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We have implemented a secure network file system called SUNDR that guarantees the integrity of data even when malicious parties control the server. SUNDR splits storage functionality between two untrusted components, a block store and a consistency server. The block store holds all file data and most metadata. Without interpreting metadata, it presents a simple interface for clients to store variable-sized data blocks and later retrieve them by cryptographic hash.

Plutus is a cryptographic storage system that enables secure file sharing without placing much trust on the file servers. In particular, it makes novel use of cryptographic primitives to protect and share files. Plutus features highly scalable key management while allowing individual users to retain direct control over who gets access to their files. We explain the mechanisms in Plutus to reduce the number of cryptographic keys exchanged between users by using filegroups, distinguish file read and write access, handle user revocation efficiently, and allow an untrusted server to authorize file writes. We have built a prototype of Plutus on OpenAFS. Measurements of this prototype show that Plutus achieves strong security with overhead comparable to systems that encrypt all network traffic.

This article presents the design, implementation, and evaluation of CATS, a network storage service with strong accountability properties. CATS offers a simple web services interface that allows clients to read and write opaque objects of variable size. This interface is similar to the one offered by existing commercial Internet storage services. CATS extends the functionality of commercial Internet storage services by offering support for strong accountability. A CATS server annotates read and write responses with evidence of correct execution, and offers audit and challenge interfaces that enable clients to verify that the server is faithful. A faulty server cannot conceal its misbehavior, and evidence of misbehavior is independently verifiable by any participant. CATS clients are also accountable for their actions on the service. A client cannot deny its actions, and the server can prove the impact of those actions on the state views it presented to other clients. Experiments with a CATS prototype evaluate the cost of accountability under a range of conditions and expose the primary factors influencing the level of assurance and the performance of a strongly accountable storage server. The results show that strong accountability is practical for network storage systems in settings with strong identity and modest degrees of write-sharing. We discuss

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Abstract. In this paper we address the problem of consistency for cryptographic file systems. A cryptographic file system protects the users ’ data from the file server, which is possibly untrusted and might exhibit Byzantine behavior, by encrypting the data before sending it to the server. The consistency of the encrypted file objects that implement a cryptographic file system relies on the consistency of the two components used to implement them: the file storage protocol and the key distribution protocol. We first define two generic classes of consistency conditions that extend and generalize existing consistency conditions. We then formally define consistency for encrypted file objects in a generic way: for any consistency conditions for the key and file objects belonging to one of the two classes of consistency conditions considered, we define a corresponding consistency condition for encrypted file objects. We finally provide, in our main result, necessary and sufficient conditions for the consistency of the key distribution and file storage protocols under which the encrypted storage is consistent. Our framework allows the composition of existing key distribution and file storage protocols to build consistent encrypted file objects and simplifies complex proofs for showing the consistency of encrypted storage. 1

We consider a set of clients collaborating through an online service provider that is subject to attacks, and hence not fully trusted by the clients. We introduce the abstraction of a fail-aware untrusted service, with meaningful semantics even when the provider is faulty. In the common case, when the provider is correct, such a service guarantees consistency (linearizability) and liveness (wait-freedom) of all operations. In addition, the service always provides accurate and complete consistency and failure detection. We illustrate our new abstraction by presenting a Fail-Aware Untrusted STorage service (FAUST). Existing storage protocols in this model guarantee so-called forking semantics. We observe, however, that none of the previously suggested protocols suffice for implementing fail-aware untrusted storage with the desired liveness and consistency properties (at least wait-freedom and linearizability when the server is correct). We present a new storage protocol, which does not suffer from this limitation, and implements a new consistency notion, called weak fork-linearizability. We show how to extend this protocol to provide eventual consistency and failure awareness in FAUST. 1

More and more users store data in “clouds ” that are accessed remotely over the Internet. We survey well-known cryptographic tools for providing integrity and consistency for data stored in clouds and discuss recent research in cryptography and distributed computing addressing these problems. Storing data in clouds Many providers now offer a wide variety of flexible online data storage services, ranging from passive ones, such as online archiving, to active ones, such as collaboration and social networking. They have become known as computing and storage “clouds. ” Such clouds allow users to abandon local storage and use online alternatives, such as Amazon S3, Nirvanix CloudNAS, or Microsoft SkyDrive. Some cloud providers utilize the fact that online storage can be accessed from any location connected to the Internet, and offer additional functionality; for example, Apple MobileMe allows users to synchronize common applications that run on multiples devices. Clouds also offer computation resources, such as Amazon EC2, which can significantly reduce the cost of maintaining such resources locally. Finally, online collaboration tools, such as Google Apps or versioning repositories for source code, make it easy to collaborate with colleagues across organizations and countries, as practiced by the authors of this paper. What can go wrong? Although the advantages of using clouds are unarguable, there are many risks involved with releasing control over your data. One concern that many users are aware of is loss of privacy. Nevertheless, the popularity of social networks and online data sharing repositories suggests that many users are willing to forfeit privacy,

We study formal security properties of a state-of-the-art protocol for secure file sharing on untrusted storage, in the automatic protocol verifier ProVerif. As far as we know, this is the first automated formal analysis of a secure storage protocol. The protocol, designed as the basis for the file system Plutus, features a number of interesting schemes like lazy revocation and key rotation. These schemes improve the protocol’s performance, but complicate its security properties. Our analysis clarifies several ambiguities in the design and reveals some unknown attacks on the protocol. We propose corrections, and prove precise security guarantees for the corrected protocol. 1.

We study formal security properties of network-attached storage (NAS) in an applied pi calculus. We model NAS as an implementation of a specification based on traditional centralized storage. We show the correctness of the implementation by proving that it is fully abstract with respect to the specification. Our result can be viewed as a strong guarantee of security for a basic network-attached storage design.