Patent Publication Number: US-7913094-B2

Title: Information reproducing apparatus and secure module

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims priority of Japanese Patent application No. 2001-397629, filed on Dec. 27, 2001, the contents being incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The present invention relates to an information reproducing apparatus and a secure module. In particular, the present invention relates to an information reproducing apparatus for reproducing information which is transmitted through a transmission medium or stored in a recording medium, and a secure module which is detachably mounted on an information reproducing apparatus and executes processing related to security when the information reproducing apparatus reproduces information which is transmitted through a transmission medium or stored in a recording medium. 
     2) Description of the Related Art 
     Recently, use of the broadband Internet and digital television are widely spreading, and right protection technology for securing safety of delivered content (mainly digital AV content) is coming to the fore. In particular, personal computers (PCs) use open architectures, and can be peeped at by basically anybody. Therefore, it has been considered that securing of safety is difficult. 
     However, the personal computers are main gates of the broadband Internet. Therefore, if safety of the personal computers can be secured, the digital AV content can be delivered to the entire Internet. That is, the securing of safety of the personal computers is very significant. 
     The conventional attempts mainly made to protect rights on personal computer software are concealment and obfuscation of algorithms for securing safety. However, once software is installed in a main memory of a personal computer, copying is easy, and the algorithms for protecting rights can be analyzed by putting much time in analysis of the copied software. That is, the conventional systems for protecting rights are considered to be very insecure. In particular, since damage from analysis of copied software in highly public systems such as broadcasting systems is considered to be serious, it is difficult to use the conventional systems for protecting rights in the highly public systems. 
       FIG. 10  is a diagram illustrating an example of a construction of a conventional system using a personal computer. As illustrated in this diagram, the conventional system comprises a personal computer  50 , a network  51 , a speaker  52 , a display device  53 , and an input device  54 . 
     The personal computer  50  comprises a CPU (central processing unit)  50   a , a ROM (read-only memory)  50   b , a RAM (random access memory)  50   c , an HDD (hard disk drive)  50   d , an MB (multimedia board)  50   f , I/Fs (interfaces)  50   g  and  50   h , and a bus  50   i . The personal computer  50  decodes information which is downloaded through the network  51 , and outputs the decoded information to the speaker  52  and the display device  53 . 
     The CPU  50   a  performs various types of processing and controls the respective portions of the apparatus, in accordance with programs stored in the HDD  50   d.    
     The ROM  50   b  stores data, programs which are executed by the CPU  50   a , and other information. 
     The RAM  50   c  temporarily stores data and programs to be executed by the CPU  50   a  when CPU  50   a  performs the various types of processing. 
     The HDD  50   d  stores data, the programs executed by the CPU  50   a , and the like. 
     The MB  50   f  decodes encoded sound data and image data which are supplied from the CPU  50   a , generates an original sound signal and an original image signal, and outputs the generated original sound signal and image signal to the speaker  52  and the display device  53 , respectively. 
     The I/F  50   g  is an interface for transmission and reception of information through the network  51 , and performs protocol conversion and data format conversion. 
     The I/F  50   h  converts data supplied from the input device  54 , into an internal format of the personal computer  50 . 
     The bus  50   i  interconnects the CPU  50   a , the ROM  50   b , the RAM  50   c , the HDD  50   d , the MB  50   f , and the I/Fs  50   g  and  50   h , and enables exchange of information between the CPU  50   a , the ROM  50   b , the RAM  50   c , the HDD  50   d , the MB  50   f , and the I/Fs  50   g  and  50   h.    
     The network  51  is constituted by, for example, the Internet, and the personal computer  50  exchanges information with a server or the like connected to the network  51 . 
     The speaker  52  converts a sound signal supplied from the MB  50   f , into sound corresponding to the sound signal, and outputs the sound. 
     The display device  53  is constituted by, for example, a CRT (cathode ray tube) monitor or a liquid crystal monitor, and displays an image corresponding to an image signal supplied from the MB  50   f.    
     The input device  54  is constituted by, for example, a mouse or a keyboard, and generates and outputs information corresponding to user manipulation. 
       FIG. 11  is a diagram illustrating a flow of information in the personal computer  50  illustrated in  FIG. 10 . 
     As illustrated in  FIG. 11 , the HDD  50   d  stores basic software, a decryption-key group, and encrypted content. The basic software is software for performing processing for decrypting the encrypted content, and other processing, and is obfuscated in order to prevent decryption by a malicious user. In the obfuscation, the following processing is performed. 
     Before obfuscation:
 
 X=X+Y  
 
     After obfuscation:
 
 X=X* 2+1 +Y *2−1
 
 X=X ÷2
 
     That is, although the results of calculations before and after the obfuscation are identical, it is difficult to analyze the algorithm after the obfuscation. 
     The decryption-key group is a group of a plurality of keys for decrypting encryptions applied to the encrypted content. In order to prevent a malicious user from easily acquiring the decryption keys, the decryption keys are confidentially scrambled, and stored in a secret place. 
     The encrypted content is content on which encryption processing is performed, and is constituted by, for example, images, sound, computer data, and the like. 
     When reproduction of the encrypted content is started, the following processing is executed. 
     (1) The obfuscated basic software is read out from the HDD  50   d , and loaded in the RAM  50   c.    
     (2) Thereafter, the basic software read out as above is stored in a secret place when necessary, and a confidentially scrambled decryption key is read out from the HDD  50   d . For example, the decryption keys are stored in three to five secret places. Further, a provision is made so that a desired key can be obtained only when secret calculation or the like is performed. 
     (3) The encrypted content is read out, and the encryption applied to the content is decrypted by using the decryption key. 
     (4) Subsequently, in the case where the decrypted content is compressed, expansion processing (such as MPEG expansion processing in the case of video content or MPEG audio expansion processing in the case of audio content) is performed. Data obtained by the processing is stored in an image buffer and a sound buffer in the RAM  50   c , and is then output to the MB  50   f.    
     (5) The MB  50   f  performs D/A conversion processing of the supplied image data so as to obtain a sound signal, executes drawing processing in accordance with the image data so as to obtain an image signal, and outputs the sound signal and the image signal to the speaker  52  and the display device  53 , respectively. 
     However, in the above processing, the basic software is loaded in the RAM  50   c  in the personal computer  50 . Therefore, there is a danger that the basic software is decrypted or copied by a malicious user. Even in the case where all of the basic software and other software which are stored in the HDD  50   d  are encrypted, when software for decrypting an encryption exists in a place in the personal computer  50 , and a place in which a decryption key is stored is determined by analyzing the software for decrypting the encryption, the basic software can be analyzed and the algorithm for protecting rights is revealed. 
     Therefore, highly public networks such as broadcasting networks require a processing method which does not allow easy decryption of content even when an algorithm is revealed. In the current hardware-dominated digital television receivers, encryptions based on MULTI2, DES (data encryption standard), and the like are used. Although the algorithms of those encryptions are well known, it is impossible to decrypt the encryptions applied to content unless “decryption keys” are known. 
     In addition, the “decryption keys” are built in hardware, and the apparatuses are configured so that the “decryption keys” cannot be read out onto software. Further, since decryption circuits and content processing circuits (e.g., MPEG video expansion circuits and MPEG audio expansion circuits) are also realized by hardware, it is very difficult to peep at details of the processing. Therefore, it is considered that hardware-dominated digital broadcast receiver is safe, and such systems are already commercialized. The receivers for SKY PerfecTV (trademark) in Japan and DirecTV (trademark) in the United States are good examples of the hardware-dominated digital broadcast receivers. 
     On the other hand, in the case of software processing, the “decryption keys,” “decryption circuits,” and “content processing circuits” (e.g., MPEG video expansion circuits and MPEG audio expansion circuits) are realized by software, and basic software realizing the processing per se and progress of calculations are loaded in the RAM  50   c , which can be easily accessed. That is, it is easy to analyze and peep at the basic software per se and the progress of calculations. 
     SUMMARY OF THE INVENTION 
     The present invention is made in view of the above problems, and the object of the present invention is to provide an information reproducing apparatus and a secure module which enable safe software processing by adding a minimum amount of hardware to an apparatus having an open architecture such as a personal computer. 
     In order to accomplish the above object, an information reproducing apparatus for reproducing information which is transmitted through a transmission medium or stored in a recording medium is provided. The information reproducing apparatus comprises: a secure module storing first information and having a structure which does not allow access to the first information from outside; a memory which can be accessed from outside; a decryption unit which is loaded in the memory, and decrypts an encryption applied to the first information by using a predetermined key; a key supply unit which is implemented in the secure module, and supplies the predetermined key to the decryption unit; an authentication unit which is implemented in the secure module, supplies second information to the decryption unit, refers to third information returned in response to the second information, and checks for authenticity of the decryption unit, where the second information is predetermined; and a key-supply stop unit which is implemented in the secure module, and stops supply of the predetermined key by the key supply unit when the authentication unit does not authenticate the decryption unit. 
     In addition, in order to accomplish the above object, a secure module is provided. The secure module can be detachably mounted on an information reproducing apparatus and executes processing related to security when the information reproducing apparatus reproduces information which is transmitted through a transmission medium or stored in a recording medium, where the information reproducing apparatus includes a memory which can be accessed from outside, and decryption unit which is loaded in the memory, and decrypts an encryption applied to the first information by using a predetermined key. The secure module comprises: a key supply unit which is implemented in the secure module, and supplies the predetermined key to the decryption unit; an authentication unit which is implemented in the secure module, supplies second information to the decryption unit, refers to third information returned in response to the second information, and checks for authenticity of the decryption unit, where the second information is predetermined; and a key-supply stop unit which is implemented in the secure module, and stops supply of the predetermined key by the key supply unit when the authentication unit does not authenticate the decryption unit. 
     The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiment of the present invention by way of example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a diagram for explaining an operational principle of the present invention; 
         FIG. 2  is a diagram illustrating an example of a construction of a first embodiment of the present invention; 
         FIG. 3  is a diagram illustrating a flow of information in the first embodiment of  FIG. 2 ; 
         FIG. 4  is a diagram illustrating a flow of information in a second embodiment of the present invention; 
         FIG. 5  is a diagram illustrating a flow of information in a third embodiment of the present invention; 
         FIG. 6  is a diagram illustrating a flow of information in a fourth embodiment of the present invention; 
         FIG. 7  is a diagram illustrating an example of a construction of a fifth embodiment of the present invention; 
         FIG. 8  is a diagram illustrating an example of a construction of a sixth embodiment of the present invention; 
         FIG. 9  is a diagram illustrating an example of a construction of a seventh embodiment of the present invention; 
         FIG. 10  is a diagram illustrating an example of a construction of a conventional system using a personal computer; and 
         FIG. 11  is a diagram illustrating a flow of information in the personal computer illustrated in  FIG. 10 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are explained below with reference to drawings. 
       FIG. 1  is a diagram for explaining an operational principle of the present invention. As illustrated in  FIG. 1 , the information reproducing apparatus according to the present invention comprises a memory  10 , a secure module  11 , an HDD  12 , and an MB (multimedia board)  13 . 
     The memory  10  is constituted by, for example, a RAM, and a decryption unit  10   a , which is realized by software, is loaded in the memory  10 . 
     The secure module  11  is constituted by, for example, a PC card. Since the secure module  11  has a TRM (tamper resistant module) structure, it is possible to prevent peeping from outside and tampering of internal data. 
     In the secure module  11 , a key supply unit  11   a , a key-supply stop unit  11   b , and an authentication unit  11   c  are implemented. 
     The key supply unit  11   a  supplies a decryption key to the decryption unit  10   a.    
     The authentication unit  11   c  supplies predetermined information to the decryption unit  10   a , refers to information which is returned as a result of the supply of the predetermined information, and checks for authentication of the decryption unit  10   a.    
     When the authentication unit  11   c  does not confirm the authenticity of the decryption unit  10   a , the key-supply stop unit  11   b  stops the supply of the key to the decryption unit  10   a.    
     The MB  13  acquires content decrypted by the decryption unit  10   a  which is loaded in the memory  10 , converts the content into audio and image signals, and supplies the audio and image signals to a speaker and a display device, respectively. 
     Next, operations of the construction illustrated in  FIG. 1  are explained. 
     The secure module  11  is a detachable module. For example, it is possible to attach the secure module  11  to a main body of the information reproducing apparatus when the key supply unit  11   a , the key-supply stop unit  11   b , and the authentication unit  11   c  are installed in the information reproducing apparatus. 
     The decryption unit  10   a  to be loaded in the memory  10  is stored in advance in the secure module  11 , and transferred to the memory  10  on startup. Alternatively, the decryption unit  10   a  is encrypted and stored in the HDD  12  in advance, transferred to the memory  10  on startup, and decrypted in the memory  10  by using a key stored in the secure module  11 . 
     In the above situation, when reproduction is started, first, the authentication unit  11   c  supplies secret information to the decryption unit  10   a . The decryption unit  10   a  acquires the secret information, obtains predetermined information by performing predetermined processing of the secret information, and supplies the predetermined information to the authentication unit  11   c.    
     The authentication unit  11   c  refers to the information returned from the decryption unit  10   a , and checks the returned information for authenticity of the decryption unit  10   a . When the authentication unit  11   c  confirms the authenticity of the decryption unit  10   a , the key supply unit  11   a  supplies a decryption key to the decryption unit  10   a.    
     On the other hand, when the authentication unit  11   c  does not confirm the authenticity of the decryption unit  10   a , the key-supply stop unit  11   b  stops the supply of the key from the key supply unit  11   a  to the decryption unit  10   a.    
     When the authentication unit  11   c  confirms the authenticity of the decryption unit  10   a , the decryption unit  10   a  decrypts the encrypted content  12   a  which is read out from the HDD  12 , by using the key supplied from the key supply unit  11   a , and supplies the decrypted content to the MB  13 . 
     The MB  13  converts audio data and image data contained in the decrypted content (supplied from the decryption unit  10   a ) into a sound signal and an image signal, respectively, and supplies the sound signal and the image signal to the speaker and the display device (which are not shown), respectively. 
     After the reproduction is started, the authentication unit  11   c  in the secure module  11  also supplies secret information to the decryption unit  10   a  at predetermined time intervals, and prevents unauthorized copying and tampering of the decryption unit  10   a.    
     As explained above, in the information reproducing apparatus according to the present invention, the secure module  11 , which cannot be peeped at from outside, is provided, authenticity of the decryption unit  10   a  loaded in the memory  10  is checked for by the operations initiated by the secure module  11 , and a key is supplied when the authenticity is confirmed. Therefore, even in personal computers using an open architecture or similar apparatuses, the security can be improved by adding a minimum amount of hardware. 
     In addition, since the authentication unit  11   c  regularly checks for the authenticity of the decryption unit  10   a , it is possible to prevent unauthorized analysis of the decryption unit  10   a  loaded in the memory  10  and falsification such as disguise (spoofing) or the like. 
     Hereinbelow, details of the embodiments of the present invention are explained. 
       FIG. 2  is a diagram illustrating an example of a construction of the first embodiment of the present invention. As illustrated in  FIG. 2 , the system including the information reproducing apparatus according to the present invention comprises a personal computer (information reproducing apparatus)  50 ′, a network  51 , a speaker  52 , a display device  53 , and an input device  54 . 
     The personal computer  50 ′ comprises a CPU (central processing unit)  50   a , a ROM (read-only memory)  50   b , a RAM (random access memory)  50   c , an HDD (hard disk drive)  50   d , a secure module  50   e , an MB (multimedia board)  50   f , I/Fs (interfaces)  50   g  and  50   h , and a bus  50   i . The personal computer  50 ′ decodes information downloaded through the network  51 , and outputs the decoded information to the speaker  52  and the display device  53 . 
     The CPU  50   a  performs various types of processing and controls the respective portions of the apparatus, in accordance with programs stored in the HDD  50   d.    
     The ROM  50   b  stores programs which are executed by the CPU  50   a , data, and the like. 
     The RAM  50   c  temporarily stores data and programs to be executed by the CPU  50   a  when the CPU  50   a  performs the various types of processing. 
     The HDD  50   d  stores data, the programs executed by the CPU  50   a , and the like. 
     The secure module  50   e  is constituted by, for example, a PC card or the like, and stores information for authentication of users. Since the secure module  50   e  has a TRM structure, it is possible to prevent peeping from outside and tampering of internal data. 
     The MB  50   f  decodes encoded sound data and image data which are supplied from the CPU  50   a , generates an original sound signal and an original image signal, and outputs the generated original audio and image signals to the speaker  52  and the display device  53 . 
     The I/F  50   g  is an interface for transmission and reception of information through the network  51 , and performs protocol conversion and data format conversion. 
     The I/F  50   h  converts data supplied from the input device  54 , into an internal format of the personal computer  50 ′. 
     The bus  50   i  interconnects the CPU  50   a , the ROM  50   b , the RAM  50   c , the HDD  50   d , the secure module  50   e , the MB  50   f , and the I/Fs  50   g  and  50   h , and enables exchange of information between the CPU  50   a , the ROM  50   b , the RAM  50   c , the HDD  50   d , the secure module  50   e , the MB  50   f , and the I/Fs  50   g  and  50   h.    
     The network  51  is constituted by, for example, the Internet, and the personal computer  50 ′ exchanges information with a content server or the like which is connected to the network  51 . 
     The speaker  52  converts a sound signal supplied from the MB  50   f , into sound corresponding to the sound signal, and outputs the sound. 
     The display device  53  is constituted by, for example, a CRT (cathode ray tube) monitor or a liquid crystal monitor, and displays an image corresponding to an image signal supplied from the MB  50   f.    
     The input device  54  is constituted by, for example, a mouse or a keyboard, and generates and outputs information corresponding to user manipulation. 
       FIG. 3  is a diagram illustrating a flow of information in the first embodiment of  FIG. 2 . 
     As illustrated in  FIG. 3 , the HDD  50   d  stores an encrypted decryption-key group, an encryption-software group, an encrypted protocol group, and encrypted content. 
     The encrypted decryption-key group is a group of keys for decrypting encryptions applied to the encrypted content. 
     The encryption-software group is a group of basic software items for decrypting encryptions. 
     The encrypted protocol group is a group of protocols which are inserted into the basic software items for authenticating the basic software items. 
     The encrypted content is content on which encryption processing is performed, and is constituted by, for example, sound data and image data. 
     In the secure module  50   e , a master-key group, transmission-and-return-reception software, and other software exist. 
     The master-key group is a group of encrypted keys for decrypting the encrypted content. 
     The transmission-and-return-reception software exchanges information with reception-and-return software loaded in the RAM  50   c , and checks for authenticity of the reception-and-return software. 
     The other software in the secure module  50   e  includes, for example, software for changing initial values in protocols, encrypted-encryption-key decryption software for decrypting encrypted encryption keys, arbitrary encryption software, arbitrary-encryption-protocol decryption software for decrypting arbitrary encrypted protocols, arbitrary-protocol insertion software for inserting an arbitrary protocol, software-downloading software for downloading software, encrypted-content decryption software for decrypting encrypted content, content-reencryption software for reencrypting content, content-decryption-key transmission software for transmitting a decryption key for decrypting content, and reencrypted-content transmission software for transmitting reencrypted content. 
     The RAM  50   c  stores secure software containing basic software, which includes the reception-and-return software, memory-region change software, content-decryption-key reception software, reencryption-content decryption software, content processing software, and decoded-image-content output software. 
     The reception-and-return software receives transmission from the secure module  50   e , and returns an appropriate response. 
     The memory-region change software receives transmission from the secure module  50   e , and changes memory regions used for storing data which require concealment (e.g., decryption keys, MPEG video/audio content after decryption, information storing regions after expansion of MPEG video/audio content, and the like). 
     The content-decryption-key reception software receives a key for decrypting an encryption applied to content. 
     The reencryption-content decryption software receives a key for decrypting content reencrypted in the secure module  50   e.    
     The content processing software is constituted by MPEG video expansion software, MPEG audio expansion software, and the like. 
     The decoded-image-content output software outputs to the MB  50   f  image content and the like obtained by the content processing software. 
     The MB  50   f  performs D/A conversion processing and the like on sound information, and drawing processing and the like on image information. 
     Next, an outline of the operations in the first embodiment is explained below. 
     The fundamental mechanism for protecting rights in the first embodiment is as follows. That is, secure software is loaded every time by the secure module  50   e  in the RAM  50   c . The loaded software includes a protocol for transmission and reception of secret numbers for confirming security of the secure software and other information. The security of the secure software is confirmed by real-time communication in accordance with the protocol. 
     At this time, in order to prevent copying of the secure software and analysis of the secret numbers for confirming the security and other information, the following measures are taken. 
     (1) A timer is provided in the secure module  50   e , and a measure such as stop of supply of a decryption key from the secure module  50   e  is taken when an interval between a transmitting operation and a receiving operation is very great. 
     (2) In order to prevent copying and disguise of secure software, a measure such as stop of supply of a decryption key from the secure module  50   e  is taken, for example, when an identical secret number or the like is returned twice. 
     (3) Further, the secure software per se is replaced every time, for example, at intervals of about an hour. In order to realize this replacement, a plurality (actually, an infinite number) of secure software items are prepared in the secure module  50   e  or the HDD  50   d , and a currently loaded secure software item is replaced with a different one. That is, although the functions as the secure software is common to all of the plurality of secure software items, the protocol for confirming security and the secret numbers are different between the plurality of secure software items. According to this configuration, even if the secure software is analyzed once, the secure software is replaced every time. Therefore, substantially the analysis of the secure software becomes impossible. Since it is considered basically difficult to analyze and falsify the secure software so as to output a calculation result as a file in a hour, sufficient security is achieved. 
     Alternatively, the secure software can be changed relatively easily, for example, by changing only transmission and reception protocols, or changing only initial values in protocols. 
     For example, transmission and reception functions for confirming security can be realized by using the following program which generates a secret number.
 
 X=X+Y*Z*X  
 
In the above formula, X is a secret number, and Y and Z are initial numbers for generating a secret number. Therefore, an infinite number of security confirmation protocols can be produced by only changing the values of X, Y, and Z arbitrarily.
 
     For example, when the initial values are set as X=1, Y=3, and Z=5, X is changed as follows.
 
 X =1→16→16+16×15→ . . .
 
     This is merely an example. In the case where the secure software is changed in the above simple manner, there is a possibility that the secure software is analyzed through a PCI bus to which the RAM  50   c  is connected even when the initial numbers are changed. Therefore, it is preferable to superimpose another formula on the above formula. Alternatively, when X becomes a certain value, processing such as a forceful conversion of the value of X to another value based on a special table is performed. In this case, even if the basic formula is leaked, the security can be maintained. 
     Further, even if the above formula is revealed, the analysis becomes very difficult by changing the secure software per se, for example, at the intervals of an hour. 
     Details of the operations of the system in the first embodiment are as follows. 
     (1) When a user wishes to view and listen to predetermined content, the user powers up the system, and clicks a predetermined icon displayed on the display device  53  by manipulating the input device  54 . 
     (2) Then, the secure module  50   e  starts working, and randomly reads out one of the group of encrypted basic software items in the HDD  50   d.    
     (3) Next, the encrypted basic software item is decrypted with a decryption key and decryption software in the secure module  50   e.    
     (4) Next, appropriate one or more of the plurality of encrypted protocols are randomly read out, and decrypted with a decryption key and decryption software in the secure module  50   e.    
     (5) The one or more of the plurality of encrypted protocols after the decryption are inserted into the decrypted basic software item so that secure software is produced. 
     (6) Initial values in the one or more of the plurality of encrypted protocols are initialized. 
     (7) The initialized secure software is loaded in (downloaded to) the RAM  50   c , and started up. Then, the secure module  50   e  refers to the timer in the secure module  50   e , and confirms that a secret number is transmitted from the secure software in accordance with a communication protocol without delay in order to prevent disguise of the secure software. 
     (8) The secure module  50   e  transmits a secret number by return. 
     (9) The secure module  50   e  waits for a return of a further secret number. 
     (10) The secure module  50   e  refers to the timer in the secure module  50   e . When it takes a predetermined time or more to return the further secret number, the secure module  50   e  determines that the secure software is falsified, sends an error signal to the secure software, and stops the operation. In addition, when more than one identical secret number is successively returned, the secure module  50   e  may determine that the secure software is falsified, send an error signal to the secure software, and stop the operation. 
     (11) On the other hand, when a normal return is received in the predetermined time, the secure module  50   e  determines that the operation of the secure software is normal. 
     (12) The secure module  50   e  reads out encrypted content from the HDD  50   d , and decrypts the encrypted content with a decryption key. 
     (13) The secure module  50   e  reencrypts the decrypted content. 
     (14) The secure module  50   e  transmits the reecrypted content to the secure software. 
     (15) The secure module  50   e  further confirms the security of the secure software by using another protocol, and then transmits to the secure software a decryption key for decrypting the reecrypted content. 
     (16) The secure module  50   e  repeats the above operations (8) to (15) until reproduction of the content is completed or the user instructs to complete the operation. 
     On the other hand, the secure software executes the following processing. 
     (1) The secure software receives the reencrypted content and the decryption key for decrypting the reencrypted content, from the secure module  50   e  through the transmission and reception of the secret numbers. 
     (2) The secure software decrypts the content by using the received decryption key for decrypting the reencrypted content. 
     (3) The secure software performs simple scrambling (e.g., using an exclusive OR operation) on the result of the decryption, and stores the scrambled result in a secret memory region determined by transmission and reception of the secret numbers and the like. In addition, leakage of information is prevented by changing the secret memory region on every occasion. Further, when registers used by the secure software are also changed on every occasion, it is possible to prevent analysis of internal operations through the registers. Furthermore, it is possible to prevent analysis of internal operations through an interruption by setting an interruption prohibit flag in transmission or reception of a secret number so as to prohibit interruptions from other software. The interruption prohibit flag can be used on every occasion when prevention of analysis is required, including every transmission or reception of a secret number. 
     (4) When the decrypted content is MPEG compressed image data, the secure software performs MPEG video expansion processing of the decrypted content stored in the secret memory region, as explained below. When the decrypted content is MPEG compressed sound data, the secure software performs MPEG sound expansion processing of the decrypted content. When the decrypted content is another type of content such as data broadcast or leader (caption) information, the secure software transfers the decrypted content to a processing routine corresponding to the type of content. 
     (5) For example, when the processed content is MPEG image data, the secure software stores the processing result in a buffer region in the RAM  50   c . At this time, if the processing result is stored in a fixed region in the RAM  50   c , the processing result can be peeped at. Therefore, the storage address is set based on communication with the secure module  50   e , and is changed on every occasion. In addition, the scrambling method may also be set based on communication with the secure module  50   e , and changed on every occasion. 
     (6) The result of the MPEG video decoding stored as above is transferred to the MB  50   f  through an AGP bus or the like by DMA (dynamic memory access) or the like. The MB  50   f  performs A/D conversion processing. Then, the result of the MPEG video decoding is converted in such a format that the result of the MPEG video decoding can be reproduced on a monitor or the like, and displayed. When the decrypted content is MPEG compressed sound data, the decrypted content is processed in a basically similar manner to the case of the MPEG compressed image data, and is finally transferred to the speaker, by which sound is reproduced. 
     As described above, in the first embodiment, the secure module  50   e  having a TRM structure checks for authenticity of the basic software loaded in the RAM  50   c . Therefore, it is possible to improve security of the system having an open architecture. 
     In addition, since the protocol for checking for the authenticity is regularly changed, the security can be ensured even when the basic software is peeped at. 
     Next, the second embodiment is explained. 
       FIG. 4  is a diagram for explaining an example of a construction of the second embodiment of the present invention. As illustrated in  FIG. 4 , the second embodiment of the present invention is characterized in that information stored in the HDD  50   d  is reduced, and instead, right protection information is stored in a memory region in the secure module  50   e.    
     In this example, only encrypted digital AV content and the like are stored in the HDD  50   d.    
     On the other hand, the secure module  50   e  contains (1) a master-key group, which is a group of encrypted keys for decrypting encrypted content, (2) software for changing initial values in a formula for calculating a secret number in each protocol, (3) a software group, (4) a protocol group, (5) software for inserting an arbitrary protocol into secure software, (6) software for downloading into the RAM  50   c  the secure software into which an arbitrary protocol is inserted, (7) software for transmission to the secure software, reception of a return from the secure software, and confirmation of security of the secure software, and (8) software for transmission of a decryption key for decrypting content. In addition, a timer (not shown) is provided in the secure module  50   e , and the secure module  50   e  measures a time from transmission to reception, and stops supply of a decryption key for decrypting the content to the secure software when the time from transmission to reception is equal to or greater than a predetermined time. 
     The RAM  50   c  stores the secure software containing basic software, which includes (1) software for receiving the basic software, (2) reception-and-return software, (3) memory-region change software for changing a memory region, (4) content-decryption-key reception software for receiving a decryption key for decrypting content, (5) encrypted-content decryption software for decrypting encrypted content, (6) content processing software, and (7) decoded-image-content output software. 
     In addition, the secure module  50   e , the HDD  50   d , and the RAM  50   c  are interconnected through a general-purpose PCI bus, and exchange information with each other. The RAM  50   c  and the MB  50   f  are connected through a local bus such as an AGP bus, where the local bus does not allow interception unlike the general-purpose bus. 
     The operations of the system in the second embodiment are as follows. 
     (1) When a user wishes to view and listen to a digital broadcast on a personal computer, the user powers up the system, and clicks an icon displayed on the display device  53  for reception of the digital broadcast. 
     (2) The secure module  50   e  starts working, and randomly reads out one of a plurality of basic software items in the secure module  50   e.    
     (3) The secure module  50   e  randomly reads out appropriate one or more of a plurality of encrypted protocols, and inserts the one or more protocols into the one of the plurality of basic software items so as to produce secure software. 
     (4) The secure module  50   e  initializes initial values in the one or more protocols. 
     (5) The secure module  50   e  loads (downloads) the initialized secure software in the RAM  50   c , and starts up the secure software. 
     (6) The secure module  50   e  refers to the timer in the secure module  50   e . When a predetermined time elapses, the secure module  50   e  confirms that a secret number is transmitted from the secure software in accordance with a communication protocol, where the predetermined time is set to a short time so as to prevent disguise of the secure software. 
     (7) The secure module  50   e  generates and transmits a secret number by return. 
     (8) The secure module  50   e  waits for a return of a secret number. 
     (9) The secure module  50   e  refers to the timer. When it takes a predetermined time or more to receive the return of the secret number, the secure module  50   e  determines that the secure software is possibly falsified, sends an error signal to the secure software, and stops the operation. 
     (10) On the other hand, when a normal return is received in the predetermined time, the secure module  50   e  determines that the operation of the secure software is normal. In addition, the secure module  50   e  further confirms the security of the secure software by using another protocol, and then transmits to the secure software a decryption key for decrypting the encrypted content. 
     (11) Thereafter, the secure module  50   e  repeats the above operations (8) to (10). 
     On the other hand, the secure software on the RAM  50   c  executes the following processing. 
     (1) The secure software receives the encrypted content from the secure module  50   e  through the transmission and reception of the secret numbers. 
     (2) The secure software receives from the secure module  50   e  the decryption key for decrypting the encrypted content. 
     (3) The secure software decrypts the content by using the received decryption key. 
     (4) The secure software performs scrambling on the result of the decryption, and stores the scrambled result in a secret memory region determined by transmission and reception of the secret numbers and the like, where the scrambling is performed by using, for example, an exclusive OR operation. It is preferable that the storage address is not fixed, and is changed on every storage operation. 
     (5) When the decrypted content is MPEG compressed image data, the secure software performs MPEG video expansion processing of the decrypted content stored in the secret memory region as explained below. When the decrypted content is MPEG compressed sound data, the secure software performs MPEG sound expansion processing of the decrypted content. When the decrypted content is another type of content such as data broadcast or leader (caption) information, the secure software transfers the decrypted content to a processing routine corresponding to the type of content. 
     (6) For example, when the processed content is MPEG image data, the secure software transfers the processing result to a buffer region in the RAM  50   c . At this time, if the processing result is stored in a fixed region in the RAM  50   c , the processing result can be peeped at. Therefore, the storage address is set based on communication with the secure module  50   e , and is changed on every occasion. In addition, the scrambling method may also be set based on communication with the secure module  50   e  on every storage operation. 
     (7) The result of the MPEG video decoding stored as above is transferred to the MB  50   f  through an AGP bus or the like by DMA or the like. The MB  50   f  performs A/D conversion processing. Then, the result of the MPEG video decoding is converted in such a format which enables reproduction on the display device  53 . 
     (8) When the decrypted content is MPEG compressed sound data, the decrypted content is processed in a basically similar manner to the case of the MPEG compressed image data, and is finally transferred to the speaker, by which sound is reproduced. 
     As described above, in the second embodiment, the information on the master-key group and the like is also stored in the secure module  50   e , while such information is stored in the HDD  50   d  in the first embodiment. Therefore, the security of the system is improved compared with the first embodiment. 
     Next, the third embodiment is explained. 
       FIG. 5  is a diagram illustrating a flow of information in the third embodiment of the present invention. As illustrated in  FIG. 5 , the third embodiment is different from the first embodiment illustrated in  FIG. 3  in that scrambling processing software is additionally stored in the RAM  50   c , and descrambling processing software is additionally stored in the MB  50   f . Since the other portions in the third embodiment are identical to the first embodiment illustrated in  FIG. 3 , the explanations on the identical portions are not repeated. 
     The scrambling processing software performs predetermined scrambling on content on which MPEG decode processing or the like is performed. 
     The descrambling processing software performs predetermined descrambling on scrambled content in the RAM  50   c , and reproduces original information. 
     Next, the operations of the third embodiment are briefly explained. 
     After transmission and reception of a key is performed between the HDD  50   d  and the secure module  50   e  in a similar manner to the aforementioned case, basic software is downloaded to and installed in the RAM  50   c.    
     The basic software downloaded to the RAM  50   c  receives encrypted content through the secure module  50   e , and decrypts the encryption. Then, the scrambling processing software performs the predetermined scrambling on the decrypted content, and transmits the scrambled content to the MB  50   f.    
     The MB  50   f  receives the scrambled content transmitted from the RAM  50   c , and corresponding descrambling software performs descrambling on the scrambled content so as to reproduce original content data. 
     In the case where the obtained content data is a sound signal, the sound signal is converted into an analog signal by D/A conversion processing, and the analog signal is output to the speaker  52 . In addition, in the case where the obtained content data is an image signal, image drawing processing is performed so as to convert the image data into a video signal, which is output to the display device  53 . 
     In the third embodiment described above, it is possible to prevent unauthorized output of the content data which is supplied from RAM  50   c  to the MB  50   f.    
     Next, the fourth embodiment is explained. 
       FIG. 6  is a diagram illustrating an example of a construction of the fourth embodiment of the present invention. The fourth embodiment illustrated in  FIG. 6  is different from the third embodiment illustrated in  FIG. 5  in that the secure module  50   e  further contains communication-protocol transmission software, and the MB  50   f  further contains communication-protocol reception software. The other portions in the fourth embodiment are identical to the third embodiment illustrated in  FIG. 5 . 
     The communication-protocol transmission software stored in the RAM  50   c  sends to the MB  50   f  information (communication-protocol information) indicating a scrambling method for content data and an order of output of content data. 
     The communication-protocol reception software stored in the MB  50   f  receives the communication-protocol information transmitted by the communication-protocol transmission software, and supplies the communication-protocol information to applicable portions of the apparatus. 
     Next, the operations of the fourth embodiment are briefly explained. 
     When reproduction of content data is started, the communication-protocol transmission software in the secure module  50   e  sends the communication-protocol information to the MB  50   f.    
     When the MB  50   f  receives the communication-protocol information, the MB  50   f  extracts descrambling processing software and information indicating the order of output of content data, from the communication-protocol information. Then, the MB  50   f  rearranges the order of the content data in accordance with the information extracted from the communication-protocol information, and performs descrambling processing of the rearranged content data. 
     The communication-protocol information is transmitted, for example, at predetermined time intervals. The descrambling method and the order of output are changed on every transmission. Therefore, even if the scrambling processing software in the RAM  50   c  is analyzed by a malicious user, different scrambling processing is performed at the next transmission timing. Thus, it is possible to prevent unauthorized use. 
     Next, the fifth embodiment is explained. 
       FIG. 7  is a diagram illustrating an example of a construction of the fifth embodiment of the present invention. As illustrated in  FIG. 7 , the fifth embodiment of the present invention is an example of a construction of a digital broadcast receiver. 
     The digital broadcast receiver  70  illustrated in  FIG. 7  comprises a digital tuner  70   a , a MULTI2 decryption unit  70   b , a B-CAS (BS conditional access systems) card  70   c , a license generation unit  70   d , a reencryption processing unit  70   e , an HDD  70   f , an MPEG decoder  70   g , an encryption decoder  70   h , a graphic processing unit  70   i , a reproduction licensing unit  70   j , an analog copyright-protection unit  70   k , and a digital copyright-protection unit  70   l . In addition, a parabola antenna  71  is externally connected to the digital broadcast receiver  70 . 
     The digital tuner  70   a  converts electromagnetic waves transmitted from a satellite and captured by the parabola antenna  71 , into electric signals (digital signals), and outputs the electric signals (digital signals). 
     The MULTI2 decryption unit  70   b  decrypts MULTI2 encryptions applied to encrypted digital signals which are output from the digital tuner  70   a , decodes the decrypted digital signals into original data, and outputs the original data. 
     The B-CAS card  70   c  is a plastic card in which an IC chip is built, and stores information for authentication of a user. 
     The license generation unit  70   d  generates license information required for reencryption, and supplies the license information to the reencryption processing unit  70   e.    
     The reencryption processing unit  70   e  reencrypts the digital signals supplied from the MULTI2 decryption unit  70   b , and supplies the reencrypted digital signals to the HDD  70   f.    
     The HDD  70   f  stores in a predetermined region the data supplied from the reencryption processing unit  70   e.    
     The encryption decoder  70   h  decodes encryptions applied to data which are read out from the HDD  70   f , and generates original data. 
     The MPEG decoder  70   g  performs MPEG decode processing of the data supplied from the encryption decoder  70   h , and generates image data and sound data. 
     The graphic processing unit  70   i  performs drawing processing based on the image data output from the MPEG decoder  70   g  and other image data, converts the image data into original image signals, and outputs the original image signals. 
     The reproduction licensing unit  70   j  supplies license information for decoding the encryptions by the encryption decoder  70   h.    
     The analog copyright-protection unit  70   k  superimposes a signal for copy protection on analog image signals, and outputs the superimposed image signals. 
     The digital copyright-protection unit  70   l  encrypts digital image signals, and outputs the encrypted digital image signals. 
     In the construction of  FIG. 7 , the functions indicated with bold lines (i.e., the license generation unit  70   d , the reencryption processing unit  70   e , and the reproduction licensing unit  70   j ) are each a TRM (tamper resistant module). That is, each of the license generation unit  70   d , the reencryption processing unit  70   e , and the reproduction licensing unit  70   j  has a structure which does not allow access to internal information. 
     In addition, the dashed lines connecting function blocks in  FIG. 7  indicate connections which are forbidden to be connected to outside for the purpose of copy protection. 
     Next, the operations of the fifth embodiment are explained. 
     The electromagnetic waves transmitted from the satellite and captured by the parabola antenna  71  are supplied to the digital tuner  70   a , converted into an MPEG-TS digital stream by the digital tuner  70   a , and output from the digital tuner  70   a.    
     In the MPEG-TS digital stream, a plurality of information items are MULTI2-encrypted and time-division multiplexed. The MULTI2 decryption unit  70   b  refers to license information including a decryption key and the like and being supplied from the B-CAS card  70   c , and decodes a program (specific digital AV content) in the MPEG-TS digital stream which is selected by a viewer. 
     The reencryption processing unit  70   e  refers to the license information supplied from the license generation unit  70   d , reencrypts the AV content, and stores the reencrypts AV content in a predetermined region of the HDD  70   f.    
     In order to reproduce encrypted digital AV content stored in the HDD  70   f , the encryption decoder  70   h  acquires from the reproduction licensing unit  70   j  license information for decoding the encryption, performs processing for decoding the encryption, and generates original AV content. 
     The MPEG decoder  70   g  performs MPEG decode processing of the AV content decoded by the encryption decoder  70   h  so as to reproduce original image data, and supplies the original image data to the graphic processing unit  70   i.    
     The graphic processing unit  70   i  performs graphic processing such as drawing processing based on the image data supplied from the MPEG decoder  70   g , generates an image signal, and supplies the image signal to the analog copyright-protection unit  70   k  and the digital copyright-protection unit  70   l.    
     The analog copyright-protection unit  70   k  superimposes a signal for copy protection on a predetermined portion of the image signal supplied from the graphic processing unit  70   i , and outputs the superimposed image signal to a monitor. 
     The digital copyright-protection unit  70   l  encrypts the image signal supplied from the graphic processing unit  70   i , and outputs the encrypted digital image signal to the monitor. 
     As mentioned before, the functions indicated with bold lines (i.e., the license generation unit  70   d , the reencryption processing unit  70   e , and the reproduction licensing unit  70   j ) in  FIG. 7  each have a TRM structure. Therefore, it is possible to prevent unauthorized use of received content by a malicious user by providing functions corresponding to the key supply unit  11   a , the key-supply stop unit  11   b , and the authentication unit  11   c  as illustrated in  FIG. 1  in the license generation unit  70   d , the reencryption processing unit  70   e , and the reproduction licensing unit  70   j , checking authenticity at predetermined time intervals, and stopping supply of a key when the authenticity cannot be confirmed. 
     Next, the sixth embodiment is explained. 
       FIG. 8  is a diagram illustrating an example of a construction of the sixth embodiment of the present invention. As illustrated in  FIG. 8 , the digital broadcast receiver  80  illustrated in  FIG. 8  comprises a digital tuner  80   a , a MULTI2 decryption unit  80   b , a B-CAS card  80   c , a reencryption-and-license-generation unit  80   d , an HDD  80   e , a software-decryption-and-reproduction-licensing unit  80   f , a software MPEG decoder  80   g , a software AAC (advanced audio coding) decoder  80   h , a graphic processing unit  80   i , a sound processing unit  80   j , and an HDCP (high-bandwidth digital content protection) LSI  80   k . In addition, a parabola antenna  81  is externally connected to the digital broadcast receiver  80 . 
     The digital tuner  80   a  converts electromagnetic waves transmitted from a satellite and captured by the parabola antenna  81 , into electric signals (digital signals), and outputs the electric signals (digital signals). 
     The MULTI2 decryption unit  80   b  decrypts MULTI2 encryptions applied to encrypted digital signals which are output from the digital tuner  80   a , decodes the decrypted digital signals into original data, and outputs the original data. 
     The B-CAS card  80   c  is a plastic card in which an IC chip is built, and stores information for authentication of a user. 
     The reencryption-and-license-generation unit  80   d  generates license information required for reencryption, and performs reencryption processing. 
     The HDD  80   e  stores in a predetermined region the data supplied from the reencryption-and-license-generation unit  80   d.    
     The software-decryption-and-reproduction-licensing unit  80   f  generates a reproduction license, decodes encryptions applied to data which are read out from the HDD  80   e , in accordance with the reproduction license, and generates original data. 
     The software MPEG decoder  80   g  performs MPEG decode processing of AV content supplied from the software-decryption-and-reproduction-licensing unit  80   f , and generates image data. 
     The software AAC decoder  80   h  decodes sound signals, and generates and outputs original sound data. 
     The graphic processing unit  80   i  performs drawing processing of the image data supplied from the software MPEG decoder  80   g , and outputs obtained image signals to the HDCP LSI  80   k.    
     The HDCP LSI  80   k  performs HDCP processing of the image signals supplied from the graphic processing unit  80   i , and outputs the processed image signals. 
     The sound processing unit  80   j  performs D/A conversion of the sound data output from the software AAC decoder  80   h , and outputs the converted result. 
     In the construction of  FIG. 8 , the functions indicated with bold lines (i.e., the B-CAS card  80   c , the reencryption-and-license-generation unit  80   d , and the software-decryption-and-reproduction-licensing unit  80   f ) are each a TRM (tamper resistant module). That is, each of the B-CAS card  80   c , the reencryption-and-license-generation unit  80   d , and the software-decryption-and-reproduction-licensing unit  80   f  has a structure which does not allow access to internal information. 
     In addition, the software-decryption-and-reproduction-licensing unit  80   f , the software MPEG decoder  80   g , and the software AAC decoder  80   h  are realized by software. 
     Therefore, as in the fifth embodiment, it is possible to prevent unauthorized use of received content by a malicious user by providing the functions of the secure module  11  illustrated in  FIG. 1  in the B-CAS card  80   c , the reencryption-and-license-generation unit  80   d , and the software-decryption-and-reproduction-licensing unit  80   f , and arranging the function blocks realized by the software in  FIG. 8  to have the functions in the memory  10  illustrated in  FIG. 1 . 
     Next, the seventh embodiment is explained. 
       FIG. 9  is a diagram illustrating an example of a construction of the seventh embodiment of the present invention. As illustrated in  FIG. 9 , the digital broadcast receiver  90  illustrated in  FIG. 9  comprises a digital tuner  90   a , a MULTI2 decryption unit  90   b , a B-CAS card  90   c , a reencryption-and-license-generation unit  90   d , an HDD  90   e , a software-decryption-and-reproduction-licensing unit  90   f , a software MPEG decoder  90   g , a software AAC decoder  90   h , a graphic processing unit  90   i , a sound processing unit  90   j , and an HDCP LSI  90   k . In addition, a parabola antenna  91  is externally connected to the digital broadcast receiver  90 . Further, in the seventh embodiment, the MULTI2 decryption unit  90   b , the reencryption-and-license-generation unit  90   d , the software-decryption-and-reproduction-licensing unit  90   f , the software MPEG decoder  90   g , the software AAC decoder  90   h , the graphic processing unit  90   i , and the sound processing unit  90   j  are sealed into an LSI. 
     The digital tuner  90   a  converts electromagnetic waves transmitted from a satellite and captured by the parabola antenna  91 , into electric signals (digital signals), and outputs the electric signals (digital signals). 
     The MULTI2 decryption unit  90   b  decrypts MULTI2 encryptions applied to encrypted digital signals which are output from the digital tuner  90   a , decodes the decrypted digital signals into original data, and outputs the original data. 
     The B-CAS card  90   c  is a plastic card in which an IC chip is built, and stores information for authentication of a user. 
     The reencryption-and-license-generation unit  90   d  generates license information required for reencryption, and performs reencryption processing. 
     The HDD  90   e  stores in a predetermined region the data supplied from the reencryption-and-license-generation unit  90   d.    
     The software-decryption-and-reproduction-licensing unit  90   f  generates a reproduction license, decodes encryptions applied to data which are read out from the HDD  90   e , in accordance with the reproduction license, and generates original data. 
     The software MPEG decoder  90   g  performs MPEG decode processing of AV content supplied from the software-decryption-and-reproduction-licensing unit  90   f , and generates image data. 
     The software AAC decoder  90   h  decodes sound signals, and generates and outputs original sound data. 
     The graphic processing unit  90   i  performs drawing processing of the image data supplied from the software MPEG decoder  90   g , and outputs obtained image signals to the HDCP LSI  90   k.    
     The HDCP LSI  90   k  performs HDCP processing of the image signals supplied from the graphic processing unit  90   i , and outputs the processed image signals. 
     The sound processing unit  90   j  performs D/A conversion of the sound data output from the software AAC decoder  90   h , and outputs the converted result. 
     In the construction of  FIG. 9 , the software-decryption-and-reproduction-licensing unit  90   f , the software MPEG decoder  90   g , and the software AAC decoder  90   h  are realized by software. 
     Therefore, as in the fifth and sixth embodiments, it is possible to prevent unauthorized use of received content by a malicious user by providing the functions of the secure module  11  illustrated in  FIG. 1  in the function blocks constituted by an LSI, and arranging the function blocks realized by the software in  FIG. 9  to have the functions in the memory  10  illustrated in  FIG. 1 . 
     In the seventh embodiment, the function blocks realized by the software are also sealed into the LSI. Therefore, the security can be further improved. 
     The above embodiments indicated by the block diagrams are merely examples, and the present invention is not limited to the above embodiments. 
     The function blocks which are realized by software in the above embodiments may be realized by hardware as appropriate. 
     As explained above, according to the present invention, an information reproducing apparatus for reproducing information which is transmitted through a transmission medium or stored in a recording medium is provided, and the information reproducing apparatus comprises: a secure module storing first information and having a structure which does not allow access to the first information from outside; a memory which can be accessed from outside; a decryption unit which is loaded in the memory, and decrypts an encryption applied to the first information by using a predetermined key; a key supply unit which is implemented in the secure module, and supplies the predetermined key to the decryption unit; an authentication unit which is implemented in the secure module, supplies second information to the decryption unit, refers to third information returned in response to the second information, and checks for authenticity of the decryption unit; and a key-supply stop unit which is implemented in the secure module, and stops supply of the predetermined key by the key supply unit when the authentication unit does not authenticate the decryption unit. Therefore, in the case where information is reproduced by an apparatus having an open architecture such as a personal computer, it is possible to construct a safe system by adding a minimum amount of hardware to the apparatus. 
     In addition, according to the present invention, a secure module is provided. The secure module can be detachably mounted on an information reproducing apparatus and executes processing related to security when the information reproducing apparatus reproduces information which is transmitted through a transmission medium or stored in a recording medium, where the information reproducing apparatus includes a memory which can be accessed from outside, and decryption unit which is loaded in the memory, and decrypts an encryption applied to the first information by using a predetermined key. Further, the secure module comprises: a key supply unit which is implemented in the secure module, and supplies the predetermined key to the decryption unit; an authentication unit which is implemented in the secure module, supplies second information to the decryption unit, refers to third information returned in response to the second information, and checks for authenticity of the decryption unit; and a key-supply stop unit which is implemented in the secure module, and stops supply of the predetermined key by the key supply unit when the authentication unit does not authenticate the decryption unit. Therefore, when the above secure module is mounted on the information reproducing apparatus, the secure module can improve the security of the system with a minimum amount of hardware. 
     The foregoing is considered as illustrative only of the principle of the present invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and applications shown and described, and accordingly, all suitable modifications and equivalents may be regarded as falling within the scope of the invention in the appended claims and their equivalents.