A method for authenticating a file, comprising steps of: (a) a server, if an authentication request for the file is acquired, from each of a first user device and a second user device, generating or supporting another device to generate a specific message digest of the file using a hash function, and transmitting the specific message digest to the first user device and the second user device; (b) the server, if a first message digest and a second message digest are acquired from each of the first user device and the second user device, wherein the first message digest is generated by encoding the specific message digest with a private key of a first user in the first user device and the second message digest is generated by encoding the specific message digest with a private key of a second user in the second user device, and if each piece of (i) first information acquired by decoding the first message digest with a public key of the first user and (ii) second information acquired by decoding the second message digest with a public key of the second user, corresponds to the specific message digest generated at the step of (a), wherein the public key of the first user is transmitted to the server with the public key of the first user being included in the authentication request from the first user device or is transmitted to the server along with the first message digest and wherein the public key of the second user is transmitted to the server with the public key of the second user being included in the authentication request from the second user device or is transmitted to the server along with the second message digest, registering or supporting another device to register a hash value of a third message digest, generated by encoding the specific message digest with the private key of the first user, the private key of the second user, and a private key of the server, in a blockchain of virtual currency; and (c) the server acquiring or supporting another device to acquire a transaction ID indicating a location of the hash value registered in the blockchain of virtual currency, wherein, at the step of (b), the third message digest is acquired by encoding a sum value with the private key of the server, in the blockchain of virtual currency, and wherein the sum value is a sum of the first message digest and the second message digest. (a) a server, if an authentication request for the file is acquired, from each of a first user device and a second user device, generating or supporting another device to generate a specific message digest of the file using a hash function, and transmitting the specific message digest to the first user device and the second user device; (b) the server, if a first message digest and a second message digest are acquired from each of the first user device and the second user device, wherein the first message digest is generated by encoding the specific message digest with a private key of a first user in the first user device and the second message digest is generated by encoding the specific message digest with a private key of a second user in the second user device, and if each piece of (i) first information acquired by decoding the first message digest with a public key of the first user and (ii) second information acquired by decoding the second message digest with a public key of the second user, corresponds to the specific message digest generated at the step of (a), wherein the public key of the first user is transmitted to the server with the public key of the first user being included in the authentication request from the first user device or is transmitted to the server along with the first message digest and wherein the public key of the second user is transmitted to the server with the public key of the second user being included in the authentication request from the second user device or is transmitted to the server along with the second message digest, registering or supporting another device to register a hash value of a third message digest, generated by encoding the specific message digest with the private key of the first user, the private key of the second user, and a private key of the server, in a blockchain of virtual currency; and (c) the server acquiring or supporting another device to acquire a transaction ID indicating a location of the hash value registered in the blockchain of virtual currency,