Patent Publication Number: US-2005135609-A1

Title: Gigabit Ethernet passive optical network for securely transferring data through exchange of encryption key and data encryption method using the same

Description:
CLAIM OF PRIORITY  
      This application claims priority to an application entitled “GIGABIT ETHERNET PASSIVE OPTICAL NETWORK FOR SECURELY TRANSFERRING DATA THROUGH EXCHANGE OF ENCRYPTION KEY AND DATA ENCRYPTION METHOD USING THE SAME,” filed in the Korean Intellectual Property Office on Dec. 18, 2003 and assigned Serial No. 2003-93277, the contents of which are hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a Gigabit Ethernet passive optical network (GE-PON) provided with an optical line terminal (OLT) at the service provider side and a plurality of optical network terminals (ONTs) at the user side, and more particularly to an encryption method for data security between one OLT and a plurality of ONTs.  
      2. Description of the Related Art  
      Nowadays, the expansion of public networks, including various wireless networks, very high-speed communication networks, etc., enables mass data to be shared online. Also widespread is the offline sharing of data through low-priced mass storage media, such as compact discs (CDs) or digital versatile discs (DVDs). Therefore, users can be provided with numerous types of data shared online/offline.  
      This online/offline sharing system is desirable to readily provide a large amount of various data to users, but has a very vulnerable security structure for various types of commercial multimedia data, or data requiring security.  
      A passive optical network (PON) is a communication network system that transfers signals to end users over an optical cable network. This PON consists of one optical line terminal (OLT) installed in a communication company and a plurality of optical network terminals (ONTs) installed near subscribers, typically a maximum of 32 ONTs connectable to one OLT.  
      The PON can provide a bandwidth of 622 Mbps in the downstream direction and a bandwidth of 155 Mbps in the upstream direction in one stand-alone system, these bandwidths being allocated among the PON users. The PON may be used as a trunk between a large-scale system, such as a cable TV system, and an Ethernet network for a neighboring building or home employing a coaxial cable.  
      In the PON, an OLT transmits a signal to an ONT via an optical cable. The ONT, which is a transfer system of the service subscriber side, is an optical network termination unit that provides a service interface to the end user. The ONT receives the signal transmitted from the OLT, processes it in a predetermined manner and then transfers the processed result to an end user. The reverse process is performed by the OLT for the signal received from the subscriber.  
      The ONT accommodates FTTC (Fiber To The Curb), FTTB (Fiber To The Building), FTTF (Fiber To The Floor), FTTH (Fiber To The Home), FTTO (Fiber To The Office), etc. to afford flexible access to the subscriber. The ONT performs an optical/electrical conversion operation to convert an optical signal from the OLT into an analog electrical signal and transmit the converted electrical signal to the subscriber, and an electrical/optical conversion operation to convert an analog electrical signal from the subscriber into an optical signal and transmit the converted optical signal to the OLT.  
       FIG. 1  shows a downstream data transmission structure of a Gigabit Ethernet passive optical network, and  FIG. 2  shows an upstream data transmission structure of the Gigabit Ethernet passive optical network.  
      As shown in  FIGS. 1 and 2 , the structure of the Gigabit Ethernet passive optical network (GE-PON) is such that one OLT  10  is connected via an optical splitter  15  with multiple ONTs  20 ,  22 ,  24  in a tree format. The GE-PON is an optical access network more inexpensive and efficient than an AON (Activity-On-Node) network.  
      As an earlier version of a PON, an asynchronous transfer mode passive optical network (ATM-PON) has been developed and standardized. The ATM-PON transmits ATM cells in the form of a block with a desired size in the upstream or downstream direction. Alternatively, an Ethernet passive optical network (E-PON) transmits packets of different sizes in the form of a block with a desired size. As a result, the E-PON has a somewhat complex control structure compared with the ATM-PON.  
      In downstream transmission, as shown in  FIG. 1 , the OLT  10  broadcasts data for reception by the optical splitter  15 , which transmits the received data to each of the ONTs  20 ,  22 ,  24 . Each of the ONTs  20 ,  22  and  24  detects data to be transferred to a corresponding one of users  30 ,  32 ,  34  and transfers that data to the corresponding user.  
      In upstream transmission, as shown in  FIG. 2 , data from the users  30 ,  32 ,  34  are transferred to the ONTs  20 ,  22 ,  24 , respectively. The ONTs  20 ,  22  and  24  transmit the data upstream to the optical splitter  15  according to a transmission permission convention from the OLT  10 , respectively. In particular, each of the ONTs  20 ,  22 ,  24  transmits received data within a respective time slot set in a TDM (Time Division Multiplexing) manner. Data collision is accordingly avoided in the optical splitter  15 .  
      With the advance of Internet technologies, service subscribers have required data services with greater bandwidths. In this connection, there has been proposed an end-to-end transmission scheme using a Gigabit Ethernet technique that is relatively low in cost and can secure a high bandwidth, as an alternative to an asynchronous transfer mode (ATM) technique that is relatively costly and limited as to bandwidth and that has to segment an Internet protocol (IP) packet. As a result, PON architecture o f the Ethernet, rather than the ATM, type has been required.  
      Churning using a 24-bit encryption key has been proposed as a packet protocol data unit (PDU) encryption scheme for the ATM-PON. This churning scheme has an encryption capability requiring key value update per second and is a relatively simple algorithm, so it can be used for high-speed support in the ATM-PON with the bit rate of 622 Mbps. Key values being periodically updated are generated in an ONT, inserted in payload fields of operation, administration and maintenance (OAM) cells and then transmitted to an OLT.  
      DOCSIS (Data Over Cable Service Interface Specification) using a DES-CBC (Data Encryption Standard with Cipher Block Chaining) encryption algorithm has been proposed as another packet PDU encryption scheme.  
      In the ATM-PON, a 3-byte churning key is inserted in an OAM cell as an encryption key owing to the limitation of encryption techniques and the necessity of high-speed support. In this case, however, there is a limitation in the capability of the encryption key itself. Since the GE-PON utilizes a higher bit rate, e.g., 622 Mbps, than the ATM-PON, it is technically inefficient for the GE-PON to adopt the encryption schemes of the ATM-PON.  
      In the DOCSIS scheme using the DES-CBC encryption algorithm, the encryption key must be updated every 12 hours so that it can be prevented from being hacked by malicious users. As a result, the application of the DES-CBC encryption algorithm to the GE-PON increases inefficiency of an OLT that must manage a plurality of ONTs in a point to multipoint architecture at a high bit rate.  
      In addition, since the point to multipoint architecture is relatively vulnerable to corruption or unauthorized intervention, it is an important issue in the GE-PON to encrypt up-link/down-link user data. For this reason, it is necessary to select a powerful and efficient encryption key scheme and effectively operate it. However, the standardizations of encryption and key management scheduling schemes of the GE-PON are merely in progress in IEEE 802.3ah and there is yet to be a determination as to encryption-related packet format.  
     SUMMARY OF THE INVENTION  
      The present invention has been made in view of the above problems, and it is an object of the present invention to provide a Gigabit Ethernet passive optical network for securely transmitting and receiving data between one OLT and a plurality of ONTs, a data encryption method using the same, and a format of an encryption key used therein.  
      It is another object of the present invention to provide a Gigabit Ethernet passive optical network which is capable of increasing data security in downstream transmission from one OLT to a plurality of ONTs, a data encryption method using the same, and a format of an encryption key used therein.  
      In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a Gigabit Ethernet passive optical network (GE-PON) comprising: an optical line terminal (OLT) configured for receiving a public key through a transmission medium, using the public key to encrypt a secret key, transmitting the encrypted secret key, using the encrypted secret key to encrypt data, and transmitting the encrypted data. The GE-PON further includes at least one optical network terminal (ONT) configured for transmitting the public key to said OLT; receiving the transmitted, encrypted secret key; using a private key to decrypt the received, encrypted secret key; and using the decrypted secret key to decrypt the encrypted data.  
      Preferably, the OLT initially transmits, to the ONT, a gate message indicating that the OLT is ready to register the ONT. The gate message includes encryption/decryption information, which is inserted in a desired portion of a reserved field contained in a format of the gate message. The encryption/decryption information includes information about whether encryption is to be performed, and information about an encryption range when the encryption is performed. The encryption range is selected from a group consisting of all data and payload data. The reserved field is 26 bytes long, and the encryption/decryption information is inserted in one byte of the reserved field.  
      Preferably, the ONT inserts the public key in a data field of a message format and transmits the resulting message to the OLT.  
      Preferably, the OLT transmits secret key encryption progress information to the ONT during use of the public key to encrypt the secret key. If the encryption of the secret key is completed, the OLT transmits to the ONT encryption completion information and the encrypted secret key. Upon receiving the secret key encryption progress information, the ONT transmits reception acknowledgement information to the OLT.  
      Preferably, the ONT transmits secret key decryption progress information to the OLT if the encrypted secret key from the OLT is received, and decryption completion information to the OLT if the decryption of the encrypted secret key is completed.  
      The public key and the private key may be a Rivest-Shamir-Adleman (RSA) public key and an RSA private key, respectively, and the secret key may be an advanced encryption standard (AES) secret key.  
      In accordance with another aspect of the present invention, there is provided a data encryption method for securely transmitting and receiving data between an OLT and at least one ONT in a GE-PON structure, comprising the steps of: a) the ONT transmitting a public key to said OLT; b) the OLT receiving said public key, using the received public key to encrypt a secret key, and transmitting the encrypted secret key to the ONT; c) the ONT using a private key to decrypt the encrypted secret key transmitted from said OLT; d) the OLT using the secret key to be encrypted to encrypt the data and transmitting the encrypted data to the ONT; and e) the ONT using the decrypted secret key to decrypt the encrypted data.  
      Preferably, the step b) includes the steps of: b-1) storing the public key transmitted from the ONT; b-2) generating the secret key for the encryption of the data if the public key is stored; and b-3) using the public key to encrypt the secret key.  
      The data encryption method may further comprise, before step a), the step of the OLT transmitting to the ONT a gate message indicating that the OLT is ready to register the ONT.  
      Preferably, the gate message includes encryption/decryption information, which is inserted in a desired portion of a reserved field contained in a format of the gate message. The encryption/decryption information includes information about whether encryption is to be performed, and information about an encryption range when the encryption is performed. The encryption range is selected from a group consisting of all data and payload data.  
      The step b) may further include the steps of: transmitting secret key encryption progress information to the ONT during use of the public key to encrypt the secret key; and, if the encryption of the secret key is completed, transmitting to the ONT encryption completion information and the encrypted secret key.  
      The step c) may include the step of, upon receiving the secret key encryption progress information, transmitting reception acknowledgement information to the OLT.  
      The step c) may further include the steps of: transmitting secret key decryption progress information to the OLT if the encrypted secret key from the OLT is received; and transmitting decryption completion information to the OLT if the decryption of the encrypted secret key is completed.  
      In a feature of the present invention, an OLT encrypts an AES secret key using an RSA public key transmitted from an ONT, transmits the encrypted AES secret key to the ONT, encrypts data using the AES secret key and transmits the encrypted data to the ONT. Therefore, it is possible to efficiently encrypt data in a GE-PON with a point to multipoint architecture. Moreover, the ONT transmits the RSA public key to the OLT to share it with the OLT, and the OLT encrypts the AES secret key for data encryption using the RSA public key and transmits the encrypted AES secret key to the ONT to share it with the ONT. Therefore, it is possible to efficiently encrypt data to be transmitted in the GE-PON with the point to multipoint architecture. Furthermore, in addition to messages which are exchanged for an initial ONT registration procedure described in IEEE 802.3ah EFM, which is an E-PON standard, various messages associated with encryption key exchange (that is, messages associated with encryption ON/OFF, encryption range, public key transfer, encrypted secret key transfer and encryption/decryption progress) are provided which have formats set to enable a secure encryption operation without violating the standard. Therefore, a device can more readily recognize an operating state of a counterpart device or an operation desired thereby by receiving an associated message from the counterpart device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which the same or similar elements are denoted by identical numerals throughout the several views:  
       FIG. 1  is a view showing a downstream data transmission structure of a Gigabit Ethernet passive optical network (GE-PON);  
       FIG. 2  is a view showing an upstream data transmission structure of the GE-PON;  
       FIG. 3  is a block diagram showing a preferred embodiment of a GE-PON for encrypting data to securely transmit and receive the data between an OLT and an ONT, according to the present invention;  
       FIG. 4  is a detailed block diagram of an OLT key manager and ONT key manager in  FIG. 3 ;  
       FIG. 5  is a flow chart illustrating a first embodiment of a data encryption method for securely transferring data between one OLT and a plurality of ONTs in a GE-PON structure, according to the present invention;  
       FIG. 6  is a flow chart illustrating a second embodiment of the data encryption method according to the present invention; and  
      FIGS.  7  to  11  are views showing the formats of messages transferred between the OLT and the ONTs in  FIG. 6 . 
    
    
     DETAILED DESCRIPTION  
      Preferred embodiments of the present invention are described below in detail with reference to the annexed drawings. In the following description, a variety of specific elements such as constituent elements of various concrete circuits are shown. The description of such elements has been made only for a better understanding of the present invention. Those skilled in the art will appreciate that the present invention can be implemented without using the above-mentioned specific elements. In the following description of the present invention, details of known functions and configurations incorporated herein are omitted for clarity of presentation.  
      A detailed description will hereinafter be given of an example of a data encryption method for securely transmitting and receiving data between one OLT and multiple ONTs in a GE-PON structure, according to the present invention. The data encryption method according to the present invention preferably employs a Rijndael algorithm or advanced encryption standard (AES) secret key algorithm using a 128-bit secret key. This secret key may be encrypted using a Rivest-Shamir-Adleman (RSA) public key algorithm employing a 1024-bit public key and private key so that it can be exchanged online between the OLT and the ONT. References below to a secret key, a public key and a private key may refer, for instance to an AES secret key, an RSA public key and RSA private key, respectively.  
      Detailed descriptions of the AES secret key algorithm and RSA public key algorithm are shown in references: R. Rivest, A. Shamir and L. Adleman, “A Method for Obtaining Digital Signatures and Public-Key Cryptosystems,” Communications of the ACM, 21(2), pp. 120-126, February 1978, and RSA Laboratories, “PKCS #1 v2.1: RSA Cryptography Standard,” June 2002.  
      As stated previously, the standard for the initial registration procedure between the OLT and ONT in the GE-PON has already been published, but there is no mention of data encryption for data transmission and reception.  
      Data encryption using the AES secret key algorithm in the GE-PON, in accordance with the instant invention, is performed with respect to all fields of a GE-PON standard packet format, except for address fields {destination address (DA) and source address (SA) fields}. In addition, in the present invention, the RSA public key algorithm is used to encrypt an AES secret key with an RSA public key. The encrypted AES secret key is inserted in a user data protocol data unit (PDU) field of an Ethernet frame and transmitted to a lower layer.  
      In an alternative embodiment of the present invention, since data must not be transmitted and received in plaintext form until the exchange of a secret key and public key between the OLT and the ONT is completed, a key exchange procedure for data encryption is incorporated, as a supplement, within the standard GE-PON registration procedure between the OLT and the ONT.  
       FIG. 3  is a block diagram showing, by way of illustrative and non-limitative example, a preferred embodiment of a GE-PON for encrypting data to securely transmit and receive the data between an OLT and an ONT, according to the present invention. For reference, in the present embodiment, data encryption is processed at a Gigabit Ethernet passive optical network media access control (GE-PON MAC) layer, or data link layer which is layer  2  of the seven layers of the open systems interconnection (OSI) communications model.  
      As shown in  FIG. 3 , the GE-PON comprises an OLT  100  and at least one ONT  400  which set up OLT-to-ONT and ONT-to-OLT channels with each other via a transmission medium  300  and transmit and receive data over the set-up channels.  
      Specifically, the OLT  100  includes a GE-PON OLT MAC module  120 , a Gigabit media independent interface (GMII) module  130 , an OLT key manager  200 , and a data encrypter  180 .  
      The GE-PON OLT MAC module  120  is adapted to support a carrier sense multiple access/collision detect (CSMA/CD) operation for input data at layer  2  of the OSI communications model. The GMII module  130  is adapted to provide an interface between a physical layer, i.e., layer  1  of the OSI communications model, and the MAC layer corresponding to layer  2  of the OSI communications model. This is an extension of a media independent interface (MII) used in a high-speed Ethernet, which supports data processing rates of 10 Mbps, 100 Mbps and 1000 Mbps. The GMII module  130  has an 8-bit independent data transmission/reception path to support both full-duplex and half-duplex.  
      A GMII layer where the GMII module  130  is located consists of three sub-layers, a physical coding sub-layer (PCS), a physical medium attachment (PMA) sub-layer and a physical medium dependent (PMD) sub-layer. Corresponding modules are provided at the sub-layers, respectively.  
      A PCS module  140  is provided at the PCS to encode and decode input data on a block-by-block basis. A PMA module  160  is provided at the PMA sub-layer to convert data, inputted from the PCS through the PCS module  140 , into serial data, and to convert data inputted from the PMD sub-layer into parallel data, respectively. A PMD module  170  is provided at the PMD sub-layer to convert an electrical signal, i.e., data sent from the PMA sub-layer through the PMA module  160 , into an optical signal and send the converted optical signal to the transmission medium  300 . The PMD module  170  also acts to convert an optical signal received from the transmission medium  300  into an electrical signal and send the converted electrical signal to the PMA sub-layer.  
      The OLT key manager  200  is adapted to, upon receiving an RSA public key transmitted from the ONT  400 , generate an AES secret key and encrypt the generated AES secret key using the received RSA public key. A copy of this encrypted AES secret key is transmitted, via the GE-PON OLT MAC module  120  and GMII module  130 , over the transmission medium  300  to the ONT  400 .  
      The data encrypter  180  is adapted to encrypt plaintext data into ciphertext data using the AES secret key. This encrypted ciphertext data is likewise transmitted, via the GE-PON OLT MAC module  120  and GMII module  130 , over the transmission medium  300  to the ONT  400 .  
      The ONT  400  includes a GE-PON ONT MAC module  420 , a GMII module  430 , an ONT key manager  500 , and a data decrypter  480 . The GE-PON ONT MAC module  420  and GMII module  430  perform the same functions as those of the GE-PON OLT MAC module  120  and GMII module  130 , respectively. The ONT key manager  500  is adapted to store the RSA public key which is used in the OLT  100  for the above-mentioned encryption of the AES secret key, and an RSA private key which is used to decrypt the encrypted AES secret key.  
      The ONT key manager  500  transmits, via the GE-PON ONT MAC module  420  and GMII module  430 , a copy of the stored RSA public key over the transmission medium  30  to the OLT  100  in order to receive a data service from the OLT  100 , e.g., the encryption of AES secret key0. Upon receiving the encrypted AES secret key, the ONT key manager  500  decrypts it using the stored RSA private key.  
      The data decrypter  480  is adapted to, upon receiving from the OLT  100  the ciphertext data encrypted with the AES secret key, decrypt the received ciphertext data using the AES secret key decrypted by the ONT key manager  500 .  
      In summary, the OLT  100  encrypts an AES secret key using an RSA public key transmitted from the ONT  400 , transmits the encrypted AES secret key to the ONT  400 , encrypts data using the AES secret key and transmits the encrypted data to the ONT  400 , thereby making it possible to efficiently encrypt data in the GE-PON with the point to multipoint architecture.  
       FIG. 4  is a detailed block diagram of the OLT key manager  200  and ONT key manager  500  in  FIG. 3 . The OLT key manager  200  includes a public key storage unit  220 , a secret key encrypter  240  and a secret key generator  260 . The public key storage unit  220  stores the RSA public key transmitted from the ONT  400 . The secret key generator  260  generates the AES secret key, if the RSA public key is being received from the ONT  400 , and provides it to the secret key encrypter  240 . The secret key encrypter  240  encrypts the AES secret key using the RSA public key stored in the public key storage unit  220 . The secret key encrypter  240  then sends the encrypted AES secret key to the GE-PON OLT MAC module  120 . The data encrypter  180  likewise encrypts input data using the AES secret key generated by the secret key generator  260  and sends the encrypted data to the GE-PON OLT MAC module  120 .  
      The ONT key manager  500  includes a public key storage unit  520 , a private key storage unit  540  and a secret key decrypter  560 . The public key storage unit  520  stores the RSA public key which is used in the OLT  100  for the encryption of the AES secret key. In order to receive a data service from the OLT  100 , the ONT key manager  500  transmits the RSA public key stored in the public key storage unit  520  to the GE-PON ONT MAC module  420 . The private key storage unit  540  stores the RSA private key which is used for the decryption of the encrypted AES secret key transmitted from the OLT  100 . The secret key decrypter  560  uses the stored RSA private key to decrypt the encrypted AES secret key, if the encrypted AES secret key is being received from the OLT  100 . The data decrypter  480 , upon receiving the encrypted data from the OLT  100 , decrypts it using the decrypted AES secret key.  
       FIG. 5  is a flow chart illustrating a first embodiment of a data encryption method for securely transferring data between one OLT and a plurality of ONTs in a GE-PON structure, according to the present invention.  
      First, in order to receive a data service from the OLT  100 , the ONT  400  sends to the OLT  100  a registration request signal and an RSA public key stored in the public key storage unit  520  (S 100 ). If the OLT  100  receives the registration request signal and RSA public key sent from the ONT  400  and then registers and stores the received RSA public key in the public key storage unit  220  (S 110 ).  
      If the RSA public key is registered and stored in the public key storage unit  220 , the secret key generator  260  generates an AES secret key and provides it to the secret key encrypter  240  (S 120 ). The secret key encrypter  240  uses the RSA public key to encrypt the provided AES secret key (S 130 ). The OLT  100  then sends the encrypted AES secret key to the ONT  400  (S 140 ).  
      The secret key decrypter  560  of the ONT  400  uses the respective RSA private key to decrypt the encrypted AES secret key and stores the decrypted AES secret key (S 150 ). If the decryption of the AES secret key is completed, the ONT  400  sends decryption completion information to the OLT  100  (S 160 ). If it receives the decryption completion information, the OLT  100  encrypts input data using the generated AES secret key and transmits the encrypted data to the ONT  400 , which then acknowledges the data transmission (S 170 ).  
      In brief, the OLT  100  and the ONT  400  share an RSA public key and AES secret key with each other. The OLT  100  encrypts data using the AES secret key and transmits the encrypted data to the ONT  400 . Therefore, it is possible to efficiently encrypt data in the GE-PON with the point to multipoint architecture, and to transmit data with more security.  
       FIG. 6  is a flow chart illustrating a second embodiment of the data encryption method according to the present invention. In this embodiment, the data encryption method is applied to an initial registration step between the OLT  100  and the ONT  400 . In  FIG. 6 , an ONT 1   400   a  and ONT 2   400   b  have the same internal configurations as that of the ONT  400  shown in  FIGS. 3 and 4 .  
      The data encryption method according to the present embodiment roughly includes an initial discovery step S 200 , a public key transmission/LLID (Logical Link IDentification) allocation step S 300 , a secret key transmission/time slot allocation step S 400 , a key shared state confirmation/bandwidth allocation step S 500  and a communication step S 600 , which will hereinafter be described in detail.  
      Upon being powered on and driven, the OLT  100  broadcasts over a communication medium, and to all ONTs connected to it via the medium, a gate signal to discover them (S 220   a  and S 220   b ). In the present embodiment, a description will be given using ONT 1   400   a  and ONT 2   400   b  as examples from among the plural ONTs.  
      The OLT  100  sends the gate signal to each of ONT 1   400   a , ONT 2   400   b  at intervals of a predetermined time until it receives back a registration request signal (S 320   a  and S 320   b ). Upon receiving the gate signal, ONT 1   400   a , ONT 2   400   b  each send the OLT  100  the registration request signal and an RSA public key stored in their respective public key storage units(S 340  and S 350 ).  
      If the OLT  100  receives the registration request signal and RSA public key sent from each of ONT 1   400   a , ONT 2   400   b , then it registers each of ONT 1   400   a , ONT 2   400   b , registers and stores the RSA public key from each of ONT 1   400   a , ONT 2   400   b  in the public key storage unit  220  and allocates an LLID to each of ONT 1   400   a , ONT 2   400   b . The OLT  100  then sends information about the registrations of ONT 1   400   a , ONT 2   400   b  and information about the LLIDs allocated thereto to ONT 1   400   a , ONT 2   400   b , respectively (S 360  and S 370 ).  
      The OLT  100  also uses the RSA public key received from each of ONT 1   400   a , ONT 2   400   b  to encrypt a generated AES secret key. It should be noted here that a certain period of time is required to perform such a process. In this regard, during this process, the OLT  100  sends information (encryption progress information or null information) indicating that the encryption of the AES secret key using the RSA public key is in progress to each of ONT 1   400   a , ONT 2   400   b  (S 420 , S 430 ). ONT 1   400   a , ONT 2   400   b  each receive the encryption progress information and send acknowledgement information (null acknowledgement information) to the OLT  100  (S 440 , S 450 ).  
      If the encryption of the AES secret key using the RSA public key is completed, the OLT  100  sends the encrypted AES secret key to each of ONT 1   400   a , ONT 2   400   b  (S 460 , S 470 ). Upon receiving the encrypted AES secret key from the OLT  100 , each of ONT 1   400   a , ONT 2   400   b  uses the corresponding RSA private key to decrypt the encrypted AES secret key and to send decryption/acknowledgement information to the OLT  100  (S 480 , S 490 ).  
      If the OLT  100  receives the decryption/acknowledgement information from each of ONT 1   400   a , ONT 2   400   b , then it sends transmission permission information to each of ONT 1   400   a , ONT 2   400   b  (S 520 , S 530 ). At this time, the transmission permission information contains information about a bandwidth allocated to a corresponding one of ONT 1   400   a , ONT 2   400   b  and information about shared states of the RSA public key and AES secret key. ONT 1   400   a , ONT 2   400   b  each receive the transmission permission information and send acknowledgement information to the OLT  100  (S 540 , S 550 ).  
      The OLT  100  and each of ONT 1   400   a , ONT 2   400   b , which share the RSA public key and AES secret key in the above manner, transmit data encrypted using the AES secret key to each other (S 560 , S 570 ).  
      As described above, in the GE-PON, the OLT  100  and each of the plural ONTs share an RSA public key and an AES secret key in one-to-one correspondence. Even though the OLT  100  encrypts data using a specific AES secret key and sends the encrypted data to the ONTs, only one of the ONTs having the specific AES secret key can decrypt the encrypted data using that key. Therefore, it is possible to efficiently encrypt data in the GE-PON with the point to multipoint architecture.  
      FIGS.  7  to  11  are views showing the formats of messages transferred between the OLT  100  and the ONTs  400   a ,  400   b  in  FIG. 6 .  
       FIG. 7  shows the format of a gate message that the OLT  100  sends to the ONTs  400   a ,  400   b  for their respective initial registrations in steps S 220   a , S 220   b , S 320   a , S 320   b  set forth in  FIG. 6 .  
      If the OLT  100  is powered on and its various devices are initialized, it sends a gate message indicating that it is ready to register ONTs  400   a ,  400   b . At this time, for encryption key exchange, the OLT  100  uses one byte, preferably Flag2, of a 26-byte reserved field contained in the existing gate message format. Here, the Flag2 includes promised information specifying whether the OLT  100  encrypts or decrypts and, if it encrypts, a range of encryption, e.g., whether the OLT encrypts all data or only payload. The promised information of the Flag2 can be classified into the following types: 
          Flag2 is “0x01” (Flag2=0x01), indicating that all data is encrypted;     Flag2 is “0x02” (Flag2=0x02), indicating that only payload is encrypted; and     Flag2 is “0x03” (Flag2=0x03), indicating that no encryption is performed.        

      Upon receiving this gate message, the ONTs  400   a ,  400   b  each can recognize an encryption/decryption operation of the OLT  100  by checking the Flag2 contained in the received message.  
       FIG. 8  shows the format of a registration request message that the ONTs  400   a ,  400   b  each send to the OLT  100  at steps S 340 , S 350  of  FIG. 6 .  
      Since the ONTs  400   a ,  400   b  which have received the gate message sent from the OLT  100  are not yet allocated time slots for data transmission from the OLT  100 , they each send a registration request message to the OLT after determining a desired delay time. In other words, if ONTs  400   a ,  400   b  receive the gate message sent from the OLT  100 , then they each read their own RSA public key from their respective public key storage unit  520 . Then, the ONTs  400   a ,  400   b  each insert the read RSA public key in a data field of the message format along with a registration request signal and send the resulting registration request message to the OLT  100 .  
       FIG. 9  shows the format of a registration information/LLID message that the OLT  100  sends to the ONTs  400   a ,  400   b  at steps S 360 , S 370  of  FIG. 6 .  
      If the  0 LT  100  receives the registration request signal/public key message from each of ONTs  400   a ,  400   b , then it sends back to each a respective registration information/LLID message containing an LLID allocated to that ONT. Upon receiving the RSA public key sent from each of ONTs  400   a ,  400   b , the OLT  100  generates an AES secret key to be used by the ONTs for encryption. The OLT  100  then uses the RSA public key sent from each of ONTs  400   a ,  400   b  to encrypt the respective generated AES secret key.  
       FIG. 10  shows exemplary formats for an encryption progress information (null information) message and encrypted secret key message that the OLT  100  sends to ONTs  400   a ,  400   b  in steps S 420 , S 430 , S 460 , S 470  of  FIG. 6 .  
      During the process of generating the AES secret key and encrypting it using the RSA public key sent from each of ONTs  400   a ,  400   b , the OLT  100  sends an encryption progress information (null information) message to each of ONTs  400   a ,  400   b  indicating that the encryption is in progress. When the encryption progress information (null information) message is sent, the OLT  100  is in the process of generating the AES secret key and encrypting it using the RSA public key and each of ONTs  400   a ,  400   b  is in a reception standby state to receive the encrypted AES secret key.  
      Thereafter, if the OLT  100  completes the process of generating the AES secret key and encrypting it using the RSA public key sent from each of ONTs  400   a ,  400   b , the OLT inserts the encrypted AES secret key in a data field of the corresponding message format and sends the resulting message to the respective ONT. When the encrypted AES secret key message is sent, the OLT  100  is in a completed state of the AES secret key generation and encryption. At that time, ONTs  400   a ,  400   b  each are in a state of receiving the encrypted AES secret key.  
      The information inserted in the flag field can be either: 
          “0x00” (Flag=0x00), indicating that RSA encryption is in progress; or     “0x01” (Flag=0x01), indicating that RSA encryption is completed and that an encrypted AES secret key is contained in a sent message.        

       FIG. 11  shows the formats of a null acknowledgement information message and decryption/acknowledgement information message that ONTs  400   a ,  400   b  each send to the OLT  100  at steps S 440 , S 450  and steps S 480 , S 490  of  FIG. 6 .  
      The null acknowledgement information message is a message indicating that a corresponding one of ONTs  400   a ,  400   b  has normally received the encryption progress information (null information) message from the OLT  100 . The “null” in the null acknowledgement information message signifies that a corresponding one of ONTs  400   a ,  400   b  is in the reception standby state to receive the encrypted AES secret key.  
      The decryption/acknowledgement information message is a message indicating that a corresponding one of ONTs  400   a ,  400   b  has normally received the AES secret key encrypted with the RSA public key from the OLT  100  and has completed the decryption of the encrypted AES secret key using its own RSA private key.  
      Points of difference between the null acknowledgement information message and the decryption/acknowledgement information message are whether the corresponding ONT is in the reception standby state to receive the encrypted AES secret key and whether the corresponding ONT has received the encrypted AES secret key and completed the decryption thereof. Also, while receiving the encrypted AES secret key and decrypting it with the RSA private key, the ONTs  400   a  and  400   b  each can notify the OLT  100  of such a situation by including null information in the corresponding message format of  FIG. 11  and sending the resulting message to the OLT  100 .  
      The null acknowledgement information message and the decryption/acknowledgement information message are distinguished from each other according to information inserted in flag fields of the message formats, which can be classified into the following types: 
          “0x00” (Flag=0x00), indicating that the corresponding ONT is in the reception standby state to receive the encrypted AES secret key or that the corresponding ONT is decrypting the encrypted AES secret key received from the OLT  100  using the RSA private key; or     “0x01” (Flag=0x01), indicating that the corresponding ONT has completed the decryption of the received, encrypted AES secret key using the RSA private key.        

      As apparent from the above description, according to the present invention, an ONT transmits an RSA public key to an OLT, which then uses the RSA public key to encrypt an AES secret key. The OLT transmits the encrypted AES secret key to the ONT, and likewise uses the AES key to encrypt data for subsequent transmission to the ONT. Therefore, it is possible to efficiently encrypt data in a GE-PON with a point to multipoint architecture.  
      Moreover, in a GE-PON with a point to multipoint architecture, an OLT shares in one-to-one correspondence with each of a plurality of ONTs an RSA public key and an AES secret key. Even though the OLT encrypts data using a specific AES secret key and sends the encrypted data to the ONTs, only the one of the ONTs having the specific AES secret key can decrypt the encrypted data using that key. Therefore, it is possible to efficiently encrypt data in the GE-PON with the point to multipoint architecture.  
      Furthermore, in addition to messages which are exchanged for an initial ONT registration procedure described in IEEE 802.3ah EFM, which is an E-PON standard, various messages associated with encryption key exchange (that is, messages associated with encryption ON/OFF, encryption range, public key transfer, encrypted secret key transfer and encryption/decryption progress) are provided which have formats set to enable a secure encryption operation without violating the standard. As a result, a device can more readily recognize an operating state of a counterpart device or an operation desired thereby by receiving an associated message from the counterpart device. Therefore, this invention provides an inevitable base to encryption in an E-PON system which is not yet standardized.  
      Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.