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.

Convolutional neural networks have made remarkable progress in the face recognition field. The more the technology of face recognition advances, the greater discriminative features into a face template. However, this increases the threat to user privacy in case the template is exposed.In this paper, we present a modular architecture for face template protection, called IronMask, that can be combined with any face recognition system using angular distance metric. We circumvent the need for binarization, which is the main cause of performance degradation in most existing face template protections, by proposing a new real-valued error-correcting-code that is compatible with real-valued templates and can therefore, minimize performance degradation. We evaluate the efficacy of IronMask by extensive experiments on two face recognitions, ArcFace and Cos-Face with three datasets, CMU-Multi-PIE, FEI, and Color-FERET. According to our experimental results, IronMask achieves a true accept rate (TAR) of 99.79% at a false accept rate (FAR) of 0.0005% when combined with ArcFace, and 95.78% TAR at 0% FAR with CosFace, while providing at least 115-bit security against known attacks.

To achieve security and privacy for data stored on the cloud, we need the ability to secure data in compute. Equality comparisons, “$x=y, x\ne y$”, have been widely studied with many proposals but there is much room for improvement for order comparisons, “$x < y,~x \leq y,~x > y \text{ and } x \geq y$”. Most protocols for order comparisons have some limitation, either leaking some information about the data or requiring several rounds of communication between client and server. In addition, little work has been done on retrieving with compound conditions, mixing several equality and order comparisons. Fully homomorphic encryption (FHE) promises the ability to compute arbitrary functions on encrypted data without sacrificing privacy and without communication, but its potential has yet to be fulfilled. Particularly, private comparisons for database queries using FHE are expensive to compute. In this article, we design an efficient private database query (PDQ) protocol which supports compound conditions with equality and order comparisons. To this end, we first present a private comparison algorithm on encrypted integers using FHE, which scales efficiently for the length of input integers, by applying techniques from finite field theory. Then, we consider a scenario for PDQ protocols, querying for values based on a conjunction of one order and four equality conditions on key columns. The proposed algorithm and protocol are implemented and tested to determine their performance in practice. The proposed comparison algorithm takes about$25.259$seconds to compare 697 pairs of 64-bit integers using Brakerski-Gentry-Vaikuntanathan's leveled FHE scheme with single instruction multiple data (SIMD) techniques at more than 138 bits of security. This yields an amortized rate of just 36 milliseconds per comparison. On top of that, we show that our techniques achieve an efficient PDQ protocol for one order and four equality comparisons, achieving an amortized time and communication cost of 57 milliseconds and 448 bytes per database element.

We solve an open question in code-based cryptography by introducing two provably secure group signature schemes from code-based assumptions. Our basic scheme satisfies the CPA-anonymity and traceability requirements in the random oracle model, assuming the hardness of the McEliece problem, the Learning Parity with Noise problem, and a variant of the Syndrome Decoding problem. The construction produces smaller key and signature sizes than the previous group signature schemes from lattices, as long as the cardinality of the underlying group does not exceed 224, which is roughly comparable to the current population of the Netherlands. We develop the basic scheme further to achieve the strongest anonymity notion, i.e., CCA-anonymity, with a small overhead in terms of efficiency. The feasibility of two proposed schemes is supported by implementation results. Our two schemes are the first in their respective classes of provably secure groups signature schemes. Additionally, the techniques introduced in this work might be of independent interest. These are a new verifiable encryption protocol for the randomized McEliece encryption and a novel approach to design formal security reductions from the Syndrome Decoding problem.

Public key encryption with equality test (PKEET) is a variant of classical public key encryption (PKE) with the special functionality of an equality test, and can be used in many applications such as in keyword search on encrypted data and for efficient management by partitioning encrypted data in the cloud. Since the original proposal of Yang et al. (CT-RSA, 2010), several subsequent proposals to improve the efficiency or functionality of PKEET have been reported. We present a PKEET construction from generic assumptions in the random oracle model . In particular, whereas previous results require number-theoretic assumptions or strictly stronger generic assumptions such as the existence of secure hierarchical identity-based encryption, our proposal requires only the existence of cryptographic hash functions and secure PKE schemes satisfying a special property , called randomness extractability . Informally, randomness extractability means that one can recover the randomness used in a ciphertext when given a secret key corresponding to a public key for the ciphertext . We investigate the fact that PKE schemes satisfying this property can be designed by the Fujisaki-Okamoto (FO) transformation, which is the widely utilized method to obtain secure PKE schemes from basic cryptographic primitives in the random oracle model . As a result, in combination with the FO transformation, we obtain a PKEET construction in the random oracle model if there exist a one-way PKE scheme, a one-time secure symmetric key encryption scheme , collision-resistant and one-way hash functions , and a pseudorandom function. In this sense, we remark that our PKEET construction is derived from fundamental generic assumptions only.

Fully homomorphic encryption (FHE) brings a paradigm shift in cryptographic engineering by enabling us to resolve various unsolved problems. Among them, this work solves the problem to design a private database query (PDQ) protocol that supports compound queries with wildcard conditions on encrypted databases using FHE. More precisely, we consider a setting where clients outsource an encrypted database using FHE to a remote server, and later request results of compound queries including a wildcard search condition-given a set of attribute values {A1; A2; ...; An} and a search pattern W, retrieve a set of all attribute values Ai's in which the pattern W occurs. To this end, we first develop an algorithm for testing whether an encrypted string contains an encrypted pattern without revealing any information of the pattern, taking auxiliary encryptions as additional inputs. Then, using this algorithm, we design PDQ protocols on encrypted databases, which support compound queries using wildcard search conditions. Finally, we demonstrate proof-of-concept implementation results of our protocols by exploiting single-instruction-multiple-data operations and multi-threading techniques.

Private query processing is a very attractive problem in the fields of both cryptography and databases. In this work, we restrict our attention to the efficiency aspect of the problem, particularly for basic queries with conditions on various combinations of equality. Without loss of generality, these conditions can be regarded as a Boolean function, and this Boolean function can then be evaluated at ciphertexts produced by a fully homomorphic encryption (FHE) scheme without decryption. From the efficiency perspective, the remaining concern is to efficiently test the equality function without severely downgrading the performance of FHE-based querying solutions. To this end, we first analyze the multiplicative depth required for an equality test algorithm with respect to the plaintext space inhabited by general FHE schemes. The primary reason for this approach is that given an equality test algorithm, its efficiency is measured in terms of the multiplicative depth required to construct its arithmetic circuit expression. Indeed, the implemented equality test algorithm dominates the entire performance of FHE-based query solutions, apart from the performance of the underlying FHE scheme. Then, we measure the multiplicative depth considering an FHE scheme that takes an extension field as its plaintext space and that supports the depth-free evaluation of Frobenius maps. According to our analysis, when the plaintext space of an FHE scheme is a field of characteristic 2, the equality test algorithm for `-bit messages requires the lowest multiplicative depth dlog`e. Furthermore, we design a set of private query protocols for conjunctive, disjunctive, and threshold queries based on the equality test algorithm. Similarly, applying the equality test algorithm over F2ℓ, our querying protocols require the minimum depths. More specifically, a multiplicative depth of [log ℓ] + [log (1 + ρ)] is required for conjunctive and disjunctive queries, and a depth of [log ℓ] + 2[log (1+ρ )] is required for threshold conjunctive queries, when their query conditions have p attributes to be compared. Finally, we provide a communication-efficient version of our solutions, though with additional computational costs, when an upper bound δ (0 ≤ δ ≤ 1) on the selectivity of a database is given. Consequently, we reduce the communication cost from n to approximately [δn] ciphertexts with [log n] additional depth when the database consists of n tuples.

In this article, we identify a flaw in Huang et al.'s public key encryption with authorized equality test (The Computer Journal, 2015). More precisely, we point out that the proof of the indistinguishability under adaptive chosen ciphertext attack (IND-CCA2) security for their scheme has a serious flaw. We illustrate this flaw by presenting a polynomial time CCA2 attack on their scheme. We also provide a solution to correct this flaw by modifying their scheme slightly. Our solution is quite efficient because it provides security against CCA2 attack by exploiting only the hash computation of a two times longer input without any increase in the sizes of ciphertexts and warrants.

Public key encryption with equality test (PKEET), which was first introduced by Yang et al. (CT-RSA, 2010), has various applications including facilitating keyword search on encrypted data and partitioning encrypted data on the cloud. It can be also applied to manage personal health records on the internet. For these reasons, there have been improvements on earlier PKEET schemes in terms of performance and functionality. We present a semi-generic method for PKEET constructions, assuming only the existence of IND-CCA2 secure traditional public key encryption (PKE) schemes, the hardness of Computational Diffie-Hellman (CDH) problems, and random oracles. Our approach has several advantages; it enables us to understand requirements for the equality test functionality more clearly. Furthermore, our approach is quite general, in that if we change the underlying PKE scheme with the identity-based encryption (IBE) scheme (and we assume the hardness of Bilinear Diffie-Hellman problems instead of CDH), then we obtain the first IBE scheme with equality test (IBEET) satisfying analogous security arguments to those of PKEET. Although an IBEET construction was recently proposed, but we note that it satisfies only weak security requirements.

We propose an efficient additive homomorphic encryption scheme. In our scheme, an encryption of a message is simply its noisy modular reduction by several different moduli. The security of our scheme relies on the hardness of a new problem, the co-Approximate Common Divisor problem. We analyze its hardness by applying all known attacks and devising dedicated attacks. These analyses are not complete, but give sufficiently plausible evidence for the hardness of this new problem.