Censorship-resistant communication systems generally use real-world cover protocols to establish a covert channel through which uncensored communication can occur. Unfortunately, many previously proposed systems use cover protocols inconsistently with the way humans normally use those protocols, leading to anomalous network traffic patterns that have been shown to be discoverable by real-world censors. In this paper, we argue that censorship-resistant communication systems should follow two behavior-based design properties: (i) behavioral independence: systems should isolate the operation of their covert channels from the operation of their cover protocols, and (ii) behavioral realism: systems should either opportunistically use existing genuine cover protocol instances or run new protocol instances that are modeled after genuine ones. These properties ensure that the behavior of a system’s users will not degrade its security. We demonstrate how to achieve these properties through the design and evaluation of Raven, a censorship-resistant messaging system that uses email cover protocols identically to the way humans use email. Raven uses a generative adversarial network that is trained on genuine email data to control the timing and sizes of the email messages it sends and receives, and these messages are transferred independently of user actions. Our evaluation shows that, compared to the state-of-the-art email-based Mailet system, Raven raises the false-positive rate from 3% to 50% when detecting covert channel usage with 100% recall.

We introduce a novel model of routing security that incorporates the ordinarily overlooked variations in trust that users have for different parts of the network. We focus on anonymous communication, and in particular onion routing, although we expect the approach to apply more broadly.

Tor is a free-software anonymizing overlay network that helps people around the world use the Internet in safety. Tor’s 2500 volunteer relays carry almost 10Gb/s of traffic for several hundred thousand users each day. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Today's software update systems have little or no defense against key compromise. As a result, key compromises have put millions of software update clients at risk. Here we identify three classes of information whose authenticity and integrity are critical for secure software updates. Analyzing existing software update systems with our framework, we find their ability to communicate this information securely in the event of a key compromise to be weak or nonexistent. We also find that the security problems in current software update systems are compounded by inadequate trust revocation mechanisms. We identify core security principles that allow software update systems to survive key compromise. Using these ideas, we design and implement TUF, a software update framework that increases resilience to key compromise.

We present Mixminion, a message-based anonymous remailer protocol with secure single-use reply blocks. Mix nodes cannot distinguish Mixminion forward messages from reply messages, so forward and reply messages share the same anonymity set. We add directory servers that allow users to learn public keys and performance statistics of participating remailers, and we describe nymservers that provide long-term pseudonyms using single-use reply blocks as a primitive. Our design integrates link encryption between remailers to provide forward anonymity. Mixminion works in a real-world Internet environment, requires little synchronization or coordination between nodes, and protects against known anonymity-breaking attacks as well as or better than other systems with similar design parameters.