This paper presents Riposte, a new system for anonymous broadcast messaging. Riposte is the first such system, to our knowledge, that simultaneously protects against traffic-analysis attacks, prevents anonymous denial-of-service by malicious clients, and scales to million-user anonymity sets. To achieve these properties, Riposte makes novel use of techniques used in systems for private information retrieval and secure multi-party computation. For latency-tolerant workloads with many more readers than writers (e.g. Twitter, Wikileaks), we demonstrate that a three-server Riposte cluster can build an anonymity set of 2,895,216 users in 32 hours.
@cite_55 consider symmetric PIR protocols, in which the servers prevent dishonest clients from learning about more than a single row of the database per query. The problem that consider is, in a way, the dual of the problem we address in , though their techniques do not appear to apply directly in our setting.
@cite_55
1619839175 2105037262 2136631923 2087207261
Private information retrieval (PIR) protocols allow a user to retrieve a data item from a database without revealing any information about the identity of the item being retrieved. Specifically, in information-theoretic k-server PIR, the database is replicated among k non-communicating servers, and each server learns nothing about the item retrieved by the user. The cost of PIR protocols is usually measured in terms of their communication complexity, which is the total number of bits exchanged between the user and the servers. However, another important cost parameter is the storage overhead, which is the ratio between the total number of bits stored on all the servers and the number of bits in the database. Since single-server information-theoretic PIR is impossible, the storage overhead of all existing PIR protocols is at least 2 (or k, in the case of k-server PIR). In this work, we show that information-theoretic PIR can be achieved with storage overhead arbitrarily close to the optimal value of 1, without sacrificing the communication complexity. Specifically, we prove that all known k-server PIR protocols can be efficiently emulated, while preserving both privacy and communication complexity but significantly reducing the storage overhead. To this end, we distribute the n bits of the database among s + r servers, each storing n s coded bits (rather than replicas). Notably, our coding scheme remains the same, regardless of the specific k-server PIR protocol being emulated. For every fixed k, the resulting storage overhead (s +r) s approaches 1 as s grows; explicitly we have equation. Moreover, in the special case k = 2, the storage overhead is only 1 + 1 s. In order to achieve these results, we introduce and study a new kind of binary linear codes, called here k-server PIR codes. Finally, we show how such codes can be constructed from multidimensional cubic, from Steiner systems, and from one-step majority-logic decodable codes. Since 1995, much work has been done creating protocols for private information retrieval (PIR). Many variants of the basic PIR model have been proposed, including such modifications as computational vs. information-theoretic privacy protection, correctness in the face of servers that fail to respond or that respond incorrectly, and protection of sensitive data against the database servers themselves. In this paper, we improve on the robustness of PIR in a number of ways. First, we present a Byzantine-robust PIR protocol which provides information-theoretic privacy protection against coalitions of up to all but one of the responding servers, improving the previous result by a factor of 3. In addition, our protocol allows for more of the responding servers to return incorrect information while still enabling the user to compute the correct result. We then extend our protocol so that queries have information-theoretic protection if a limited number of servers collude, as before, but still retain computational protection if they all collude. We also extend the protocol to provide information-theoretic protection to the contents of the database against collusions of limited numbers of the database servers, at no additional communication cost or increase in the number of servers. All of our protocols retrieve a block of data with communication cost only O(lscr) times the size of the block, where lscr is the number of servers. Finally, we discuss our implementation of these protocols, and measure their performance in order to determine their practicality. The goal of Private Information Retrieval (PIR) is the ability to query a database successfully without the operator of the database server discovering which record(s) of the database the querier is interested in. There are two main classes of PIR protocols: those that provide privacy guarantees based on the computational limitations of servers (CPIR) and those that rely on multiple servers not colluding for privacy (IT-PIR). These two classes have different advantages and disadvantages that make them more or less attractive to designers of PIR-enabled privacy enhancing technologies. Private information retrieval (PIR) protocols allow a user to retrieve a data item from a database while hiding the identity of the item being retrieved. Specifically, in information-theoretic, k-server PIR protocols the database is replicated among k servers, and each server learns nothing about the item the user retrieves. The cost of such protocols is measured by the communication complexity of retrieving one out of n bits of data. For any fixed k, the complexity of the best protocols prior to our work was O(n sup 1 2k-1 ). Since then several methods were developed in an attempt to beat this bound, but all these methods yielded the same asymptotic bound. In this paper, this barrier is finally broken and the complexity of information-theoretic k-server PIR is improved to n sup O(log log k k log k) . The new PIR protocols can also be used to construct k-query binary locally decodable codes of length exp(n sup O(log log k k log k) ), compared to exp(n sup 1 k-1 ) in previous constructions. The improvements presented in this paper apply even for small values of k: the PIR protocols are more efficient than previous ones for every k spl ges 3, and the locally decodable codes are shorter for every k spl ges 4.