A method for secure transmission of user-authenticating information is provided. The method includes steps of: a service-providing server (a) determining whether a public key of the user is registered in a blockchain network, and instructing a user-authenticating information generator to generate user-authenticating information for reference, instructing an encrypting engine to encrypt the generated user-authenticating information for reference by using the public key of the user retrieved from the blockchain network, and instructing a message-sending part to transmit the encrypted user-authenticating information for reference to a mobile device of the user; and (b) in case that user-authenticating information for comparison is acquired from the user device and if the user-authenticating information for comparison is determined as identical to the user-authenticating information for reference, providing the user device with the service desired by the user.