Jan Henrik Ziegeldorf

San Antonio, Texas

In the summer semester of 2014, I taught a¬†Research Focussed Class (RFC)*¬†on¬†Selected Topics of Security and Privacy. That’s when I first came into contact with Bitcoin**: A student of mine was interested in the anonymity properties of Bitcoin or the lack of it. One approach to (re-)establish privacy in Bitcoin is¬†Mixing, i.e., shuffling Bitcoins between the members of a group such that transactions become untraceable when viewed from the outside – the bigger the group the higher the anonymity of the participating users***.

When we started work, a few commercial mixing services existed as well as a handful of academic approaches. All of them (try to) tackle the same two core problems:

  • preventing theft: some services are outright scams and will steal your coins
  • providing anonymity:¬†centralized services know how coins are mixed while decentralized approaches scaled only to small number of users thereby limiting the anonymity set***

Our idea was to use¬†Secure Multi-Party Computation (SMC)¬†for mixing. In a nutshell, SMC allowed us to build a mixing service¬†that was secure, achieved higher anonymity, and completely oblivious, i.e., not even the mixing service could tell how coins were mixed. Eventually, our work resulted in the paper “CoinParty: Secure Multi-Party Mixing of Bitcoins”¬†[1] which I presented at CODASPY’15¬†in San Antonio, Texas. A year later, CoinParty got me invited to Europe’s financial melting pot, London. But that’s a different story and a different half-marathon.

At first, I must admit, I was really skeptical about running in an american metropolis, let alone one in Texas where people prefer pickup trucks or cowboy boots over running shoes. In retrospect, I must admit, I was right to be skeptical. But I was still highly motivated to build up my half-marathon-sight-running tradition and so I pulled through.

Total distance: 21.27 km
Max elevation: 212 m
Total climbing: 162 m

Where to run?¬†San Antonio is a big city in Texas. There’s no good place to run! Period!¬†I put all my desperate hope in the San Antonio Riverwalk. It’s rather creek than a river and there rarely is any green around it. But beggars can’t be choosers.

  • Mission Concepci√≥n:¬†I set off southwards on the riverwalk. Mission Concepci√≥n was my turning point. The course was flat and boring – it also didn’t get more exciting on the way back.
  • The Alamo:¬†On my return, I took a junction of the Riverwalk through the most touristic part of San Antonio. Just a few strides off my course lay the Misi√≥n San Antonio de Valero, known as the Alamo, a pivotal element in the Texas Revolution.
  • The Haunted Emily Morgen Hotel:¬†Right next to the Alamo lies the Haunted Emily Morgen Hotel, listed as the¬†third most haunted hotel in the world¬†by USA Today. I didn’t book a tour but it certainly presents a nice view.

When to run?¬†March, when I ran, was quite agreeable.¬†Avoid summer, I guess. And while you’re at it, avoid San Antonio altogether and, for that matter, Houston. Do yourself a favor and go straight to Austin which is what I did after the conference.

 * RFCs are a flexible teaching format at COMSYS with the goal to bring talented students (interested in pursuing a PhD after their Master’s) into first contact with “real” research. Incidentally, RFCs are also a document series by the Internet Engineering Task Force (IETF) that standardize many of the protocols that run today’s Internet, e.g., IP, UDP, TCP, TLS …

 ** One Bitcoin was valued at 350 – 450 ‚ā¨ in mid 2014 when I started my work on Bitcoin. Prices peaked at 16724,12 ‚ā¨ in December 16, 2017 – just one week after I finished my PhD. Talk about correlation vs. causality.

 *** Figuratively, a user hides within the mixing group, therefore often called the anonymity set. It is easy to imagine that you can hide better in a large group of people than in a small group.

[1] [pdf] [doi] J. H. Ziegeldorf, F. Grossmann, M. Henze, N. Inden, and K. Wehrle, “CoinParty: Secure Multi-Party Mixing of Bitcoins,” in Proceedings of the 5th ACM Conference on Data and Application Security and Privacy, New York, NY, USA, 2015, pp. 75-86.
  Acceptancerate = {21 %},
  Acmid = {2699100},
  Address = {New York, NY, USA},
  Author = {Ziegeldorf, Jan Henrik and Grossmann, Fred and Henze, Martin and Inden, Nicolas and Wehrle, Klaus},
  Booktitle = {Proceedings of the 5th ACM Conference on Data and Application Security and Privacy},
  Date-Added = {2018-10-06 16:11:11 +0000},
  Date-Modified = {2018-10-10 05:27:51 +0000},
  Doi = {10.1145/2699026.2699100},
  Isbn = {978-1-4503-3191-3},
  Keywords = {anonymity, bitcoin, secure multi-party computation},
  Location = {San Antonio, Texas, USA},
  Numpages = {12},
  Pages = {75--86},
  Publisher = {ACM},
  Series = {CODASPY '15},
  Title = {CoinParty: Secure Multi-Party Mixing of Bitcoins},
  Url = {http://doi.acm.org/10.1145/2699026.2699100},
  Year = {2015},
  Bdsk-Url-1 = {http://doi.acm.org/10.1145/2699026.2699100},
  Bdsk-Url-2 = {https://doi.org/10.1145/2699026.2699100}}

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