David Isaac Wolinsky

Ewa Syta, Iulia Tamas, Dylan Visher et al.

The secret keys of critical network authorities - such as time, name, certificate, and software update services - represent high-value targets for hackers, criminals, and spy agencies wishing to use these keys secretly to compromise other hosts. To protect authorities and their clients proactively from undetected exploits and misuse, we introduce CoSi, a scalable witness cosigning protocol ensuring that every authoritative statement is validated and publicly logged by a diverse group of witnesses before any client will accept it. A statement S collectively signed by W witnesses assures clients that S has been seen, and not immediately found erroneous, by those W observers. Even if S is compromised in a fashion not readily detectable by the witnesses, CoSi still guarantees S's exposure to public scrutiny, forcing secrecy-minded attackers to risk that the compromise will soon be detected by one of the W witnesses. Because clients can verify collective signatures efficiently without communication, CoSi protects clients' privacy, and offers the first transparency mechanism effective against persistent man-in-the-middle attackers who control a victim's Internet access, the authority's secret key, and several witnesses' secret keys. CoSi builds on existing cryptographic multisignature methods, scaling them to support thousands of witnesses via signature aggregation over efficient communication trees. A working prototype demonstrates CoSi in the context of timestamping and logging authorities, enabling groups of over 8,000 distributed witnesses to cosign authoritative statements in under two seconds.

John Maheswaran, Daniel Jackowitz, David Isaac Wolinsky et al.

Social networking sites supporting federated identities offer a convenient and increasingly popular mechanism for cross-site authentication. Unfortunately, they also exacerbate many privacy and tracking risks. We propose Crypto-Book, an anonymizing layer enabling cross-site authentication while reducing these risks. Crypto-Book relies on a set of independently managed servers that collectively assign each social network identity a public/private keypair. Only an identity's owner learns all the private key shares, and can therefore construct the private key, while all participants can obtain any user's public key, even if the corresponding private key has yet to be retrieved. Having obtained an appropriate key set, a user can then leverage anonymous authentication techniques such as linkable ring signatures to log into third-party web sites while preserving privacy. We have implemented a prototype of Crypto-Book and demonstrate its use with three applications: a Wiki system, an anonymous group communication system, and a whistleblower submission system. Our results show that for anonymity sets of size 100, Crypto-Book login takes 0.56s for signature generation by the client, 0.38s for signature verification on the server, and requires 5.6KB of communication bandwidth.

David Isaac Wolinsky, Bryan Ford

Despite the attempts of well-designed anonymous communication tools to protect users from tracking or identification, flaws in surrounding software (such as web browsers) and mistakes in configuration may leak the user's identity. We introduce Nymix, an anonymity-centric operating system architecture designed "top-to-bottom" to strengthen identity- and tracking-protection. Nymix's core contribution is OS support for nym-browsing: independent, parallel, and ephemeral web sessions. Each web session, or pseudonym, runs in a unique virtual machine (VM) instance evolving from a common base state with support for long-lived sessions which can be anonymously stored to the cloud, avoiding de-anonymization despite potential confiscation or theft. Nymix allows a user to safely browse the Web using various different transports simultaneously through a pluggable communication model that supports Tor, Dissent, and a private browsing mode. In evaluations, Nymix consumes 600 MB per nymbox and loads within 15 to 25 seconds.

David Isaac Wolinsky, Ewa Syta, Bryan Ford

Some anonymity schemes might in principle protect users from pervasive network surveillance - but only if all messages are independent and unlinkable. Users in practice often need pseudonymity - sending messages intentionally linkable to each other but not to the sender - but pseudonymity in dynamic networks exposes users to intersection attacks. We present Buddies, the first systematic design for intersection attack resistance in practical anonymity systems. Buddies groups users dynamically into buddy sets, controlling message transmission to make buddies within a set behaviorally indistinguishable under traffic analysis. To manage the inevitable tradeoffs between anonymity guarantees and communication responsiveness, Buddies enables users to select independent attack mitigation policies for each pseudonym. Using trace-based simulations and a working prototype, we find that Buddies can guarantee non-trivial anonymity set sizes in realistic chat/microblogging scenarios, for both short-lived and long-lived pseudonyms.

Henry Corrigan-Gibbs, David Isaac Wolinsky, Bryan Ford

The DC-nets approach to anonymity has long held attraction for its strength against traffic analysis, but practical implementations remain vulnerable to internal disruption or "jamming" attacks requiring time-consuming tracing procedures to address. We present Verdict, the first practical anonymous group communication system built using proactively verifiable DC-nets: participants use public key cryptography to construct DC-net ciphertexts, and knowledge proofs to detect and detect and exclude misbehavior before disruption. We compare three alternative constructions for verifiable DC-nets, one using bilinear maps and two based on simpler ElGamal encryption. While verifiable DC-nets incurs higher computation overheads due to the public-key cryptography involved, our experiments suggest Verdict is practical for anonymous group messaging or microblogging applications, supporting groups of 100 clients at 1 second per round or 1000 clients at 10 seconds per round. Furthermore, we show how existing symmetric-key DC-nets can "fall back" to a verifiable DC-net to quickly identify mis- behavior improving previous detections schemes by two orders of magnitude than previous approaches.