Enciphering method, deciphering method, recording and reproducing method, deciphering device, deciphering unit device, recording medium, recording-medium manufacturing method, and key control method

Abstract: On a recording medium, first information obtained by enciphering data with the first key and second information obtained by enciphering the first key with each of the predetermined second keys are recorded. A deciphering method is characterized by comprising the steps of inputting the first and second information (S34, S32), deciphering the first key using at least one of the second keys (S33), determining by a specific method that the obtained first key is correct (S33), and then deciphering the data using the first key to obtain the data (S35).

Description: The present invention relates to an enciphering method, deciphering method, recording and reproducing device, deciphering device, deciphering unit device, recording medium, recording-medium manufacturing method, and key control method which are for preventing the digitally recorded data from being copied from a recording medium. Compact disks and laser disks have been available as recording mediums that record digitized data (e.g., documents, sound, images, or programs). Floppy disks and hard disks have been used as recording mediums for computer programs and data. In addition to those recording mediums, a DVD (digital video disk), which is a large-capacity recording medium, has been developed. Since the aforementioned various digital recording mediums record the digital data (including the compressed or encoded data, which can be decoded later) as it is, the recorded data can be copied easily to another recording medium without impairing the quality of sound or the quality of image, which enables a large number of reproductions to be made, contributing to literary piracy. In summary, when the data is copied from a digital recording medium, the data can be copied with the sound quality and picture quality of the mater remaining unchanged, or without the deterioration of sound quality of picture quality. This has caused the problem of permitting the wrongful conduct of making unauthorized copies of the original and selling them without paying a royalty. US patent number 5,319,705 discloses a technique for securely transferring files from a software distribution processor to a user processor. According to this reference, a first encryption means encrypts an ith file (FILEi) with an ith file encryption key (Kdi) to obtain an ith encrypted file. A second encryption means encrypts the ith file encryption key (Kdi) with a second customer key (KC2) to obtain an encrypted ith file encryption key (eKC2(KDi)). US patent number 4,683,968 discloses a system for enabling a protected program to run on only selected computers by utilising a triple encrypted key E FK [E Ki [E FK [Ki]]]. Ki is the unencrypted key, E FK is an encryption procedure using a fixed key and E Ki is an encryption procedure using key Ki. The selected computer has an attached module with the fixed key for decrypting E FK and a program in the computer, which contains key Ki and performs a decryption procedure on E Ki The triple encrypted key is sent first to the attached module for the first decryption with the fixed key, then to the program in the computer for decryption with key Ki then back to the attached module for another decryption using the fixed key before unencrypted key Ki can be obtained. It is an object of the present invention to provide an enciphering method, deciphering method, recording and reproducing device, deciphering device, deciphering unit device, recording medium, recording-medium manufacturing method, and key control method which are for preventing an unauthorized copy of digital recording mediums. According to one aspect of the present invention, there is provided an enciphering method characterized by comprising the steps of keeping a plurality of second keys (M Ki ); enciphering (303) data with a first key (DATA); and enciphering (301) said first key (S K ) with a p number af second keys (MK i to MK p ); where p is an integer greater than or equal to two, of the kept plurality of second keys (M Ki ) to obtain a p number of enciphered first keys (E Mki (S K )), respectively and recording said p number of enciphered first keys on a recording medium with said enciphered data. According to another aspect of the present invention, there is provided a deciphering method characterized by comprising recording at least part of a p number of second keys, where p is an integer greater than or equal to two, in a secret area; inputting first information (E SK (DATA)) composed of enciphered data obtained by enciphering data with a first key (S K ) and second information (E Mki (S K )) composed of a p number of enciphered first keys, where p is an integer greater than or equal to two, obtained by enciphering said first key (S k ) with said p number of second keys (MK i to MK p ), respectively; deciphering at least one of said p number of enciphered first keys using at least part of a recorded p number of second keys to obtain said first key; confirming that the obtained first key is correct; and deciphering said enciphered data using said obtained first key after the confirmation to obtain said data. According to another aspect of the present invention, there is provided a recording medium having information items recorded thereon, said information items characterized by comprising: first information (E SK (DATA)) composed of enciphered data obtained by enciphering data with a first key (S K ); and second information (E Mki (S K )) composed of a p number of enciphered first keys, where p is an integer greater than or equal to two, obtained by enciphering said first key (S K ) with a p number of second keys (MK i to MK p ), respectively, wherein said second information composed of the p number of enciphered first keys is recorded in a key recording area of said recording medium. According to another aspect of the present invention, there is provided a recording medium manufacturing apparatus characterized by comprising means (221, 222 or 223) for enciphering first information (DATA) with a first key (S K ); means (221, 222 or 223) for generating second information (E Mki (S K )) composed of a p number of enciphered first keys, where p is an integer greater than or equal to two, by enciphering said first key (S K ) with a p number of second keys of a plurality of second keys (M Ki ), respectively; and means (221, 222 or 223) for recording said first and second information on a recording medium. According to another aspect of the present invention, there is provided a deciphering device characterized by comprising storage means for recording at least part of a p number of second keys, where p is an integer greater than or equal to two, in a secret area; input means for inputting first information (E SK (DATA) composed of enciphered data obtained by enciphering data with a first key (S K ) and second information (E Mki (S K )) composed of a p number of enciphered first keys, where p is an integer greater than or equal to two, obtained by enciphering said first key (S K ) with said p number of second keys (MK i to MK p ) respectively; and deciphering means for deciphering at least one of said p number of enciphered first keys of said second information (E Mki (S K )) inputted from said input means using at least one of a recorded p number of second keys to obtain said first key, confirming that the obtained first key is correct, and deciphering said enciphered data of said first information (E SK (DATA)) using said first key (S K ) after the confirmation to obtain said data. According to an example useful for understanding the present invention, there is provided a recording and reproducing device characterized by comprising storage means for recording at least part of a p number of second keys, where p is an integer greater than or equal to two, in a secret area; reading means for reading first information composed of enciphered data obtained by enciphering data with a first key and second information composed of a p number of enciphered first keys, where p is an integer greater than or equal to two, obtained by enciphering said first key with a p number of second keys from a recording medium on which said first information and said second information have been stored, respectively; and deciphering means for deciphering at least one of said p number of enciphered first keys of said second information read by said reading means using at least one of the p number of second keys in said storage means, confirming by a specific method that the obtained first key is correct, and deciphering said enciphered data of said first information using said first key after the confirmation to obtain said data. According to another example useful for understanding the present invention, there is provided key control method characterized by comprising the steps of causing a first caretaker to take custody of a plurality of second keys (MK i ); causing a second caretaker to take custody of first information (E SK (DATA)) composed of enciphered data obtained by enciphering data with a first key and second information (E Mki (S K )) composed of a p number of enciphered first keys, where p is an integer greater than or equal to two, obtained by enciphering said first key with a p number of second keys (MK i to MK p ) of said plurality of second keys, respectively; and causing a third caretaker to take custody of at least one of said plurality of second keys (M Ki ), said at least part of said plurality of second keys being recorded in a secret area of a device provided by said third caretaker. In each of the above categories, the data may include at least one of key information, documents, sound, images, and programs. With the present invention, only the correct party having at least one of the second keys can get the first key and therefore can get the plain data of the data enciphered using the first key. As a result, the wrongful conduct of making unauthorized copies and selling the thus copied mediums can be prevented, thereby protecting copyrights. Moreover, with the present invention, even if the data flowing over the signal line connecting the enciphering unit to the deciphering unit is stored, the stored data cannot be reproduced or used, because the data is the enciphered data. In addition, because the information necessary for enciphering the data is created on the basis of, for example, random numbers, and cannot be reproduced later, the stored data cannot be reproduced or used, even if the second key (master key) in the deciphering unit has been broken. As a result, the wrongful conduct of making unauthorized copies and selling the thus copied mediums can be prevented, thereby protecting copyrights. Still furthermore, with the present invention, because the enciphering unit and deciphering unit can be designed separately from the essential portion of the reproducing section of the digital recording and reproducing apparatus, even if the cipher is broken, the enciphering unit and deciphering unit have only to be replaced to overcome this problem. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a block diagram of a system according to an example useful for understanding the present invention; FIG. 2 is a flowchart for the operation of the example of FIG. 1; FIG. 3 illustrates an example of a format in which the enciphered key and the enciphered data are stored on a recording medium; FIG. 4 is a diagram to help explain a case where the data is stored from the CPU BUS; FIG. 5 is a block diagram of a system according to a first embodiment of the present invention; FIGS. 6A and 6B show examples of the internal structure of the key judging section; FIG. 7 is a flowchart for the operation of the first embodiment; FIG. 8 is a flowchart for the operation of the first embodiment; FIG. 9 is a block diagram of a system according to a second embodiment of the present invention; FIG. 10 is a flowchart for the operation of the second embodiment; FIG. 11 is a diagram to help explain the key control method; and FIG. 12 is a diagram to help explain the enciphering operation. Hereinafter, referring to the accompanying drawings, embodiments of the present invention and examples useful for understanding the present invention will be explained. In the examples and embodiments, the operation of enciphering a certain data item a using key K is expressed as E K (a) and the operation of deciphering a certain data item a using key K is expressed as D K (a). By this way of expression, the operation of enciphering and deciphering a certain data item a using key K is expressed as D K (E K (a)), for example. In the examples and embodiments, there is a case where a certain data item is first deciphered and then the deciphered data item is enciphered to restore the original data item. This is based on the fact that the deciphering of the data has the same function as the enciphering of the data. Specifically, to return the enciphered data to the original data, the key used for deciphering must be known. Once the key is known, enciphering the deciphered data produces the original data that was first deciphered. If the cipher key is x and the data item is y, the operation will be expressed as: E X (D X (y)) = y In the examples and embodiments, explanation will be given using an example of a system that reads the image data compressed and enciphered according to the MPEG 2 data compression standard from a DVD and enciphers, decodes, and reproduce the read-out data. Hereinafter, an example useful for understanding the present invention will be explained. FIG. 1 is a block diagram of a system according to an example useful for understanding the present invention. FIG. 2 is a flowchart for the operation of this example. The system related to the example is connected to the CPU BUS of the CPU (not shown) used for reproduction in a computer, such as a personal computer. The system is designed to allow the enciphered data (E SK (Data) explained later) to flow over the CPU BUS. FIG. 1 shows only the sections related to the CPU used for reproduction. As shown in FIG. 1, the system of the first embodiment comprises a DVD driving unit (not shown) that reads the data from a DVD 101, an enciphering unit 107 that is connected to the DVD driving unit without the CPU BUS or is built in the DVD driving unit, and a deciphering unit 114. The enciphering unit 107 and deciphering unit 114 are connected to the CPU BUS 110. The deciphering unit 114 outputs the data via, for example, an I/O port, not via the CPU BUS. That is, in the example, the input and output of the data is carried out without the CPU BUS, whereas the CPU BUS is used for the data transfer between the enciphering unit 107 and the deciphering unit 114. The enciphering unit 107 includes a demodulation/error correction circuit 117, a demodulation/error correction circuit 118, and an enciphering circuit 104. Although in FIG. 1, the enciphering unit 107 has two enciphering circuits 104, it is assumed that it actually has one enciphering circuit. The enciphering unit 107 is assumed to be composed of a single independent IC chip. The demodulation/error correction circuit 117 and demodulation/error correction circuit 118 may be provided in the unit (the DVD driving unit) in the preceding stage, not in the enciphering unit 107. The deciphering unit 114 includes a deciphering circuit 112 and a session key creation circuit 111 that creates a second session key S K '. In the example, the deciphering unit 114 is assumed to include an MPEG decoder circuit 115 and a converter circuit 116 that converts the digital enciphered image data into analog data. Although in FIG. 1, the deciphering unit 114 has four deciphering circuits 112, it is assumed that it actually has one deciphering circuit. The deciphering unit 114 is assumed to be composed of a single independent IC chip. In each of the enciphering unit 107 and deciphering unit 114, a master key, explained later, has been registered. It is assumed that the master key has been recorded in a secret area in each of the enciphering unit chip and the deciphering unit chip so that the user cannot externally take out the master key. A control section (not shown) is assumed to control the entire system. The control section is realized by, for example, executing a program on the CPU in the computer. Concrete examples of control by the control section include an instruction to read the data from a DVD, the specification of data transfer destination, and an instruction to output the data from the deciphering unit 114. The control section may be triggered, for example, by the user via a user interface, or by a process in an application program. In the above example, a first session key is represented by S K , a second session key S K ', the master key M K , and image data (i.e., the data to be enciphered) Data. In FIG. 1, numeral 102 indicates E MK (S K ) created by enciphering the first session key S K using the master key M K , 103 E SK (Data) created by enciphering the image data Data using the first session key S K ,] 105 the master key M K , 106 a second session key S K ', 108 D MK (S K ') created by deciphering the second session key S K ' using the master key M K , 109 E SK '(E MK (S K )) created by enciphering the first session key E MK (S K ) enciphered with the master key M K using the second session key S K ', and 113 the first session key S K . As shown in FIG. 3, it is assumed that on the DVD 101, E MK (S K ) created by enciphering the first session key S K using the master key M K is recorded in the key recording area (lead-in area) in the innermost circumference portion and the E SK (Data) created by enciphering the image data Data using the first session key S K is recorded in the data recording area (data area). Hereinafter, the operation of the first embodiment will be explained by reference to the flowchart of FIG. 2. At step S1, the first session key E MK (S K ) enciphered using the master key M K is read from the DVD 101, on which the DVD driving unit (not shown) has recorded the first session key, and then is loaded into the enciphering unit 107. At that time, the demodulation/error correction circuit 117 performs demodulation and data error correction. At step S2, in the deciphering unit 114, the session key creation circuit 111 creates a second session key S K ' using random numbers, such as time data from a clock (not shown). Then, the deciphering circuit 112 deciphers the created second session key S K ' using the master key M K to create D MK (S K ') and sends it to the enciphering unit 107 via the CPU BUS 110. As the timing of generating random numbers (e.g., the timing of inputting time information), for example, the timing with which the signal indicating that the DVD 101 has been loaded into the DVD driving unit is asserted may be used. The session creation circuit 111 may be composed of a random-number generator that is as long as the key. When a key is created using random numbers all of whose bits may take 0s or 1s, it is necessary to perform a check process to prevent all of the bits from taking 0s or 1s. At step S3, using the master key M K , the enciphering circuit 104 of the enciphering unit 107 enciphers D MK (S K ') received via the CPU BUS 110. Namely, from E MK (D MK (S K ')) = S K ' a second session key S K ' created at the session key creation circuit 111 of the deciphering unit 114 can be obtained. The second session key S K ' created at the session key creation circuit 111 is designed to prevent its contents from being known even if it is stolen on the CPU BUS 110. Then, at step S4, using the second session key S K ', the enciphering unit 107 enciphers the enciphered first session key E MK (S K ) recorded on the DVD 101 to create E SK '(E MK (S K )), and sends this to deciphering unit 114. Then, at step S5, the deciphering circuit 112 of the deciphering unit 114 deciphers E SK '(E MK (S K )) received via the CPU BUS 110 using the second session key S K ' and produces: D SK ' (E SK '(E MK (S K ))) = E MK (S K ) Furthermore, E MK (S K ) obtained at the deciphering circuit 112 is deciphered using the master key M K to produce: D MK (E MK (S K )) = S K Thus, this gives the first session key S K . After the first session key S K has been obtained as described above, at step S6, the image data E SK (Data) enciphered using the first session key S K recorded on the DVD 101 by the DVD driving unit (not shown) is read out and loaded into the enciphering unit 107. At that time, the demodulation/error correction circuit 118 performs demodulation and corrects errors in the data. Then, E SK (Data) is sent to the enciphering unit 107 via the CPU BUS 110. At step S7, the deciphering circuit 112 of the deciphering unit 114 deciphers E SK (Data) received via the CPU BUS 110 using the first session key S K and produces: D SK (E SK (Data) = Data Then, the enciphered image data is deciphered to produce Data. Then, step S6 and step S7 are repeated until for example, the process of the data to be deciphered (i.e., E SK (Data)) has been completed or the stop of the process has been requested. When the image data Data thus obtained has been compressed according to, for example, the MPEG2 data compression standard, the image data is decoded at an MPEG decoder circuit 115. After the decoded signal has been converted by a D/A converter circuit 116 into an analog signal, the analog signal is sent to an imaging device (not shown), such as a television, which reproduces the image. Step 1 may be executed before or after step S2 and step S3. Step S6 and step S7 may be executed by the method of carrying out the steps in units of E SK (Data), the method of reading a specific number of E SK (Data) at step S6, storing the read-out data in a buffer temporarily, and then deciphering E SK (Data) in the buffer at step S7, or the method of carrying out step S6 and step S7 in a pipeline processing manner. Moreover, the deciphering circuit 112 may transfer the image data E SK (Data) to the MPEG decoder circuit 115 in units of one Data item or a specific number of Data items. As described in the above example, when the data is reproduced from a medium on which the digitized data has been enciphered and recorded (when the enciphered data is deciphered), the deciphered data is prevented from flowing over the CPU BUS of the computer and the second session key S K ' used to encipher the first session key necessary for deciphering the enciphered data flowing over the CPU BUS is created on the basis of information that changes each time the data is reproduced, such as time information. Therefore, even when the data flowing over the CPU BUS 110 is stored from signal lines 210 into a digital storage medium 211 as shown in FIG. 4, the data cannot be reproduced of used. As a result, the wrongful conduct of making unauthorized copies and selling the thus copied mediums can be prevented, thereby protecting copyrights. Furthermore, with the above example, as seen from FIG. 1, because the circuits used for enciphering and deciphering can be designed separately from the essential portion of the reproducing section of the digital recording and reproducing apparatus, such as a DVD, even if the cipher is broken, the deciphering unit 114 (or the enciphering unit 107 and deciphering unit 114) has only to be replaced to overcome this problem. While in the above example, the enciphering unit 107 has one enciphering circuit, it may have two enciphering circuits. Moreover, although in the above example, the deciphering unit 114 has one deciphering circuit, it may have two, three, or four deciphering circuits. In these cases, it is desirable that the enciphering circuits should be paired with the corresponding deciphering circuits and each pair be used independently or in a shared manner. When a set of an enciphering circuit and the corresponding deciphering circuit is used independently, an enciphering method different from that in another enciphering circuit and deciphering circuit may be used in the enciphering circuit and its corresponding deciphering circuit in the independent set. (First Embodiment) Hereinafter, a first embodiment of the present invention will be explained. What will be explained in the first embodiment is an example suitable for a case where a plurality of predetermined master keys are prepared and one or more of them are allocated to deciphering unit makers (or DVD makers and distributors) FIG. 5 is a block diagram of the system according to the first embodiment of the present invention. An example of the operation of the first embodiment is shown in the flowchart of FIGS. 7 and 8. The system related to the first embodiment is connected to the CPU BUS of the CPU (not shown) used for reproduction in a computer, such as a personal computer. The system is designed to allow the enciphered data (E SK (Data)) to flow over the CPU BUS. FIG. 5 shows only the sections related to the CPU used for reproduction. As shown in FIG. 5, the system of the first embodiment comprises a DVD driving unit (not shown) that reads the data from a DVD 101, an enciphering unit 107 that is connected to the DVD driving unit without the CPU BUS or is built in the DVD driving unit, and a deciphering unit 114a. The enciphering unit 107 and deciphering unit 114a are connected to the CPU BUS 110. The deciphering unit 114a outputs the data via, for example, an I/O port, not via the CPU BUS. That is, in the first embodiment, the input and output of the data is carried out without the CPU BUS, whereas the CPU BUS is used for the data transfer between the enciphering unit 107 and the deciphering unit 114a. The enciphering unit 107 includes a demodulation/error correction circuit 117, a demodulation/error correction circuit 118, and an enciphering circuit 104. Although in FIG. 5, the enciphering unit 107 has two enciphering circuits 104, it is assumed that it actually has one enciphering circuit. The enciphering unit 107 is assumed to be composed of a single independent IC chip. The demodulation/error correction circuit 117 and demodulation/error correction circuit 118 may be provided in the unit (the DVD driving unit) in the preceding stage, not in the enciphering unit 107. The deciphering unit 114a includes a deciphering circuit 112 and a session key creation circuit 111 that creates a second session key S K ', and a key judging circuit 120. FIGS. 6A and 6B show examples of the structure of the key judging circuit 120. The key judging circuit 120 includes a deciphering circuit 112, a comparison circuit 121, and a gate circuit 122. In the second embodiment, it is assumed that the deciphering unit 114a incorporates an MPEG decoder circuit 115 and a conversion circuit 116 that converts the deciphered digital image data into analog image data. Although in FIG. 5 and FIGS. 6A and 6B, the deciphering unit 114a has a total of five deciphering circuits 112, including the two deciphering circuits 112 in the key judging circuit 120, it is assumed that it actually has one deciphering circuit. The deciphering unit 114a is composed of a single independent IC chip. In each of the enciphering unit 107 and deciphering unit 114a, master keys, explained later, have been registered. It is assumed that the master keys have been recorded in a secret area in each of the enciphering unit chip and the deciphering unit chip so that the user cannot externally take out the master keys. A control section (not shown) is assumed to control the entire system. The control section is realized by, for example, executing a program on the CPU in the computer. Concrete examples of control by the control section include an instruction to read the data from a DVD, the specification of data transfer destination, and an instruction to output the data from the deciphering unit 114a. The control section may be triggered, for example, by the user via a user interface, or by a process in an application program. In the first embodiment, there are an n number of types of master keys. A first session key is represented by S K , a second session key S K ', the n-th master key M Kt (t s in the range of 1 to n), and image data (i.e., the data to be enciphered) Data. In FIG. 5, numeral 102-1 indicates E MKi (S K ) created by enciphering the first session key S K using the master key M Ki , 102-2 E SK (S K ) created by enciphering the first session key S K using the first session key S K itself, 103 E SK (Data) created by enciphering the image data Data using the first session key S K , 105 the master key M Ki , 106 a second session key S K ', 108 D MKj (S K ') created by deciphering the second session key S K ' using the master key M Kj , 109-1 E SK '(E MKi (S K )) created by enciphering the first session key E MKi (S K ) enciphered with the master key M Ki using the second session key S K ', 109-2 E SK '(E SK (S K )) created by enciphering the first session key E SK (S K ) enciphered with the first session key S K itself using the second session key S K ' and 113 the first session key S K . Several methods can be considered as to how to set the number of types of E MKi (S K ) created by enciphering the first session key S K recorded on the DVD 101 using the master key M Ki and how to set the number of types of master key M Kj the deciphering unit 114a has in it. For example, they are as follows. (Method 1) One enciphered first session key E MKi (S K ) (i is in the range of 1 to n) is recorded on the DVD 101. The deciphering unit 114a has an n number of master keys M Kj (j = 1 to n) in it. Method 1 is an example useful for understanding the present invention. (Method 2) An n number of enciphered first session keys E MKi (S K ) (i = 1 to n) are recorded on the DVD 101. The deciphering unit 114a has one master key M Kj (j is in the range of 1 to n) in it. (Method 3) This is an expansion of Method 2. An n number of enciphered first session keys E MKi (S K ) (i = 1 to n) are recorded on the DVD 101. The deciphering unit 114a has an m (2 < m < n) number of master keys M Kj (j = 1 to n) in it. The m number of master keys have been selected from the n number of master keys beforehand. As a concrete example, n = 100 or n = 400 and m = 2, 3, 4, or 10. The present invention is not limited to these values. (Method 4) This is the reverse of Method 3. An m (2 < m < n) number of enciphered first session keys E MKi (S K ) (i = 1 to n) are recorded on the DVD 101. The m number of master keys have been selected from an n number of master keys M Kj (j = 1 to n) beforehand. The deciphering unit 114a has an n number of master keys M Kj (j = 1 to n) in it. (Method 5) An n number of enciphered first session keys E MKi (S K ) (i = 1 to n) are recorded on the DVD 101. The deciphering unit 114a has an n number of master key M Kj (j = 1 to n) in it. Method 3 to Method 5 have the same deciphering procedure. As shown in FIG. 3, it is assumed that on the DVD 101, one (in the case of Method 1) or more (in the case of Method 2 to Method 5) E MKi (S K ) created by enciphering the first session key S K using the master key M Ki are recorded in the key recording area (lead-in area) in the innermost circumference portion and E SK (Data) created by enciphering the image data Data using the first session key S K is recorded in the data recording area (data area). It is assumed that an n number of master keys M Kj (in the case of Method 1, Method 4, or Method 5), one master key M Kj (in the case of Method 2), or an m number of master keys M Kj (in the case of Method 3) have been registered in the deciphering unit 114a. A predetermined master key is assumed to have been registered in the deciphering unit 107. Hereinafter, Method 1, Method 2, and Method 3 to Method 5 will be explained in that order. First, the operation of the first embodiment in the case of Method 1 will be explained by reference to the flowcharts of FIGS. 7 and 8. At step S11, the first session key E SK (S K ) enciphered using the first session key S K itself is read from the DVD 101, on which the DVD driving unit (not shown) has recorded the first session key, and then is loaded into the enciphering unit 107. At that time, the demodulation/error correction circuit 117 performs demodulation and data error correction. At step S12, the first session key E MKi (S K ) (i in the range of 1 to n, where i is unknown here) enciphered using the master key M Ki is read from the DVD 101, on which the DVD driving unit (not shown) has recorded the master key, and then is loaded into the enciphering unit 107. At that time, the demodulation/error correction circuit 117 performs demodulation and data error correction. At step S13, the session key creation circuit 111 of the deciphering unit 114 creates a second session key S K ' using random numbers, such as time data from a clock (not shown). Then, the deciphering circuit 112 deciphers the created second session key S K ' using the master key M Kj (j is in the range of 1 to n, where j is predetermined) to create D MKj (S K ') and sends it to the enciphering unit 107 via the CPU BUS 110. As the timing of generating random numbers (e.g., the timing of inputting time information), for example, the timing with which the signal indicating that the DVD 101 has been loaded into the DVD driving unit is asserted may be used. The session creation circuit 111 may be composed of a random-number generator that is as long as the key, for example. When a key is created using random numbers all of whose bits may take 0s or 1s, it is necessary to perform a check process to prevent all of the bits from taking 0s or 1s. At step S14, using the master key M Kj (j has a predetermined value in the range of 1 to n), the enciphering circuit 104 of the enciphering unit 107 enciphers D MKj (S K ') received via the CPU BUS 110. Namely, from E MKj (D MKj (S K ')) = S K ' a second session key S K ' created at the session key creation circuit 111 of the deciphering unit 114a can be obtained. The second session key S K ' created at the session key creation circuit 111 is designed to prevent its contents from being known even if it is stolen on the CPU BUS 110. Then, at step S15, using the thus obtained second session key S K ', the enciphering unit 107 enciphers the enciphered first session key E SK (S K ) recorded on the DVD 101 to create E SK '(E SK (S K )), and sends this to deciphering unit 114a in via CPU BUS 110. Similarly, at step S16, using the thus obtained second session key S K ', the enciphering unit 107 enciphers the enciphered first session key E MKi (S K ) recorded on the DVD 101 to create E SK '(E MKi (S K )), and sends this to deciphering unit 114a. Then, at step S17, the deciphering circuit 112 of the deciphering unit 114a deciphers E SK '(E SK (S K )) received via the CPU BUS 110 using the second session key S K ' and produces: D SK ' (E SK ' (E SK (S K ))) = E SK (S K ) Similarly, at step S18, the deciphering circuit 112 of the deciphering unit 114a deciphers E SK '(E MKi (S K )) received via the CPU BUS 110 using the second session key S K ' and produces: D SK ' (E SK ' (E MKi (S K ))) = E MKi (S K ) Because the master key M Ki used in creating E MKi (S K ) is unknown, the first session key S K is found using the key judging circuit 120 as follows. First, the principle of the key judging process will be explained. When E MKi (S K ) is deciphered using all of the master keys M Kj (j = 1 to n), this gives: S Kij = D MKj (E MKi (S K )) (j = 1 to n) Of these, one S Kij (j = 1 to n) is the first session key SK. Using the E SK (S K ), it is determined which one of the created S Kij (j = 1 to n) is the first session key S K . Then, when E SK (S K ) is deciphered using all of the candidates S Kij (j = 1 to n) of the first session key, this gives: S K ''(i, j) = D SKij (E SK (S K )) Here, when the same master key M Kj as the master key M Ki used in creating E MKi (S K ) is used in the deciphering unit, or when i = j, this gives S K "(i, j) = S Kij = S K Therefore, when a check is made to see if S K "(i, j) = S Kij (j = 1 to n) holds for each S Kij (j = 1 to n), this gives S Kij that meets S K "(i, j) = S Kij (j = 1 to n) as the first session key S K . The one corresponding to j giving the S Kij is the master key used in the present session. The operation is expressed in C language notation as follows: The second line in the above procedure indicates the operation of deciphering E MKi (S K ) using M Ki and substituting the result into DS1[i]. The third line in the procedure indicates the operation of deciphering E SK (S K ) using S Ki and substituting the result into DS2[i]. The fourth line in the procedure indicates the operation of judging whether nor not DS1[i] coincides with DS2[i]. The ninth line in the procedure indicates the operation executed when DS1[i] does not coincide with DS2[i]. For example, in FIGS. 6A and 6B, the deciphering circuit 112 in the key judging circuit 120 deciphers E MKi (S K ) for j = 1 using master key M Kj , giving: S Kij = D MKj (E MKi (S K )) Then, the deciphering circuit 112 deciphers E SK (S K ) using S Kij , giving: S K '' = D SKij (E SK (S K )) Next, the comparison circuit 121 compares S K " with S Kij . If they coincide with each other, the gate circuit 122 will be controlled so as to output the stored S Kij (FIG. 6A) or S K " (FIG. 6B) as the first session key S K . If they do not coincide, j is incremented by one and the same operation will be carried out until the first session key S K has been obtained. After the first session key S K has been obtained as described above, at step S20, the image data E SK (Data) enciphered using the first session key S K recorded on the DVD 101 by the DVD driving unit (not shown) is read out and loaded into the enciphering unit 107. At that time, the demodulation/error correction circuit 118 performs demodulation and corrects errors in the data. Then, E SK (Data) is sent to the enciphering unit 107 via the CPU BUS 110. At step S21, the deciphering circuit 112 of the deciphering unit 114a deciphers E SK (Data) received via the CPU BUS 110 using the first session key S K and produces: D SK (E SK (Data) = Data Then, the enciphered image data is deciphered to produce Data. Then, step S20 and step S21 are repeated until for example, the process of the data to be deciphered (i.e., E SK (Data)) has been completed or the stop of the process has been requested. When the image data Data thus obtained has been compressed according to, for example, the MPEG2 data compression standard, the image data is decoded at an MPEG decoder circuit 115. After the decoded signal has been converted by a D/A converter circuit 116 into an analog signal, the analog signal is sent to an imaging device (not shown), such as a television, which reproduces the image. Any one of step S11, step S12, and steps S13 and S4 may be executed first. Moreover, either step S15 and step S17 or step S16 and S18 may be executed first. Step S20 and step S21 may be executed by the method of carrying out the steps in units of E SK (Data), the method of reading a specific number of E SK (Data) at step S20, storing the read-out data in a buffer temporarily, and then deciphering E SK (Data) in the buffer at step S21, or the method of carrying out step S20 and step S21 in a pipeline processing manner. Moreover, the deciphering circuit 112 may transfer the image data E SK (Data) to the MPEG decoder circuit 115 in units of one Data item or a specific number of Data items. As described above, with the first embodiment, even when the data flowing over the CPU BUS 110 is stored, the data cannot be reproduced of used, as in the first example useful for understanding the invention. As a result, the wrongful conduct of making unauthorized copies and selling the thus copied mediums can be prevented, thereby protecting copyrights. Furthermore, with the first embodiment, the information that directly indicates the master key used to encipher the first session key recorded on the recording medium is not necessary, which enables a suitable master key to be selected and used in a predetermined range in recording the data on a DVD. In addition, the first embodiment has the advantage that it can allocate master keys in a specific unit, such as a DVD maker or a DVD distributor. With the first embodiment, because the circuits used for enciphering and deciphering can be designed separately from the essential portion of the reproducing section of the digital recording and reproducing apparatus, such as a DVD, even if the cipher is broken, the deciphering unit 114a (or the enciphering unit 107 and deciphering unit 114a) has only to be replaced to overcome this problem. While in the first embodiment, the enciphering unit 107 has one enciphering circuit, it may have two enciphering circuits. Moreover, although in the embodiment, deciphering unit 114a has one deciphering circuit, it may have two, three, four, or five deciphering circuits. In these cases, it is desirable that the enciphering circuits should be paired with the corresponding deciphering circuits and each pair be used independently. When a set of an enciphering circuit and its corresponding deciphering circuit is used independently, an enciphering method different from that in another enciphering circuit and deciphering circuit may be used in the enciphering circuit and its corresponding deciphering circuit in the independent set. Next, the operation of the first embodiment in the case of Method 2 where an n number of E MKi (S K ) (i = 1 to n) have been recorded on the DVD 101 and the deciphering unit 114a includes one M Kj (j has a value in the range of 1 to n) will be explained by reference to the flowcharts of FIGS. 7 and 8. At step S11, the first session key E SK (S K ) enciphered using the first session key S K itself is read from the DVD 101, on which the DVD driving unit (not shown) has recorded the first session key, and then is loaded into the enciphering unit 107. At that time, the demodulation/error correction circuit 117 performs demodulation and data error correction. At step S12, the first session key E MKi (S K ) (i = 1 to n) enciphered using the master key M Ki is read from the DVD 101, on which the DVD driving unit (not shown) has recorded the master key, and then is loaded into the enciphering unit 107. At that time, the demodulation/error correction circuit 117 performs demodulation and data error correction. At step S13, the session key creation circuit 111 of deciphering unit 114a creates a second session key S K ' using random numbers, such as time data from a clock (not shown). Then, the deciphering circuit 112 deciphers the created second session key S K ' using the master key M Kj (j has a predetermined value in the range of 1 to n) to create D MKj (S K ') and sends it to the enciphering unit 107 via the CPU BUS 110. As the timing of generating random numbers (e.g., the timing of inputting time information), for example, the timing with which the signal indicating that the DVD 101 has been loaded into the DVD driving unit is asserted may be used. At step S14, using the master key M Kj (j has a predetermined value in the range of 1 to n), the enciphering circuit 104 of the enciphering unit 107 enciphers D MKj (S K ') received via the CPU BUS 110. Namely, from E MKj (D MKj (S K ')) = S K ' a second session key S K ' created at the session key creation circuit 111 of the deciphering unit 114a can be obtained. The second session key S K ' created at the session key creation circuit 111 is designed to prevent its contents from being known even if it is stolen on the CPU BUS 110. Then, at step S15, using the thus obtained second session key S K ', the enciphering unit 107 enciphers the enciphered first session key E SK (S K ) recorded on the DVD 101 to create E SK '(E SK (S K )), and sends this to deciphering unit 114a. Similarly, at step S16, using the thus obtained second session key S K ', the enciphering unit 107 enciphers an n number of enciphered first session keys E MKi (S K ) recorded on the DVD 101 to create E SK '(E MKi (S K )), and sends these to deciphering unit 114a via the CPU BUS 110. Then, at step S17, the deciphering circuit 112 of the deciphering unit 114a deciphers E SK '(E SK (S K )) received via the CPU BUS 110 using the second session key S K ' and produces: D SK ' (E SK ' (E SK (S K ))) = E SK (S K ) Similarly, at step S18, the deciphering circuit 112 of the deciphering unit 114a deciphers E SK '(E MKi (S K )) received via the CPU BUS 110 using the second session key S K ' and produces: D SK ' (E SK ' (E MKi (S K ))) = E MKi (S K ) Because the master key M Ki used in creating each of the n number of E MKi (S K ) (i = 1 to n) recorded on the DVD 101 is unknown, it cannot be known whether the master key M Ki corresponds to the master key M Kj in the deciphering unit 114a. At step S19, the first session key S K is found using the key judging circuit 120 as follows. First, the principle of the key judging process will be explained. When all of E MKi (S K ) (i = 1 to n) are deciphered using the master key M Kj , this gives: S Kij = D MKj (E MKi (S K )) (i = 1 to n) Of these, one S Kij (i is in the range of 1 to n) is the first session key S K . Using the E SK (S K ), it is determined which one of the created S Kij (i = 1 to n) is the first session key S K . Then, when E SK (S K ) is deciphered using all of the candidates S Kij (i = 1 to n) of the first session key, this gives: S K ''(i, j) = D SKij (E SK (S K )) Here, when the same master key M Kj as the master key M Ki used in creating E MKi (S K ) is used in the deciphering unit, or when i = j, this gives S K "(i, j) = S Kij = S K . Therefore, when a check is made to see if S K "(i, j) = S Kij (j = 1 to n) holds for each S Kij (i = 1 to n), this gives S Kij that meets S K "(i, j) = S Kij (j = 1 to n) as the first session key S K . The one corresponding to i giving the S Kij is the master key used in the present session. For example, in FIGS. 6A and 6B, the deciphering circuit 112 in the key judging circuit 120 deciphers E MKi (S K ) for i = 1 using master key M Kj , giving: S Kij = D MKj (E MKi (S K )) Then, the deciphering circuit 112 deciphers E SK (S K ) using S Kij , giving: S K '' = D SKij (E SK (S K )) Next, the comparison circuit 121 compares S K " with S Kij . If they coincide with each other, the gate circuit 122 will be controlled so as to output the stored S Kij (FIG. 6A) or S K " (FIG. 6B) as the first session key S K . If they do not coincide, i is incremented by one and the same operation will be carried. This will be continued until the first session key S K has been obtained. After the first session key S K has been obtained as described above, at steps S20 to S22, the image data Data is extracted from the image data E SK (Data) enciphered using the first session key S K . As described earlier, the image data Data is decoded at the MPEG decoder circuit 115. After the decoded signal has been converted by the D/A converter circuit 116 into an analog signal, the analog signal is sent to the imaging device (not shown), such as a television, which reproduces the image. In Method 2, too, any one of step S11, step S12, and step S13 and step S14 may be executed first. Moreover, either step S15 and step S17 or step S16 and S18 may be executed first. Furthermore, steps S12, S16, S18, and S19 may be executed in a batch processing manner using all the n number of (enciphered) master keys recorded on the DVD or using a specific number of master keys at a time. They may be executed one after another for each master key. When they are executed sequentially every third master key, the second session key S K ' may be created for each master key. Step S20 and step S21 may be executed by the method of carrying out the steps in units of E SK (Data), the method of reading a specific number of E SK (Data) at step S20, storing the read-out data in a buffer temporarily, and then deciphering E SK (Data) in the buffer at step S21, or the method of carrying out step S20 and step S21 in a pipeline processing manner. Moreover, the deciphering unit 114a may transfer the image data E SK (Data) to the MPEG decoder circuit 115 in units of one Data item or a specific number of Data items. As described above, with the second embodiment, even when the data flowing over the CPU BUS 110 is stored, the data cannot be reproduced or used, as in the example useful for understanding the invention. As a result, the wrongful conduct of making unauthorized copies and selling the thus copied mediums can be prevented, thereby protecting copyrights. Furthermore, with the first embodiment, because the first session keys enciphered using more than one master key and the first session key enciphered with the first session key itself are stored on the recording medium, the master keys built in the deciphering unit can be allocated in a specific unit, such as to each unit manufacturer. With the second embodiment, because the circuits used for enciphering and deciphering can be designed separately from the essential portion of the reproducing section of the digital recording and reproducing apparatus, such as a DVD, as seen from FIG. 1, even if the cipher is broken, the deciphering unit 114b (or the enciphering unit 107 and deciphering unit 114b) has only to be replaced to overcome this problem. While in the second embodiment, the enciphering unit 107 has one enciphering circuit, it may have two enciphering circuits. Moreover, although in the embodiment, the deciphering unit 114a has one deciphering circuit, it may have two, three, four, or five deciphering circuits. In these cases, it is desirable that the enciphering circuits should be paired with the corresponding deciphering circuits and each pair be used independently or be shared. When a set of an enciphering circuit and its corresponding deciphering circuit is used independently, an enciphering method different from that in another enciphering circuit and deciphering circuit may be used in the enciphering circuit and its corresponding deciphering circuit in the independent set. Next, explanation will be given about Method 3 where an n number of E MKi (S K ) (i = 1 to n) have been recorded on the DVD 101 and the deciphering unit 114a includes an m number of M Kj (j takes m values in the range of 1 to n (m < n)). Since Method 3 is the same as Method 2 in basic configuration, operation, and effect, only the difference between them will be explained. While in Method 2, the deciphering unit 114a includes one predetermined master key M Kj (j has a value in the range of 1 to n), in Method 3, the deciphering unit 114a includes an m number of predetermined master keys M Kj (m ≧ 2). The order in which the m number of master keys M Kj (j takes m values in the range of 1 to n) are used in the key judgment has been determined. Because an n number of E MKi (S K ) (i = 1 to n) have been recorded on the DVD 101, using the master key first in order of use in the deciphering unit 114b produces the first session key S K . Therefore, in this case, the operation is the same as in Method 2. With Method 3, if one of the master keys is broken, the master key is made unusable. From this time on, E MKi (S K ) corresponding to the unusable master key is not allowed to be recorded on the DVD 101. This case will be explained below. When the unusable master key is not the master key first in order of use, the first session key S K can be obtained. In this case, too, the operation is the same as in Method 2. When the master key first in order of use is made unusable, E MKi (S K ) corresponding to the unusable master key has not been recorded on the DVD 101. Even if the master key first in order of use is used, the first session key S K cannot be obtained in step S19. In such a case, when the deciphering unit 114a carries out the same operation using the master key second in order of use as in Method 2, this produces the first session key S K , provided that this master key is not unusable. Even when the master key r-th in order of use is made unusable, the first session key S K can be obtained similarly, provided that one of the master keys (r + 1)-th or later in order of use is not unusable. In this way, the deciphering unit 114a can be used until the predetermined m number of master keys (m ≧ 2) in the deciphering unit 114a have all been made unusable. The operation of Method 5 is the same as that of Method 3. Because in Method 4, the information corresponding to all the master keys has not been stored on the DVD 101, when the information corresponding to the master key selected in the deciphering unit has not been recorded on the DVD 101, deciphering cannot be effected as in the case where the master key is unusable. In this case, the master key next in order of use is selected and deciphering is tried. Therefore, the operation of Method 4 is also the same as that of Method 3. In the embodiment, to encipher the information and transfer it safely over the CPU BUS 110, the second session key S K ' has been used. The second session key S K ' is created in the deciphering unit 114a and is transferred to the enciphering unit 107 through the procedure of using master keys. At that time, one predetermined master key is supposed to have been registered in the enciphering unit 107. Instead, a plurality of master keys may be registered in the enciphering unit 107 and the second session key S K ' may be transferred from the deciphering unit 114a to the enciphering unit 107, using the procedure as described in Method 1 to Method 5 using key judgment. For example, when the same master key as that registered in the deciphering unit 114a is also registered in the enciphering unit 107, the operation is the same as that of Method 5. When part of the master keys registered in the deciphering unit 114a are registered in the enciphering unit 107, the operation is the same as that of Method 3. When one master key is registered in the enciphering unit 107, the procedure of Method 2 can be used. In these cases, however, in the procedure of each of Method 1 to Method 5, enciphering is replaced with deciphering. Specifically, D MKi (S K ) and D SK (S K ) are transferred from the deciphering unit 114a to the enciphering unit 107. In addition to the configuration using the master key, various suitable configurations may be used as the configuration that safely transfers the second session key S K ' from the deciphering unit 114a to the enciphering unit 107 over the CPU BUS 110. For example, the techniques disclosed in Nikkei Electronics, No. 676, Nov. 18, 1996, pp. 13-14. In this case, it is not necessary to register a master key in the enciphering unit 107. (Second Embodiment) Hereinafter, a second embodiment of the present invention will be explained. The second embodiment is, for example, a single DVD player. FIG. 9 is a block diagram of a system according to the second embodiment of the present invention. An example of the operation of the second embodiment is shown in the flowchart of FIG. 10. The second embodiment is what is obtained by eliminating from the configuration of the first embodiment the portion related to the operation of exchanging an enciphered key between the enciphering unit and deciphering unit by use of the second session key. As shown in FIG. 9, the system of the second embodiment comprises a DVD driving unit (not shown) that reads the data from a DVD 101 and a deciphering unit 114b. The deciphering unit 114b includes a deciphering circuit 112, a key judging circuit 120, a demodulation/error correction circuit 117, and a demodulation/error correction circuit 118. In the second embodiment, the deciphering unit 114b is assumed to include an MPEG decoder circuit 115 and a conversion circuit 116 that converts the digital deciphered data into analog data. As shown in FIGS. 6A and 6B, the key judging circuit 120 includes a deciphering circuit 112, a comparison circuit 121, and a gate circuit 122. Although in FIG. 9 and FIGS. 6A and 6B, the deciphering unit 114b has a total of three deciphering circuits 112, including the two deciphering circuits 112 in the key judging circuit 120, it is assumed that it actually has one deciphering circuit. Each of the demodulation/error correction circuit 117 and the demodulation/error correction circuit 118 may be provided in the unit in the preceding stage, not in the enciphering unit 107. The deciphering unit 114b is composed of a single independent IC chip. In the deciphering unit 114b, a master key, explained later, has been registered. It is assumed that the master key has been recorded in a secret area in the deciphering unit chip so that the user cannot externally take out the master key. In the second embodiment, there are an n number of master keys. A first session key is represented by S K , a second session key S K ', the i-th master key M Ki (i is in the range of 1 to n), and image data (i.e., the data to be enciphered) Data. In FIG. 9, numeral 102-1 indicates E MKi (S K ) created by enciphering the first session key SK using the master key M Ki , 102-2 E SK (S K ) created by enciphering the first session key S K using the first session key S K itself, 103 E SK (Data) created by enciphering the image data Data using the first session key S K , 105 the master key M Ki , and 113 the first session key S K . As in the first embodiment, several methods can be considered as to how to set the number of types of E MKi (S K ) created by enciphering the first session key SK recorded on the DVD 101 using the master key M Ki and how to set the number of types of master key M Kj the deciphering unit 114b has in it. For example, they are as follows. (Method 1) One master key E MKi (S K ) (i is in the range of 1 to n) is recorded on the DVD 101. The deciphering unit 114b has an n number of master keys M Kj (j = 1 to n) in it. (Method 2) An n number of master keys E MKi (S K ) (i = 1 to n) are recorded on the DVD 101. The deciphering unit 114b has one master key M Kj (j has a value in the range of 1 to n) in it. (Method 3) An n number of master keys E MKi (S K ) (i = 1 to n) are recorded on the DVD 101. The deciphering unit 114b has an m (2 < m < n) number of master keys M Kj (j is in the range of 1 to n) in it. (Method 4) An m (2 < m < n) number of master keys E MKi (S K ) (i is in the range of 1 to n) are recorded on the DVD 101. The deciphering unit 114b has an n number of master keys M Kj (j = 1 to n) in it. (Method 5) An n number of master keys E MKi (S K ) (i = 1 to n) are recorded on the DVD 101. The deciphering unit 114b has an n number of master key M Kj (j = 1 to n) in it. As shown in FIG. 3, it is assumed that on the DVD 101, one (in the case of Method 1) or more (in the case of Method 2 to Method 5) E MKi (S K ) created by enciphering the first session key S K using the master key M Ki are recorded in the key recording area (lead-in area) in the innermost circumference portion and the E SK (Data) created by enciphering the image data Data using the first session key S K is recorded in the data recording area (data area). Next, the operation of the second embodiment will be explained by reference to the flowchart of FIG. 10. The operation of the second embodiment is what is obtained by eliminating from the operation of the first embodiment the portion related to the operation of exchanging an enciphered key between the enciphering unit and deciphering unit by use of the second session key. At step S31, the first session key E SK (S K ) enciphered using the first session key S K itself is read from the DVD 101, on which the DVD driving unit (not shown) has recorded the first session key, and then is loaded into the deciphering unit 114b. At that time, the demodulation/error correction circuit 117 performs demodulation and data error correction. At step S32, the first session key E MKi (S K ) enciphered using the master key M Ki is read from the DVD 101, on which the DVD driving unit (not shown) has recorded the master key, and then is loaded into the deciphering unit 114b. At that time, the demodulation/error correction circuit 117 performs demodulation and data error correction. At step S33, the first session key S K is obtained using the key judging circuit 120. The operation of obtaining the first session key S K differs depending on Method 1, Method 2, or Method 3 to Method 5. Each case is the same as explained in the first embodiment, so explanation of them will not be given. After the first session key S K has been obtained, the image data Data is extracted from the enciphered image data E SK (Data) using the first session key S K at steps S34 to S36. The operation at step S34 to S36 are the same as that of steps S20 to S22 explained in the first embodiment (i.e., that of steps S6 to S8 explained in the example useful for understanding the invention) except that there is no exchange of the image data Data between the units via the CPU BUS. As described earlier, the image data Data is decoded at the MPEG decoder circuit 115. After the decoded signal has been converted by the D/A converter circuit 116 into an analog signal, the analog signal is sent to the imaging device (not shown), such as a television, which reproduces the image. In Method 3, too, step S31 may be executed before step S32 or vice versa. Furthermore, in method 2 and in method 3 to method 5, step S32 and step S33 may be executed in a batch processing manner using all the n number of (enciphered) master keys (in the case of Methods 2, 3, and 5) or all the m number of (enciphered) master keys (in the case of Method 4) recorded on the DVD or using a specific number of master keys at a time. They may be executed one after another for each master key. Step S34 and step S35 may be executed by the method of carrying out the steps in units of E SK (Data), the method of reading a specific number of E SK (Data) at step S34, storing the read-out data in a buffer temporarily, and then deciphering E SK (Data) in the buffer at step S35, or the method of carrying out step S34 and step S35 in a pipeline processing manner. Moreover, the deciphering unit 114b may transfer the image data E SK (Data) to the MPEG decoder circuit 115 in units of one Data item or a specific number of Data items. With the second embodiment, the wrongful conduct of making unauthorized copies and selling the thus copied mediums can be prevented, thereby protecting copyrights. Furthermore, with the second embodiment, it is possible to select and use a suitable master key in a predetermined range in recording the data on a DVD. The master keys can be allocated in a specific unit, such as to a DVD player maker, a DVD maker, or a DVD distributor. Still furthermore, with thesecond embodiment, because the circuits used for enciphering and deciphering can be designed separately from the essential portion of the reproducing section of the digital recording and reproducing apparatus, such as a DVD, as seen from FIG. 1, even if the cipher is broken, the deciphering unit 114b has only to be replaced to overcome this problem. While in the second embodiment, the deciphering unit 114b has one deciphering circuit, it may have two or three deciphering circuits. In these cases, it is desirable that the enciphering circuits should be paired with the corresponding deciphering circuits and each pair be used independently or be shared. When a set of an enciphering circuit and its corresponding deciphering circuit is used independently, an enciphering method different from that in another enciphering circuit and deciphering circuit may be used in the enciphering circuit and its corresponding deciphering circuit in the independent set. Until now, the first embodiment (specifically, the three types of configuration), and the second embodiment (specifically, the three types of configuration) have been explained. The present invention is not limited to these embodiments, but may be practiced or embodied in still other ways without departing from the scope of the appended claims. Although the embodiments have been explained using a DVD as information recording medium, the present invention may be applied to other recording mediums, such as CD-ROMs. While in the embodiments, the image data has been used as the information to be deciphered, the present invention may be applied to reproducing devices of other types of information, such as sound, text, or programs. While in the embodiments, the data Data is image data, the configuration may be designed to use key information S Kt as the data Data. Specifically, E SK (S Kt ) and E SKt (Data) may be recorded on a recording medium, such as a DVD, beforehand in place of E SK (Data), then S Kt is first obtained at the deciphering units 114, 114a, 114b through the procedure in each of the embodiments, and E SKt (Data) is deciphered using the S Kt to produce the actual contents of the data. The hierarchization of keys may be carried out over any number of levels of hierarchy. While in the embodiments, the information to be deciphered has been compressed according to the MPEG2 standard, the present invention is not restricted to this. The data may be compressed or enciphered according to another standard. In this case, a decoder circuit corresponding to another standard has to be provided instead of the MPEG decoder circuit 115. The data may not be enciphered. In this case, the MPEG decoder circuit 115 is eliminated. To output any data items compressed by various methods (or data items requiring no deciphering), several types of decoder circuits may be provided and switched suitably. In this case, a method can be considered which reads an identifier indicating the decoder to be used from a recording medium, such as a DVD, and selects a suitable decoder circuit according to the identifier. The configurations of the key judging circuit 120 shown in FIGS. 6A and 6B in the second and third embodiments are illustrative and not restrictive. Other configurations of the key judging circuit may be considered. Various types of the configuration that uses E SK (S K ) as key judgment information may be considered. For instance, D SK (S K ) is used as information used for key judgment. The key judging circuit 120 deciphers E MKi (S K ) read from a recording medium, such as a DVD, using master key M Kj to produce S Kij = D MKj (E MKi (S K )), deciphers the S Kij using the S Kij itself to produce S K ''' = D SKij (S Kij ), and compares the S K '' with D SK (S K ) read from a recording medium, such as a DVD. When they coincide with each other, the key judging circuit judges that the first session key S K = S Kij is correct and outputs it. As other examples of key judgment information, the one enciphered or deciphered twice or more times, such as E SK (E SK (S K )) or D SK (D SK (S K )) may be considered. In addition, E MKi (E MKi (S K )) may be provided for each E MKi (S K ). In the embodiments, on the basis of the key judgment information, a judgment is made through the procedure shown in each of Method 1 to Method 5 as to whether the key obtained by deciphering is the correct first session key. However, the key judgment information, key judging procedure, and the structure for key judgment can be eliminated by recording all the E MKi (S K ) on a recording medium, such as a DVD, in order of i and registering them in the deciphering unit in such a manner that i corresponds to M Ki . When M Ki for a certain i becomes unusable, it is desirable that information indicating invalidity should be stored on a recording medium, such as a DVD, in place of E MKi (S K ). A key control method followed by disk makers (assumed to be makers that produce DVDs for writings, including movies and music), player makers (assumed to be makers that produce DVD players), and a key control organization that controls master keys will be described taking a DVD-ROM as example, by reference to FIG. 11 as an example useful for understanding the present invention. Here, in addition to the contents, Data may be key information, as described earlier (explanation of the case where enciphering or deciphering is done using key information S Kt when Data is key information S Kt will be omitted). In FIG. 11, a computer used for processing is not shown. FIG. 12 is a diagram to help explain a system for deciphering. Enciphering circuits 301, 312, 303 in FIG. 12 may be on the same unit (e.g., a computer) or on different units (e.g., computers). In the latter case, information is exchanged between the units. The enciphering circuits 301, 312, 303 may be constructed in hardware or in software. Explanation will be given about a case where an n number of master keys E MKi (S K ) (i = 1 to n) are recorded on a DVD. A DVD player (a deciphering unit 114b) has an m (2 < m < n) number of master keys M Kj (j is in the range of 1 to n) in it. The m number of master keys have been selected from the n number of master keys beforehand. The master keys M Kj are assumed to be allocated exclusively to the DVD player maker. It is assumed that n = 100 and m = 10. A method of recording E SK (S K ) on a DVD as key judgment information is used (the section indicated by numeral 302 in FIG. 12 uses E SK (S K ) as key judgment information). A key control organization 200 keeps master keys M Ki (i = 1 to 100). It is desirable that the number of master keys should be set at a larger value than necessary in preparation for the entry of a new player maker or in case a master key is broken. The key control organization 200 exclusively allocates the master keys M Ki (i = 1 to 100) to the individual player makers 201 to 203. For example, as shown in FIG. 11, it allocates master keys M Ki (i = 10 to 19) to player maker A, master keys M Ki (i = 20 to 29) to player maker B, and master keys M Ki (i = 30 to 39) to player maker C. The key control organization 200 sends the allocated master keys to the individual player makers by means of communication mediums or recording mediums. At that time, it is desirable that they should be exchanged safely by enciphered communication. Each player maker controls the master keys allocated by the key control organization 200. Using the allocated master keys, each player maker manufactures DVD players with the configuration as shown in the third embodiment and sells the resulting products. It is assumed that the key control organization 200 does not give the plain data on the master keys to disk makers 221 to 223. First, each disk maker (e.g., maker a) determines the first session key S K (e.g., for each disk) by itself, and gives the first session key S K to the key control organization 200. The key control organization 200 enciphers the received first session key S K using all the master keys M Ki (i = 1 to 100) to produce E MKi (S K ) (i = 1 to 100) (using the enciphering unit 301 of FIG. 12). Then, the key control organization 200 gives E MKi (S K ) (i = 1 to 100) to disk maker a. It is desirable that the exchange of the allocated master keys between the key control organization 200 and the disk maker should be made by means of communication mediums or recording mediums through enciphered communication. Disk maker a records E MKi (S K ) (i = 1 to 100), E SK (S K ), and E SK (Data) on a DVD 231. The operation of enciphering S K with S K itself to produce E SK (S K ) is carried out by the disk maker side or by the key control organization 200 side (using the enciphering circuit 321 of FIG. 12) in the case of enciphering with a mater key. It is assumed that at least the enciphering of the contents is done at the disk maker side (using the enciphering circuit 303 of FIG. 12). Disk maker a controls the received E MKi (S K ), key judgment information E SK (S K ), and E SK (Data) (or Data) for S K , for example. The same is true for the other disk makers. In case it is found that the master key has been broken, from that time on, DVDs are manufactured without using the broken master key. For example, if the master key for i = 19 has been broken, ninety-nine E MKi (S K ) corresponding to i = 1 to 18 and 20 to 100 are recorded on a DVD. In case it is found that the master key has been broken, it is desirable that the player maker to which the broken master key has been allocated should manufacture and sell DVD players excluding the broken master key. For example, if the master key for i = 19 has been broken, player maker A manufactures DVD players using the master keys for i = 10 to 18 and sells the resulting products. The already sold DVD player having the master key for i = 19 may be used without any modification. It may be modified so as not to have the master key for i = 19. Consequently, the master keys can be controlled safely and effectively. In addition, the risk of the master key being deciphered in an unauthorized manner can be dispersed and even after the deciphering of the master key, the system can function safely and effectively. As described in detail, only the correct maker having at least one of a plurality of second keys can get the first key and therefore can get the plain data of the data enciphered using the first key. As a result, the wrongful conduct of making unauthorized copies and selling the thus copied mediums can be prevented, thereby protecting copyrights.

Claims: An enciphering method characterized by comprising the steps of: keeping a plurality of second keys (M Ki ); enciphering (303) data with a first key (E SK ); and enciphering (301) said first key (S K ) with a p number of second keys (MK i to MK p ); where p is an integer greater than or equal to two, of the kept plurality of second keys (M Ki ) to obtain a p number of enciphered first keys (E Mki (S K )), respectively and recording said p number of enciphered first keys on a recording medium with said enciphered data. An enciphering method according to claim 1, characterized in that said data includes at least one of key information, documents, sound, images, and programs. An enciphering method according to claim 1, further comprising the step of enciphering (312) said first key (S K ) with said first key itself (E SK (S K )) and recording said enciphered key on the recording medium. A recording medium (101) having information items recorded thereon, said information items characterized by comprising: first information (E SK (DATA)) composed of enciphered data obtained by enciphering data with a first key (S K ); and second information (E Mki (S K )) composed of a p number of enciphered first keys, where p is an integer greater than or equal to two, obtained by enciphering said first key (S K ) with a p number of second keys (MK i to MK p ), respectively, wherein said second information composed of the p number of enciphered first keys is recorded in a key recording area of said recording medium. A recording medium according to claim 4, characterized by further comprising: third information (E SK (S K )) composed of enciphered first key obtained by enciphering said first key (S K ) with first key itself. A recording medium manufacturing apparatus characterized by comprising: means (221,222 or 223) for enciphering first information (DATA) with a first key (S K ); means (221, 222 or 223) for generating second information (E Mki (S K )) composed of a p number of enciphered first keys, where p is an integer greater than or equal to two, by enciphering said first key (S K ) with a p number of second keys of a plurality of second keys (M Ki ), respectively; and means (221, 222 or 223) for recording said first and second information on a recording medium. A recording medium manufacturing apparatus according to claim 6, characterized by further comprising: means (221, 222 or 223) for producing third information (E SK (S K )) composed of enciphered first key obtained by enciphering said first key (S K ) with first key itself; and means (221, 222 or 223) for recording said third information on the recording medium. A deciphering method characterized by comprising: recording at least part of a p number of second keys, where p is an integer greater than or equal to two, in a secret area; inputting (S32, S34) first information (E SK (DATA)) composed of enciphered data obtained by enciphering data with a first key (S K ) and second information (E Mki (S K )) composed of a p number of enciphered first keys, where p is an integer greater than or equal to two, obtained by enciphering said first key (S k ) with said p number of second keys (MK i to MK p ), respectively; deciphering (S33) at least one of said p number of enciphered first keys using at least part of a recorded p number of second keys to obtain said first key; confirming (S33) that the obtained first key is correct; and deciphering (S35) said enciphered data using said obtained first key after the confirmation to obtain said data. A deciphering method according to claim 8, characterized in that said data includes at least one of key information, documents, sound, images, and programs. A deciphering method according to claim 8, characterized in that : said inputting step (S32, S34) includes the step of inputting (S31) third information (E SK (S K )) composed of enciphered first key obtained by enciphering said first key (S K ) with first key itself, and said confirming step (S33) includes the steps of deciphering the third information with the obtained first key, and comparing a deciphered result of the third information with the obtained first key. A deciphering method according to claim 10, characterized by further comprising the step of: transferring (S15, S16, S20) the first, second, and third information, from a first unit (107) built in a driving unit of a recording medium or connected to the driving unit of the recording medium to a second unit (114a) via a CPU bus (110) of a computer. A deciphering device characterized by comprising: storage means (105) for recording at least part of a p number of second keys, where p is an integer greater than or equal to two, in a secret area; input means (112) for inputting first information (E SK (DATA) composed of enciphered data obtained by enciphering data with a first key (S K ) and second information (E Mki (S K )) composed of a p number of enciphered first keys, where p is an integer greater than or equal to two, obtained by enciphering said first key (S K ) with said p number of second keys (MK i to MK p ) respectively; and deciphering means (112, 120) for deciphering at least one of said p number of enciphered first keys of said second information (E Mki (S K )) inputted from said input means using at least one of a recorded p number of second keys to obtain said first key, confirming that the obtained first key is correct, and deciphering said enciphered data of said first information (E SK (DATA)) using said first key (S K ) after the confirmation to obtain said data. A deciphering device according to claim 12, characterized in that : said input means (112) includes means for inputting third information (E SK (S K ) composed of enciphered first key obtained by enciphering said first key (S K ) with first key itself; and said deciphering means (112, 120) includes means for deciphering the third information (E SK (S K )) with the obtained first key, and comparing a deciphered result of the third information with the obtained first key. A deciphering device according to claim 13, characterized in that : said input means (112) includes means for receiving the first, second, and third information from a unit built in a driving unit of a recording medium or connected to the driving unit of the recording medium via a CPU bus of a computer.

Inventors: ENDOH NAOKI, HIRAYAMA KOICHI, KATO TAKEHISA, KOJIMA TADASHI, UNNO HIROAKI, ENDOH, NAOKI, HIRAYAMA, KOICHI, KATO, TAKEHISA, KOJIMA, TADASHI, UNNO, HIROAKI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1997
Main classification: G11B20
IPC codes: G06F21, G06F12, H04L9, G11B20, G11B23, H04N5, G09C1