Authentication using dynamic, client information based PIN

An improved PIN-based authentication technique for authenticating the user of a client machine to a server automatically generates a personal identification number (PIN) for the user based on user-specific authentication information, such as encrypted cookie information. The server provides user-specific authentication information to a client machine. When the user submits an authentication request, user-specific authentication information is collected and uploaded to the server. The user-specific authentication information is processed to form a PIN, and authentication of the user proceeds based on the PIN and any other authentication factors provided. Since the disclosed techniques compute PINs automatically based on information exchanged between a client machine and a server, the user is relieved of any burden associated with registering and remembering a PIN.