ^T ) / 2 ^T (3)
ceil _T (x) = [x · 2 ^T ] / 2 ^T (4)

  When the chaotic sequence is generated by a computer, it is necessary to consider the selection of the above function. In the following description, in order to generate a chaotic sequence, equation (3) of the above three equations is used.

  If functions for accuracy and approximation are defined, the chaotic sequence can be generated independently on the computer. Such a digital chaotic sequence also has initial value sensitivity. Therefore, even if the difference between the initial values is extremely small, the error is accumulated by repeating the logistic mapping. And error propagation is not linear. This makes the chaotic trajectory unpredictable.

Here, in order to analyze the chaotic trajectory, a slight error is added to x _i in the equation (1), and the propagation of the error is examined. From equation (1), error propagation is expressed as follows.
x _{i + 1} + e _{i + 1} = 4 (x _i + e _i ) (1−x _i −e _i )
= 4x _i (1−x _i ) + 4e _i (1-2x _i −e _i )

Here, the magnitude of e _i ² is much smaller than e _i (1-2x _i ) and can be ignored. Therefore, the following formula is established.
x _{i + 1} + e _{i + 1} ? 4x _i (1-x _i ) +4 e _i (1-2x _i ) (5)

In the analysis of the logistic map, it is known as an accurate solution that x _i = sin ² 2 ⁱ θ. Because,
_{4x i (1-x i)} = 4sin 2 2 i θ (1-sin 2 2 i θ)
^{= 4sin 2 2 i θcos 2 2} i θ
= Sin ² 2 ^{i + 1} θ
= _{Xi + 1} (6)
That's why.

From the above equation (5) and x _i = sin ² 2 ⁱ θ, the error e _{i + 1} can be expressed as follows.
e _{i + 1} = 4 e _i (1-2sin ² 2 ⁱ θ)
= 4e _i cos2 ^{i + 1} θ (7)

From the above examination, the error e _i propagated from the slight error e ₀ which is the initial value can be estimated by the following equation.

When the bit length of the error was examined with a computer based on the above equation (8), the result was as shown in FIG. FIG. 4 shows the correspondence between the number of repetitions of logistic mapping and the bit length (influence) of the error. In the figure, the initial value e ₀ of the error is ^2-300 . From the simulation result by the computer, the transition of the bit length having the influence of the error on the number of repetitions of the mapping is linear, and the slope of the graph in the figure is approximately 1. Therefore, the following assumptions can be made:
(Assumption 1)... The effect of error propagates to the higher one bit each time the logistic mapping is repeated.

In the content distribution system 1, the expiration date of the user key is adjusted using the above-described error propagation property. Specifically, in the content distribution system 1 adopting the common key cryptosystem, the provider-side server 2 uses only the upper S bits of the chaotic sequence with x as an initial value as the encryption key, and the user-side information It is assumed that the terminal 3 uses only the upper S bits of the chaotic sequence (user key) having (x + e ₀ ) as an initial value as a decryption key. In this case, as shown in FIG. 5 (a), in the initial state, only the lower k bits indicated by the diagonal lines in the user key sequence having (x + e ₀ ) as the initial value are the error initial value e _0. receiving an impact, but the higher the S bit of the portion 59 to be used as a decryption key is not affected by the initial value e ₀ of the error. Next, as shown in FIG. 5B, when logistic mapping is performed only once with (x + e ₀ ) as an initial value, an error occurs every time logistic mapping is performed once according to (Assumption 1). Is propagated to the upper 1 bit, the portion affected by the error is expanded to the lower (k + 1) bit portion indicated by hatching. If logistic mapping is repeatedly performed on a chaotic sequence having x as an initial value and a user key having (x + e ₀ ) as an initial value, the influence of the error propagates to the upper 1 bit each time the logistic mapping is repeated. Therefore, as shown in FIG. 5 (c), when the number of logistic mapping iterations exceeds a predetermined number, the upper S bit portion 59 used as a decryption key is affected by an error. End up. By utilizing this, it is possible to share a key for a certain period (only within the expiration date) between the provider side and the user side.

A mechanism for sharing the key only within the validity period as described above will be described below. First, the processing when the provider side server 2 sets the initial value K ₀ (corresponding to the initial value x in FIG. 5) of the chaotic sequence will be described. First, the provider side server 2 selects the initial value sk cryptographic key bit lengths T _S. Then, a sequence of encryption keys extended by the logistic mapping is calculated using the initial value sk. Here, it should be noted that the range of x _i is (0 <x _i <1) in the logistic mapping formula shown in the above formula (1), so that the initial value sk of the encryption key is It cannot be used directly for logistic mapping. In order to make the initial value sk of the encryption key a decimal number within the range of (0 ≦ x _i ≦ 1), a bit shift operation is necessary. The number K ₀ resulting from the bit shift operation on this sk is the initial value of the logistic map.

Specifically, if the binary representation of sk is {sk ₀ sk ₁ sk ₂ ... Sk _Ts−1 }, an initial value K ₀ of the logistic map is obtained by the following equation.

Based on the initial value K ₀ , a chaotic sequence K _t (t = 1, 2, 3,...) Is generated as follows.
K _t = f ^t (K ₀ ) (10)

T bits are assigned to represent the initial value K ₀ of the logistic map. Here, it should be noted that the t-th chaotic sequence K _t can also be the initial value at that time. Because chaotic sequence generated using the K _t is also accompanied by the original chaotic sequence, because to maintain the same properties as the original chaotic sequence. For example, when a slight error in the K _t is applied, the error by repeating the logistic map, are accumulated.

Next, a user key generation method using the above properties will be described. Generally, in an actual broadcast type content distribution system, a user tries to register as a user when he / she is interested. Therefore, it is desirable that the provider side can issue the user key at any time. In the content distribution system 1, the provider server 2 can generate a user key with an expiration date when a user requests user registration based on the nature of the chaotic sequence. To make such a user key, the initial value of the user key with minor errors in the latest chaotic sequence K _t at that time. Due to the characteristics of the chaotic sequence, due to the slight error between the chaotic sequence K _t and the user key U _t at the initial value, the error between these sequences caused by repeated generation of the original chaotic sequence and the user key is Is cumulative. When a certain period of time elapses (when a certain number of logistic maps are repeated), the error becomes large, and the prediction of the next key becomes extremely difficult. Even when the initial value of the user key is generated based on the chaotic sequence K _t at any point in time, error propagation occurs based on the characteristics of the chaos. Therefore, the provider side server 2, at any time, it is possible to generate an initial value of the user key based on the latest chaotic sequence K _t at that time.

Specifically, when the user-side information terminal 3 transmits user registration request data to the provider-side server 2, the provider-side server 2 first selects a random number r having a bit length _Tu . Then, a binary expression {r ₀ r ₁ r ₂ ... R _Tu−1 } of the random number r is prepared. If the sequence number of the latest chaotic sequence at that time (the number of encryption key update processes) is t, the user key U _t is calculated as follows.

Here, T _p is a parameter of the period, and when the bit length of U _t is T,
T = T _s + T _p + T _u (12)
Is established.

An image of the above user key generation is shown in FIG. FIG. 7 shows the relationship among T, T _s , T _p , and _Tu shown in the above equation (12). In FIG. 7, “key” and “key ′” indicate an encryption key and a decryption key, respectively. In addition, in the drawing, since the logistic mapping is performed α times in one encryption key update process in the provider-side server 2 and one decryption key update process in the user-side information terminal 3, The original chaotic sequence and the user key that is the source of the decryption key are represented by K _αt and U _αt .

Finally, the sequence number t and the user key U _t shown in the equation (11) are transmitted to the user side information terminal 3.

The bit length T of the original chaotic sequence K _t and the user key U _t should be increased to some extent so that the expiration date can be secured to some extent. Further, when the bit length T is constant, the provider side may adjust the expiration date by the bit length T _u of the random number r. If longer the bit length T _u of the random number r, the expiration date is short, and if shorter the bit length T _u of the random number r, the expiration date is long. Accordingly, by carefully selecting the random number r, the expiration date can be set to a length that actually matches. However, the bit length T _u of the random number r, in order to deal with the analysis by brute force attack from an unauthorized person about the chaotic sequence K _t of the provider side, must be a certain degree of length.

Next, the common key generation process in the provider side server 2 and the user side information terminal 3 will be described. As described above, the chaotic sequence in the content distribution system 1 has a bit length of T bits, and its value is a positive decimal number smaller than 1. For the generation of the common key in the provider side server 2 and the user-side information terminal 3 which, higher T _S bits from chaotic sequence of T bit is extracted. The provider-side server 2 uses the initial value K ₀ of the secret chaos sequence to generate the latest chaos sequence and repeat the update of the encryption key every time a predetermined time elapses. In order to generate the current latest encryption key “key”, first, the latest chaotic sequence K _αt is obtained by repeating the logistic mapping for the immediately preceding chaotic sequence K _{α (t−1)} α times. That is,
K _αt = f ^α (K _{α (t−1)} ) (13)
Perform the operation.

Next, the upper T _s bits of this latest chaotic sequence K _αt are extracted by the following calculation.
key = round _Ts (K _αt ) (14)

Here, the function round _Ts (·) is a function that outputs a higher _{T s} bits of the input value. Since the above chaotic sequence K _αt is generated based on the initial value K ₀ , the above encryption key “key” is also expressed as follows.
key = round _Ts (f ^αt (K ₀ )) (15)

On the other hand, since the initial value U _t of the user key is slightly different from the value of the chaotic sequence K _t that is the original user key, the user key sequence is totally different from the original chaotic sequence. For example, when the initial value of the user key of the user A is U _tA and the chaos sequence _from which the user A is based is K _tA , the user side information terminal 3 obtains the decryption key of the user A as follows: .

Here, the validity of key _A depends on the expiration date. If αt-t _A is equal to or less than T _p (see FIG. 7), key = key _A , and if αt-t _A is greater than T _p , key = key _A is not satisfied. Eventually, the common key is shared between the provider side and the user during the validity period.

Next, analysis of safety in the content distribution system 1 will be described. The following description describes the difficulty of predicting the provider's chaotic sequence from the user key sent to the user. Chaos has been analyzed for decades, but from a mathematical point of view it is difficult to predict chaotic orbits. Therefore, the following assumptions can be made.
(Assumption 2) The chaotic sequence is randomly distributed, and the prediction of the chaotic trajectory is difficult unless the initial value is known.

Since the error is propagated to the higher one bit each time the logistic mapping is repeated, if the number of repeated mappings for each update increases, the expiration date of the user key decreases. However, considering the generation of the encryption key, if the number of repetitions per update is small, a malicious user can attack the current user key by a brute force attack even after the expiration date. The next user key can be predicted. For example, if the number of mapping iterations α per update is 1, the number of round-robin attacks is only two. This is because, from (Assumption 1), the difference between the user key and the chaotic sequence of the provider is only 1 bit. Therefore, the number of attempts needed to brute force attack is a 2 ^α. In order to defend against brute force attacks, the number of repetitions α of mapping per update needs to be a plurality of times. In consideration of safety, it is desirable that the number of repetitions α of mapping is a large number.

Here, in the content distribution system 1, the following theorem holds.
(Theorem 1) The analysis of the valid common key key from the user key U _t immediately after the expiration date is successful with a probability of 2- ^α .

The proof of the above theorem is as follows. That is, in the content distribution system 1, an effective common key having a bit length T _s is used for encrypted communication based on the secret key encryption system. Therefore, only the upper T _s bits of the generated chaotic sequence are used as the encryption key, and information regarding the remaining bits of the original chaotic sequence does not leak to the user even if the key sharing operation is performed. In the content distribution system 1, the chaotic sequence is generated by a logistic map. When the original chaotic sequence K _t leaks to the user, the mechanism of the content distribution system 1 collapses. However, only T _s bits of the original chaotic sequence K _t , that is, only the common key, only leaks to the user during key sharing. Therefore, the prediction of the original chaotic sequence K _t is difficult for the user due to lack of information.

Here, due to the characteristics of the chaotic sequence, if the sequence includes an error, it is difficult to accurately predict the trajectory. And error accumulation is inevitable. Since only the upper T _s bits are leaked, the attacker cannot analyze the original chaotic sequence K _t from the user key U _t . After a valid decryption key is last generated, the next decryption key generated based on the user key includes an α-bit error, as shown in FIG. If α is sufficiently large, the analysis of the decryption key becomes difficult. Then, in the brute force attack of this ^case, 2 α trials are required. Therefore, the above (Theorem 1) has been proved.

One important feature of the content distribution system 1 is the expiration date of the user key. From (Assumption 1) and (11), the user can generate a user key approximate to the original chaotic sequence T _p times. However, considering security, not all (T _p ) user keys can be used to generate a common key. The number of decryption keys extracted from the user key must be T _p / α.

In generating the user key U _t , the random number r is added to the latest chaotic sequence K _t at that time. The latest chaotic sequence K _t at that time can always be the initial value of the user key. Accordingly, user key generation can be performed at any time.

In general, an encryption system collapses when a secret key leaks, and it is very difficult to recover the system. However, the content distribution system 1 can be restored with a simple operation. When the secret key leaks, the provider server 2 refreshes the key by adding a random number to the latest chaotic sequence at that time. In this case, in order to guarantee the validity of the user key, the bit length of the random number should be _Tu . The only operations necessary to restore the system are the above operations. In order to enhance safety, it is desirable that the provider-side server 2 periodically adds a random number to the latest chaotic sequence at that time.

Next, the relationship between each parameter shown in FIG. 7 and the expiration date will be described with specific numerical examples. For example, the bit length T _s = 128 of the encryption key and the decryption key, the bit length T _u = 100 of the initial value of the error, the number of repetitions of mapping per update of the chaotic sequence K _αt and the user key U _αt α = 80 _{Assuming that} the entire bit length T of the chaotic sequence K _αt and the user key U _αt is 2048, the expiration date is obtained by the following equation.
(2048-100-128) /80=1820/80=22.75 (times)

Therefore, if the chaos sequence K _αt and the user key U _αt are updated once a day, the user side and the provider side can share the key for 22 days.

Considering resistance to brute force attacks, it is preferable to set the number of repetitions α of mapping per update of the chaotic sequence K _αt and the user key U _αt to 80 or more as described above. In addition, in order to set the user key expiration date to a certain long period while setting the number of repetitions α to be able to withstand a brute force attack, as described above, the entire chaos sequence K _αt and the user key U _αt It is desirable to set the bit length T to 2048.

It is also possible to change the expiration date of the user key for each user. For example, the mapping per one update of the bit length T _s = 128 of the encryption key and the decryption key, the chaotic sequence K _αt and the user key U _αt I want to be able to use the user key (decryption key) only 10 times (for example, only for 10 days) when the number of repetitions α = 80 and the overall bit length T = 2048 of the chaotic sequence K _αt and the user key U _αt when is set to the bit length of 12 times the updated by an amount to fill (T _u corresponding to the updated bit length T _u of 12 times (-10 times 22 times) of the initial value of the error shown in FIG. 7 ). That is, the bit length T _u of the initial value of the error,
{100+ (22-10) X80} = 1060
Set to. As a result, the user key of this user can be made unusable after the expiration date of 10 times (for example, 10 days) has passed.

  Next, referring to the timing charts shown in FIG. 9 and FIG. 10 and the flowcharts shown in FIG. 11 and FIG. Processing performed in the information terminal 3 will be described. First, at the time of transmission / reception of the ciphertext of the content 6, as shown in FIG. 9, the provider-side server 2 encrypts the content 6 using the encryption key generated in the encryption key generation process (S1) described later. (S2), the ciphertext 7 of the content 6 is transmitted to the user side information terminal 3 (S3). On the other hand, when the user side information terminal 3 has already been registered as a user and already has the decryption key generated in the decryption key generation process (S4) described later, the user side information terminal 3 transmits it from the provider side server 2. The ciphertext 7 of the content 6 is received (S5), and the ciphertext 7 of the content 6 is decrypted using the decryption key generated in the process of S4 (S6).

  As shown in FIG. 10, when the user side information terminal 3 transmits user registration request data to the provider side server 2 during user registration (S11), the provider side server 2 sends the user registration request data. (S12), a random number for each user is added to the latest chaotic sequence at that time to generate a user key (S13). And this user key is transmitted to the user side information terminal 3 (S14). When receiving the user key, the user-side information terminal 3 performs a decryption key generation process to be described later based on the user key (S15). When this decryption key generation process ends, the corresponding user-side information terminal 3 can decrypt the ciphertext 7 of the content 6. Accordingly, the corresponding user can view the content 6.

  Next, the encryption key generation process will be described with reference to the flowchart of FIG. At the time of generating the first encryption key, the provider-side server 2 generates an initial value of a chaotic sequence represented by a binary number (S21), and the upper n bits (for example, 128 bits) of the initial value of the chaotic sequence ) Are extracted to generate an encryption key (S22). On the other hand, the second and subsequent encryption key generation processes are performed as follows. Whenever a predetermined time (for example, 24 hours = 1 day) elapses from the generation of the previous encryption key (YES in S23), the provider-side server 2 performs logistic mapping represented by the above equation (1) α times. (E.g., 80 times) to calculate the latest chaos sequence (S24), extract only the upper n bits of the latest chaos sequence, and repeat the process of generating the encryption key. Thereby, the encryption key is updated every time a predetermined time elapses.

  Next, the decryption key generation process will be described with reference to the flowchart of FIG. When the user-side information terminal 3 receives the user key generated in S13 in FIG. 10 from the provider-side server 2 (S31), only the upper n bits (for example, 128 bits) of the received user key are received. The first decryption key is generated by extracting (S32). On the other hand, the second and subsequent encryption key generation processes are performed as follows. The user-side information terminal 3 uses the user key received from the provider-side server 2 as an initial value at the same interval as the update of the chaos sequence in the provider-side server 2 (for example, every 24 hours elapses) (S33 YES), the logistic mapping shown in the above equation (1) is performed α times to calculate the latest user key sequence (S34), and only the upper n bits of the latest user key sequence are extracted. The process of generating a decryption key is repeated. As a result, the decryption key is updated every time a predetermined time elapses.

  In order to make the decryption key on the user side the same key as the encryption key on the provider side in accordance with the update timing of the encryption key and the decryption key, the provider side informs the user side, for example, “every day at 12:00. Then, the encryption key is updated, so we want you to update the decryption key accordingly. " The provider side and the user side perform the encryption key update process and the decryption key update process every day at 12:00 in accordance with the notification content, so that the user side and the provider are within the expiration date of the user key. The key can be shared without performing preliminary communication with the side. That is, when viewed from the provider side, the provider-side server 2 transmits a user key to the user-side information terminal 3 at the time of user registration. It is possible to share the key with the user without performing preliminary communication with the user side information terminal 3 only by performing the update process, and after the end of the user key issuance process (transmission process) There is no need to manage user keys or decryption keys.

  Next, a mechanism for charging the content distribution fee to the user in the content distribution system 1 will be described. In the content distribution system 1, the provider charges the user at the time of user registration. That is, the user cannot issue the user key from the provider unless the provider has paid the content distribution fee to the provider with electronic money or a credit card. When the user issues a user key, the same key (decryption key) as the encryption key on the provider side can be generated for a certain period (expiration date) based on this user key. By decrypting the ciphertext 7 of the content 6 transmitted from the provider side with this decryption key, the content 6 transmitted from the provider side can be viewed. However, when the expiration date has passed, the user cannot view the content 6 unless he / she registers again and has the provider reissue the user key. In order to perform user registration again, the user needs to pay the content distribution fee to the provider again. Therefore, when viewed from the provider side, every time a predetermined time elapses, the user again performs user registration and pays the content distribution fee. Charges can be made. Even if a malicious user illegally distributes a user key, an accurate decryption key cannot be generated using the corresponding user key after a certain period (expiration date) has passed. There is no need to manage keys.

  In the content distribution system 1, it is also easy to make the content distribution fee necessary for the second and subsequent user registrations lower than the content distribution fee necessary for the first user registration. Specifically, when the user-side information terminal 3 expires before the expiration date of the user key when the expiration date of the user key expires, the user key calculated last ( 1) The logistic map shown in the equation is repeated β times (β> 1) to obtain a new chaotic sequence, and then the upper n bits (n is equal to the bit length of the decryption key) of the chaotic sequence are extracted. Thus, proof data for re-registering user is generated. Then, the user side information terminal 3 transmits the proof data to the provider side server 2 together with the initial value and expiration date of the user key received from the provider side server 2 at the time of user registration. Since the effect of the random number included in the initial value of the user key appears in the proof data, the provider side server 2 receives the initial value of the user key, the expiration date, the proof data received from the user side information terminal 3 Can be determined based on the initial value of the user key to determine whether or not the user-side information terminal 3 has actually generated the corresponding proof data. When the provider side server 2 records the initial value of the user key issued to each user on the hard disk 21, the user side information terminal 3 sends only the above proof data to the provider side server 2. By transmitting, the provider side server 2 can determine whether or not the user side information terminal 3 has been able to generate the corresponding certification data. This makes it possible to discount the content distribution fee (registration fee) only for users who have registered in the past.

  As described above, according to the content distribution system 1 and the user key management method thereof according to the present embodiment, the logistic map used for calculating the latest chaos sequence in the provider side server 2 and the latest in the user side information terminal 3 are used. The logistic map used for calculating the user key is the same map, and the error propagation speed is substantially constant. Therefore, since the initial value of the user key is a value obtained by adding a random number to the latest chaotic sequence at that time, the value of the latest chaotic sequence and the value of the latest user key each time a predetermined time elapses. The error increases. For this reason, the encryption key generated based on the upper n bits of the latest chaotic sequence matches the decryption key generated based on the upper n bits of the latest user key until a predetermined period elapses. Therefore, the encrypted content can be decrypted. However, when a predetermined period elapses, the upper n bits of the latest chaotic sequence and the upper n bits of the latest user key do not match. The key does not match and the encrypted content cannot be decrypted. This is substantially the same as providing an expiration date for the user key.

  By doing as described above, even when the user registration time limit (the expiration date of the user key) has expired, the provider-side server 2 performs processing such as communication with the user-side information terminal 3 of the corresponding user, It becomes unnecessary to make it impossible to use the user key of the corresponding user. Further, the provider-side server 2 can encrypt the content to be transmitted to all the user-side information terminals 3 using the same encryption key generated based on the latest chaotic sequence at that time, so that each user There is no need to send different content ciphertexts for each side information terminal. That is, the process necessary for the provider-side server 2 to perform user management can be only the user key generation process at the time of user registration. Accordingly, the cost required for user management can be reduced as compared with a broadcast type content distribution system employing a conventional broadcast encryption system.

  In addition, as described above, by providing an expiration date for the user key, the decryption key generated based on the user key cannot be used after a predetermined period of time has passed since the user key was issued. Even if it is distributed illegally, the impact can be minimized. In addition, by providing an expiration date for the user key as described above, the unauthorized distributor must frequently distribute the user key to unregistered users, so the provider detects unauthorized distribution of the user key. The possibility that it can be increased. That is, it is possible to effectively deal with the problem of illegal distribution of the decryption key.

  The present invention is not limited to the above embodiment, and various modifications can be made. For example, in the above embodiment, the chaotic map used for updating the chaotic sequence and the user key is a logistic map in which the error propagation speed is almost constant, but the chaotic map used for updating the chaotic sequence and the user key is The chaotic map is not limited to this, and any error propagation speed may be used. As described in the description of FIG. 1, the user side information terminal is a digital television broadcast signal receiver, and the provider side server digitally transmits the encrypted content in the form of a digital television broadcast signal using radio waves. You may make it transmit to a television broadcast signal receiver. Thus, the present invention can be applied to distribution systems such as terrestrial digital television broadcasting and BS digital broadcasting.

The figure which shows the outline of the process in the broadcast type content delivery system by one Embodiment of this invention. The hardware block diagram of the said content delivery system. The graph which shows the difference in the value of the chaotic series which the error of an initial value gives when the logistic map used for the said content delivery system is repeated. The graph which shows the correspondence of the repetition frequency of the said logistic map, and the bit length of an error. (A), (b), and (c) show the bit lengths of errors in the initial state, when the mapping is performed only once using the logistic mapping, and when the mapping is performed t times using the mapping, respectively. Figure. The figure which shows the image of the user key generation in the said content delivery system. Explanatory drawing of the expiration date of the user key in the said content delivery system. Explanatory drawing of safety in the content distribution system. The timing chart which shows the process at the time of transmission / reception of the ciphertext of the content in the said content delivery system. The timing chart which shows the process at the time of user registration in the said content delivery system. 10 is a flowchart of encryption key generation processing in FIG. 9. 10 is a flowchart of decryption key generation processing in FIG. 9.

Explanation of symbols

1 Content distribution system 2 Provider side server (provider side computer)
3 User side information terminal 5 Network 12 Encryption key generation PG (initial value generation means, chaos sequence update means, encryption key generation means)
13 User key generation PG (user key generation means)
14 Encrypted PG (encryption means)
17 Decryption PG (Decoding means)
20 Microprocessor (user key generation means, initial value generation means, chaos sequence update means, encryption key generation means, encryption means, determination means)
23 Transmission / reception unit (content transmission means, user key transmission means)
30 Microprocessor (user key update means, decryption key generation means, decryption means, expiration determination means, re-registration certification data generation means)
33 Transmission / reception unit (registration request transmission means, user key reception means, content reception means, proof data transmission means)

Claims (14)

A provider side computer installed on the provider side of content such as digital television broadcasting, and a user side connected to the provider side computer via a network and installed on the user side who is the viewer of the content In a broadcast-type content distribution system that is configured by an information terminal and that allows only a user registered in the provider-side computer to restore and view the content distributed from the provider-side computer,
The provider-side computer includes an initial value generating means for generating an initial value of a chaotic sequence that is a generation source of a content encryption key;
The initial value generated by the initial value generating means is subjected to α times (where α is an integer equal to or greater than 1) of the chaotic maps, the error propagation speed of which is substantially constant, to obtain the latest chaos. Chaos sequence updating means for obtaining the latest chaos sequence by repeating the same chaos map as the chaos map for the latest chaos sequence at that time every predetermined time after obtaining the sequence ,
The latest chaotic sequence obtained by the chaotic sequence updating means is expressed in binary, and a predetermined number or more bits are extracted from the upper n bits (n is an integer of 1 or more) constituting this binary number. An encryption key generating means for generating an encryption key for encrypting the content,
Encryption means for encrypting content using the encryption key generated by the encryption key generation means;
Content transmitting means for transmitting the content encrypted by the encryption means to the user side information terminal;
User key generation means for generating a user key by adding a random number to the latest chaos sequence at that time, which is obtained by the chaos sequence update means upon reception of user registration request data from the user side information terminal; ,
User key transmission means for transmitting the user key generated by the user key generation means to the user side information terminal,
The user side information terminal includes a registration request transmitting means for transmitting user registration request data to the provider side computer,
User key receiving means for receiving a user key transmitted from the provider computer;
Using the user key received by the user key receiving means as the latest user key in the initial state, every time a predetermined time elapses, the same chaos mapping as the chaos mapping is repeated α times for the latest user key at that time. User key update means for obtaining the latest user key,
The latest user key obtained by the user key update unit is expressed in binary number, and the binary number is subjected to bit extraction processing having the same contents as the bit extraction processing in the encryption key generation unit and encrypted. Decryption key generation means for generating a decryption key for decrypting the content;
Content receiving means for receiving encrypted content transmitted from the provider computer;
Decrypting means for decrypting the encrypted content received by the content receiving means using the decryption key generated by the decryption key generating means,
The encryption means performs the encryption processing by a common key encryption method,
While the decryption key generated by the decryption key generation means matches the encryption key generated by the encryption key generation means, the decryption means enables decryption, and the decryption key generated by the decryption key generation means However, when the encryption key generated by the encryption key generation unit does not match, the broadcast type is characterized in that the user key has an expiration date by disabling the decryption by the decryption unit Content distribution system.   2. The broadcast type content distribution system according to claim 1, wherein the chaotic map used by the chaotic sequence updating means is a logistic map.   The encryption key generation means represents the latest chaos sequence obtained by the chaos sequence update means in binary number, extracts the upper n bits constituting the binary number, and encrypts the content. The broadcast type content distribution system according to claim 1 or 2, wherein:   3. The broadcast type content distribution system according to claim 2, wherein the initial value generated by the initial value generating means is represented by a binary number having a bit length of 256 or more.   5. The broadcast type content distribution system according to claim 4, wherein α, which is the number of repetitions of chaotic mapping by the chaotic sequence updating means, is 16 or more. The user key transmitting means of the provider side computer transmits data representing the expiration date of the user key together with the user key to the user side information terminal,
A user key receiving means of the user side information terminal receives data representing an expiration date of the user key;
The user side information terminal is
Expiration determination means for determining whether or not the expiration date of the user key has expired based on the data representing the expiration date of the user key;
When it is determined that the expiration date of the user key has expired by the expiration determination unit, the user key calculated at the end of the user keys calculated by the user key update unit before the expiration date expires, The same chaotic mapping as that used by the user key updating means is repeated β times (where β is an integer of 1 or more) to obtain a new chaotic sequence, and then the upper n bits (however, , N is an integer of 1 or more), and re-registration proof data generation means for generating proof data indicating that the user is a re-registration
Proof data transmission means for transmitting the proof data generated by the re-registration proof data generation means to the provider side computer,
The provider-side computer determines, based on the proof data transmitted from the user-side information terminal, whether or not the user who transmitted the proof data is a user who has registered in the past. The broadcast type content distribution system according to claim 1, further comprising:   The user-side information terminal is a digital television broadcast signal receiver, and the content transmission unit is configured to transmit the content encrypted by the encryption unit in the form of a digital television broadcast signal using radio waves. 7. The broadcast type content distribution system according to claim 1, wherein the broadcast type content distribution system transmits the signal to a signal receiver. A provider side computer installed on the provider side of content such as digital television broadcasting, and a user side connected to the provider side computer via a network and installed on the user side who is the viewer of the content In a user key management method applied to a broadcast-type content distribution system composed of information terminals,
The provider computer generating an initial value of a chaotic sequence from which a content encryption key is generated;
Each time the provider computer uses the generated initial value, a map having a substantially constant error propagation speed among chaos maps is expressed α times (where α is An integer greater than or equal to 1) to obtain the latest chaotic sequence,
The provider-side computer expresses the latest chaotic sequence in binary number, and extracts a predetermined number or more bits from the upper n bits (where n is an integer of 1 or more) constituting the binary number. Generating an encryption key for encrypting the content;
The provider computer encrypts the content using the encryption key;
The provider-side computer transmitting the encrypted content to the user-side information terminal;
The user-side information terminal transmitting user registration request data to the provider-side computer;
When the provider computer receives user registration request data from the user information terminal, it adds a random number to the latest chaotic sequence at that time to generate a user key;
The provider-side computer transmitting the user key to the user-side information terminal;
The user side information terminal receiving a user key transmitted from the provider side computer;
The user side information terminal uses the received user key as an initial value, and every time a predetermined time elapses, the same chaotic map as that used for calculation of the latest chaotic sequence in the provider side computer Step α to obtain the latest user key,
The user-side information terminal represents the latest user key in binary number, and the bit extraction of the same content as the bit extraction processing performed in the step of generating the encryption key in the provider-side computer for the binary number Performing a process to generate a decryption key for decrypting the encrypted content;
The user-side information terminal receiving encrypted content transmitted from the provider-side computer; and the encryption received by the user-side information terminal from the provider-side computer using the decryption key Decrypting the encrypted content,
The encryption processing in the step of encrypting is performed by a common key cryptosystem,
While the decryption key generated in the step of generating the decryption key matches the encryption key generated in the step of generating the encryption key, decryption of the encrypted content received from the provider computer is performed. The encryption key received from the provider computer when the decryption key generated in the step of generating the decryption key does not match the encryption key generated in the step of generating the encryption key. User key management method applied to a broadcast-type content distribution system, characterized in that an expiration date is provided for the user key by disabling decryption of the content that has been made.   9. The user key management method applied to the broadcast type content distribution system according to claim 8, wherein the chaotic map used in the step of obtaining the latest chaotic sequence is a logistic map.   In the step of generating the encryption key, the latest chaotic sequence obtained in the step of obtaining the latest chaotic sequence is represented by a binary number, and the upper n bits constituting the binary number are extracted to encrypt the content. The user key management method applied to the broadcast type content distribution system according to claim 8 or 9, wherein an encryption key for generating the encryption key is generated.   The user key management applied to the broadcast type content distribution system according to claim 9, wherein the initial value generated in the step of generating the initial value is represented by a binary number having a bit length of 256 or more. Method.   12. The user key management method applied to the broadcast type content distribution system according to claim 11, wherein the number of repetitions α of the chaotic map in the step of obtaining the latest chaotic sequence is 16 or more. In the step of transmitting the user key, the provider-side computer transmits data representing an expiration date of the user key together with the user key to the user-side information terminal,
In the step of receiving the user key, the user side information terminal receives data representing an expiration date of the user key,
Determining whether the user key has expired based on the data representing the expiration date of the user key, the user side information terminal;
When the user side information terminal determines that the expiration date of the user key has expired in the determining step, the user key calculated at the end of the user keys calculated before the expiration date expires. , Repeating the same chaotic mapping as that used in the step of obtaining the latest user key β times (where β is an integer of 1 or more) to obtain a new chaotic sequence, generating proof data for re-registering user by extracting n bits (where n is an integer equal to or greater than 1);
The user side information terminal transmitting the certification data;
Determining whether the provider computer is based on the proof data transmitted from the user side information terminal, and whether the user who transmitted the proof data is a user who has registered in the past; The user key management method applied to the broadcast type content distribution system according to claim 8, further comprising:   The user side information terminal is a digital television broadcast signal receiver, and in the step of transmitting the content, the encrypted content is transmitted in the form of a digital television broadcast signal by radio waves in the digital television broadcast signal receiver. 14. The user key management method applied to the broadcast type content distribution system according to claim 8, wherein the user key management method is applied to the broadcast type content distribution system.

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