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 including: receiving a first message including a first attached file, generating at least one encryption key for encrypting the first message by taking into account a type of the first attached file, encrypting the first attached file of the first message by using the encryption key, adding sender information of the first message to the first message, and transmitting the first message including the sender information to a message server, may be provided.