I am a Ph.D. Candidate in Computer Science and Engineering at the University of Michigan, where I am advised by J. Alex Halderman. I graduated from the University of Michigan in 2015 as a Bachelor of Science, Engineering in Computer Science with a Mathematics Minor. I can be reached at firstname.lastname@example.org, and my CV is available here.
I am interested in Computer Security and Privacy broadly. My research interests are problems that affect policy discussions and the use of measurement as a tool to make precise claims about the Internet in these discussions. Currently, I am working on providing additional insight into the phenomenon of Internet censorship. Previous work includes a meta-measurement study of the HTTPS certificate ecosystem and the proposal of a new, malicious cryptocurrency. As part of this, I am a contributer to the Censys Search Engine.
Internet censorship and network interference have become widespread in many countries, and an emerging body of work has sought to bring transparency to these practices through techniques that can remotely measure connectivity disruptions. Remote measurement tools can now detect DNS- and IP-based blocking at global scale; however, a major unmonitored form of interference is blocking triggered by deep packet inspection of application-layer data. We close this gap by introducing a scalable, remote measurement system that can efficiently detect application-layer interference. We show that our technique can effectively detect application-layer blocking triggered on HTTP and TLS headers, and it is flexible enough to support many other diverse protocols. In experiments, we test for blocking across 4,458 autonomous systems, an order of magnitude larger than provided by country probes used by OONI. We also test a corpus of 100,000 keywords from vantage points in 40 countries to produce detailed national blocklists. Finally, we analyze the keywords we find blocked to provide insight into the application-layer blocking ecosystem and compare countries’ behavior. We find that the most consistently blocked services are related to circumvention tools, pornography, and gambling, but that there is significant country-to-country variation.
The HTTPS certificate ecosystem has been of great interest to the measurement and security communities. Without any ground truth, researchers have attempted to study this PKI from a variety of fragmented perspectives, including passively monitored networks, scans of the popular domains or the IPv4 address space, search engines such as Censys, and Certificate Transparency (CT) logs. In this work, we comparatively analyze all these perspectives. We find that aggregated CT logs and Censys snapshots have many properties that complement each other, and that together they encompass over 99% of all certificates found by any of these techniques. However, they still miss 1.5% of certificates observed in a crawl of all domains in .com, .net, and .org. We go on to illustrate how this combined perspective affects results from previous studies. In light of these findings, we have worked with the operators of Censys to incorporate CT log data into its results going forward, and we recommend that future HTTPS measurement adopt this new vantage.
Since its creation in 2009, Bitcoin has used a hash-based proof-of-work to generate new blocks, and create a single public ledger of transactions. The hash-based computational puzzle employed by Bitcoin is instrumental to its security, preventing Sybil attacks and making double-spending attacks more difficult. However, there have been concerns over the efficiency of this proof-of-work puzzle, and alternative “useful” proofs have been proposed. In this paper, we present DDoSCoin, which is a cryptocurrency with a malicious proof-of-work. DDoSCoin allows miners to prove that they have contributed to a distributed denial of service attack against specific target servers. This proof involves making a large number of TLS connections to a target server, and using cryptographic responses to prove that a large number of connections has been made. Like proof-of-work puzzles, these proofs are inexpensive to verify, and can be made arbitrarily difficult to solve.
We investigate the security of Diffie-Hellman key exchange as used in popular Internet protocols and find it to be less secure than widely believed. First, we present a novel flaw in TLS that allows a man-in-the-middle to downgrade connections to “export-grade” Diffie-Hellman. To carry out this attack, we implement the number field sieve discrete log algorithm. After a week-long precomputation for a specified 512-bit group, we can compute arbitrary discrete logs in this group in minutes. We find that 82% of vulnerable servers use a single 512-bit group, allowing us to compromise connections to 7% of Alexa Top Million HTTPS sites. In response, major browsers are being changed to reject short groups. We go on to consider Diffie-Hellman with 768- and 1024-bit groups. A small number of fixed or standardized groups are in use by millions of TLS, SSH, and VPN servers. Performing precomputations on a few of these groups would allow a passive eavesdropper to decrypt a large fraction of Internet traffic. In the 1024-bit case, we estimate that such computations are plausible given nation-state resources, and a close reading of published NSA leaks shows that the agency’s attacks on VPNs are consistent with having achieved such a break. We conclude that moving to stronger key exchange methods should be a priority for the Internet community.