User terminals, and methods and computer-readable recording mediums storing computer programs for transmitting and receiving messages

1. A non-transitory computer-readable recording medium storing computer-readable instructions that, when executed by a first user terminal, cause the first user terminal to perform a method comprising:

receiving a first message including a first attached file;

generating a first encryption key and a second encryption key, the first encryption key being a key for encrypting the first message, the second encryption key being a key for encrypting the first attached file, the second encryption key being one selected from a plurality of encryption key types based on file type of the first attached file;

encrypting the first message using the first encryption key and the first attached file using the second encryption key;

adding sender information to the first message, the sender information associated with the first message; and

transmitting the first message including the sender information to a message server,

wherein the transmitting transmits the first message including the sender information to the message server while not sharing information associated with the encrypting with the message server.

2. The non-transitory computer-readable recording medium of claim 1 , wherein the generating comprises:

generating a hash value of the first attached file as the second encryption key for the first attached file.

3. The non-transitory computer-readable recording medium of claim 1 , wherein

the first attached file is a video file; and

the generating includes generating a hash value of the first attached file as the second encryption key for the first attached file.

4. The non-transitory computer-readable recording medium of claim 1 , wherein the generating comprises:

generating a plurality of first encryption keys including the first encryption key, for a plurality of second user terminals for transmitting the first message to the plurality of second user terminals.

5. The non-transitory computer-readable recording medium of claim 1 , wherein the method further comprises:

receiving a second message and an index associated with a second attached file, the second message transmitted by a second user terminal via the message server;

extracting the index;

calling the second attached file from the message server using the index; and

decrypting the second attached file using a decryption key received from the second user terminal.

6. The non-transitory computer-readable recording medium of claim 1 , wherein the method further comprises:

receiving a first authentication key from a second user terminal;

deciding whether the first authentication key is identical to a second authentication key that has been transmitted to the message server from a third user terminal; and

transmitting one or more messages that have been provided through the second user terminal to the third user terminal in response to a result of the deciding indicating that the first authentication key matches the second authentication key,

wherein the one or more messages sent to the third user terminal are encrypted using an encryption key generated based on the first authentication.

7. A user terminal comprising:

a memory storing computer-readable instructions; and

at least one processor configured to execute the computer-readable instructions such that the at least one processor is configured to:

receive a first message including a first attached file, the first message including sender information:

generate a first encryption key and a second encryption key, the first encryption key being a key for encrypting the first message, the second encryption key being a key for encrypting the first attached file, the second encryption key being one selected from a plurality of encryption key types based on file type of the first attached file; and

encrypt the first message using the first encryption key and the first attached file using the second encryption key; and

a transceiver configured to transmit the first message to a message server.

8. The user terminal of claim 7 , wherein, when a size of the first attached file is greater than or equal to a threshold value, the at least one processor is configured to execute the computer-readable instructions such that the at least one processor is further configured to generate a hash value of the first attached file as the second encryption key for the first attached file.

9. The user terminal of claim 7 , wherein

the first attached file is a video file; and

the at least one processor is configured to execute the computer-readable instructions such that the at least one processor is further configured to generate a hash value of the first attached file as the second encryption key for the first attached file.

10. The user terminal of claim 7 , wherein the at least one processor is configured to execute the computer-readable instructions such that the at least one processor is further configured to generate a plurality of first encryption keys including the first encryption key for a plurality of other user terminals for transmitting the first message to the plurality of other user terminals.

11. The user terminal of claim 7 , wherein the at least one processor is configured to execute the computer-readable instructions such that the at least one processor is further configured to

receive a second message and an index associated with a second attached file, the second message received from a second user terminal via the message server;

extract an index;

call the second attached file from the message server using the index; and

decrypt the second attached file using a decryption key received from the second user terminal.

12. The user terminal of claim 7 , wherein the at least one processor is configured to execute the computer-readable instructions such that the at least one processor is further configured to,

receive a first authentication key from a second user terminal,

decide whether the first authentication key is identical to a second authentication key that has been transmitted to the message server from a third user terminal, and

transmit one or more messages that have been provided through the second user terminal to the third user terminal in response to a decided result indicating that the first authentication key matches the second authentication key,

wherein the one or more messages sent to the third user terminal are encrypted using an encryption key generated based on the first authentication.

13. The user terminal of claim 12 , wherein the transceiver is further configured to

transmit the second authentication key from the second user terminal to the message server;

receive a validation result with regard to the second authentication key from the message server in response to transmitting the second authentication key; and

transmit the first message to the second user terminal based on the validation result.

14. The user terminal of claim 12 , wherein

the at least one processor is further configured to execute the computer-readable instructions such that the at least one processor is further configured to encrypt the first message using the first authentication key; and

the transceiver is further configured to transmit the encrypted first message to the second user terminal.