UPPRESSO: Untraceable and Unlinkable Privacy-PREserving Single Sign-On Services

TL;DR

UPPRESSO introduces a privacy-preserving SSO scheme that generates untraceable ephemeral identities, preventing identity tracking and profile linking across relying parties while maintaining compatibility with existing protocols.

Contribution

It proposes an identity-transformation approach for untraceable pseudo-identities in SSO, enhancing privacy without requiring additional client-side software.

Findings

Protects user identities against curious IdPs and colluding RPs.

Works with existing SSO protocols and browsers without plug-ins.

Achieves reasonable performance overheads.

Abstract

Single sign-on (SSO) allows a user to maintain only the credential for an identity provider (IdP) to log into multiple relying parties (RPs). However, SSO introduces privacy threats, as (a) a curious IdP could track a user's all visits to RPs, and (b) colluding RPs could learn a user's online profile by linking her identities across these RPs. This paper presents a privacypreserving SSO scheme, called UPPRESSO, to protect an honest user's online profile against (a) an honest-but-curious IdP and (b) malicious RPs colluding with other users. UPPRESSO proposes an identity-transformation approach to generate untraceable ephemeral pseudo-identities for an RP and a user from which the target RP derives a permanent account for the user, while the transformations also provide unlinkability. This approach protects the identities of the user and the target RPs in a login flow, while working compatibly with widely-deployed SSO protocols and providing services accessed from a commercial-off-the-shelf browser without plug-ins or extensions. We built a prototype of UPPRESSO on top of MITREid Connect, an open-source SSO system. The extensive evaluations show that it fulfills the security and privacy requirements of SSO with reasonable overheads.