Privacy-Preserving Web Single Sign-On: Formal Security Analysis and Design

Guido Schmitz

Research output: ThesisDoctoral Thesis


Web-based single sign-on (SSO) systems enable Web sites, so-called relying parties (RPs), to outsource user authentication to other entities, so-called identity providers (IdPs). Such systems are widely deployed in the Web, e.g., Facebook Login or Google Sign-in. RPs do not need to maintain authentication data of their users, and users can log in at RPs in a convenient way. Fundamental to SSO is security: The SSO protocol must not permit an attacker to impersonate anyone else, nor must it allow a false identity to be imposed on anyone. If this is not the case, attacks are possible that have devastating effects on the security of RPs and their users. While aiming at security, most SSO systems, however, neglect privacy. IdPs can track their users as they (by design) learn at which RP a user logs in. This lack of privacy allows IdPs to create extensive user profiles and might cause some users not to use SSO at all. Moreover, IdPs are enabled to decide ad-hoc whether they allow a user to log in at a specific RP. Therefore, privacy-preserving systems, which do not reveal to IdPs to which RP a user would like to log in or has logged in, are highly desirable in many situations. The design of such systems, however, is very challenging because privacy can easily be compromised. So far, only one SSO system has been proposed with this kind of privacy in mind: Mozilla's BrowserID (a.k.a. Mozilla Persona). In this thesis, we use the Web Infrastructure Model (WIM) to analyze the security of SSO protocols. The WIM is the most comprehensive formal model of the Web infrastructure to date, which applies to a wide range of Web applications and standards. We also extend the WIM to be able to analyze privacy. We use the extended WIM to, for the first time, carry out a systematic and rigorous formal analysis of privacy for Web SSO systems. Using our approach, we analyze the Web SSO system BrowserID. As a result of this first rigorous analysis of an SSO system in the Web infrastructure, we find severe attacks. These attacks not only affect the security of BrowserID but also show that BrowserID's unique privacy claim does not hold. We propose fixes for BrowserID and prove that the fixed system provides security. Regarding privacy, we show that BrowserID, unfortunately, is broken beyond repair. Inspired by BrowserID's goal, we propose the first privacy-preserving Web SSO system, called SPRESSO (for Secure Privacy-REspecting Single Sign-On). SPRESSO is easy to use, decentralized and based solely on native Web features. We design SPRESSO within the WIM right from the start and prove that SPRESSO satisfies strong security and privacy guarantees.
Original languageEnglish
Awarding Institution
  • University of Stuttgart
  • Küsters, Ralf, Supervisor, External person
Award date30 Oct 2019
Publication statusPublished - 2019
Externally publishedYes

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