Abstract: An Online Brokering Service provides user authentication and billing services to allow users to anonymously and securely purchase online services from Service Providers (SP) sites (e.g., World Wide Web sites) over a distributed public network, which may be an untrusted public network such as the Internet. Users and SP sites initially register with the Brokering Service, and are provided with respective client and server software components for using the Brokering Service. In one embodiment, when a user initially connects to an SP site, the SP site transmits a challenge message over the public network to the user computer, and the user computer generates and returns and cryptographic response message (preferably generated using a password of the user). The SP site then passes the response message to the Brokering Service, which in-turn looks up the user's password and authenticates the response message.