Paper 2021/325

Spectrum: High-Bandwidth Anonymous Broadcast

Zachary Newman, Sacha Servan-Schreiber, and Srinivas Devadas


We present Spectrum, a high-bandwidth, metadata-private file broadcasting system. In Spectrum, a small number of broadcasters share a file with many subscribers via two or more non-colluding broadcast servers. Subscribers generate cover traffic by sending dummy files, hiding which users are broadcasters and which users are only consumers. Spectrum optimizes for a setting with few broadcasters and many subscribers—as is common to many real-world applications—to drastically improve throughput over prior work. Malicious clients are prevented from disrupting broad- casts using a novel blind access control technique that allows servers to reject malformed requests. Spectrum also prevents deanonymization of broadcasters by malicious servers deviating from protocol. Our techniques for providing malicious security are applicable to other systems for anonymous broad- cast and may be of independent interest. We implement and evaluate Spectrum. Compared to the state-of-the-art in cryptographic anonymous communication systems, Spectrum’s peak throughput is 4–120,000× faster (and commensurately cheaper) in a broadcast setting. Deployed on two commodity servers, Spectrum allows broad- casters to share 1GB (two full-length 720p documentary movies) in 13h 20m with an anonymity set of 10,000 (for a total cost of about $6.84). These costs scale roughly linearly in the size of the file and total number of users, and Spectrum parallelizes trivially with more hardware.

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Publication info
Published elsewhere. 19th USENIX Symposium on Networked Systems Design and Implementation (NSDI 22)
Contact author(s)
zjn @ mit edu
3s @ mit edu
devadas @ csail mit edu
2022-03-04: last of 3 revisions
2021-03-11: received
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      author = {Zachary Newman and Sacha Servan-Schreiber and Srinivas Devadas},
      title = {Spectrum: High-Bandwidth Anonymous Broadcast},
      howpublished = {Cryptology ePrint Archive, Paper 2021/325},
      year = {2021},
      note = {\url{}},
      url = {}
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