Spectrum: High-Bandwidth Anonymous Broadcast with Malicious Security Zachary Newman Sacha Servan-Schreiber Srinivas Devadas MIT CSAIL MIT CSAIL MIT CSAIL
[email protected] [email protected] [email protected] Abstract researchers, digital mass surveillance by U.S. government agencies [12]. Political philosophers debate [2, 23] the ethics We present Spectrum, a high-bandwidth, metadata-private of whistleblowing, but agree it often has a positive impact. file broadcasting system with malicious security guarantees. In Spectrum, a small number of publishers broadcast to many Motivation for this work. Whistleblowers take on great subscribers via two or more non-colluding servers. Sub- personal risks in bringing misdeeds to light. The luckiest scribers generate indistinguishable cover traffic, hiding which enjoy legal protections [80] or financial reward [81]. But users are publishers, for full metadata privacy. many face exile [12], incarceration [39,60,66], or risk their Spectrum builds on prior work that uses DC-nets for anony- lives [79]. More recently, political activist Alexei Navalny mous broadcast. Existing anonymous broadcast systems do was detained and sentenced to prison following the release not optimize for a setting where there are fewer publishers of documents accusing Russian president Vladimir Putin of compared to subscribers – a common situation in real-world corruption and embezzlement [72]. broadcasts. To prevent disruption by malicious clients sending To mitigate these risks, many whistleblowers turn to tech- malformed requests, we develop a blind authentication proto- nology to protect themselves [36]. Secure messaging apps col that allows servers to reject malicious requests. We also Signal [17] and SecureDrop [4] have proven to be an im- ensure security against malicious servers deviating from pro- portant resource to whistleblowers and journalists [31, 76].