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, a message obtained by encrypting the message or the file attached to the message; and

when receiving the message from the another user terminal, request the server device to decrypt the message or the file attached to the message, and receive the decrypted message or file from the server device, and the server device includes:

second processing circuitry configured to:

generate a private key corresponding to the identification information about the recipient of the message; and

when receiving a request for decryption of the message or the file attached to the message from the user terminal, decrypt the message or the file attached to the message using the private key generated, and transmit the decrypted message or file to the user terminal that has made the request for decryption.

2 . The communication system according to claim 1 ,

wherein the first processing circuitry is further configured to perturb the message or the file attached to the message encrypted.

3 . 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.

4 . 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, a message obtained by encrypting the message or the file attached to the message; and

when receiving the message from the another user terminal, request a server device to decrypt the message or the file attached to the message, and receive the decrypted message or file from the server device.

5 . 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, transmitting, to the another user terminal, a message obtained in the encryption in which the message or the file attached to the message has been encrypted;

by the user terminal, when receiving the message from the another user terminal, requesting the server device to decrypt the message or the file attached to the message;

by the server device, generating a private key corresponding to the identification information about the recipient of the message; and

by the server device, when receiving a request for decryption of the message or the file attached to the message from the user terminal, decrypting the message or the file attached to the message using the private key generated, and transmitting the decrypted message or file to the user terminal that has made the request for decryption.

6 . (canceled)