JP2003143126A - Security maintaining method, its execution system and its processing process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide technique capable of detecting and excluding completely pretending equipment. SOLUTION: A plurality of sets of equipment obtained by connecting center equipment and home equipment is set with respect to the single equipment in a network security maintaining method. The method has a step for setting an access number and a ciphering key different by each set of the equipments obtained by connecting the center equipment and the home equipment, a step of updating the access number by a prescribed sequence every ciphering processing and deciphering processing using the ciphering key, which are performed when transmitting or receiving data in the set of the equipments obtained by connecting the center equipment and the home equipment, and a step of giving notice of the access number updated in the set of the equipments obtained by connecting the center equipment and the home equipment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an authentication system for devices connected to a network, and more particularly to preventing intrusion into the network by spoofing in a system in which a plurality of devices are connected to the network such as a cable television. It relates to technology that is effective when applied.

[0002]

2. Description of the Related Art A conventional device authentication system is disclosed in Japanese Patent Application No. 2001-62962 or Japan CA.
TV Technical Council Standard JCTEA STA-005
CATV high-speed data transmission equipment of BP-1.0 As shown in privacy, the unique numerical value (Mac: Media access control address etc.) of the equipment and the key distributed to the equipment (common key, baseline in the above patent) In privacy, device authentication is performed based on the public and private keys. In such a system, if the device-specific numerical value and the key are stolen, it is possible to easily spoof. If it is merely spoofed and you can use the network for free, it will not cause much damage, but if a device connected to the network is hacked, even if serious damage occurs, You may not be able to identify intruders.

Further, in order to obtain the numerical value and key unique to the apparatus, there is a case where an unauthorized intrusion, a physical theft, or a case where a legitimate user hands a copy or the like to an unauthorized person regardless of whether there is malicious intent.

Note that device authentication is usually performed automatically without human intervention. Therefore, biometrics such as passwords and fingerprints used for personal authentication cannot be used.

[0005]

The present inventor has found the following problems as a result of examining the above-mentioned prior art.

As described above, the conventional device authentication system does not have a mechanism for detecting a unique value of a device and a duplication of a key. Therefore, there is a problem that a spoofed device cannot be detected or excluded. .

It is an object of the present invention to provide a technique capable of detecting and eliminating a spoofing device.

The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

[0009]

Among the inventions disclosed in the present application, a brief description will be given to the outline of typical ones.
It is as follows.

In a network security system in which a plurality of sets of devices connecting a center device and a home device are set for a single center device, a set of devices connecting the center device and the home device. Setting means for setting a different access number and encryption key for each, and encryption processing using the encryption key each time data is transmitted or received within a set of devices connecting the center device and the home device And a means for performing a decryption process, an updating means for updating the access number in a predetermined procedure for each of the encryption process and the decryption process, and the update in a set of devices connecting the center device and the home device. And a notification means for notifying the access number.

According to the above-mentioned means, first, the setting means sets a different access number and encryption key for each set of devices connecting the center device and the home device. Here, the encryption processing and the decryption processing means perform the encryption processing and the decryption processing using the encryption key each time the data is transmitted or received within the set of devices connecting the center device and the home device. The updating means updates the access number in a predetermined procedure for each of the encryption processing and the decryption processing, and the notifying means notifies the updated access number in the set of devices connecting the center device and the home device. Therefore, when the spoofing terminal exists on the network, the access number of the center apparatus changes every time data is transmitted and received to and from both the legitimate home apparatus and the spoofing terminal.

On the other hand, since the access numbers of the legitimate home device and the spoofing terminal change only when data is transmitted and received between each device and the center device, the notified access number And the access number updated in the device have different values, and spoofing on the network can be detected. At this time, the device that accesses the center device with the same encryption key is a device group including the spoofed terminal. Therefore, by setting a new encryption key and access number for the legitimate home device set, the spoofed terminal is set. Can be excluded from the network. As a result, the security of the network is further improved.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to the drawings together with an embodiment (example) of the invention.

In all the drawings for explaining the embodiments of the invention, components having the same function are designated by the same reference numeral, and the repeated description thereof will be omitted.

FIG. 1 is a diagram for explaining a schematic configuration of a security system according to an embodiment of the present invention.
In particular, it shows a case where it is applied to a cable television network in which a center station and subscribers are connected by a tree-like network. Further, in the following description, the case where the number of subscriber devices 110 and 120 connected to one center station 100 is two will be described, but the number of subscriber devices 110 and 120 is not limited to two, and two or more subscriber devices 110 and 120 are not limited thereto. Needless to say,

In FIG. 1, 100 is a center station (center device), 101 is a headend modem, 102 is an encryption / decryption device, 103 is a key management device, 104 is an authentication device, 105 is a master key table, and 110 is a first key. 1 subscriber device (first home device), 111 a cable modem, 112 encryption / decryption device, 113 encryption key storage means,
114 is a key manager, 115 is a key access number table, 116 is a personal computer (PC), 12
0 indicates a second subscriber device (second home device), and 130 indicates a network. However, in the present embodiment, as in the conventional cable television network, the center station 100 is configured to be connected to other center stations or information processing devices (not shown) via a dedicated line or internet line not shown. .

As shown in FIG. 1, the center station 100 is
Headend modem 101 and encryption / decryption device 102
A key management device 103, an authentication device 104, and a master key table 105.

The headend modem 101 is means for modulating / demodulating an encrypted frame and transmitting / receiving it to / from each of the subscriber devices 110 and 120 via a network 130 such as a cable television network.

The encryption / decryption device 102 is the center station 1
00, the Mac address of the destination subscriber device 110, 120 is set to the destination address of the frame transmitted from 00, and the Mac address of the center station 100 is set to the source address, and the frame is set to the key management device 1
It is means for encrypting with the work key Kw corresponding to the subscriber devices 110 and 120 from 03. In addition, the encryption / decryption device 102 first transmits the encrypted frame from the first subscriber device 110, which is the sender of the encrypted frame transmitted from the subscriber devices 110 and 120 to the center station 100.
It is means for decrypting with the work key Kw corresponding to the subscriber device 110 of. However, as will be described later, at the time of authentication, encryption and decryption are performed using the master key obtained by the subscriber devices 110 and 120. A plurality of keys may be used for both encryption and decryption during authentication.

Based on the request from the authentication device 104, the key management device 103 stores information about the work key such as the work key Kw, the applicable period of the work key Kw, and the current time, and the subscriber devices 110 and 120. Is a means for generating a random number as an access number of the master key stored in the encryption key storage means (key (A)) 113 of the master key table 105.
Is a means of storing as. Also, the key management device 103
Is a means for monitoring access to the master key (master key table 105) and sequentially incrementing the key access number corresponding to the master key that has been accessed for each access.

The authentication device 104 includes the subscriber devices 110,
It is verified whether the destination address in the encrypted frame transmitted from 120 to the center station 100 is equal to the address of the center station 100, and the correct subscriber device 110, 1
It is a means to confirm that the communication is with 20. Also,
As will be described later, the authentication device 104 generates an authentication request each time a new subscriber device is connected and a preset period (for example, a valid period of a work key) or a monitoring device (not shown) detects an abnormality. Based on the authentication request, the center station 100 makes an authentication request to the subscriber device for which the authentication request is generated. However, the detailed operation at the time of the authentication request will be described later.

However, the center station 100 is replaced by the headend modem 101, the encryption / decryption device 102, and the key management device 1.
03 and the program for functioning as the authentication device 104 are recorded in a recording medium such as a CD-ROM and stored in a magnetic disk or the like, and then loaded into a memory and executed. The recording medium for recording this program may be a recording medium other than the CD-ROM. Further, this program may be installed in an information processing apparatus from a recording medium and used, or the recording medium may be accessed through a network to use this program.

Each subscriber device 110, 120 comprises a cable modem 111, an encryption / decryption device 112, an encryption key storage means 113, a key manager 114, a key access number table 115, and a personal computer 116. Has become. However, each subscriber device 110,
Each of the 120 is connected to the center station 100 via a network 130 of a cable television network formed in a tree shape. In particular, it goes without saying that the personal computer 116 does not have to be arranged in every subscriber device. That is, the center station 10
When the digital image (digital broadcast) is distributed to the subscriber device with 0 as the head end, and the digital image distributed by the subscriber device is reproduced and viewed, the present invention is applicable to a personal computer. Computer 116
Is unnecessary.

The cable modem 111 is means for modulating / demodulating an encrypted frame and transmitting / receiving it to / from the center station 100 via a network 130 such as a cable television network.

The encryption / decryption unit 112 is used by the subscriber unit 110.
To the destination address of the frame transmitted from the center station 1
00 is set, the Mac address of the subscriber device 110 is set as the sender address, and the frame is encrypted with the work key Kw of the subscriber device 110. Further, the encryption / decryption device 112 has a work key Kw of the subscriber device 110 which is a destination of the encrypted frame transmitted from the center station 100 to the subscriber device 110.
Is a means for decrypting the encrypted frame. However, as will be described later, at the time of authentication, encryption and decryption are performed using the master key obtained by each of the subscriber devices 110 and 120. A plurality of keys may be used for both encryption and decryption during authentication.

The encryption key storage means 113 is, for example, a well-known I
It is a means consisting of a C card, etc., and is issued by the operator of the cable TV. However, the encryption key storage unit 113 is not limited to the IC card and may be another storage medium.

The key manager 114 is used by the encryption / decryption device 1
This is a means for monitoring the access to the encryption key storage means 113 by 12, ie, the access to the master key, and successively incrementing the key access number stored in the key access number table 115 for each access to the master key. Further, the key manager 114 monitors access to the encryption key storage means 113, that is, access to the master key,
In order to prevent the master key from being easily read from the outside (such as another device connected via the center station 100), it is a means of denying access from other devices.

The subscriber devices 110 and 120 have means (not shown) for acquiring the work key Kw for encrypting and decrypting the frame including the destination Mac address and the source Mac address from the center station 100 and storing the work key Kw. Have. Further, the subscriber devices 110 and 120 are provided with conversion means of a well-known interface for converting data from the well-known personal computer 116 to the encryption / decryption 112 or data from the encryption / decryption 112 to the personal computer 116. .

The subscriber unit 110 is connected to the cable modem 11
The program for functioning as 1, the encryption / decryption unit 112, the key manager 114, and the like is recorded in a recording medium such as a ROM and executed. The recording medium for recording this program may be a recording medium other than the ROM. Further, the program may be installed in the information processing apparatus from the recording medium and used, or the recording medium may be accessed through the network to use the program.

As described above, in the security system of this embodiment, the center station 100, the subscriber device 110,
When transmitting and receiving a frame including data to and from 120, a frame including a destination Mac address and a source Mac address is encrypted and decrypted using the work key Kw, thereby improving network security. ing. It should be noted that the transmission and reception of this frame (cipher frame) is performed by the same applicant as Japanese Patent Application No.
The operation is similar to that of 62962.

Further, in the present embodiment, the authentication operation is repeatedly executed every predetermined period, and the number of accesses to the master key used in this authentication operation is determined by the center station 1.
00 and the subscriber devices 110 and 120 are synchronized with each other, and the number of accesses is compared to prevent fraud such as spoofing.

FIG. 2 is a diagram for explaining the operation of the security system of this embodiment, and FIG.
The operation related to the authentication will be described based on.

A user (subscriber) who decides to join the cable television network needs to obtain a cable modem and a key (eg, a master key). Here, the master key and the cable modem can be used only after being registered in the center station of the cable operator. However, although it is possible to register with a public key, in a system such as the cable television network of the present embodiment, which is applied to the limited use of subscribers in a specific area such as one area, the common key is used. It can be easily configured by using. Therefore, in the following description, an initial registration procedure when a common key is used as an encryption key and an authentication procedure executed at predetermined time intervals will be described.

First, an example of the procedure of initial registration is as follows. However, the following description will be given for the case of the subscriber device 110.

1. Cable modem 1 of subscriber device 110
11 is connected to the network 130.

2. The master key stored in the cable modem 111 and the encryption key storage means 113 is used as the center station 100.
Register with. Here, to register the cable modem 111, a Mac address may be used as an ID unique to the cable modem. However, the registration method is the following i ~ iv
There are various possibilities as shown in. In addition, as a method of mounting the master key, there are a case where it is mounted on an IC card including a master key and encryption / decryption processing (this method is used in BS broadcasting), and a case where all are mounted on the cable modem 111. There is. When mounted on the cable modem 111, the master key needs to be stored in a non-volatile memory so that the master key can be rewritten.

If the cable operator delivers the subscriber device 110 including the already registered cable modem 111 and the master key to the subscriber, the registration is completed.

Ii. When an unregistered IC card key is used as the encryption key storage means 113 in the subscriber device 110 equipped with the registered cable modem 111, the IC card is sent to the cable operator for registration of the master key. Bring in and re-register, or obtain a new IC card that has already been registered from the cable company.

Iii, the cable modem 1 obtained by the subscriber
In the case of the subscriber device 110 (unregistered) including 11 and the master key (IC card) registered with the cable operator,
The cable modem 111 is registered. Cable modem 1
The Mac address of 11 can also be read by the center station 100 by connecting the cable modem 111 to the network 130. It is sufficient to associate this Mac address with the registered master key. In this case, the Mac address may be transmitted to the network as raw data and may be known by an eavesdropper. Therefore, it is possible to register by another method such as telephone.

Iv, if the subscriber device 110 having the cable modem 111 and the master key are both unregistered, the key integrated cable modem records the registration key. The recording of the registration key may be carried out by bringing the cable modem 111 to the cable operator or bringing the key registration device to the subscriber's house. The key registration device may be connected to the PC 116 of the subscriber's house for recording. When the master key is mounted on the IC card which is the encryption key storage means 113, the IC card is brought to the cable operator or a new IC card is obtained from the cable operator for registration of the master key. . When the key registration is completed, the cable modem 111 is registered according to the procedure of iii.

3. Work key Kw for encrypting communication data
To set. The work key Kw is not limited to one, but as described above, in the present embodiment, one case will be described in order to simplify the description. However, the system can be realized by using the master key itself which is the registered key, but if one key is used, the damage will be serious. Therefore, it is common practice to use multiple keys.

After the registration of the cable modem 111 and the master key (including the center station 100), the physical layer of the headend modem 101 and the cable modem 111 is established. This operation is performed by, for example, the Japan Catv Technology Association Standard JCTEA STD-005-1.0 "C
ATV High Speed Data Transmission Equipment Asymmetric Cable Modem "
It can be realized by using the method described in.

After that, the Mac layer is set.
The operation in this Mac layer is as follows. However, the operation in the Mac layer can be triggered from the center station 100, but in the following description, the cable modem 111 on the subscriber side is used to operate the center station 1.
The case of applying a trigger to 00 will be described.

A registration confirmation request is output from the cable modem 111. M of the cable modem 111 is used for the registration confirmation request.
It may include an ac address, a service ID request for communication permission in the security mode (encryption mode), and a password for certifying that the subscriber is legitimate. These pieces of information (registration request information) are stored in the network 1
It is transmitted to the headend modem 101 via 30.

In the Mac layer, after the preamble signal, the destination Mac address (DA: Destinati
on Address), sender Mac address (S
A: Source Address) is attached and data is sent. However, at this point, the headend modem 101
Since the Mac address (DA) is unknown, the subscriber device 110 may send it as a broadcast (all 1s) or as a predetermined address.

When the center station 100 identifies this address, it can recognize that it is in the initial registration / re-registration mode. Since the cable modem 111 sends the registration request information to the headend modem 101, which is always in a pair, all the cable modems 11 are connected.
When sending from 1, the destination Mac address may always be the broadcast address (1). At this time, in the present embodiment, these data (frames) including DA and SA are sent from the cable modem 111 after being encrypted by the encryption / decryption device 112 with the master key. The data (frame) sent in this manner is detected by the subscriber station cable modem on the side of the center station 100.
Encrypts DA and SA with the master key in advance, and when this data is sent, identifies that it is from the cable modem 111 of the subscriber unit 110. However, DA and SA are 48-bit data. Therefore, for the 48-bit raw data, an encryption mode is used that outputs 48-bit encrypted data.

Next, in the center station 100, the work key K
In addition to w, the applicable period of the work key Kw, information about the work key such as the current time, a random number is generated as the access number of the master key. However, in FIG. 1, for example, 73864ah3 is generated as a 4-byte numerical value. These pieces of information are generated by the key management device 103 and stored in the master key table 105.
Further, the encryption / decryption device 102 creates Mac data as data for these pieces of information, and after encrypting them with a master key including DA and SA, the headend modem 10
1 is transmitted to the downlink. The cable modem 111 and the encryption / complexer 112 decrypt all Mac data, or encrypt the Mac address of the cable modem 111 with a master key in advance, and detect a DA that matches the encrypted data. , The cable modem 1
It is detected as the information addressed to 11.

Here, the information taken in by the cable modem 111 is decrypted by the encryption / decryption device 112 and output to the key manager 114. The key manager 114 stores the work key Kw, the applicable period of the work key Kw, information about the work key such as the current time, and the key access number which is a random number as the access number of the master key in the key access number table 115. To do.

In this way, the same work key Kw and key access number are set in the center station 100 and the subscriber device 110 at the subscriber's house. Thus, the initial registration operation shown in step 201 is completed. However, as described above, the key access number is sequentially changed because the key manager 114 increments or decrements the numerical value by a certain method each time the master key is accessed.

Immediately after the registration, the device authentication phase shown in step 202 is entered. Although the device authentication is performed once in the initial registration, the device authentication in step 202 includes the following operations including confirmation of whether the normal authentication operation is normally performed.

1. Device authentication request The cable modem 111 sends out a device authentication request encrypted with the work key Kw. From the subscriber's point of view, it is a request for making the work key Kw usable. The center station 100 side prohibits normal use of the work key Kw until the key management device 103 completes the device authentication. Also,
The key management device 103 prohibits the use of expired work keys.

2. The authentication response center station 100 permits the device authentication (encrypted with the work key Kw). It should be noted that this permission is granted to the authentication device 104.
Is generated by.

3. The data obtained by encrypting the Mac address of the authentication data sending cable modem 111 with the master key and the result of calculating the key access number with the one-way function such as the hash function are encrypted with Kw and sent. At this time, since the master key is accessed, the key manager 114 changes the key access number by one in accordance with the rule. Note that the key access number may also be encrypted with the master key, but in this case, access to the master key does not occur, that is, when the Mac address of the cable modem 111 is encrypted with the master key, the master key is accessed by the access. This is because it can be obtained.

4. The center station 100 unlocks the data with the work key Kw, confirms the Mac address of the cable modem and the master key, and determines the key access number corresponding to the master key stored (recorded) in the master key table 105 in the station. The result calculated by the directional function is compared with the sent data, and if they match, the device authentication is completed.
However, in this comparison, the key management device 103 first reads out the key access number corresponding to the master key from the master key table, and then calculates this key access number in the encryption / decryption device 102 as a one-way function. Then, the calculation result is compared with the key access number calculated by the one-way function sent from the subscriber device 110. At this time, since the master key recorded in the center station 100 is accessed, the key manager 114 changes the key access number by one like the subscriber side. As a result, the key access number stored in the key access number table 115 of the subscriber device 110 and the key access number stored in the master key table 105 of the center station 100 are kept at the same number (information). .

After the device authentication in step 202 is completed, the normal cryptographic operation using the work key shown in step 203 is started. However, in the cryptographic operation shown in step 203, as described above, in transmitting and receiving a frame including data between the center station 100 and the subscriber device 110, the frame including the destination Mac address and the source Mac address is used as the work key Kw. The encrypted frame is sent to the network 130 after being encrypted and decrypted.

Here, when the work key Kw expires or some abnormality is detected by the authentication device 104 or the key management device 103 of the center station 100, the key management device 103 of the center station 100 normally uses the work key Kw. stop. In such a case, that is, the cable modem 11
When 1 detects the suspension of use of the work key Kw, the subscriber device 110 immediately enters the device authentication phase of step 202 described above.

In this device authentication phase, as shown in step 205, if a spoofed terminal exists in the network 130, the subscriber device 110 of the legitimate subscriber is registered even if the key access numbers happen to match.
Will also enter the device authentication phase. As a result, device authentication with the same master key is required from the spoofing terminal and the subscriber device 110 of the authorized subscriber to the center station 100, so that duplication of the key access number for the same master key is performed by the center. Detected at station 100,
The existence of the spoofing terminal and the subscriber device to which unauthorized access is made by the spoofing terminal are identified.

When the duplication of the key access numbers for the same master key is found to be the duplication of the key access numbers for the same master key, the center station 100 determines in step 2
Move to 04. In this step 204, immediately stop the communication and enter the re-registration phase, or if the re-registration is impossible immediately at night or the like, the communication is still prohibited, or the operation of the abnormal terminal is put into a monitoring system to make an abnormality. Unless the operation is performed, the operation is preselected by the cable operator, such as whether to continue to use it as it is.

Then, the re-registration phase of step 201 is entered, and the excluded subscriber device is re-registered together with the spoofing terminal by the procedure described above.

However, the reason for entering the re-registration phase is considered to be a device defect such as the presence of a spoofing terminal, a work key Kw or key access number synchronization deviation.
If the device proves to be defective, the master key does not need to be changed. However, for security reasons, it is desirable to change the master key.

The authentication of each subscriber device 110, 120 on the side of the center station 100 described above is all performed by the authentication device 1.
Conduct at 04.

In the security system of this embodiment, in the security operation in step 203, the frame including DA and SA is used for the transmission and reception of the frame including the data between the center station 100 and the subscriber devices 110 and 120. The key Kw is used to perform encryption and decryption, but the present invention is also applicable to conventional frame transmission / reception in which DA and SA are not encrypted. Even in this case, the spoofing terminal can be detected and excluded by the procedure described above.

In the security system of the present embodiment, the device authentication is performed when the work key Kw expires or some abnormality is detected. It is needless to say that the key access number may be exchanged between the center station 100 and each of the subscriber devices 110 and 120 every time, and either one or both of them may determine the coincidence.

Further, in this embodiment, the center station 10
0 and each of the subscriber devices 110 and 120 are connected to each other in a tree-like network, the center station 100 and each of the subscriber devices 110 and 120 are connected at 1: N (where N is a natural number). A connection form such as a star shape may be used.

Further, it goes without saying that the network 130 is not limited to the cable television network, and can be applied to other networks such as a wireless LAN.

As described above, the invention made by the present inventor is
Although the specific description has been given based on the embodiment of the invention, the invention is not limited to the embodiment of the invention and can be variously modified without departing from the scope of the invention. .

[0067]

The effects obtained by the typical ones of the inventions disclosed in the present application will be briefly described as follows.

By receiving random numbers etc. from the center in a secure manner, changing the above numbers etc. with each other by the same rule every time the key is used, sending the changed numbers etc., and checking whether they agree with each other. Since it is possible to detect spoofing on the network, it is possible to construct a higher level security system.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a schematic configuration of a security system according to an embodiment of the present invention.

FIG. 2 is a diagram for explaining the operation of the security system according to the present embodiment.

[Explanation of symbols]

100 ... Center station, 101 ... Headend modem, 1
02 ... Encryption / decryption device, 103 ... Key management device, 104
... authentication device, 105 ... master key table, 110 ... first subscriber device, 111 ... cable modem, 112 ... encryption / decryption device, 113 ... encryption key storage means, 114 ... key manager, 115 ... key access number table, 116 ...
Personal computer (PC), 120 ... Second subscriber device, 130 ... Network.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Tatsuei Ishii             2-31-19 Shiba, Minato-ku, Tokyo Communications and Broadcasting Organization             Within (72) Inventor Shin Ito             2-31-19 Shiba, Minato-ku, Tokyo Communications and Broadcasting Organization             Within F term (reference) 5B085 AE09 AE29 BA06 BG07                 5J104 AA01 AA04 DA03 EA01 FA07                       KA02 NA02 NA11

Claims (12)

[Claims] 1. A method for maintaining security of a network in which a plurality of sets of devices connecting a center device and a home device are set for a single center device, wherein the center device and the home device are connected. A step of setting a different access number and an encryption key for each set of devices, and an encryption using the encryption key when transmitting or receiving data within a set of devices connecting the center device and the home device The method further comprises a step of updating the access number in a predetermined procedure for each process and a decryption process, and a step of notifying the updated access number in a set of devices connecting the center device and the home device. How to maintain security. 2. The security maintaining method according to claim 1, wherein the step of notifying the updated access number in a set of devices connecting the center device and the home device,
A method for maintaining security, comprising the step of encrypting the updated access number with a one-way function and / or the encryption key and notifying the encrypted access number. 3. The security maintaining method according to claim 1, wherein each set of devices connecting the center device and the home device is set every preset period or each time the center device detects an abnormality. A method for maintaining security, further comprising the step of newly setting an access number and an encryption key. 4. The security maintaining method according to claim 1, wherein the encryption process sets a Mac address of a device as a transmission destination to a transmission destination address of a frame to be transmitted, There is a step of setting a Mac address of a device which is a transmission source in a transmission source address of the frame, and encrypting the frame with an encryption key of the device which is the transmission source. A method for maintaining security, comprising the step of decrypting the encrypted frame with an encryption key corresponding to the device that is the sender of the. 5. In a network security system, wherein a plurality of sets of devices connecting a center device and a home device are set for a single center device, and a device connecting the center device and the home device. Setting means for setting a different access number and encryption key for each group, and encryption using the encryption key each time data is transmitted or received within a group of devices connecting the center device and the home device. Within a set of a device that connects the center device and the home device, and a unit that performs an encryption process and a decryption process, an updating unit that updates the access number in a predetermined procedure for each of the encryption process and the decryption process. A security system, comprising: a notification unit that notifies the updated access number. 6. The security system according to claim 5, wherein the setting unit encrypts the updated access number with a one-way function and / or the encryption key, and notifies the encrypted access number. A security system characterized by: 7. The security system according to claim 5, wherein the center device includes a counting unit that counts a preset period or a monitoring unit that monitors the abnormality, and the center unit detects whether the preset period or abnormality occurs. A security system comprising means for newly setting an access number and an encryption key for each set of devices connecting the center device and the home device each time it is detected. 8. The security system according to any one of claims 5 to 7, wherein the encryption processing and decryption processing means sets a destination address of a frame to be transmitted to a Mac address of a device as a transmission destination. Along with the setting, the source address of the frame is set to the Mac address of the source device, the frame is encrypted with the encryption key of the source device, and the source of the received encrypted frame is set. And a unit for decrypting the encrypted frame with an encryption key corresponding to the device. 9. A program for causing a computer to function as a network security system configured by setting a plurality of devices, each of which is configured by connecting a center device and a home device, to a single center device. Setting means for setting a different access number and encryption key for each set of devices connected to the device, and the setting means for transmitting or receiving data within the set of devices connecting the center device and the home device The means for performing the encryption processing and the decryption processing using the encryption key, the updating means for updating the access number in a predetermined procedure for each of the encryption processing and the decryption processing, and the center device and the home device are connected. A program that causes a computer to function as a notification means for notifying the updated access number within a set of devices. Beam. 10. The program according to claim 9, wherein the setting unit encrypts the updated access number with a one-way function and / or the encryption key, and notifies the encrypted access number. A program characterized by being a thing. 11. The program according to claim 9, wherein the center device includes a counting unit that counts a preset period or a monitoring unit that monitors an abnormality, and a preset period or an abnormality is detected. A program that is a means for newly setting an access number and an encryption key for each set of devices that connect the center device and the home device each time. 12. The program according to claim 9, wherein the encryption processing and decryption processing means sets a Mac address of a device as a transmission destination to a transmission destination address of a frame to be transmitted. At the same time, the source address of the frame is set to the Mac address of the device which is the source, and the frame is encrypted by the encryption key of the device which is the source, and the source of the received encrypted frame. A program which is means for decrypting the encrypted frame with an encryption key corresponding to the device.