TU Berlin

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Peer-to-Peer Computing

Peer-to-peer computing is an interesting networking paradigm as it offers a high degree of scalability by exploiting the resources of the participants and avoids single-points of failures. Due to these desirable properties, peer-to-peer computing plays a crucial role in many networking applications beyond file-sharing, and the underlying ideas are also discussed as a design principle for the future Internet. Our research is concerned with the question of whether peer-to-peer is mature enough to step outside its "comfort zone". We conduct measurements of state-of-the-art peer-to-peer networks such as Kad and investigate the robustness, e.g., to Sybil attacks or selfish behavior. For example, we implemented the proof-of-concept BitTorrent client "BitThief" which provides evidence that despite the tit-for-tat incentive mechanism, free-riding is possible in BitTorrent. We develop algorithms to improve the performance of peer-to-peer systems: we devise peer-to-peer networks which are robust to worst-case churn (see e.g., our IPTPS paper), which allow for efficient joins and leaves (see e.g., our SHELL system at ICALP), or which are robust to denial of service attacks (see e.g., our Chameleon system at SPAA). Some of these algorithms were successfully implemented in the online storage tool Wuala and the streaming tool Streamforge, two Swiss startups.


  • Stefan Schmid

Selected Publications

Cryptree: A Folder Tree Structure for Cryptographic File Systems
Zitatschlüssel GMSW-CFTSCFS-06
Autor Grolimund, Dominik and Meisser, Luzius and Schmid, Stefan and Wattenhofer, Roger
Buchtitel 25th IEEE Symposium on Reliable Distributed Systems (SRDS)
Seiten 189–198
Jahr 2006
ISBN 0-7695-2677-2
ISSN 1060-9857
DOI http://dx.doi.org/10.1109/SRDS.2006.15
Ort Leeds, United Kingdom
Monat October
Zusammenfassung We present Cryptree, a cryptographic tree structure which facilitates access control in file systems operating on untrusted storage. Cryptree leverages the file system's folder hierarchy to achieve efficient and intuitive, yet simple, access control. The highlights are its ability to recursively grant access to a folder and all its subfolders in constant time, the dynamic inheritance of access rights which inherently prevents scattering of access rights, and the possibility to grant someone access to a file or folder without revealing the identities of other accessors. To reason about and to visualize Cryptree, we introduce the notion of cryptographic links. We describe the Cryptrees we have used to enforce read and write access in our own file system. Finally, we measure the performance of the Cryptree and compare it to other approaches.
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