A method for storing an encrypted file by a server is provided. The server receives a first encrypted file identifier from a client. The server generates a random number and transmits the random number to the client, when the first encrypted file identifier is present in a first database. The server generates a first verification value using the random number. In addition, the server confirms whether or not the client possesses a first encrypted file corresponding to the first encrypted file identifier among encrypted files stored in a second database by comparing the first verification value and a second verification value based on the random number with each other, when receiving the second verification value from the client.