In an authentication system, a first computer system provides a desired service and a second computer system provides a dialog-based interactive protocol service (e.g., an instant messaging service). Users of a second computer system can be authenticated by the first system using a mechanism separate from the dialog based interactive protocol system, so the users can then access the first system using the dialog based interactive protocol systems (even though the systems are not owned or necessarily trusted by the first system). The authentication system enables communication between the first and second computer systems by establishing the dialog session between the computer systems and transmitting to the second computer system a link to a site used to authenticate this computer system. The authentication system associates authentication information (e.g., a unique identifier and/or an authorization token, etc.) with the dialog and uses this information to enable the second computer system to have authorized communications with the first.