While many cloud storage systems allow users to protect their data by making use of encryption, only few support collaborative editing on that data. A major challenge for enabling such collaboration is the need to enforce cryptographic access control policies in a secure and efficient manner. In this paper, we introduce IBBE-SGX, a new cryptographic access control extension that is efficient both in terms of computation and storage even when processing large and dynamic workloads of membership operations, while at the same time offering zero knowledge guarantees.IBBE-SGX builds upon Identity-Based Broadcasting Encryption (IBBE). We address IBBE's impracticality for cloud deployments by exploiting Intel Software Guard Extensions (SGX) to derive cuts in the computational complexity. Moreover, we propose a group partitioning mechanism such that the computational cost of membership update is bound to a fixed constant partition size rather than the size of the whole group. We have implemented and evaluated our new access control extension. Results highlight that IBBE-SGX performs membership changes 1.2 orders of magnitude faster than the traditional approach of Hybrid Encryption (HE), producing group metadata that are 6 orders of magnitude smaller than HE, while at the same time offering zero knowledge guarantees.
Using public cloud services for storing and sharing confidential data requires end users to cryptographically protect both the data and the access to the data. In some cases, the identity of end users needs to remain confidential against the cloud provider and fellow users accessing the data. As such, the underlying cryptographic access control mechanism needs to ensure the anonymity of both data producers and consumers.We introduce A-SKY, a cryptographic access control extension capable of providing confidentiality and anonymity guarantees, all while efficiently scaling to large organizations. A-SKY leverages trusted execution environments (TEEs) to address the impracticality of anonymous broadcast encryption (ANOBE) schemes, achieving faster execution times and shorter ciphertexts. The innovative design of A-SKY limits the usage of the TEE to the narrow set of data producing operations, and thus optimizes the dominant data consumption actions by not requiring a TEE. Furthermore, we propose a scalable implementation for A-SKY leveraging micro-services that preserves strong security guarantees while being able to efficiently manage realistic large user bases. Results highlight that the A-SKY cryptographic scheme is 3 orders of magnitude better than state of the art ANOBE, and an end-to-end system encapsulating A-SKY can elastically scale to support groups of 10 000 users while maintaining processing costs below 1 second.
Much research has focused during the last years on the security and privacy concerns of public cloud storages. Cryptographic primitives are commonly used to ensure user data confidentiality, authenticity and integrity. Confidentiality has been addressed by the use of symmetric-key encryption algorithms, while integrity and authenticity have been achieved by using message authentication codes, secure hashes or digital signatures. The choice of a specific configuration for securing an untrusted cloud storage highly depends on the expected security level, the size and type of data to store and the access pattern to these data. In this work, we are interested in overcoming the lack of comprehensive comparison of the costs and effectiveness of cryptographic primitives for securing public cloud storage, and ease an informed choice between them based on target usage conditions. We describe the results of an independent experimental study of six cryptographic schemes, representative of the principal design alternatives. Our practical experience report reveals that the best scheme for a given situation, such as a write-heavy workload of mostly small files, is not necessarily the most appropriate for a different situation such as a read-only workload of large files. We identify the scheme characteristics that are correlated with these differences and discuss the pros and cons of each design. Our experimental framework and results are available in the open for use by the community.
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