Recently, a forward secure identity-based encryption with equality test (FS-IBEET) scheme was proposed for secure data sharing in healthcare systems and was claimed to achieve indistinguishability against adaptive chosen-identity and chosen-ciphertext attacks (IND-ID-CCA). In this letter, we show that the proposed scheme fails to provide the claimed IND-ID-CCA security. We present a practical chosen-ciphertext attack that exploits a structural design flaw. In particular, achieving INDID-CCA security appears infeasible under the current framework, where the ciphertext publicly reveals the time slot in conjunction with the 0/1-encodings. This vulnerability is critical, as it fundamentally compromises the confidentiality guarantees essential to healthcare IoT environments, potentially leading to the corruption or leakage of patient records.

Recently, Yang et al. proposed a reputation incentive committee consensus-based matchmaking scheme (IEEE Transactions on Services Computing, 2024), claiming to achieve indistinguishability under adaptive chosen ciphertext attacks (IND-CCA2). In this work, we present a plaintext recovery attack against their scheme under the adaptive chosen ciphertext attack model, analyze the design and proof flaws enabling the attacks, and suggest a countermeasure to achieve the IND-CCA2 security.

Abstract The authors present a classical polynomial‐time attack against the branching program obfuscator of Fernando–Rasmussen–Sahai (for short FRS, Asiacrypt’17) (with one zerotest parameter), which is robust against all known classical cryptanalyses on obfuscators when instantiated with the CLT13 multilinear map. The first step is to recover a plaintext modulus of the CLT13 multilinear map. To achieve the goal, the Coron and Notarnicola (Asiacrypt’19) algorithm is applied. However, because of parameter issues, the algorithm cannot be used directly. In order to detour the issue, the authors convert an FRS obfuscator into a new programme containing a small message space. Through the conversion, the authors obtain two zerotest parameters and encodings of zero except for two non‐zero slots. Then, they are used to mitigate parameter constraints of the message space recovering algorithm. Then, a cryptanalysis of the FRS obfuscation based on the recovered message space is proposed. The authors show that there exist two functionally equivalent programmes such that their obfuscated programmes are computationally distinguishable. Thus, the FRS scheme does not satisfy the desired security without any additional constraints.

Abstract A simultaneous Diophantine approximation (SDA) algorithm takes instances of the partial approximate common divisor (PACD) problem as input and outputs a solution. While several encryption schemes have been published and their securities depend on the presumed hardness of variant of the PACD problem, fewer studies have attempted to extend the SDA algorithm to be applicable to these variants. In this study, the SDA algorithm is extended to solve the general PACD problem. In order to proceed, first the variants of the PACD problem are classified and how to extend the SDA algorithm for each is suggested. Technically, the authors show that a short vector of some lattice used in the SDA algorithm gives an algebraic relation between secret parameters. Then, all the secret parameters can be recovered by finding this short vector. It is also confirmed experimentally that this algorithm works well.