Usually, a proof of a theorem contains more knowledge than the mere fact that the theorem is true. For instance, to prove that a graph is Hamiltonian it suffices to exhibit a Hamiltonian tour in it; however, this seems to contain more knowledge than the single bit Hamiltonian/non-Hamiltonian.In this paper a computational complexity theory of the “knowledge” contained in a proof is developed. Zero-knowledge proofs are defined as those proofs that convey no additional knowledge other than the correctness of the proposition in question. Examples of zero-knowledge proof systems are given for the languages of quadratic residuosity and 'quadratic nonresiduosity. These are the first examples of zero-knowledge proofs for languages not known to be efficiently recognizable.

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The knowledge complexity of interactive proof-systems

In this paper, we define and explore proofs of retrievability (PORs). A POR scheme enables an archive or back-up service (prover) to produce a concise proof that a user (verifier) can retrieve a target file F, that is, that the archive retains and reliably transmits file data sufficient for the user to recover F in its entirety.A POR may be viewed as a kind of cryptographic proof of knowledge (POK), but one specially designed to handle a large file (or bitstring) F. We explore POR protocols here in which the communication costs, number of memory accesses for the prover, and storage requirements of the user (verifier) are small parameters essentially independent of the length of F. In addition to proposing new, practical POR constructions, we explore implementation considerations and optimizations that bear on previously explored, related schemes.In a POR, unlike a POK, neither the prover nor the verifier need actually have knowledge of F. PORs give rise to a new and unusual security definition whose formulation is another contribution of our work.We view PORs as an important tool for semi-trusted online archives. Existing cryptographic techniques help users ensure the privacy and integrity of files they retrieve. It is also natural, however, for users to want to verify that archives do not delete or modify files prior to retrieval. The goal of a POR is to accomplish these checks without users having to download the files themselves. A POR can also provide quality-of-service guarantees, i.e., show that a file is retrievable within a certain time bound.

PORs: Proofs of Retrievability for Large Files

Searchable symmetric encryption (SSE) allows a party to outsource the storage of its data to another party (a server) in a private manner, while maintaining the ability to selectively search over it. This problem has been the focus of active research in recent years. In this paper we show two solutions to SSE that simultaneously enjoy the following properties: Both solutions are more efficient than all previous constant-round schemes. In particular, the work performed by the server per returned document is constant as opposed to linear in the size of the data. Both solutions enjoy stronger security guarantees than previous constant-round schemes. In fact, we point out subtle but serious problems with previous notions of security for SSE, and show how to design constructions which avoid these pitfalls. Further, our second solution also achieves what we call adaptive SSE security, where queries to the server can be chosen adaptively (by the adversary) during the execution of the search; this notion is both important in practice and has not been previously considered.Surprisingly, despite being more secure and more efficient, our SSE schemes are remarkably simple. We consider the simplicity of both solutions as an important step towards the deployment of SSE technologies.As an additional contribution, we also consider multi-user SSE. All prior work on SSE studied the setting where only the owner of the data is capable of submitting search queries. We consider the natural extension where an arbitrary group of parties other than the owner can submit search queries. We formally define SSE in the multi-user setting, and present an efficient construction that achieves better performance than simply using access control mechanisms.

/pdf/searchable-symmetric-encryption-improved-definitions-and-19ss3wg07l.pdf

Searchable symmetric encryption: improved definitions and efficient constructions

We consider the problem of building a secure cloud storage service on top of a public cloud infrastructure where the service provider is not completely trusted by the customer We describe, at a high level, several architectures that combine recent and non-standard cryptographic primitives in order to achieve our goal We survey the benefits such an architecture would provide to both customers and service providers and give an overview of recent advances in cryptography motivated specifically by cloud storage

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Cryptographic cloud storage

In a proof-of-retrievability system, a data storage center convinces a verifier that he is actually storing all of a client's data. The central challenge is to build systems that are both efficient and provably secure--that is, it should be possible to extract the client's data from any prover that passes a verification check. In this paper, we give the first proof-of-retrievability schemes with full proofs of security against arbitrary adversaries in the strongest model, that of Juels and Kaliski. Our first scheme, built from BLS signatures and secure in the random oracle model, has the shortest query and response of any proof-of-retrievability with public verifiability. Our second scheme, which builds elegantly on pseudorandom functions (PRFs) and is secure in the standard model, has the shortest response of any proof-of-retrievability scheme with private verifiability (but a longer query). Both schemes rely on homomorphic properties to aggregate a proof into one small authenticator value.

/pdf/compact-proofs-of-retrievability-2cuaz82nov.pdf

Compact Proofs of Retrievability

The paradigm of cloud computing is currently revolutionizing our way of communicating and computing. It impacts and redefines how we store, retrieve, process, and share information in business and private settings. The fact that increasingly many systems and services rely on this paradigm increases our dependence on the security, privacy, availability, and robustness of the involved operations, components, and systems.

The CCSW workshop aims to provide insights into the latest research and development activities on this topic. Its goal is to enable the sharing of information and knowledge on the latest advances in secure cloud and utility computing, outsourced computations, trusted computing and virtualization, among many other topics.

In response to the call for papers, the program committee received a total of fifty-two high quality submissions. Due to the constraints of a one-day workshop, only thirteen out of the fifty-two were accepted.

In addition to the technical program, the workshop featured keynotes by leading experts on cloud security: Burt Kaliski, Senior Vice President and Chief Technology Officer at Verisign; Kristin Lauter, Principal Researcher and Head of the Cryptography Group at Microsoft Research; and Howard Shrobe, Program Manager at DARPA.

Proceedings of the 2012 ACM Workshop on Cloud computing security workshop

Peer-to-peer (p2p) file sharing accounts for the most uplink bandwidth use in the Internet. Therefore, in the past few decades, many solutions tried to come up with better proposals to increase the social welfare of the participants. Social welfare in such systems are categorized generally as average download time or uplink bandwidth utilization. One of the most influential proposals was the BitTorrent. Yet, soonafter studies showed that BitTorrent has several problems that incentivize selfish users to game the system and hence decrease social welfare.

DogFish: Decentralized Optimistic Game-theoretic FIle SHaring

The ubiquitous growth in the popularity of public cloud computing platforms as seen today entails an inherent risk: the shared nature of data center networks (DCNs) renders co-hosted tenants susceptible to attacks from within the network. In this paper, we discuss the security mechanisms offered by popular cloud service providers at present, and explore the extent to which existing data center networks might be vulnerable to internal denial-of-service attacks. We describe two categories of attacks: the first comprising those that are easy to mount, but are ill-disguised and conform to traditional attack patterns (overt attacks), and the second comprising hard-to-detect covert attacks that involve a greater complexity of deployment, but also show greater impacts. Finally, we discuss attack-mitigation schemes that employ common network mechanisms, such as TCP pacing and Network Virtualization, and also present a new source-based filtering scheme to regulate network communications.

/pdf/attacking-data-center-networks-from-the-inside-1nka41zed0.pdf

Attacking Data Center Networks from the Inside

Distributed hash tables (DHT) are a fundamental building block in the design of distributed systems with applications ranging from content distribution networks to off-chain storage networks for blockchains and smart contracts. When DHTs are used to store sensitive information, system designers use end-to-end encryption in order to guarantee the confidentiality of their data. A prominent example is Ethereum’s off-chain network Swarm. In this work, we initiate the study of end-to-end encryption in DHTs and the many systems they support. We introduce the notion of an encrypted DHT and provide simulation-based security definitions that capture the security properties one would desire from such a system. Using our definitions, we then analyze the security of a standard approach to storing encrypted data in DHTs. Interestingly, we show that this “standard scheme” leaks information probabilistically, where the probability is a function of how well the underlying DHT load balances its data. We also show that, in order to be securely used with the standard scheme, a DHT needs to satisfy a form of equivocation with respect to its overlay. To show that these properties are indeed achievable in practice, we study the balancing properties of the Chord DHT—arguably the most influential DHT—and show that it is equivocable with respect to its overlay in the random oracle model. Finally, we consider the problem of encrypted DHTs in the context of transient networks, where nodes are allowed to leave and join. ∗archita_agarwal@brown.edu †seny@brown.edu

/pdf/encrypted-distributed-hash-tables-1o33wacy4q.pdf

Encrypted Distributed Hash Tables

Adversaries in cryptography have traditionally been modeled as either semi-honest or malicious. Over the years, however, several lines of work have investigated the design of cryptographic protocols against rational adversaries. The most well-known example are covert adversaries in secure computation (Aumann & Lindell, TCC ’07 ) which are adversaries that wish to deviate from the protocol but without being detected. To protect against such adversaries, protocols secure in the covert model guarantee that deviations are detected with probability at least ε which is known as the deterrence factor. In this work, we initiate the study of contracts in cryptographic protocol design. We show how to design, use and analyze contracts between parties for the purpose of incentivizing honest behavior from rational adversaries. We refer to such contracts as adversarial level agreements (ALA). The framework we propose can result in more efficient protocols and can enforce deterrence in covert protocols; meaning that one can guarantee that a given deterrence factor will deter the adversary instead of assuming it. We show how to apply our framework to two-party protocols, including secure two-party computation (2PC) and proofs of storage (PoS). In the 2PC case, we integrate ALAs to publicly-verifiable covert protocols and show, through a game-theoretic analysis, how to set the parameters of the ALA to guarantee honest behavior. We do the same in the setting of PoS which are two-party protocols that allow a client to efficiently verify the integrity of a file stored in the cloud. ∗marilyn_george@brown.edu †seny@brown.edu

Seny Kamara

Papers

Proceedings of the 2012 ACM Workshop on Cloud computing security workshop

DogFish: Decentralized Optimistic Game-theoretic FIle SHaring

Attacking Data Center Networks from the Inside

Encrypted Distributed Hash Tables

Adversarial Level Agreements for Two-Party Protocols.