A method and an apparatus for distinguishing humans from computers. During user registration, a computer prompts a human user to provide a spoken response to certain authentication information for registration. The computer obtains registration voice data from the spoken response and establishes a registration voiceprint of the human user. During user logon, the computer identifies the user requesting to logon by the user's logon credentials, provides authentication information for logon to the user, and prompts the user to provide a spoken response to the authentication information for logon. The computer obtains logon voice data from the spoken response, and establishes a logon voiceprint of the user. The computer then determines whether the user requesting to logon is human by comparing the logon voiceprint with the registration voiceprint.