JP2000059352A - Encryption communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the encryption communication system where a transmitter side encrypts information efficiently and the entire system transmits an encrypted text efficiently in the case of transmitting information to plural receivers via a repeater. SOLUTION: A transmitter side 1 transmits a text f(M, K) resulting from encrypting information M by a common key K and plural texts f(K, Ki) resulting from encrypting the common key K with a public key Ki for each of receiver side devices 3-5 to a repeater 2. The repeater 2 transmits the texts f(M, K), f(K, Ki) in pairs to the receivers 3-5. For example, a couple of the texts f(M, K) and f(K, K3) is sent to the receiver 3. The receiver 3 decodes the text f(K, K3) by using a private key to obtain the common key K and decodes the text f(M, K) by using the common key K.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an encryption communication system for encrypting and transmitting information from a transmitting device to a plurality of receiving devices via a relay device.

[0002]

2. Description of the Related Art In communication, in order to prevent leakage of information during transmission, information has been encrypted for a long time for communication. Various encryption methods have been developed. One method of encryption is a common encryption key method. In this scheme, one common key K is secretly possessed by both the transmitting device and the receiving device. The transmitting device encrypts the information to be sent using the common key K and transmits it. The receiving device decrypts the received message using the same common key K as the transmitting device and obtains the information. The key K in this method
May be called a secret key, but here it is called a common key.

In this common encryption key system, high confidentiality can be maintained if the secret of the common key is maintained in both the transmitting side and the receiving side. However, since a common key is shared by both the transmitting side and the receiving side, the information of the common key must be secretly passed from one side to the other in some form. There is also a drawback that the management load is large because the common key is stored on both sides.

[0004] In recent years, for example, an encryption method based on a public encryption key system, such as that found in RSA, has come to be widely used. In this public encryption key system, a pair of keys K and S
Is prepared, encrypted by one key K and decrypted by the other key S. Of the two keys, the key K used for encryption is made public, and the key S used for decryption is kept secret. Therefore, the key K is called a public key, and the key S is called a secret key.

When sending information, the information to be sent is encrypted with the public key K and sent. Public key K used at this time
Corresponds to the secret key S held secretly in the receiver. The receiving device that has received the encrypted information can obtain the original information by decrypting the information with the private key S held therein.

[0006] In such a public encryption key system, the encrypted information cannot be decrypted with the public key K, and the private key S is required for decryption. Therefore, as long as the secret key S does not leak, the information encrypted using the public key K cannot be decrypted, and confidentiality on the transmission path can be guaranteed.
Also, since the information to be kept secret is only the secret key S,
It also has the advantage of being easy to manage.

[0007] On the other hand, as one of the communication systems, a broadcast communication for transmitting the same information to a plurality of receivers is used. In this case, the transmitting device individually communicates with a number of receiving devices. In this case, the transmitting side device is occupied, and if there are many remote side receiving devices, the communication cost increases. In order to avoid this, the transmitting-side machine has been using relay broadcasting, in which the transmitting side transmits to a distant repeater and broadcasts from the repeater to a nearby receiving-side machine. In addition, intermediate relay broadcasting is also performed from the repeater via the repeater.

[0008] In such a relay broadcast communication or a middle relay broadcast communication, a case where encrypted information is transmitted will be considered.
FIG. 7 is an explanatory view showing one mode of transmitting information after encrypting it in the conventional relay broadcast communication. 1 is the transmitting device, 2
Is a repeater, and 3 to 5 are receivers. In this example, the transmitting device 1 transmits information only to the relay device 2,
Shows an example in which information is transmitted to the three receivers 3 to 5 by broadcast communication.

[0009] In the example shown in FIG. 7A, the transmitting device 1 and the receiving devices 3 to 5 all have a common key K.
In this case, the transmitting device 1 transmits the information M to be transmitted to the common key K.
Then, a message f (M, K) is created by encryption and transmitted to the relay device 2. The relay device 2 transmits the received message f (M, K) to the receiving devices 3 to 5 as it is. This function is a normal broadcast function. Each of the receivers 3 to 5 sends a message f
Since (M, K) is received, information can be obtained by decrypting with the common key K.

The same applies to the case where the public encryption key system is used. When all of the receiving side devices 3 to 5 publish the same public key K, the transmitting side device 1 encrypts the information M to be sent with the public key K to create a message f (M, K), 2. The relay device 2 transmits the received message f (M, K) to the receiving devices 3 to 5 as it is, and the receiving devices 3 to 5 decrypt the received message f (M, K) using the secret key S, respectively. I just need.

However, in this example, it is necessary that either the transmitting device and all the receiving devices have the common key K, or that all the receiving devices have published the same public key K. for that reason,
There is a problem that only the group sharing the same key can perform relay broadcast communication.

In the example shown in FIG. 7B, the receivers 3-5
Respectively hold common keys K3, K4, and K5, and the transmitting device 1 holds all of the common keys K3, K4, and K5 corresponding to the receiving devices 3 to 5, respectively. In this case, in the conventional system, the transmitting side machine 1 encrypts the information M to be sent using the common key K3 common to the receiving side machine 3 to create a message f (M, K3). The information M is encrypted using the common key K4 common to the terminal device 5 to generate a message f (M, K4), and the information M is encrypted using the common key K5 common to the receiving device 5 and the message f (M, K4). Create K5). Then, the generated telegrams f (M, K3), f (M, K4), and f (M, K5) are specified and transmitted to the relay device 2 by specifying the respective receiving side devices. The repeater 2 sends the received message f (M, K3) to the receiver 3
4) is transmitted to the receiver 4 and the message f (M, K5) is transmitted to the receiver 5. The receivers 3 to 5 respectively receive the received message f (M,
K3), f (M, K4) and f (M, K5) are decrypted with common keys S3, S4 and S5, respectively, to obtain information.

The receiving devices 3 to 5 respectively have public keys K3, K4,
The same applies to the public encryption key system that discloses K5,
The transmitting device 1 encrypts the information M to be transmitted using the public key K3 of the receiving device 3 to create a message f (M, K3), and similarly uses the public key K4 of the receiving device 4 to generate the information M. And encrypt the message f (M, K
4) is created, and the information M is encrypted using the public key K5 of the receiver 5 to create a message f (M, K5). Then, the generated telegrams f (M, K3), f (M, K4), and f (M, K5) are specified and transmitted to the relay device 2 by specifying the respective receiving side devices. In the repeater 2, the received message f (M, K3) is transmitted to the receiver 3 and the message f (M, K4) is transmitted to the receiver 4.
, The message f (M, K5) is transmitted to the receiver 5. Receiver 3
5, the received messages f (M, K3), f (M, K4), f (M,
K5) is decrypted with the respective secret keys S3, S4, S5 to obtain information.

In the case of the example shown in FIG. 7B, even if the receivers have different keys, information is transmitted by transmitting a message corresponding to each receiver. Can be. However, in the case of this example, the transmitting side machine 1 sends the telegrams f (M, K3), f (M, K4) and f (M, K5) to the relay machine 2. Therefore, the amount of transmission data hardly changes as compared with the case where data is directly transmitted to each of the receiving devices 3 to 5, and the advantage of transmitting via the relay device 2 is diminished. In addition, since the transmitting device 1 must create messages corresponding to each of the receiving devices 3 to 5, there is also a problem that the processing amount for encryption increases.

[0015]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and when transmitting information to a plurality of receiving devices via a repeater, the transmitting device efficiently transmits the information. It is an object of the present invention to provide an encrypted communication system capable of transmitting an encrypted message as well as efficiently encrypting the entire system.

[0016]

SUMMARY OF THE INVENTION The present invention relates to a cryptographic communication system for transmitting encrypted information from a transmitting side device to a plurality of receiving side devices via a repeater.
Are transmitted to the relay device by encrypting the message f (M, K) with the common key K and the plurality of messages f (K, Ki) obtained by encrypting the common key K with the public key Ki for each receiving device. The relay device is characterized in that the message f (M, K) and f (K, Ki) are transmitted to each receiving device in pairs. The receiver receives a pair of the message f (M, K) and the message f (K, Ki) corresponding to the receiver, so the message f (K, Ki)
Is decrypted with the secret key Si to obtain the common key K, and the message f (M, K) may be decrypted into the plaintext using the common key K.

In order to realize such an encryption communication system, the transmitting side machine transmits a message f obtained by encrypting the information M with the common key K.
It is only necessary to have a function of transmitting (M, K) and a plurality of electronic messages f (K, Ki) obtained by encrypting the common key K with the public key Ki for each of the receiving devices to the relay device. The repeater transmits a message f (M, K) and a plurality of messages f (K,
Ki), and has a function of pairing the message f (M, K) and the message f (K, Ki) corresponding to that receiver for each receiver, and transmitting them to each receiver. It should just be. The receiving device sends a message f
(M, K) and f (K, Ki) are received, and the message f (K, Ki) is decrypted with the secret key Si corresponding to the public key Ki to obtain a common key K. It is only necessary to have a function of obtaining the information M by decrypting the electronic message f (M, K) into a plaintext.

Thus, even when relay broadcasting is performed to a receiving device having a different public key, the transmitting device transmits the information to be transmitted to the relay device with a message encrypted with a common key. Only a plurality of messages in which the common key is encrypted with the public key need be transmitted. Therefore, the burden of the encryption process on the transmitting side device is reduced, and the communication cost to the relay device can be reduced. In this method, the common key K to be kept secret is transmitted. However, since the common key K itself is encrypted, there is no problem in confidentiality.
Conversely, the receiving device need not have the common key K,
The transmission side device can perform encryption using an arbitrary common key K.

When the information M is to be sent to the repeater, the electronic message f (K, KC) encrypted with the repeater's public key KC is also transmitted to the repeater. The repeater uses its own secret key SC to send a message f (K, K
C) to obtain a common key K, and use the common key K to send a message f
By decoding (M, K), information M is obtained.

If the repeater has a function of relaying information to another repeater, intermediate relay broadcast can be realized. Corresponds to one or more receivers of the message f (M, K) sent from the transmitter or another relay and one or more of the messages f (K, Ki) that communicate after the other relay. The message f (K, Ki) that encrypted the public key K and the message that encrypted the information M
f (M, K) may be transmitted to another repeater.

[0021]

FIG. 1 is a block diagram showing a first embodiment of a cryptographic communication system according to the present invention. The reference numerals in the figure are the same as those in FIG. Here, the receiving side devices 3 to 5 publish the public keys K3 to K5, and decrypt using the corresponding secret keys S3 to S5.

The transmitting device 1 has a function of encrypting information and a function of transmitting information. When encrypting the information M, a common encryption key method is used. That is, the information M is encrypted using an arbitrary common key K to create a message f (M, K). In the common encryption key system, since the same common key K must exist in the receiving devices 3 to 5, the common key K is encrypted and sent to the receiving devices 3 to 5. A public encryption key method is used as an encryption method for that purpose. The common key K is encrypted with the public keys K3 to K5 of the receivers 3 to 5, and the message f (K, K3)
, F (K, K4) and f (K, K5). Then, the message f (M,
K), f (K, K3), f (K, K4) and f (K, K5) are transmitted to the repeater 2.

The repeater 2 receives the encrypted message from the transmitter 1 or another repeater, and distributes the encrypted message to the receivers 3 to 5 or another repeater. Of course, it may have a function of decrypting an encrypted electronic message sent to itself and extracting information. In this example, the message f (M, K), f (K, K3)
, F (K, K4) and f (K, K5). Then, the message f (M, K) and the message f (K, K3) are transmitted to the receiving device 3. Also, it transmits the electronic message f (M, K) and the electronic message f (K, K4) to the receiver 4. Further, the message f (M, K) and the message f (K, K5) are transmitted to the receiver 5. In this way, each receiving device 3
To 5, the message f (M, K) and the common key K when the message was encrypted are stored in a message f (K, Ki) (i = 3 to 5) will be transmitted as a pair.

Each of the receivers 3 to 5 has a function of receiving an encrypted message from the relay 2 and decrypting the message using a secret key to obtain the original information. For example, the receiver 3 transmits a message f (M, K) obtained by encrypting the information M from the exchange 2 with the common key K.
And a message f (K, K) obtained by encrypting the common key K with its own public key K3.
3) Receive. Then, first, the message f (K, K3) is
To obtain a common key K.
Further, using the common key K, the received message f (M, K) is decrypted. As a result, the information M to be transmitted by the transmission side device 1 can be obtained. The same applies to the receiving-side devices 4 and 5. When the common key K is obtained, the information may be decrypted using the respective secret keys, and the information M may be decrypted using the obtained common key K.

The information M transmitted by the transmitting device 1 in this way
Is transmitted to each of the receiving side devices 3 to 5 via the relay device 2. Since the information M is encrypted on the transmission line from the transmitting side device 1 to the receiving side devices 3 to 5, confidentiality is guaranteed and the information M can be transmitted safely. Further, the common key K used when encrypting the information M is also encrypted, and can be sent to the receivers 3 to 5 in secret.

As described above, in this system, the transmitting device 1
Transmits to the relay device 2 a message f (M, K) obtained by encrypting the information M with the common key K and a common key K for decrypting the message using the public key of each of the receivers 3 to 5. Message f (K, K
3), f (K, K4) and f (K, K5). Normally, since the data amount of the common key K is smaller than the information M to be sent, it is necessary to encrypt the information M rather than to encrypt the information M using the public keys K3 to K5 corresponding to the respective receivers 3 to 5. The load and the amount of data to be transmitted are reduced. Further, by encrypting the common key K using the public keys of the receiving side devices 3 to 5, even if the receiving side devices 3 to 5 have different public keys as in the above-described example, the relaying operation is performed. Broadcast communication can be used. Furthermore, a common key for encrypting and decrypting the information M can be set arbitrarily without depending on the relay device 2 and the receiving devices 3 to 5. In addition, it is also possible to improve the confidentiality by using the fact that a key can be set arbitrarily, for example, by changing the key each time information is transmitted.

Hereinafter, an example of the operation of each of the transmitter, the repeater, and the receiver will be described. The transmitting device 1
It has a function of encrypting information and a function of transmitting information as described above. When encrypting information, a public key for encrypting the common key K is required for each receiver. In order to obtain the information, for example, a destination table having information for specifying a receiving side device to which information is to be transmitted and public key information published by the receiving side device is created.

FIG. 2 is an explanatory diagram of an example of the destination table. In this example, a dial number is included as information for specifying the receiver. A public key is associated with each dialed number. In addition, as shown in FIG. 2, it may also have a dial number and a public key KC corresponding to the repeater 2. When the receiving side to which information is to be transmitted is specified by the dial number, the destination table can be searched using the specified dial number, and the public key of the receiving side can be obtained. In addition,
The information for specifying the receiving side device is not limited to the dial number, but may be, for example, a mail address or the like, and may be any information as long as the receiving side device can be specified.

This destination table can be held in the transmitting device 1. Alternatively, the relay device 2 may have the destination table, and the relay device 2 may transmit the public key list to the transmitting device 1 by specifying the receiving device from the transmitting device 1. Of course, the sender 1
And the repeater 2 may have a destination table. Alternatively, another device may have the destination table, and the public key may be obtained from that device.

FIG. 3 shows a first example of the cryptographic communication system of the present invention.
9 is a flowchart illustrating an example of the operation of the transmission side device according to the embodiment. In S31, a common key K for encoding the information M is arbitrarily generated. The generation method is also arbitrary. In S32, the information M to be transmitted is
A message f (M, K) is generated by encrypting with the common key K.

Next, the common key K is encrypted with the public key corresponding to each receiver. For the i-th receiver, S
At 33, a public key Ki is obtained. This public key Ki is
It can be obtained directly or indirectly from the destination table as shown in FIG. Using the obtained public key Ki, the common key K is encrypted to create a message f (K, Ki). In S35, it is determined whether or not a message of the encrypted common key K corresponding to all the receivers transmitting the information M has been created,
If there is an unprocessed receiver, the process returns to step S33 to encrypt the common key K corresponding to the receiver.

When the encryption processing of the corresponding common key K is completed for all the receivers transmitting the information M, the process proceeds to S3.
6, the message f (M, K) generated in S32 and a plurality of messages obtained by encrypting the common key K with a public key corresponding to each receiver.
Send f (K, Ki) to the repeater. In addition, message f
Add information to distinguish each message, such as which (K, Ki) corresponds to which receiver or which group, or which is encrypted with which public key It is good to keep.

The repeater transmits information M to be transmitted from the transmitting side machine.
Is encrypted with a common key K, and a plurality of messages f (K, Ki) are generated by encrypting the common key K used for the encryption with the public key Ki of each receiver. To receive. In addition, information for specifying the receiving side, which is the destination, is also sent from the transmitting side. For example, the dial number and address of the receiving side device may be directly instructed, or a plurality of receiving side devices may be registered in advance in a repeater as a group, and an instruction to specify the group may be issued from the receiving side device. The repeater sends a message to the receiver specified by the transmitter in this way.
A message f (M, K) and a common key encrypted with the public key of the receiver f
(K, Ki) is transmitted to the receiver side as a pair.

FIG. 4 shows a first example of the cryptographic communication system of the present invention.
9 is a flowchart illustrating an example of an operation of the relay device according to the embodiment. When the repeater receives the message f (M, K) and the plurality of messages f (K, Ki) sent from the transmitter, the processes of S41 and S42 are performed for each receiver. That is, in S41, a common message f (M, Ki) is shared with the message f (K, Ki) encrypted using the public key Ki published by a certain receiver.
K) and a pair. Then, in S42, the paired two messages are transmitted to the receiver. In S43,
It is determined whether or not the message has been transmitted to all the receiving devices designated by the transmitting device. If there is an untransmitted receiving device, the process returns to step S41 to transmit the received device. Perform processing. By transmitting the message to all the receiving-side devices instructed to transmit, the process of the relay device is terminated.

When a message f (K, KC) in which the transmitting device specifies the relay device and the common key K is encrypted using the public key KC published by the relay device is transmitted. The repeater can perform the same processing as that of the next receiver to acquire the information M sent from the transmitter.

FIG. 5 shows a first example of the cryptographic communication system of the present invention.
9 is a flowchart showing an example of the operation of the receiving device according to the embodiment. The receiving device receives, from the repeater, a message f (M, K) in which the information M is encrypted with the common key K and a message f (K, Ki) in which the common key K is encrypted with the public key Ki of the own device. I do. Message f (M,
K) is the secret key Si corresponding to the public key Ki that is disclosed by the device itself.
Cannot be decrypted. Therefore, first in step S51, the electronic message f (K, Ki) is decrypted with the secret key Si. As a result, the common key K used when encrypting the message f (M, K) at the transmitting side device is obtained. In S52, the received message f (M, K) is decrypted using the common key K obtained in S51. In this way, the receiver can obtain the information M.

FIG. 6 shows a second example of the cryptographic communication system of the present invention.
FIG. 2 is a configuration diagram showing an embodiment. In the figure, 21 to 24 are repeaters. In this example, a system is shown that transmits encrypted information from the transmitting device 1 to the receiving devices 3 to 5 via a plurality of relay devices. Here, the repeater 21 transmits a message to the repeaters 22 and 23. The relay 22 transmits a message to the receivers 3 and 4, and the relay 23 transmits a message to the receiver 5, another relay 24, and the like. It is assumed that each of the repeaters 21 to 24 is set in advance to transmit from which repeater to which receiver.

The relay device 21 transmits a message f (M, K) obtained by encrypting information M to be sent from the transmitting device 1 with an arbitrary common key K, and a public key K3 disclosed by the receiving devices 3-5. A plurality of electronic messages f (K, Ki) obtained by encrypting the common key K with .about.K5 are received. Of the plurality of messages, a message f (K, Ki) corresponding to the receiving-side device transmitting via the relay device 22, in this example, the messages f (K, K3) and f (K, K4)
Then, a message f (M, K) obtained by encrypting the information M is transmitted to the relay device 22. In addition, a message f (K, Ki) corresponding to the receiving side device transmitted via the relay device 23, a message f (K, K5) in this example, and a message f (M, K) obtained by encrypting the information M are represented by: Transmit to the repeater 23.

The repeaters 22 and 23 receive a common message f (M, K) and a message f (K, Ki) corresponding to each receiving device from the repeater 21, respectively. The processing for the received message is the same as that of the repeater 2 in the first embodiment. The relay device 22 transmits the message f (K, K3) to the receiver 3 together with the message f (M, K). Further, it transmits the electronic message f (M, K) and the electronic message f (K, K4) to the receiver 4. Repeater 23
Are sent to the receiver 5 in the same manner as the message f (M, K) and the message f (K, K
5) Send. In addition, the repeater 23 transmits the message f (K, Ki) corresponding to the receiving device that receives the message after the other repeater 24 in the same manner as the repeater 21 for the transmission via the other repeater 24. What is necessary is just to transmit the common message f (M, K) to the other repeaters.

As described above, the present invention can be applied to the intermediate relay broadcasting that transmits data via two or more relays. In this case as well, the transmitting device 1
1, at the time of transmission from the repeater 21 to the repeater 22 or the repeater 23, and from the repeater 23 to another repeater 24, only one message f (M, K) in which the information M to be transmitted is encrypted is provided. It is. Therefore, as compared with the case of transmitting a plurality of messages f (M, Ki) in which the information M is encrypted corresponding to each of the receiving side devices, the transmission data amount between the transmitting side device 1 and the repeater and the repeater can be reduced. It can be significantly reduced.

Each of the repeaters may have a case where the transmission destination is the repeater and a case where the transmission destination is the reception-side device like the repeater 23. Also, if each repeater also serves as a receiver, and has received from the transmitter a message in which the common key K has been encrypted with the public key published by the repeater, the message is transmitted using its own secret key. The information M may be decrypted and received.

The cryptographic communication system of the present invention as described above includes, for example, facsimile communication, an electronic mail system,
The present invention can be applied to any system including communication, such as and a composite system of these.

[0043]

As is apparent from the above description, according to the present invention, even when relay broadcast communication or middle relay broadcast communication is performed for a receiver having a different public key, the transmission side The machine encrypts the information M to be sent with the common key K to form one message f (M,
K) and a message f (K, Ki) for each receiving device obtained by encrypting a common key K for decrypting the message with the public key Ki of each receiving device.
May be transmitted to the repeater. Information M you want to send in this way
Since only one message that is common to each receiver is required to be encrypted, even if the data amount of the information M is large, the load required for the encryption process can be reduced. Also, the amount of data transmitted to the repeater can be reduced. Further, since the common key K for encrypting the information M can be set arbitrarily, it can be set, for example, every time communication is performed. Furthermore, since decryption and re-encryption are not performed in the repeater, confidentiality can be maintained in the repeater. of course,
By transmitting a message in which the common key K is encrypted with the repeater's public key KC to the repeater, the information M can also be sent to the repeater. Further, in the case of transmission via a plurality of repeaters, there is an effect that the amount of data to be transmitted can be reduced even in communication between the repeaters.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of a cryptographic communication system of the present invention.

FIG. 2 is an explanatory diagram of an example of a destination table.

FIG. 3 is a flowchart illustrating an example of an operation of a transmission side device in the first embodiment of the cryptographic communication system of the present invention.

FIG. 4 is a flowchart illustrating an example of an operation of the relay device in the first embodiment of the cryptographic communication system of the present invention.

FIG. 5 is a flowchart illustrating an example of an operation of the receiving side device in the first embodiment of the cryptographic communication system of the present invention.

FIG. 6 is a configuration diagram showing a second embodiment of the cryptographic communication system of the present invention.

FIG. 7 is an explanatory diagram showing one mode of transmitting information after encrypting it in the conventional relay broadcast communication.

[Explanation of symbols]

Reference numeral 1 denotes a transmitter, 2, 21 to 24: a relay, and 3 to 5: a receiver.

Claims (3)

[Claims] 1. A cryptographic communication system for transmitting encrypted information from a transmitting side device to a plurality of receiving side devices via a relay device, wherein the transmitting side device encrypts information M with a common key K. f (M, K) and the public key Ki for each receiving device
And transmits a plurality of messages f (K, Ki) to the relay device, and the relay device pairs the messages f (M, K) and f (K, Ki) to each of the reception-side devices. A cryptographic communication system for transmitting. 2. The transmitting device according to claim 1, wherein said transmitting device has a public key KC.
The cryptographic communication system according to claim 1, wherein the electronic message f (K, KC) encrypted in (1) is also transmitted to the relay device, and the information device M is also decrypted in the relay device. 3. The relay device further has a function of relaying information to another relay device, and among the received message f (M, K) and a plurality of messages f (K, Ki), 2. A message f (K, Ki) and a message f (M, K) corresponding to one or more receivers communicating with the other repeater and thereafter are transmitted to the other repeater. 3. The cryptographic communication system according to 1.