A method is provided for providing a notary service for a file, the method includes the steps in which: (a) when a notary service request for a specific file is obtained, server generates, by using a hash function, or supports the generation of, a message digest of the specific file; and (b) if a predetermined condition is satisfied, the server registers, in a database, or supports the registration of, a representative hash value or a value obtained by processing the representative hash value, the representative hash value being generated by calculating at least one neighboring hash value that matches a specific hash value, wherein the specific hash value is a hash value of the result of encrypting the message digest with a private key of a first user, a private key of a second user and a private key of the server.