Token-based Web Single Signon with Enabled Clients

We study a type of web single signon recently introduced by one of four proposed standard protocols of the Liberty Alliance. In contrast to the other three Liberty protocols and prior protocols like Microsoft Passport and the SAML standard, the client is not only a browser, but aware of the protocol, for instance a web-service client. We investigate how this protocol differs from standard three-party authentication, and possible benefits. We call the new protocol class token-based web single signon with enabled clients. We show a man-in-the-middle attack on the original Liberty V1.0 protocol and countermeasures against it. (Such a countermeasure was now added as an erratum, and no deployed implementation will use V1.0.) We also give general guidance for designing secure protocols in this class.