To securely share the data between users, encryption schemes with keyword searches in various settings have been proposed. Many studies design schemes in a designated receiver setting where a data owner specifies which receivers could download the data in advance at the time the data are uploaded. In this setting, it is not easy to extend the scheme to support environments with multiple data owners. Moreover, there was no scheme considering the situation in which a newly enrolled user accesses data that were uploaded prior to his enrollment. On the other hand, schemes designed in an undesignated receiver setting support multiple data owners and allow data to be accessed by all users in the system, regardless of the time the data were uploaded. However, most of them are not secure against collusion attacks involving an untrusted server and revoked users. In this paper, we propose a full‐accessible multiparty searchable encryption (FA‐MPSE) scheme for data‐sharing systems. Our scheme supports the property that we call full-accessibility , and any users in the system can access all data in the storage. In addition, our scheme is secure against collision attacks so that the revoked users who collaborate with the server can not access the stored data. Furthermore, our scheme provides all the essential properties of MPSE, such as query privacy, query unforgeability, full‐revocability, and unlinkability, and its security is proven in a formal security model. We provide the comparison result with the related schemes to show that our scheme has a comparative advantage.

Homomorphic Encryption (HE) makes it possible to compute on encrypted data without decryption. In lattice-based HE, a ciphertext contains noise, which accumulates along with homomorphic computations. Bootstrapping refreshes the noise and it is possible to perform arbitrary-depth computations on HE with bootstrapping, which we call Fully Homomorphic Encryption (FHE). In this article, we propose a new general bootstrapping technique for RLWE-based schemes and its practical instantiation for FHE. It can be applied to all three RLWE-based leveled FHE schemes: Brakerski-Gentry-Vaikuntanathan (BGV), Brakerski/Fan-Vercauteren (BFV), and Cheon-Kim-Kim-Song (CKKS) with minor deviations in the algorithms. Our new construction of bootstrapping extracts a noiseless ciphertext for a part of the input, scales it, and finally removes it. In contrast with previous bootstrapping algorithms, the proposed method consumes only 1–2 levels and uses smaller parameters. For BGV and BFV, our new bootstrapping does not have any restrictions on a plaintext modulus unlike typical cases of the previous methods. The error introduced by our approach for CKKS is comparable to a rescaling error, allowing us to preserve a large amount of precision after bootstrapping.