COMMUNICATION SYSTEM, USER TERMINAL, COMMUNICATION METHOD, AND COMMUNICATION PROGRAM

1 . A communication system comprising: a user terminal that transmits and receives a message; and a server device that manages a public key and a private key, wherein

the user terminal includes:

first processing circuitry configured to:

when transmitting the message to another user terminal, acquire a public key corresponding to identification information about a recipient of the message, and encrypt the message or a file attached to the message, using the acquired public key;

transmit, to the another user terminal, the message encrypted or the file attached to the message;

when receiving the message from the another user terminal, request the server device to provide a private key for decrypting the message or the file attached to the message, and receive the private key from the server device; and

decrypt the message or the file attached to the message, using the private key received, and

the server device includes

second processing circuitry configured to:

when accepting a request for the private key from the user terminal, generate the private key corresponding to the identification information about the recipient of the message, and transmit the private key to the user terminal.

2 . The communication system according to claim 1 , wherein the first processing circuitry is further configured to encrypt the message or the file attached to the message, with policy information included in the message or the file, the policy information indicating a condition for allowing decryption.

3 . A user terminal comprising:

processing circuitry configured to:

when transmitting a message to another user terminal, acquire a public key corresponding to identification information about a recipient of the message, and encrypt the message or a file attached to the message, using the acquired public key;

transmit, to the another user terminal, the message encrypted or the file attached to the message;

when receiving the message from the another user terminal, request a server device to provide a private key for decrypting the message or the file attached to the message, and receive the private key from the server device; and

decrypt the message or the file attached to the message, using the private key received.

4 . A communication method implemented by a communication system that includes: a user terminal that transmits and receives a message; and a server device that manages a public key and a private key,

the communication method comprising:

by the user terminal, when transmitting the message to another user terminal, acquiring a public key corresponding to identification information about a recipient of the message, and encrypting the message or a file attached to the message, using the acquired public key;

by the user terminal, to transmitting, to the another user terminal, the message encrypted or the file attached to the message;

by the user terminal, when receiving the message from the another user terminal, requesting the server device to provide a private key for decrypting the message or the file attached to the message, and receiving the private key from the server device;

by the server device, when accepting a request for the private key from the user terminal, generating the private key corresponding to the identification information about the recipient of the message, and transmitting the private key to the user terminal; and

by the user terminal, decrypting the message or the file attached to the message, using the private key received.

5 . (canceled)