Communication device, server device, concealed communication system, methods for the same, and program

A server device as a component of a concealed communication network that provides concealed communication to N communication devices, wherein n communication devices out of the N communication devices belong to a group, and each of the communication devices includes processing circuitry configured to implement a secret key generation unit that generates a secret key using m partial secret keys included in K partial secret keys generated by K key generation devices, respectively, a signature encryption unit in which (1-1) if the communication device itself belongs to the group, the signature encryption unit encrypts input information with the secret key and transmits the information to the server device, and (1-2) if the communication device itself does not belong to the group, the signature encryption unit transmits a dummy message to the server device, a signature decryption unit in which (2-1) if the communication device itself belongs to the group, the signature decryption unit downloads, from the server device, encrypted n−1 pieces of the input information and N−n number of the dummy messages transmitted from the other communication devices and decrypts the encrypted n−1 pieces of the input information with the secret key, and (2-2) if the communication device itself does not belong to the group, the signature decryption unit downloads encrypted n pieces of the input information and N−n−1 number of the dummy messages transmitted from the other communication devices, a session key generation unit that transmits session key generation information to the server device via the signature encryption unit, the session key generation information being information for use in generation of a session key, and generates the session key using n−1 pieces of session key generation information acquired via the signature decryption unit and session key generation information of the communication device itself, and a message sending and receiving unit that transmits a cipher text encrypted with the session key via the signature encryption unit to the server device and decrypts n−1 cipher texts acquired via the signature decryption unit with the session key, the server device comprising:

• processing circuitry configured to implement
• a session key generation information saving unit that receives encrypted n pieces of the session key generation information and N−n number of the dummy messages and stores them in a downloadable manner;
• and
• a message saving unit that receives encrypted n number of the cipher texts and N−n number of the dummy messages and stores them in a downloadable manner.

A concealed communication system comprising:

• a server device;
• K key generation devices;
• and
• N communication devices, wherein
• n communication devices out of the N communication devices belong to a group,
• each of the communication devices includes processing circuitry configured to implement
• a secret key generation unit that generates a secret key using m partial secret keys included in K partial secret keys generated by the K key generation devices, respectively,
• a signature encryption unit in which (1-1) if the communication device itself belongs to the group, the signature encryption unit encrypts input information with the secret key and transmits the information to the server device, and (1-2) if the communication device itself does not belong to the group, the signature encryption unit transmits a dummy message to the server device,
• a signature decryption unit in which (2-1) if the communication device itself belongs to the group, the signature decryption unit downloads, from the server device, encrypted n−1 pieces of the input information and N−n number of the dummy messages transmitted from the other communication devices and decrypts the encrypted n−1 pieces of the input information with the secret key, and (2-2) if the communication device itself does not belong to the group, the signature decryption unit downloads encrypted n pieces of the input information and N−n−1 number of the dummy messages transmitted from the other communication devices,
• a session key generation unit that transmits session key generation information to the server device via the signature encryption unit, the session key generation information being information for use in generation of a session key, and generates the session key using n−1 pieces of session key generation information acquired via the signature decryption unit and session key generation information of the communication device itself, and
• a message sending and receiving unit that transmits a cipher text encrypted with the session key via the signature encryption unit to the server device and decrypts n−1 cipher texts acquired via the signature decryption unit with the session key, and the server device includes processing circuitry configured to implement
• a session key generation information saving unit that receives encrypted n pieces of the session key generation information and N−n number of the dummy messages and stores them in a downloadable manner, and
• a message saving unit that receives encrypted n number of the cipher texts and N−n number of the dummy messages and stores them in a downloadable manner.

A concealed communication method using a communication device that performs communication with other communication devices via a concealed communication network including a server device, comprising:

• a secret key generation step of generating a secret key using m partial secret keys included in K partial secret keys generated by K key generation devices, respectively;
• a signature encryption step in which given that N communication devices including the communication device itself utilize concealed communication provided by the concealed communication network and that n communication devices out of the N communication devices belong to a group, (1-1) if the communication device itself belongs to the group, the signature encryption step includes encrypting input information with the secret key and transmitting the information to the server device, and (1-2) if the communication device itself does not belong to the group, the signature encryption step includes transmitting a dummy message to the server device;
• a signature decryption step in which (2-1) if the communication device itself belongs to the group, the signature decryption step includes downloading, from the server device, encrypted n−1 pieces of the input information and N−n number of the dummy messages transmitted from the other communication devices and decrypting the encrypted n−1 pieces of the input information with the secret key, and (2-2) if the communication device itself does not belong to the group, the signature decryption step includes downloading encrypted n pieces of the input information and N−n−1 number of the dummy messages transmitted from the other communication devices;
• a session key generation step of transmitting session key generation information to the server device via the signature encryption step, the session key generation information being information for use in generation of a session key, and generating the session key using n−1 pieces of session key generation information acquired via the signature decryption step and session key generation information of the communication device itself;
• and
• a message sending and receiving step of transmitting a cipher text encrypted with the session key via the signature encryption step to the server device and decrypting n−1 cipher texts acquired via the signature decryption step with the session key.

A concealed communication method using a server device as a component of a concealed communication network that provides concealed communication to N communication devices, wherein n communication devices out of the N communication devices belong to a group, each of the communication devices includes a secret key generation unit that generates a secret key using m partial secret keys included in K partial secret keys generated by K key generation devices, respectively, a signature encryption unit in which (1-1) if the communication device itself belongs to the group, the signature encryption unit encrypts input information with the secret key and transmits the information to the server device, and (1-2) if the communication device itself does not belong to the group, the signature encryption unit transmits a dummy message to the server device, a signature decryption unit in which (2-1) if the communication device itself belongs to the group, the signature decryption unit downloads, from the server device, encrypted n−1 pieces of the input information and N−n number of the dummy messages transmitted from the other communication devices and decrypts the encrypted n−1 pieces of the input information with the secret key, and (2-2) if the communication device itself does not belong to the group, the signature decryption unit downloads encrypted n pieces of the input information and N−n−1 number of the dummy messages transmitted from the other communication devices, a session key generation unit that transmits session key generation information to the server device via the signature encryption unit, the session key generation information being information for use in generation of a session key, and generates the session key using n−1 pieces of session key generation information acquired via the signature decryption unit and session key generation information of the communication device itself, and a message sending and receiving unit that transmits a cipher text encrypted with the session key via the signature encryption unit to the server device and decrypts n−1 cipher texts acquired via the signature decryption unit with the session key, and concealed communication method comprising:

• a session key generation information saving step of receiving encrypted n pieces of the session key generation information and N−n number of the dummy messages and storing them in a downloadable manner, and
• a message saving step of receiving encrypted n number of the cipher texts and N−n number of the dummy messages and storing them in a downloadable manner.

A concealed communication method using a server device, K key generation devices, and N communication devices, wherein n communication devices out of the N communication devices belong to a group, the concealed communication method comprising:

• a secret key generation step in which each of the communication devices generates a secret key using m partial secret keys included in K partial secret keys generated by the K key generation devices, respectively;
• a signature encryption step in which (1-1) if the communication device itself belongs to the group, the communication device encrypts input information with the secret key and transmits the information to the server device, and (1-2) if the communication device itself does not belong to the group, the communication device transmits a dummy message to the server device;
• a signature decryption step in which (2-1) if the communication device itself belongs to the group, the communication device downloads, from the server device, encrypted n−1 pieces of the input information and N−n number of the dummy messages transmitted from the other communication devices and decrypts the encrypted n−1 pieces of the input information with the secret key, and (2-2) if the communication device itself does not belong to the group, the communication device downloads encrypted n pieces of the input information and N−n−1 number of the dummy messages transmitted from the other communication devices;
• a step in which the communication device transmits session key generation information to the server device via the signature encryption step, the session key generation information being information for use in generation of a session key;
• a session key generation information saving step in which the server device receives encrypted n pieces of the session key generation information and N−n number of the dummy messages and stores them in a downloadable manner;
• a session key generation step in which the communication device generates the session key using n−1 pieces of session key generation information acquired via the signature decryption step and session key generation information of the communication device itself;
• a step in which the communication device transmits a cipher text encrypted with the session key via the signature encryption step to the server device;
• a message saving step in which the server device receives encrypted n number of the cipher texts and N−n number of the dummy messages and stores them in a downloadable manner;
• and
• a message sending and receiving step in which the communication device decrypts n−1 cipher texts acquired via the signature decryption step with the session key.