The lack of transparency for Internet communication prevents effective mitigation of today's security threats: i) Source addresses cannot be trusted and enable untraceable reflection attacks. ii) Malicious communication is opaque to all network entities, except for the receiver; and although ISPs are control points that can stop such attacks, effective detection and mitigation requires information that is available only at the end hosts. We propose TRIS, an architecture that bootstraps transparency for Internet communication. TRIS enables the definition of misbehavior according to the unique requirements of hosts, and then it constructs verifiable evidence of misbehavior. First, hosts express desired traffic properties for incoming traffic; a deviation from these properties signifies misbehavior. Second, ISPs construct verifiable evidence of misbehavior for the traffic they forward. If misbehavior is detected, it can then be proven to the ISPs of the communicating hosts. We implement our architecture on commodity hardware and demonstrate that verifiable proof of misbehavior introduces little overhead with respect to bandwidth and packet processing in the network: our prototype achieves line-rate performance for common packet sizes, saturating a 10 Gbps link with a single CPU core. In addition, we tackle incremental deployment issues and describe interoperability with today's Internet architecture.

Motivated by the weaknesses of today's TLS public-key infrastructure (PKI), recent studies have proposed numerous enhancements to fortify the PKI ecosystem. Deploying one particular enhancement is no panacea, since each one solves only a subset of the problems. At the same time, the high deployment barrier makes the benefit-cost ratio tilt in the wrong direction, leading to disappointing adoption rates for most proposals.

We make a case for packet-replay suppression at the network layer, a concept that has been generally neglected. Our contribution is twofold. First, we demonstrate a new attack, the router-reflection attack, that can be launched using compromised routers. In this attack, a compromised router degrades the connectivity of a remote Internet region just by replaying packets. The attack is feasible even if all packets are attributed to their sources, i.e., source authentication is in place, and our evaluation shows that the threat is pervasive---candidate routers for compromise are in the order of hundreds or thousands. Second, we design an in-network mechanism for replay suppression. We start by showing that designing such a mechanism poses unsolved challenges and simple adaptations of end-to-end solutions are not sufficient. Then, we devise, analyze, and implement a highly efficient protocol that suppresses replayed traffic at the network layer without global time synchronization. Our software-router prototype can saturate a 10 Gbps link using only two CPU cores for packet processing.

In an ideal Internet, every packet would be attributable to its sender, while host identities and transmitted content would remain private. Designing such a network is challenging because source accountability and communication privacy are typically viewed as conflicting properties. In this paper, we propose an architecture that guarantees source accountability and privacy-preserving communication by enlisting ISPs as accountability agents and privacy brokers. While ISPs can link every packet that originates from their network to their customers, customer identity remains unknown to the rest of the Internet. In our architecture, network communication is based on Ephemeral Identifiers (EphIDs)---cryptographic tokens that can be linked to a source only by the source's ISP. We demonstrate that EphIDs can be generated and processed efficiently, and we analyze the practical considerations for deployment.

SDN promises to make networks more flexible, programmable, and easier to manage. Inherent security problems in SDN today, however, pose a threat to the promised benefits. First, the network operator lacks tools to proactively ensure that policies will be followed or to reactively inspect the behavior of the network. Second, the distributed nature of state updates at the data plane leads to inconsistent network behavior during reconfigurations. Third, the large flow space makes the data plane susceptible to state exhaustion attacks. This paper presents SDNsec, an SDN security extension that provides forwarding accountability for the SDN data plane. Forwarding rules are encoded in the packet, ensuring consistent network behavior during reconfigurations and limiting state exhaustion attacks due to table lookups. Symmetric-key cryptography is used to protect the integrity of the forwarding rules and enforce them at each switch. A complementary path validation mechanism allows the controller to reactively examine the actual path taken by the packets. Furthermore, we present mechanisms for secure link-failure recovery.

Although TLS is used on a daily basis by many critical applications, the public-key infrastructure that it relies on still lacks an adequate revocation mechanism. An ideal revocation mechanism should be inexpensive, efficient, secure, and privacypreserving. Moreover, rising trends in pervasive encryption pose new scalability challenges that a modern revocation system should address. In this paper, we investigate how network nodes can deliver certificate-validity information to clients. We present RITM, a framework in which middleboxes (as opposed to clients, servers, or certification authorities) store revocation-related data. RITM provides a secure revocation-checking mechanism that preserves user privacy. We also propose to take advantage of content-delivery networks (CDNs) and argue that they would constitute a fast and cost-effective way to disseminate revocations. Additionally, RITM keeps certification authorities accountable for the revocations that they have issued, and it minimizes overhead at clients and servers, as they have to neither store nor download any messages. We also describe feasible deployment models and present an evaluation of RITM to demonstrate its feasibility and benefits in a real-world deployment.

The act of communication on the Internet inevitably leaks information. In particular, network headers reveal information (e.g., source address, flow information); yet, protecting the header has proven challenging. Past research successfully protected certain fields of the headers (e.g., source address), but no proposal has attempted to eliminate flow information from the header so that packets cannot be linked to flows; flow information is systematically used to subvert privacy. Hence, we investigate the following questions: Can we design an architecture that eliminates flow-packet linkability? Can we do so without imposing impractical requirements on the network infrastructure? Our proposed architecture is based on per-packet One Time Address (OTA)-an address that a host uses to send or receive exactly one packet. Furthermore, the architecture eliminates any implicit (e.g., the standard five-tuple in TCP/UDP packets) or explicit (e.g., flow identifier) flow information from packet headers. Yet, the architecture allows the communicating hosts to demultiplex seemingly unrelated packets to flows. We have implemented the proposed architecture, and our evaluation shows that it can satisfy today's packet forwarding requirements.

With the growing incidents of flash crowds and sophisticated DDoS attacks mimicking benign traffic, it becomes challenging to protect Internet-based services solely by differentiating attack traffic from legitimate traffic. While fair-sharing schemes are commonly suggested as a defense when differentiation is difficult, they alone may suffer from highly variable or even unbounded waiting times. We propose RainCheck Filter (RCF), a lightweight primitive that guarantees bounded waiting time for clients despite server flooding without keeping per-client state on the server. RCF achieves strong waiting time guarantees by prioritizing clients based on how long the clients have waited - as if the server maintained a queue in which the clients lined up waiting for service. To avoid keeping state for every incoming client request, the server sends to the client a raincheck, a timestamped cryptographic token that not only informs the client to retry later but also serves as a proof of the client's priority level within the virtual queue. We prove that every client complying with RCF can access the server in bounded time, even under a flash crowd incident or a DDoS attack. Our large-scale simulations confirm that RCF provides a small and predictable maximum waiting time while existing schemes cannot. To demonstrate its deployability, we implement RCF as a Python module such that web developers can protect a critical server resource by adding only three lines of code.