Dos and Don'ts of Client Authentication on the Web

Kevin Fu


Client authentication has been a continuous source of problems on the Web. Although many well-studied techniques exist for authentication, Web sites continue to use extremely weak authentication schemes, especially in non-enterprise environments such as store fronts. These weaknesses often result from careless use of authenticators within Web cookies. Of the twenty-seven sites we investigated, we weakened the client authentication on two systems, gained unauthorized access on eight, and extracted the secret key used to mint authenticators from one.

We provide a description of the limitations, requirements, and security models specific to Web client authentication. This includes the introduction of the interrogative adversary, a surprisingly powerful adversary that can adaptively query a Web site.

We propose a set of hints for designing a secure client authentication scheme. Using these hints, we present the design and analysis of a simple authentication scheme secure against forgeries by the interrogative adversary. In conjunction with SSL, our scheme is secure against forgeries by the active adversary.

Gates 4B (opposite 490), Monday 3/18/02, 4:30 PM