Abstract Multi-factor authentication (MFA) is extensively employed in mobile applications to enhance security, including Internet of Vehicles, healthcare systems, smart homes, etc. Traditional MFA requires users to present specific factors, which can be inconvenient if certain factors are unavailable. To address this, $ (t, n) $-threshold MFA (T-MFA) allows users to select any $ t $ out of $ n $ registered factors for authentication. However, existing T-MFA solutions face four key issues: (i) reliance on $ n-1 $ devices, which may be impractical; (ii) susceptibility to denial of service when the mandatory factor fails; (iii) limited factor types, reducing user flexibility; and (iv) increasing client-side computational costs with higher $ t $. In this work, we propose a veritable $ (t, n) $-threshold multi-factor authenticated key exchange protocol that addresses these challenges. Utilizing oblivious programmable pseudorandom functions (OPPRF) as main tools, we eliminate dependence on multiple devices, mandatory factors, and restricted factor types, achieving what we called veritable. We present a new construction of batched OPPRF to reduce client-side costs from $ O(t) $ to $ O(1) $, with 2 exponentiations cost by the client and $ t+1 $ by the server. We implement it with JavaScript to validate its flexibility and efficiency, making it highly suitable for mobile device applications.

Cloud-edge computing has been widely-adopted for large-scale data sharing and processing. In practical data sharing systems, data are very sensitive and typically encrypted, such as health records. Unauthorized users may attempt to decrypt ciphertexts to recover the data. Due to mistakes or malice, some users might try to share sensitive information with others who do not have access. Clearly, strong access control should be employed to restrict the read and write privilege of users. There was a rich literature on mandatory fine-grained information flow control for such scenarios, but three important issues remain. First, payload privacy was often neglected. Most of the known solutions focused on the protection ciphertext header, but ignored the payload, i.e. encrypted data, which may leak information by a malicious sender. Second, no guarantee of the encrypted data. Ill-formed ciphertexts, e.g. encrypted garbage data, can pass the global policy check, causing decryption failures or disseminating bad information, hence are incapable of content distribution. Finally, the heavy computation cost of sender authentication impedes the practical deployment. In this work, we introduce Hodor, a robust fine-grained information flow control scheme that not only guards the transmission channel with mandatory fine-grained access control for massive data, but also protects whole data traffic, checks ciphertext well-formedness, and efficiently authenticates the sender. In particular, Hodor considers full data traffic protection of both the ciphertext header and encrypted payload to resist information leakage, completely verifies the consistency between the claimed access structure and the actual access structure, and achieves efficient sender authentication with a succinct challenge-response protocol. We present a formal model and give detailed proofs. We also implement and evaluate Hodor using various optimization techniques to boost its performance. The results demonstrate the efficiency and practicality of Hodor for cloud-edge data sharing.

Searchable symmetric encryption (SSE) schemes allow a client to store encrypted data with a storage provider and retrieve corresponding documents without revealing the content or search keywords to the provider. However, achieving efficient SSE schemes often comes at the cost of statistical information leakage, including search, access and size patterns. The known solutions from fully homomorphic encryption or oblivious RAM often admit poor performances due to significant computational and communication overheads. Additionally, the demand for rich search expressiveness, such as Boolean queries, further complicates the design. In this paper, we introduce NEMO, a novel SSE achieving a good balance between efficiency, security and query expressiveness. NEMO utilizes function secret sharing (FSS) and replicated secret sharing-based multi-party computation (MPC) protocol, but is highly optimized for large database. For functionality, NEMO supports arbitrary Boolean queries and enables dynamic updates in a multi-user setting. For security, NEMO achieves minimal leakage by eliminating all search, access, and size patterns, while only allowing the leakage of Boolean formulas in queries. Regarding efficiency, we propose a new FSS for multi-point functions, effectively batching multiple distributed point functions, and an infix-to-postfix conversion algorithm for Boolean formula to reduce the communication rounds in the MPC protocol. A proof-of-concept implementation of NEMO demonstrates its efficiency, with a search latency of approximately 622 ms for a conjunction query with 8 keywords, even with a dataset exceeding 1 million documents.

Personally identifiable information (PII) refers to any information that links to an individual. Sharing PII is extremely useful in public affairs yet hard to implement due to the worries about privacy violations. Building a PII retrieval service over multi-cloud, which is a modern strategy to make services stable where multiple servers are deployed, seems to be a promising solution. However, three major technical challenges remain to be solved. The first is the privacy and access control of PII. In fact, each entry in PII can be shared to different users with different access rights. Hence, flexible and fine-grained access control is needed. Second, a reliable user revocation mechanism is required to ensure that users can be revoked efficiently, even if few cloud servers are compromised or collapse, to avoid data leakage. Third, verifying the correctness of received PII and locating a misbehaved server when wrong data are returned is crucial to guarantee user's privacy, but challenging to realize. In this paper, we propose Rainbow, a secure and practical PII retrieval scheme to solve the above issues. In particular, we design an important cryptographic tool, called Reliable Outsourced Attribute Based Encryption (ROABE) which provides data privacy, flexible and fine-grained access control, reliable immediate user revocation and verification for multiple servers simultaneously, to support Rainbow. Moreover, we present how to build Rainbow with ROABE and several necessary cloud techniques in real world. To evaluate the performance, we deploy Rainbow on multiple mainstream clouds, namely, AWS, GCP and Microsoft Azure, and experiment in browsers on mobile phones and computers. Both theoretical analysis and experimental results indicate that Rainbow is secure and practical.