1. A software distribution system comprising: an entry device signing up a client through a secured communications line; a key database preliminarily storing a correspondence table for key information and a plurality of machine identifiers of clients; a key information assignment device assigning key information uniquely corresponding to a machine identifier of the client in a sign-up process; and an encryption device enciphering at least one of a password and software contents on Internet using the key information to distribute software contents through the Internet.