International Association
for Cryptologic Research

In this paper, we study the design of efficient signature and public-key encryption (PKE) schemes in the presence of both leakage and tampering attacks. Firstly, we formalize the strong leakage and tamper-resilient (sLTR) security model for signature, which provides strong existential unforgeability, and deals with bounded leakage and restricted tampering attacks, as a counterpart to the sLTR security introduced by Sun et al. (ACNS 2019) for PKE. Then, we present direct constructions of signature and chosen-ciphertext attack (CCA) secure PKE schemes in the sLTR model, based on the matrix decisional Diffie-Hellman (MDDH) assumptions (which covers the standard symmetric external DH (SXDH) and k-Linear assumptions) over asymmetric pairing groups. Our schemes avoid the use of heavy building blocks such as the true-simulation extractable non-interactive zero-knowledge proofs (tSE-NIZK) proposed by Dodis et al. (ASIACRYPT 2010), which are usually needed in constructing schemes with leakage and tamper-resilience. Especially, our SXDH-based signature and PKE schemes are more efficient than the existing schemes in the leakage and tamper-resilient setting: our signature scheme has only 4 group elements in the signature, which is about 5×~8× shorter, and our PKE scheme has only 6 group elements in the ciphertext, which is about 1.3×~3.3× shorter. Finally, we note that our signature scheme is the {\it first} one achieving strong existential unforgeability in the leakage and tamper-resilient setting, where strong existential unforgeability has important applications in building more complex primitives such as signcryption and authenticated key exchange.

We propose a novel KZG-based sum-check scheme, dubbed $\mathsf{Losum}$, with \emph{optimal} efficiency. Particularly, its proving cost is \emph{one} multi-scalar-multiplication of size $k$---the number of non-zero entries in the vector, its verification cost is \emph{one} pairing plus one group scalar multiplication, and the proof consists of only \emph{one} group element. Using $\mathsf{Losum}$ as a component, we then construct a new lookup argument, named $\mathsf{Locq}$, which enjoys a smaller proof size and a lower verification cost compared to the state of the arts $\mathsf{cq}$, $\mathsf{cq}$+ and $\mathsf{cq}$++. Specifically, the proving cost of $\mathsf{Locq}$ is comparable to $\mathsf{cq}$, keeping the advantage that the proving cost is independent of the table size after preprocessing. For verification, $\mathsf{Locq}$ costs four pairings, while $\mathsf{cq}$, $\mathsf{cq}$+ and $\mathsf{cq}$++ require five, five and six pairings, respectively. For proof size, a $\mathsf{Locq}$ proof consists of four $\mathbb{G}_1$ elements and one $\mathbb{G}_2$ element; when instantiated with the BLS12-381 curve, the proof size of $\mathsf{Locq}$ is $2304$ bits, while $\mathsf{cq}$, $\mathsf{cq}$+ and $\mathsf{cq}$++ have $3840$, $3328$ and $2944$ bits, respectively. Moreover, $\mathsf{Locq}$ is zero-knowledge as $\mathsf{cq}$+ and $\mathsf{cq}$++, whereas $\mathsf{cq}$ is not. $\mathsf{Locq}$ is more efficient even compared to the non-zero-knowledge (and more efficient) versions of $\mathsf{cq}$+ and $\mathsf{cq}$++.

Recently, researchers have proposed many LWE estimators, such as lattice-estimator (Albrecht et al, Asiacrypt 2017) and leaky-LWE-Estimator (Dachman-Soled et al, Crypto 2020), while the latter has already been used in estimating the security level of Kyber and Dilithium using only BKZ. However, we prove in this paper that solving LWE by combining a lattice reduction step (by LLL or BKZ) and a target vector searching step (by enumeration or sieving), which we call a Two-step mode, is more efficient than using only BKZ. Moreover, we give a refined LWE estimator in Two-step mode by analyzing the relationship between the probability distribution of the target vector and the solving success rate in a Two-step mode LWE solving algorithm. While the latest Two-step estimator for LWE, which is the “primal-bdd” mode in lattice-estimator1, does not take into account some up-to-date results and lacks a thorough theoretical analysis. Under the same gate-count model, our estimation for NIST PQC standards drops by 2.1∼3.4 bits (2.2∼4.6 bits while considering more flexible blocksize and jump strategy) compared with leaky-LWE-Estimator. Furthermore, we also give a conservative estimation for LWE from the Two-step solving algorithm. Compared with the Core-SVP model, which is used in previous conservative estimations, our estimation relies on weaker assumptions and outputs higher evaluation results than the Core-SVP model. For NIST PQC standards, our conservative estimation is 4.17∼8.11 bits higher than the Core-SVP estimation. Hence our estimator can give a closer estimation for both upper bound and lower bound of LWE hardness.

Kyber and Dilithium are both lattice-based post-quantum cryptography (PQC) algorithms that have been selected for standardization by the American National Institute of Standards and Technology (NIST). NIST recommends them as two primary algorithms to be implemented for most use cases. As the applications of RISC-V processors move from specialized scenarios to general scenarios, efficient implementations of PQC algorithms on general-purpose RISC-V platforms are required. In this work, we present an optimized hardware-software co-design for Kyber and Dilithium on the industry’s first RISC-V System-on-Chip (SoC) Field Programmable Gate Array (FPGA) platform. The performance of both algorithms is enhanced through the utilization of hardware acceleration and software optimization, while a certain level of flexibility is still maintained. The polynomial arithmetic operations in Kyber and Dilithium are accelerated by the customized accelerators. We employ a unified high-level architecture to depict their shared characteristics and design dedicated underlying modular multipliers to explore their distinctive features. The hashing functions are optimized using RISC-V assembly instructions, resulting in improved performance and reduced code size without additional hardware resources. For other operations involving matrices and vectors, we present a multi-core acceleration scheme based on the multi-core RISC-V Microprocessor Sub-System (MSS). Combining these acceleration and optimization methods, experimental results show that the overall performance of Kyber and Dilithium across different security levels improves by 3 to 5 times, while the utilized FPGA resources account for less than 5% of the total resources provided by the platform.

Functional encryption (FE for short) can be used to calculate a function output of a message, without revealing other information about the message. There are mainly two types of security definitions for FE, exactly simulation-based security (SIM-security) and indistinguishability-based security (IND-security). The two types of security definitions both suffer from their own drawbacks: FE with SIM-security supporting all circuits cannot be constructed for unbounded number of ciphertext and/or key queries, while IND-security is sometimes not enough: there are examples where an FE scheme is IND-secure but not intuitively secure. In this paper, we present a new security definition which can avoid the drawbacks of both SIM-security and IND-security, called indistinguishability-based security against probabilistic queries (pIND-security for short), and we give an FE construction for all circuits which is secure for unbounded key/ciphertext queries under this new security definition. We prove that this new security definition is strictly between SIM-security and IND-security, and provide new applications for FE which were not known to be constructed from IND-secure or SIM-secure FE.

(Asymmetric) Password-based Authenticated Key Exchange ((a)PAKE) protocols allow two parties establish a session key with a pre-shared low-entropy password. In this paper, we show how Encrypted Key Exchange (EKE) compiler [Bellovin and Merritt, S&P 1992] meets tight security in the Universally Composable (UC) framework. We propose a strong 2DH variant of EKE, denoted by 2DH-EKE, and prove its tight security in the UC framework based on the CDH assumption. The efficiency of 2DH-EKE is comparable to the original EKE, with only O(\lambda) bits growth in communication (\lambda the security parameter), and two (resp., one) extra exponentiation in computation for client (resp., server). We also develop an asymmetric PAKE scheme 2DH-aEKE from 2DH-EKE. The security reduction loss of 2DH-aEKE is N, the total number of client-server pairs. With a meta-reduction, we formally prove that such a factor N is inevitable in aPAKE. Namely, our 2DH-aEKE meets the optimal security loss. As a byproduct, we further apply our technique to PAKE protocols like SPAKE2 and PPK in the relaxed UC framework, resulting in their 2DH variants with tight security from the CDH assumption.

In this paper, we propose a new type of non-interactive zero-knowledge (NIZK), called Fine-grained Verifier NIZK (FV-NIZK), which provides more flexible and more fine-grained verifiability of proofs than standard NIZK that supports public verifiability and designated-verifier NIZK (DV-NIZK) that supports private verifiability. FV-NIZK has two statistically equivalent verification approaches: -- a master verification using the master secret key msk; -- a fine-grained verification using a derived secret key sk_d, which is derived from msk w.r.t. d (which may stand for user identity, email address, vector, etc.). We require unbounded simulation soundness (USS) of FV-NIZK to hold, even if an adversary obtains derived secret keys sk_d with d of its choices, and define proof pseudorandomness which stipulates the pseudorandomness of proofs for adversaries that are not given any secret key. We present two instantiations of FV-NIZK for linear subspace languages, based on the matrix decisional Diffie-Hellman (MDDH) assumption. One of the FV-NIZK instantiations is pairing-free and achieves almost tight USS and proof pseudorandomness. We illustrate the usefulness of FV-NIZK by showing two applications and obtain the following pairing-free schemes: -- the first almost tightly multi-challenge CCA (mCCA)-secure inner-product functional encryption (IPFE) scheme without pairings; -- the first public-key encryption (PKE) scheme that reconciles the inherent contradictions between public verifiability and anonymity. We formalize such PKE as Fine-grained Verifiable PKE (FV-PKE), which derives a special key from the decryption secret key, such that for those who obtain the derived key, they can check the validity of ciphertexts but the anonymity is lost from their views (CCA-security still holds for them), while for others who do not get the derived key, they cannot do the validity check but the anonymity holds for them. Our FV-PKE scheme achieves almost tight mCCA-security for adversaries who obtain the derived keys, and achieves almost tight ciphertext pseudorandomness (thus anonymity) for others who do not get any derived key.

In this paper, we consider tight multi-user security under adaptive corruptions, where the adversary can adaptively corrupt some users and obtain their secret keys. We propose generic constructions for a bunch of primitives, and the instantiations from the matrix decision Diffie-Hellman (MDDH) assumptions yield the following schemes: (1) the first digital signature (SIG) scheme achieving almost tight strong EUF-CMA security in the multi-user setting with adaptive corruptions in the standard model; (2) the first public-key encryption (PKE) scheme achieving almost tight IND-CCA security in the multi-user multi-challenge setting with adaptive corruptions in the standard model; (3) the first signcryption (SC) scheme achieving almost tight privacy and authenticity under CCA attacks in the multi-user multi-challenge setting with adaptive corruptions in the standard model. As byproducts, our SIG and SC naturally derive the first strongly secure message authentication code (MAC) and the first authenticated encryption (AE) schemes achieving almost tight multi-user security under adaptive corruptions in the standard model. We further optimize constructions of SC, MAC and AE to admit better efficiency. Furthermore, we consider key leakages besides corruptions, as a natural strengthening of tight multi-user security under adaptive corruptions. This security considers a more natural and more complete "all-or-part-or-nothing" setting, where secret keys of users are either fully exposed to adversary ("all"), or completely hidden to adversary ("nothing"), or partially leaked to adversary ("part"), and it protects the uncorrupted users even with bounded key leakages. All our schemes additionally support bounded key leakages and enjoy full compactness. This yields the first SIG, PKE, SC, MAC, AE schemes achieving almost tight multi-user security under both adaptive corruptions and leakages.

In this work, we construct the {\it first} digital signature (SIG) and public-key encryption (PKE) schemes with almost tight multi-user security under adaptive corruptions based on the learning-with-errors (LWE) assumption in the standard model. Our PKE scheme achieves almost tight IND-CCA security and our SIG scheme achieves almost tight strong EUF-CMA security, both in the multi-user setting with adaptive corruptions. The security loss is quadratic in the security parameter, and independent of the number of users, signatures or ciphertexts. Previously, such schemes were only known to exist under number-theoretic assumptions or in classical random oracle model, thus vulnerable to quantum adversaries. To obtain our schemes from LWE, we propose new frameworks for constructing SIG and PKE with a core technical tool named {\it probabilistic} quasi-adaptive hash proof system (pr-QA-HPS). As a new variant of HPS, our pr-QA-HPS provides {\it probabilistic} public and private evaluation modes that may toss coins. This is in stark contrast to the traditional HPS [Cramer and Shoup, Eurocrypt 2002] and existing variants like approximate HPS [Katz and Vaikuntanathan, Asiacrypt 2009], whose public and private evaluations are deterministic in their inputs. Moreover, we formalize a new property called evaluation indistinguishability by requiring statistical indistinguishability of the two probabilistic evaluation modes, even in the presence of the secret key. The evaluation indistinguishability, as well as other nice properties resulting from the probabilistic features of pr-QA-HPS, are crucial for the multi-user security proof of our frameworks under adaptive corruptions. As for instantiations, we construct pr-QA-HPS from the LWE assumption and prove its properties with almost tight reductions, which admit almost tightly secure LWE-based SIG and PKE schemes under our frameworks. Along the way, we also provide new almost-tight reductions from LWE to multi-secret LWE, which may be of independent interest.

Zero-Knowledge Virtual Machines (ZKVMs) have gained traction in recent years due to their potential applications in a variety of areas, particularly blockchain ecosystems. Despite tremendous progress on ZKVMs in the industry, no formal definitions or security proofs have been established in the literature. Due to this lack of formalization, existing protocols exhibit significant discrepancies in terms of problem definitions and performance metrics, making it difficult to analyze and compare these advancements, or to trust the security of the increasingly complex ZKVM implementations. In this work, we focus on random-access memory, an influential and expensive component of ZKVMs. Specifically, we investigate the state-of-the-art protocols for validating the correct functioning of memory, which we refer to as the \emph{memory consistency checks}. Isolating these checks from the rest of the system allows us to formalize their definition and security notion. Furthermore, we summarize the state-of-the-art constructions using the Polynomial IOP model and formally prove their security. Observing that the bottleneck of existing designs lies in sorting the entire memory trace, we break away from this paradigm and propose a novel memory consistency check, dubbed $\mathsf{Permem}$. $\mathsf{Permem}$ bypasses this bottleneck by introducing a technique called the address cycle method, which requires fewer building blocks and---after instantiating the building blocks with state-of-the-art constructions---fewer online polynomial oracles and evaluation queries. In addition, we propose $\mathsf{gcq}$, a new construction for the lookup argument---a key building block of the memory consistency check, which costs fewer online polynomial oracles than the state-of-the-art construction $\mathsf{cq}$.

Privacy-Preserving Authenticated Key Exchange (PPAKE) provides protection both for the session keys and the identity information of the involved parties. In this paper, we introduce the concept of robustness into PPAKE. Robustness enables each user to confirm whether itself is the target recipient of the first round message in the protocol. With the help of robustness, a PPAKE protocol can successfully avoid the heavy redundant communications and computations caused by the ambiguity of communicants in the existing PPAKE, especially in broadcast channels. We propose a generic construction of robust PPAKE from key encapsulation mechanism (KEM), digital signature (SIG), message authentication code (MAC), pseudo-random generator (PRG) and symmetric encryption (SE). By instantiating KEM, MAC, PRG from the DDH assumption and SIG from the CDH assumption, we obtain a specific robust PPAKE scheme in the standard model, which enjoys forward security for session keys, explicit authentication and forward privacy for user identities. Thanks to the robustness of our PPAKE, the number of broadcast messages per run and the computational complexity per user are constant, and in particular, independent of the number of users in the system.

Mimblewimble is a privacy-preserving cryptocurrency, providing the functionality of transaction aggregation. Once certain coins have been spent in Mimblewimble, they can be deleted from the UTXO set. This is desirable: now storage can be saved and computation cost can be reduced. Fuchsbauer et al. (EUROCRYPT 2019) abstracted Mimblewimble as an Aggregate Cash System (ACS) and provided security analysis via game-based definitions. In this paper, we revisit the ACS, and focus on {\em Non-interactive} ACS, denoted as NiACS. We for the first time propose a simulation-based security definition and formalize an ideal functionality for NiACS. Then, we construct a NiACS protocol in a hybrid model which can securely realize the ideal NiACS functionality in the Universal Composition (UC) framework. In addition, we propose a building block, which is a variant of the ElGamal encryption scheme that may be of independent interest. Finally, we show how to instantiate our protocol, and obtain the first NiACS system with UC security.

Double-block Hash-then-Sum (\textsf{DbHtS}) MACs are a class of MACs that aim for achieving beyond-birthday-bound security, including \textsf{SUM-ECBC}, \textsf{PMAC\_Plus}, \textsf{3kf9} and \textsf{LightMAC\_Plus}. Recently Datta et al. (FSE'19), and then Kim et al. (Eurocrypt'20) prove that \textsf{DbHtS} constructions are secure beyond the birthday bound in the single-user setting. However, by a generic reduction, their results degrade to (or even worse than) the birthday bound in the multi-user setting. In this work, we revisit the security of \textsf{DbHtS} MACs in the multi-user setting. We propose a generic framework to prove beyond-birthday-bound security for \textsf{DbHtS} constructions. We demonstrate the usability of this framework with applications to key-reduced variants of \textsf{DbHtS} MACs, including \textsf{2k-SUM-ECBC}, \textsf{2k-PMAC\_Plus} and \textsf{2k-LightMAC\_Plus}. Our results show that the security of these constructions will not degrade as the number of users grows. On the other hand, our results also indicate that these constructions are secure beyond the birthday bound in both single-user and multi-user setting without additional domain separation, which is used in the prior work to simplify the analysis. Moreover, we find a critical flaw in \textsf{2kf9}, which is proved to be secure beyond the birthday bound by Datta et al. (FSE'19). We can successfully forge a tag with probability 1 without making any queries. We go further to show attacks with birthday-bound complexity on several variants of \textsf{2kf9}.

With the renaissance of deep learning, the side-channel community also notices the potential of this technology, which is highly related to the profiling attacks in the side-channel context. Many papers have recently investigated the abilities of deep learning in profiling traces. Some of them also aim at the countermeasures (e.g., masking) simultaneously. Nevertheless, so far, all of these papers work with an (implicit) assumption that the number of time samples in raw traces can be reduced before the profiling, i.e., the position of points of interest (PoIs) can be manually located. This is arguably the most challenging part of a practical black-box analysis targeting an implementation protected by masking. Therefore, we argue that to fully utilize the potential of deep learning and get rid of any manual intervention, the end-to-end profiling directly mapping raw traces to target intermediate values is demanded.In this paper, we propose a neural network architecture that consists of encoders, attention mechanisms and a classifier, to conduct the end-to-end profiling. The networks built by our architecture could directly classify the traces that contain a large number of time samples (i.e., raw traces without manual feature extraction) while whose underlying implementation is protected by masking. We validate our networks on several public datasets, i.e., DPA contest v4 and ASCAD, where over 100,000 time samples are directly used in profiling. To our best knowledge, we are the first that successfully carry out end-to-end profiling attacks. The results on the datasets indicate that our networks could get rid of the tricky manual feature extraction. Moreover, our networks perform even systematically better (w.r.t. the number of traces in attacks) than those trained on the reduced traces. These validations imply our approach is not only a first but also a concrete step towards end-to-end profiling attacks in the side-channel context.

Deep learning (DL)-based techniques have recently proven to be very successful when applied to profiled side-channel attacks (SCA). In a real-world profiled SCA scenario, attackers gain knowledge about the target device by getting access to a similar device prior to the attack. However, most state-of-the-art literature performs only proof-of-concept attacks, where the traces intended for profiling and attacking are acquired consecutively on the same fully-controlled device. This paper reminds that even a small discrepancy between the profiling and attack traces (regarded as domain discrepancy) can cause a successful single-device attack to completely fail. To address the issue of domain discrepancy, we propose a Cross-Device Profiled Attack (CDPA), which introduces an additional fine-tuning phase after establishing a pretrained model. The fine-tuning phase is designed to adjust the pre-trained network, such that it can learn a hidden representation that is not only discriminative but also domain-invariant. In order to obtain domain-invariance, we adopt a maximum mean discrepancy (MMD) loss as a constraint term of the classic cross-entropy loss function. We show that the MMD loss can be easily calculated and embedded in a standard convolutional neural network. We evaluate our strategy on both publicly available datasets and multiple devices (eight Atmel XMEGA 8-bit microcontrollers and three SAKURA-G evaluation boards). The results demonstrate that CDPA can improve the performance of the classic DL-based SCA by orders of magnitude, which significantly eliminates the impact of domain discrepancy caused by different devices.

For Key Encapsulation Mechanism (KEM) deployed in a multi-user setting, an adversary may corrupt some users to learn their secret keys, and obtain some encapsulated keys due to careless key managements of users. To resist such attacks, we formalize Enhanced security against Chosen Plaintext/Ciphertext Attack (ECPA/ECCA), which ask the pseudorandomness of unrevealed encapsulated keys under uncorrupted users. This enhanced security for KEM serves well for the security of a class of Authenticated Key Exchange protocols built from KEM. In this paper, we study the achievability of tight ECPA and ECCA security for KEM in the multi-user setting, and present an impossibility result and an optimal security loss factor that can be obtained. The existing meta-reduction technique due to Bader et al. (EUROCRYPT 2016) rules out some KEMs, but many well-known KEMs, e.g., Cramer-Shoup KEM (SIAM J. Comput. 2003), Kurosawa-Desmedt KEM (CRYPTO 2004), run out. To solve this problem, we develop a new technique tool named rank of KEM and a new secret key partitioning strategy for meta-reduction. With this new tool and new strategy, we prove that KEM schemes with polynomially-bounded ranks have no tight ECPA and ECCA security from non-interactive complexity assumptions, and the security loss is at least linear in the number n of users. This impossibility result covers lots of well-known KEMs, including the Cramer-Shoup KEM, Kurosawa-Desmedt KEM and many others. Moreover, we show that the linear security loss is optimal by presenting concrete KEMs with security loss Θ(n). This is justified by a non-trivial security reduction with linear loss factor from ECPA/ECCA security to the traditional multi-challenge CPA/CCA security.

Persistence fault analysis (PFA) is a novel fault analysis technique proposed in CHES 2018 and demonstrated with rowhammer-based fault injections. However, whether such analysis can be applied to traditional fault attack scenario, together with its difficulty in practice, has not been carefully investigated. For the first time, a persistent fault attack is conducted on an unprotected AES implemented on ATmega163L microcontroller in this paper. Several critical challenges are solved with our new improvements, including (1) how to decide whether the fault is injected in SBox; (2) how to use the maximum likelihood estimation to pursue the minimum number of ciphertexts; (3) how to utilize the unknown fault in SBox to extract the key. Our experiments show that: to break AES with physical laser injections despite all these challenges, the minimum and average number of required ciphertexts are 926 and 1641, respectively. It is about 38% and 28% reductions of the ciphertexts required in comparison to 1493 and 2273 in previous work where both fault value and location have to be known. Furthermore, our analysis is extended to the PRESENT cipher. By applying the persistent fault analysis to the penultimate round, the full PRESENT key of 80 bits can be recovered. Eventually, an experimental validation is performed to confirm the accuracy of our attack with more insights. This paper solves the challenges in most aspects of practice and also demonstrates the feasibility and universality of PFA on SPN block ciphers.

We revisit the method of designing public-key puncturable encryption schemes and present a generic conversion by leveraging the techniques of distributed key-distribution and revocable encryption. In particular, we first introduce a refined version of identity-based revocable encryption, named key-homomorphic identity-based revocable key encapsulation mechanism with extended correctness . Then, we propose a generic construction of puncturable key encapsulation mechanism from the former by merging the idea of distributed key-distribution. Compared to the state-of-the-art, our generic construction supports unbounded number of punctures and multiple tags per message, thus achieving more fine-grained revocation of decryption capability. Further, it does not rely on random oracles , not suffer from non-negligible correctness error, and results in a variety of efficient schemes with distinct features. More precisely, we obtain the first scheme with very compact ciphertexts in the standard model, and the first scheme with support for both unbounded size of tags per ciphertext and unbounded punctures as well as constant-time puncture operation. Moreover, we get a comparable scheme proven secure under the standard DBDH assumption, which enjoys both faster encryption and decryption than previous works based on the same assumption, especially when the number of tags associated with the ciphertext is large.

We propose a generic construction of 2-pass authenticated key exchange (AKE) scheme with explicit authentication from key encapsulation mechanism (KEM) and signature (SIG) schemes. We improve the security model due to Gjosteen and Jager [Crypto2018] to a stronger one. In the strong model, if a replayed message is accepted by some user, the authentication of AKE is broken. We define a new security notion named ''IND-mCPA with adaptive reveals'' for KEM. When the underlying KEM has such a security and SIG has unforgeability with adaptive corruptions, our construction of AKE equipped with counters as states is secure in the strong model, and stateless AKE without counter is secure in the traditional model. We also present a KEM possessing tight ''IND-mCPA security with adaptive reveals'' from the Computation Diffie-Hellman assumption in the random oracle model. When the generic construction of AKE is instantiated with the KEM and the available SIG by Gjosteen and Jager [Crypto2018], we obtain the first practical 2-pass AKE with tight security and explicit authentication. In addition, the integration of the tightly IND-mCCA secure KEM (derived from PKE by Han et al. [Crypto2019]) and the tightly secure SIG by Bader et al. [TCC2015] results in the first tightly secure 2-pass AKE with explicit authentication in the standard model.

Robustly reusable Fuzzy Extractor (rrFE) considers reusability and robustness simultaneously. We present two approaches to the generic construction of rrFE. Both of approaches make use of a secure sketch and universal hash functions. The first approach also employs a special pseudo-random function (PRF), namely unique-input key-shift (ui-ks) secure PRF, and the second uses a key-shift secure auxiliary-input authenticated encryption (AIAE). The ui-ks security of PRF (resp. key-shift security of AIAE), together with the homomorphic properties of secure sketch and universal hash function, guarantees the reusability and robustness of rrFE. Meanwhile, we show two instantiations of the two approaches respectively. The first instantiation results in the first rrFE from the LWE assumption, while the second instantiation results in the first rrFE from the DDH assumption over non-pairing groups.

We propose the concept of quasi-adaptive hash proof system (QAHPS), where the projection key is allowed to depend on the specific language for which hash values are computed. We formalize leakage-resilient(LR)-ardency for QAHPS by defining two statistical properties, including LR-$$\langle \mathscr {L}_0, \mathscr {L}_1 \rangle $$-universal and LR-$$\langle \mathscr {L}_0, \mathscr {L}_1 \rangle $$-key-switching.We provide a generic approach to tightly leakage-resilient CCA (LR-CCA) secure public-key encryption (PKE) from LR-ardent QAHPS. Our approach is reminiscent of the seminal work of Cramer and Shoup (Eurocrypt’02), and employ three QAHPS schemes, one for generating a uniform string to hide the plaintext, and the other two for proving the well-formedness of the ciphertext. The LR-ardency of QAHPS makes possible the tight LR-CCA security. We give instantiations based on the standard k-Linear (k-LIN) assumptions over asymmetric and symmetric pairing groups, respectively, and obtain fully compact PKE with tight LR-CCA security. The security loss is $${{O}}(\log {Q_{{e}}})$$ where $${Q_{{e}}}$$ denotes the number of encryption queries. Specifically, our tightly LR-CCA secure PKE instantiation from SXDH has only 4 group elements in the public key and 7 group elements in the ciphertext, thus is the most efficient one.

Selective opening security (SO security) is desirable for public key encryption (PKE) in a multi-user setting. In a selective opening attack, an adversary receives a number of ciphertexts for possibly correlated messages, then it opens a subset of them and gets the corresponding messages together with the randomnesses used in the encryptions. SO security aims at providing security for the unopened ciphertexts. Among the existing simulation-based, selective opening, chosen ciphertext secure (SIM-SO-CCA secure) PKEs, only one (Libert et al. Crypto’17) enjoys tight security, which is reduced to the Non-Uniform LWE assumption. However, their public key and ciphertext are not compact.In this work, we focus on constructing PKE with tight SIM-SO-CCA security based on standard assumptions. We formalize security notions needed for key encapsulation mechanism (KEM) and show how to transform these securities into SIM-SO-CCA security of PKE through a tight security reduction, while the construction of PKE from KEM follows the general framework proposed by Liu and Paterson (PKC’15). We present two KEM constructions with tight securities based on the Matrix Decision Diffie-Hellman assumption. These KEMs in turn lead to two tightly SIM-SO-CCA secure PKE schemes. One of them enjoys not only tight security but also compact public key.