Sarah Meiklejohn – Security and Cryptography

Sarah Meiklejohn As a child, Sarah Meiklejohn thought she might become a linguist, largely because she was so strongly interested in the work being done to decode the ancient Greek writing systems Linear A and Linear B.

“I loved all that stuff,” she says. “And then I started doing mathematics.” At that point, with the help of Simon Singh’s The Code Book, she realised the attraction was codebreaking rather than human languages themselves. Simultaneously, security and privacy were increasingly in the spotlight.

“I’m a very private person, and so privacy is near and dear to my heart,” she says. “It’s an important right that a lot of people don’t seem interested in exercising, but it’s still a right. Even if no one voted we would still agree that it was important for people to be able to vote.”

It was during her undergraduate years at Brown, which included a fifth-year Masters degree, that she made the transition from mathematics to cryptography and began studying computer science. She went on to do her PhD at the University of California at San Diego. Her appointment at UCL, which is shared between the Department of Computer Science and the Department of Crime Science, is her first job.

Probably her best-known work is A Fistful of Bitcoins: Characterizing Payments Among Men with No Names (PDF), written with Marjori Pomarole, Grant Jordan, Kirill Levchenko, Damon McCoy, Geoffrey M. Voelker, and Stefan Savage and presented at USENIX 2013, which studied the question of how much anonymity bitcoin really provides.

“The main thing I was trying to focus on in that paper is what bitcoin is used for,” she says. The work began with buying some bitcoin (in 2012, at about £3 each), and performing some transactions with them over a period of months. Using the data collected this way allowed her to uncover some “ground truth” data.

“We developed these clustering techniques to get down to single users and owners.” The result was that they could identify which addresses belonged to which exchanges and enabled them to get a view of what was going on in the network. “So we could say this many bitcoins passed through this exchange per month, or how many were going to underground services like Silk Road.”

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Just how sophisticated will card fraud techniques become?

In late 2009, my colleagues and I discovered a serious vulnerability in EMV, the most widely used standard for smart card payments, known as “Chip and PIN” in the UK. We showed that it was possible for criminals to use a stolen credit or debit card without knowing the PIN, by tricking the terminal into thinking that any PIN is correct. We gave the banking industry advance notice of our discovery in early December 2009, to give them time to fix the problem before we published our research. After this period expired (two months, in this case) we published our paper as well explaining our results to the public on BBC Newsnight. We demonstrated that this vulnerability was real using a proof-of-concept system built from equipment we had available (off-the shelf laptop and card reader, FPGA development board, and hand-made card emulator).

No-PIN vulnerability demonstration

After the programme aired, the response from the banking industry dismissed the possibility that the vulnerability would be successfully exploited by criminals. The banking trade body, the UK Cards Association, said:

“We believe that this complicated method will never present a real threat to our customers’ cards. … Neither the banking industry nor the police have any evidence of criminals having the capability to deploy such sophisticated attacks.”

Similarly, EMVCo, who develop the EMV standards said:

“It is EMVCo’s view that when the full payment process is taken into account, suitable countermeasures to the attack described in the recent Cambridge Report are already available.”

It was therefore interesting to see that in May 2011, criminals were caught having stolen cards in France then exploiting a variant of this vulnerability to buy over €500,000 worth of goods in Belgium (which were then re-sold). At the time, not many details were available, but it seemed that the techniques the criminals used were much more sophisticated than our proof-of-concept demonstration.

We now know more about what actually happened, as well as the banks’ response, thanks to a paper by the researchers who performed the forensic analysis that formed part of the criminal investigation of this case. It shows just how sophisticated criminals could be, given sufficient motivation, contrary to the expectations in the original banking industry response.

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UCL Code Breaking Competition

6689260_sModern security systems frequently rely on complex cryptography to fulfil their goals and so it is important for security practitioners to have a good understanding of how cryptographic systems work and how they can fail. The Cryptanalysis (COMPGA18/COMPM068) module in UCL’s MSc Information Security provides students with the foundational knowledge to analyse cryptographic systems whether as part of system development in industry or as academic research.

To give students a more realistic (and enjoyable) experience there is no written exam for this module; instead the students are evaluated based on coursework and a code breaking competition.

UCL has a strong tradition of experimental research and we have been running many student competitions and hacking events in the past. In March 2013 a team directed by Dr Courtois won the UK University Cipher Challenge 2013 award, held as part of the UK Cyber Security Challenge.

This year the competition has been about finding cryptographically significant events in a real-life financial system. The competition (open both to UCL students and those of other London universities) requires the study of random number generators, elliptic curve cryptography, hash functions, exploration of large datasets, programming and experimentation, data visualisation, graphs and statistics.

We are pleased to announce the winners of the competition:

  • Joint 1st prize: Gemma Bartlett. Grade obtained 92/100.
  • Joint 1st prize: Vasileios Mavroudis.  Grade obtained 92/100.
  • 2nd prize: David Kohan Marzagão.  Grade obtained 82/100.

About the winners:

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  • Gemma Bartlett (left) is in her final year at UCL studying for an M.Eng. in Mathematical Computation with a focus on Information Security. Her particular interests include digital forensics. She will be starting a job in this field after graduation.
  • Vasilios Mavroudis (middle) received his B.Sc. in Applied Informatics from the University of Macedonia, Greece in 2012.  He is currently pursuing an M.Sc. in Information Security at UCL. In the past, he has worked as a security researcher in Deutsche Bank, University of California Santa Barbara and at the Centre for Research and Technology Hellas (CERTH). His research interests include network and systems security, malware, and applied cryptography.
  • David Kohan Marzagão (right) is currently undertaking a PhD in Computer Science under the supervision of Peter McBurney at King’s College London.  In 2014, he received his BSc in Mathematics at the University of São Paulo, Brazil. His research interests include cryptography, multi-agent systems, graph theory, and random walks.

Teaching cybersecurity to criminologists

I recently had the pleasure of teaching my first module at UCL, an introduction to cybersecurity for students in the SECReT doctoral training centre.

The module had been taught before, but always from a fairly computer-science-heavy perspective. Given that the students had largely no background in computer science, and that my joint appointment in the Department of Security and Crime Science has given me at least some small insight into what aspects of cybersecurity criminologists might find interesting, I chose to design the lecture material largely from scratch. I tried to balance the technical components of cybersecurity that I felt everyone needed to know (which, perhaps unsurprisingly, included a fair amount of cryptography) with high-level design principles and the overarching question of how we define security. Although I say I designed the curriculum from scratch, I of course ended up borrowing heavily from others, most notably from the lecture and exam material of my former supervisor’s undergraduate cybersecurity module (thanks, Stefan!) and from George’s lecture material for Introduction to Computer Security. If anyone’s curious, the lecture material is available on my website.

As I said, the students in the Crime Science department (and in particular the ones taking this module) had little to no background in computer science.  Instead, they had a diverse set of academic backgrounds: psychology, political science, forensics, etc. One of the students’ proposed dissertation titles was “Using gold nanoparticles on metal oxide semiconducting gas sensors to increase sensitivity when detecting illicit materials, such as explosives,” so it’s an understatement to say that we were approaching cybersecurity from different directions!

With that in mind, one of the first things I did in my first lecture was to take a poll on who was familiar with certain concepts (e.g., SSH, malware, the structure of the Internet), and what people were interested in learning about (e.g., digital forensics, cryptanalysis, anonymity). I don’t know what I was expecting, but the responses really blew me away! The students overwhelmingly wanted to hear about how to secure themselves on the Internet, both in terms of personal security habits (e.g., using browser extensions) and in terms of understanding what and how things might go wrong. Almost the whole class specifically requested Tor, and a few had even used it before.

This theme of being (pleasantly!) surprised continued throughout the term.  When I taught certificates, the students asked not for more details on how they work, but if there was a body responsible for governing certificate authorities and if it was possible to sue them if they misbehave. When I taught authentication, we played a Scattergories-style game to weigh the pros and cons of various authentication mechanisms, and they came up with answers like “a con of backup security questions is that they reveal cultural trends that may then be used to reveal age, ethnicity, gender, etc.”

There’s still a month and a half left until the students take the exam, so it’s too soon to say how effective it was at teaching them cybersecurity, but for me the experience was a clear success and one that I look forward to repeating and refining in the future.

One-out-of-Many Proofs: Or How to Leak a Secret and Spend a Coin

I’m going to EUROCRYPT 2015 to present a new zero-knowledge proof that I’ve developed together with Markulf Kohlweiss from Microsoft Research. Zero-knowledge proofs enable you to demonstrate that a particular statement is true without revealing anything else than the fact it is true. In our case the statements are one-out-of-many statements, intuitively that out of a number of items one of them has a special property, and we greatly reduce the size of the proofs compared to previous works in the area. Two applications where one-out-of-many proofs come in handy are ring signatures and Zerocoin.

Ring signatures can be used to sign a message anonymously as a member of a group of people, i.e., all a ring signature says is that somebody from the group signed the message but not who it was. Consider for instance a whistleblower who wants to leak her company is dumping dangerous chemicals in the ocean, yet wants to remain anonymous due to the risk of being fired. By using a ring signature she can demonstrate that she works for the company, which makes the claim more convincing, without revealing which employee she is. Our one-out-of-many proofs can be used to construct very efficient ring signatures by giving a one-out-of-many proof that the signer holds a secret key corresponding to a public key for one of the people in the ring.

Zerocoin is a new virtual currency proposal where coins gain value once they’ve been accepted on a public bulletin board. Each coin contains a commitment to a secret random serial number that only the owner knows. To anonymously spend a coin the owner publishes the serial number and gives a one-out-of-many proof that the serial number corresponds to one of the public coins. The serial number prevents double spending of a coin; nobody will accept a transaction with a previously used serial number. The zero-knowledge property of the one-out-of-many proof provides anonymity; it is not disclosed which coin the serial number corresponds to. Zerocoin has been suggested as a privacy enhancing add-on to Bitcoin.

The full research paper is available on the Cryptology ePrint Archive.