Mitigating Intersection Attacks in Anonymous Microblogging

Anonymous microblogging systems are known to be vulnerable to intersection attacks due to network churn. An adversary that monitors all communications can leverage the churn to learn who is publishing what with increasing confidence over time. In this paper, we propose a protocol for mitigating intersection attacks in anonymous microblogging systems by grouping users into anonymity sets based on similarities in their publishing behavior. The protocol provides a configurable communication schedule for users in each set to manage the inevitable trade-off between latency and bandwidth overhead. In our evaluation, we use real-world datasets from two popular microblogging platforms, Twitter and Reddit, to simulate user publishing behavior. The results demonstrate that the protocol can protect users against intersection attacks at low bandwidth overhead when the users adhere to communication schedules. In addition, the protocol can sustain a slow degradation in the size of the anonymity set over time under various churn rates

Sub-session hijacking on the web: Root causes and prevention

Since cookies act as the only proof of a user identity, web sessions are particularly vulnerable to session hijacking attacks, where the browser run by a given user sends requests associated to the identity of another user. When n > 1 cookies are used to implement a session, there might actually be n sub-sessions running at the same website, where each cookie is used to retrieve part of the state information related to the session. Sub-session hijacking breaks the ideal view of the existence of a unique user session by selectively hijacking m sub-sessions, with m < n. This may reduce the security of the session to the security of its weakest sub-session. In this paper, we take a systematic look at the root causes of sub-session hijacking attacks and we introduce sub-session linking as a possible defense mechanism. Out of two flavors of sub-session linking desirable for security, which we call intra-scope and inter-scope sub-session linking respectively, only the former is relatively smooth to implement. Luckily, we also identify programming practices to void the need for inter-scope sub-session linking. We finally present Warden, a server-side proxy which automatically enforces intra-scope sub-session linking on incoming HTTP(S) requests, and we evaluate it in terms of protection, performances, backward compatibility and ease of deployment

Token meets Wallet: Formalizing Privacy and Revocation for FIDO2

The FIDO2 standard is a widely-used class of challenge-response type protocols that allows to authenticate to an online service using a hardware token. Barbosa et al. (CRYPTO ‘21) provided the first formal security model and analysis for the FIDO2 standard. However, their model has two shortcomings: (1) It does not include privacy, one of the key features claimed by FIDO2. (2) It only covers tokens that store all secret keys locally. In contrast, due to limited memory, most existing FIDO2 tokens either derive all secret keys from a common seed or store keys on the server (the latter approach is also known as key wrapping). In this paper, we revisit the security of the WebAuthn component of FIDO2 as implemented in practice. Our contributions are as follows. (1) We adapt the model of Barbosa et al. so as to capture authentication tokens using key derivation or key wrapping. (2) We provide the first formal definition of privacy for the WebAuthn component of FIDO2. We then prove the privacy of this component in common FIDO2 token implementations if the underlying building blocks are chosen appropriately. (3) We address the unsolved problem of global key revocation in FIDO2. To this end, we introduce and analyze a simple revocation procedure that builds on the popular BIP32 standard used in cryptocurrency wallets and can efficiently be implemented with existing FIDO2 servers

Token meets Wallet: Formalizing Privacy and Revocation for FIDO2

The FIDO2 standard is a widely-used class of challenge-response type protocols that allows to authenticate to an online service using a hardware token. Barbosa et al. (CRYPTO `21) provided the first formal security model and analysis for the FIDO2 standard. However, their model has two shortcomings: (1) It does not include privacy, one of the key features claimed by FIDO2. (2) It only covers tokens that store {all secret keys locally}. In contrast, due to limited memory, most existing FIDO2 tokens either derive all secret keys from a common seed or store keys on the server (the latter approach is also known as {key wrapping}). In this paper, we revisit the security of the WebAuthn component of FIDO2 as implemented in practice. Our contributions are as follows. (1) We adapt the model of Barbosa et al. so as to capture authentication tokens using key derivation or key wrapping. (2) We provide the {first formal definition of privacy for the WebAuthn component of FIDO2}. We then prove the privacy of this component in common FIDO2 token implementations if the underlying building blocks are chosen appropriately. (3) We address the unsolved problem of {global key revocation} in FIDO2. To this end, we introduce and analyze a simple revocation procedure that builds on the popular BIP32 standard used in cryptocurrency wallets and can efficiently be implemented with existing FIDO2 servers