What a lovely hat

In this work, we provide new, tighter proofs for the $T_{RH}$-transformation by Jiang et al. (ASIACRYPT 2023), which converts OW-CPA secure PKEs into KEMs with IND-1CCA security, a variant of typical IND-CCA security where only a single decapsulation query is allowed. Such KEMs are efficient and have been shown sufficient for real-world applications by Huguenin-Dumittan and Vaudenay at EUROCRYPT 2022. We reprove Jiang et al.'s $T_{RH}$-transformation in both the random oracle model (ROM) and...

We propose a new NTRU-based Public-Key Encryption (PKE) scheme called $\mathsf{NTRU+}\mathsf{PKE}$, which effectively incorporates the Fujisaki-Okamoto transformation for PKE (denoted as $\mathsf{FO}_{\mathsf{PKE}}$) to achieve chosen-ciphertext security in the Quantum Random Oracle Model (QROM). While $\mathsf{NTRUEncrypt}$, a first-round candidate in the NIST PQC standardization process, was proven to be chosen-ciphertext secure in the Random Oracle Model (ROM), it lacked corresponding...

Non-Interactive Timed Commitment schemes (NITC) allow to open any commitment after a specified delay $t_{\mathrm{fd}}$. This is useful for sealed bid auctions and as primitive for more complex protocols. We present the first NITC without repeated squaring or theoretical black box algorithms like NIZK proofs or one-way functions. It has fast verification, almost arbitrary delay and satisfies IND-CCA hiding and perfect binding. Our protocol is based on isogenies between supersingular elliptic...

In 2018, Mouha et al. (IEEE Trans. Reliability, 2018) performed a post-mortem investigation of the correctness of reference implementations submitted to the SHA3 competition run by NIST, finding previously unidentified bugs in a significant portion of them, including two of the five finalists. Their innovative approach allowed them to identify the presence of such bugs in a black-box manner, by searching for counterexamples to expected cryptographic properties of the implementations under...

Assuming the hardness of LWE and the existence of IO, we construct a public-key encryption scheme that is IND-CCA secure but fails to satisfy even a weak notion of indistinguishability security with respect to selective opening attacks. Prior to our work, such a separation was known only from stronger assumptions such as differing inputs obfuscation (Hofheinz, Rao, and Wichs, PKC 2016). Central to our separation is a new hash family, which may be of independent interest. Specifically,...

We present a formally verified proof of the correctness and IND-CCA security of ML-KEM, the Kyber-based Key Encapsulation Mechanism (KEM) undergoing standardization by NIST. The proof is machine-checked in EasyCrypt and it includes: 1) A formalization of the correctness (decryption failure probability) and IND-CPA security of the Kyber base public-key encryption scheme, following Bos et al. at Euro S&P 2018; 2) A formalization of the relevant variant of the Fujisaki-Okamoto transform in...

The One-Way to Hiding (O2H) theorem, first given by Unruh (J ACM 2015) and then restated by Ambainis et al. (CRYPTO 2019), is a crucial technique for solving the reprogramming problem in the quantum random oracle model (QROM). It provides an upper bound $d\cdot\sqrt{\epsilon}$ for the distinguisher's advantage, where $d$ is the query depth and $\epsilon$ denotes the advantage of a one-wayness attacker. Later, in order to obtain a tighter upper bound, Kuchta et al. (EUROCRYPT 2020) proposed...

The Module-NTRU problem, introduced by Cheon, Kim, Kim, Son (IACR ePrint 2019/1468), and Chuengsatiansup, Prest, Stehlé, Wallet, Xagawa (ASIACCS ’20), generalizes the versatile NTRU assump- tion. One of its main advantages lies in its ability to offer greater flexibil- ity on parameters, such as the underlying ring dimension. In this work, we present several lattice-based encryption schemes, which are IND-CPA (or OW-CPA) secure in the standard model based on the Module-NTRU and...

At EUROCRYPT’23, Castryck and Decru, Maino et al., and Robert present efficient attacks against supersingular isogeny Diffie-Hellman key exchange protocol (SIDH). Drawing inspiration from these attacks, Andrea Basso, Luciano Maino, and Giacomo Pope introduce FESTA, an isogeny-based trapdoor function, along with a corresponding IND-CCA secure public key encryption (PKE) protocol at ASIACRYPT’23. FESTA incorporates either a diagonal or circulant matrix into the secret key to mask torsion...

X-Wing is a hybrid key-encapsulation mechanism based on X25519 and ML-KEM-768. It is designed to be the sensible choice for most applications. The concrete choice of X25519 and ML-KEM-768 allows X-Wing to achieve improved efficiency compared to using a generic KEM combiner. In this paper, we introduce the X-Wing hybrid KEM construction and provide a proof of security. We show (1) that X-Wing is a classically IND-CCA secure KEM if the strong Diffie-Hellman assumption holds in the X25519...

The size of the authentication tag represents a significant overhead for applications that are limited by bandwidth or memory. Hence, some authenticated encryption designs have a smaller tag than the required privacy level, which was also suggested by the NIST lightweight cryptography standardization project. In the ToSC 2022, two papers have raised questions about the IND-CCA security of AEAD schemes in this situation. These papers show that (a) online AE cannot provide IND-CCA security...

Non-interactive zero-knowledge (NIZK) proof systems are a cornerstone of modern cryptography, but their security has received little attention in the quantum settings. Motivated by improving our understanding of this fundamental primitive against quantum adversaries, we propose a new definition of security against quantum adversary. Specifically, we define the notion of quantum simulation soundness (SS-NIZK), that allows the adversary to access the simulator in superposition. We show a...

In 2023, Basso, Maino, and Pope proposed FESTA (Fast Encryption from Supersingular Torsion Attacks), an isogeny-based public-key encryption (PKE) protocol that uses the SIDH attack for decryption. In the same paper, they proposed a parameter for that protocol, but the parameter requires high-degree isogeny computations. In this paper, we introduce QFESTA (Quaternion Fast Encapsulation from Supersingular Torsion Attacks), a new variant of FESTA that works with better parameters using...

We give a tighter security proof for authenticated key exchange (AKE) protocols that are generically constructed from key encapsulation mechanisms (KEMs) in the quantum random oracle model (QROM). Previous works (Hövelmanns et al., PKC 2020) gave reductions for such a KEM-based AKE protocol in the QROM to the underlying primitives with square-root loss and a security loss in the number of users and total sessions. Our proof is much tighter and does not have square-root loss. Namely, it only...

When modelling how public key encryption can enable secure communication, we should acknowledge that secret information, such as private keys or the randomness used for encryption, could become compromised. Intuitively, one would expect unrelated communication to remain secure, yet formalizing this intuition has proven challenging. Several security notions have appeared that aim to capture said scenario, ranging from the multi-user setting with corruptions, via selective opening attacks...

In this paper, we present NEV -- a faster and smaller NTRU Encryption using Vector decoding, which is provably IND-CPA secure in the standard model under the decisional NTRU and RLWE assumptions over the cyclotomic ring $R_q = \mathbb{Z}_q[X]/(X^n+1)$. Our main technique is a novel and non-trivial way to integrate a previously known plaintext encoding and decoding mechanism into the provably IND-CPA secure NTRU variant by Stehl\'e and Steinfeld (Eurocrypt 2011). Unlike the original NTRU...

In this work, we construct the 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...

Isogeny-based cryptography is one of the candidates for post-quantum cryptography. In 2023, Kani's theorem breaks an isogeny-based scheme SIDH, which was considered a promising post-quantum scheme. Though Kani's theorem damaged isogeny-based cryptography, some researchers have been trying to dig into the applications of this theorem. A FESTA trapdoor function is an isogeny-based trapdoor function that is one trial to apply Kani's theorem to cryptography. This paper claims that there is an...

Selective opening (SO) security is a security notion for public-key encryption schemes that captures security against adaptive corruptions of senders. SO security comes in chosen-plaintext (SO-CPA) and chosen-ciphertext (SO-CCA) variants, neither of which is implied by standard security notions like IND-CPA or IND-CCA security. In this paper, we present the first SO-CCA secure encryption scheme that combines the following two properties: (1) it has a constant ciphertext expansion...

Hofheinz et al. (TCC 2017) proposed several key encapsulation mechanism (KEM) variants of Fujisaki-Okamoto (\textsf{FO}) transformation, including $\textsf{FO}^{\slashed{\bot}}$, $\textsf{FO}_m^{\slashed{\bot}}$, $\textsf{QFO}_m^{\slashed{\bot}}$, $\textsf{FO}^{\bot}$, $\textsf{FO}_m^\bot$ and $\textsf{QFO}_m^\bot$, and they are widely used in the post-quantum cryptography standardization launched by NIST. These transformations are divided into two types, the implicit and explicit rejection...

The Fujisaki-Okamoto (\textsf{FO}) transformation (CRYPTO 1999 and Journal of Cryptology 2013) and its KEM variants (TCC 2017) are used to construct \textsf{IND-CCA}-secure PKE or KEM schemes in the random oracle model (ROM). In the post-quantum setting, the ROM is extended to the quantum random oracle model (QROM), and the \textsf{IND-CCA} security of \textsf{FO} transformation and its KEM variants in the QROM has been extensively analyzed. Grubbs et al. (EUROCRYPTO 2021) and Xagawa...

We introduce FESTA, an efficient isogeny-based public-key encryption (PKE) protocol based on a constructive application of the SIDH attacks. At its core, FESTA is based on a novel trapdoor function, which uses an improved version of the techniques proposed in the SIDH attacks to develop a trapdoor mechanism. Using standard transformations, we construct an efficient PKE that is IND-CCA secure in the QROM. Additionally, using a different transformation, we obtain the first isogeny-based PKE...

User authentication and message confidentiality are the basic security requirements of high-end applications such as multicast communication and distributed systems. Several efficient signature-then-encrypt cryptographic schemes have been proposed to offer these security requirements with lower computational cost and communication overhead. However, signature-then-encryption techniques take more computation time than signcryption techniques. Signcryption accomplishes both digital signature...

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...

Recently, in post-quantum cryptography migration, it has been shown that an IND-1-CCA-secure key encapsulation mechanism (KEM) is required for replacing an ephemeral Diffie-Hellman (DH) in widely-used protocols, e.g., TLS, Signal, and Noise. IND-1-CCA security is a notion similar to the traditional IND-CCA security except that the adversary is restricted to one single decapsulation query. At EUROCRYPT 2022, based on CPA-secure public-key encryption (PKE), Huguenin-Dumittan and Vaudenay...

Kyber is a key-encapsulation mechanism (KEM) that was recently selected by NIST in its PQC standardization process; it is also the only scheme to be selected in the context of public-key encryption (PKE) and key establishment. The main security target for KEMs, and their associated PKE schemes, in the NIST PQC context has been IND-CCA security. However, some important modern applications also require their underlying KEMs/PKE schemes to provide anonymity (Bellare et al., ASIACRYPT 2001)....

The Fujisaki-Okamoto (FO) transform (CRYPTO 1999 and JoC 2013) turns any weakly (i.e., IND-CPA) secure public-key encryption (PKE) scheme into a strongly (i.e., IND-CCA) secure key encapsulation method (KEM) in the random oracle model (ROM). Recently, the FO transform re-gained momentum as part of CRISTAL-Kyber, selected by the NIST as the PKE winner of the post-quantum cryptography standardization project. Following Fischlin (ICALP 2005), we study the complete non-malleability of KEMs...

The quantum resistance Key Encapsulation Mechanism (PQC-KEM) design aims to replace cryptography in legacy security protocols. It would be nice if PQC-KEM were faster and lighter than ECDH or DH for easy migration to legacy security protocols. However, it seems impossible due to the temperament of the secure underlying problems in a quantum environment. Therefore, it makes reason to determine the threshold of the scheme by analyzing the maximum bandwidth the legacy security protocol can...

Extending work leveraging program obfuscation to instantiate random-oracle-based transforms (e.g., Hohenberger et al., EUROCRYPT 2014, Kalai et al., CRYPTO 2017), we show that, using obfuscation and other assumptions, there exist standard-model hash functions that suffice to instantiate the classical RO-model encryption transforms OAEP (Bellare and Rogaway, EUROCRYPT 1994) and Fujisaki-Okamoto (CRYPTO 1999, J. Cryptology 2013) for specific public-key encryption (PKE) schemes to achieve...

Timed commitment schemes, introduced by Boneh and Naor (CRYPTO 2000), can be used to achieve fairness in secure computation protocols in a simple and elegant way. The only known non-malleable construction in the standard model is due to Katz, Loss, and Xu (TCC 2020). This construction requires general-purpose zero knowledge proofs with specific properties, and it suffers from an inefficient commitment protocol, which requires the committing party to solve a computationally expensive...

We say a public-key encryption is plaintext-extractable in the random oracle model if there exists an algorithm that given access to all inputs/outputs queries to the random oracles can simulate the decryption oracle. We argue that plaintext-extractability is enough to show the indistinguishably under chosen ciphertext attack (IND-CCA) of OAEP+ transform (Shoup, Crypto 2001) when the underlying trapdoor permutation is one-way. We extend the result to the quantum random oracle model...

If you had a time machine, what cryptography would you be able to break? In this work, we investigate the notion of time travel, formally defining models for adversaries equipped with a time machine, and exploring the consequences for cryptography. We find that being able to rewind time breaks some cryptographic schemes, and being able to freely move both forwards and backwards in time breaks even more schemes. We look at the impacts of time travel on encryption and signatures in...

In this paper, we present the first side-channel attack on a higher-order masked implementation of an IND-CCA secure lattice-based key encapsulation mechanism (KEM). Our attack exploits a vulnerability in the procedure for the arithmetic to Boolean conversion which we discovered. On the example of Saber KEM, we demonstrate successful message and secret key recovery attacks on the second- and third-order masked implementations running on a different device than the profiling one. In our...

Selective opening (SO) security is one of the most important security notions of public key encryption (PKE) in a multi-user setting. Even though messages and random coins used in some ciphertexts are leaked, SO security guarantees the confidentiality of the other ciphertexts. Actually, it is shown that there exist PKE schemes which meet the standard security such as indistinguishability against chosen ciphertext attacks (IND-CCA security) but do not meet SO security against chosen...

We present $\BAT$ -- an IND-CCA secure key encapsulation mechanism (KEM) that is based on NTRU but follows an encryption/decryption paradigm distinct from classical NTRU KEMs. It demonstrates a new approach of decrypting NTRU ciphertext since its introduction 25 years ago. Instead of introducing an artificial masking parameter $p$ to decrypt the ciphertext, we use 2 linear equations in 2 unknowns to recover the message and the error. The encryption process is therefore close to the GGH...

Fully homomorphic encryption (FHE) is a powerful tool in cryptography that allows one to perform arbitrary computations on encrypted material without having to decrypt it first. There are numerous FHE schemes, all of which are expanded from somewhat homomorphic encryption (SHE) schemes, and some of which are considered viable in practice. However, while these FHE schemes are semantically (IND-CPA) secure, the question of their IND-CCA1 security is much less studied. In this paper, we group...

The main protection against side-channel attacks consists in computing every function with multiple shares via the masking countermeasure. For IND-CCA secure lattice-based encryption schemes, the masking of the decryption algorithm requires the high-order computation of a polynomial comparison. In this paper, we describe and evaluate a number of different techniques for such high-order comparison, always with a security proof in the ISW probing model. As an application, we describe the full...

In 2016, Galbraith et al. presented an adaptive attack on the SIDH key exchange protocol. In SIKE, one applies a variant of the Fujisaki-Okamoto transform to force Bob to reveal his encryption key to Alice, which Alice then uses to re-encrypt Bob's ciphertext and verify its validity. Therefore, Bob can not reuse his encryption keys. There have been two other proposed countermeasures enabling static-static private keys: k-SIDH and its variant by Jao and Urbanik. These countermeasures are...

The aim of this document is to clarify the DFR (Decoding Failure Rate) claims made for BIKE, a third round alternate candidate KEM (Key Encapsulation Mechanism) to the NIST call for post-quantum cryptography standardization. For the most part, the material presented here is not new, it is extracted from the relevant scientific literature, in particular [V21]. Even though a negligible DFR is not needed for a KEM using ephemeral keys (e.g. TLS) which only requires IND-CPA security, it seems...

This paper investigates __anonymity__ of all NIST PQC Round 3 KEMs: Classic McEliece, Kyber, NTRU, Saber, BIKE, FrodoKEM, HQC, NTRU Prime (Streamlined NTRU Prime and NTRU LPRime), and SIKE. We show the following results: * NTRU is anonymous in the quantum random oracle model (QROM) if the underlying deterministic PKE is strongly disjoint-simulatable. NTRU is collision-free in the QROM. A hybrid PKE scheme constructed from NTRU as KEM and appropriate DEM is anonymous and robust. (Similar...

Combining several primitives together to offer greater security is an old idea in cryptography. Recently, this concept has resurfaced as it could be used to improve trust in new Post-Quantum (PQ) schemes and smooth the transition to PQ cryptography. In particular, several ways to combine key exchange mechanisms (KEMs) into a secure hybrid KEM have been proposed. In this work, we observe that most PQ KEMs are built using a variant of the Fujisaki-Okamoto (FO) transform. Thus, we propose...

Key dependent message (KDM) security is a security notion that guarantees confidentiality of communication even if secret keys are encrypted. KDM security has found a number of applications in practical situations such as hard-disk encryption systems, anonymous credentials, and bootstrapping of fully homomorphic encryptions. Recently, it also found an application in quantum delegation protocols as shown by Zhang (TCC 2019). In this work, we investigate the KDM security of existing practical...

Code-based public key cryptosystems are one of the main techniques available in the area of Post-Quantum Cryptography. This work aims to propose a key encapsulation mechanism (KEM) with short ciphertext and secret key. Our goal is achieved in two steps. We first present a public key encryption (PKE) scheme, basicPKE, using a parity check matrix of Maximum Distance Separable (MDS) code as the public key matrix. In our construction, we exploit the structure of a companion matrix to obtain an...

Bounded IND-CCA security (IND-qCCA) is a notion similar to the traditional IND-CCA security, except the adversary is restricted to a constant number q of decryption/decapsulation queries. We show in this work that IND-qCCA is easily obtained from any passively secure PKE in the (Q)ROM. That is, simply adding a confirmation hash or computing the key as the hash of the plaintext and ciphertext holds an IND-qCCA KEM. In particular, there is no need for derandomization or re-encryption as in...

In this work, we propose generic and novel side-channel assisted chosen-ciphertext attacks on NTRU-based key encapsulation mechanisms (KEMs). These KEMs are IND-CCA secure, that is, they are secure in the chosen-ciphertext model. Our attacks involve the construction of malformed ciphertexts. When decapsulated by the target device, these ciphertexts ensure that a targeted intermediate variable becomes very closely related to the secret key. An attacker, who can obtain information about the...

A core goal of the NIST PQC competition is to produce public-key encryption (PKE) schemes which, even if attacked with a large-scale quantum computer, maintain the security guarantees needed by applications. The main security focus in the NIST PQC context has been IND-CCA security, but other applications demand that PKE schemes provide 'anonymity' (Bellare et al., ASIACRYPT 2001), and 'robustness' (Abdalla et al., TCC 2010). Examples of such applications include anonymous communication...

The KEM BIKE is a Round-3 alternative finalist in the NIST Post-Quantum Cryptography project. It uses the FO$^{\not \bot}$ transformation so that an instantiation with a decoder that has a DFR of $2^{-128}$ will make it IND-CCA secure. The current BIKE design does not bind the randomness of the ciphertexts (i.e., the error vectors) to a specific public key. We propose to change this design, although currently, there is no attack that leverages this property. This modification can be...

At Asiacrypt 2020, Moriya et al. introduced two new IND-CPA secure supersingular isogeny based Public Key Encryption (PKE) protocols: SiGamal and C-SiGamal. Unlike the PKEs canonically derived from SIDH and CSIDH, the new protocols provide IND-CPA security without the use of hash functions. SiGamal and C-SiGamal are however not IND-CCA secure. Moriya et al. suggested a variant of SiGamal that could be IND-CCA secure, but left its study as an open problem. In this paper, we revisit the...

In this paper, we present the first side-channel attack on a first-order masked implementation of IND-CCA secure Saber KEM. We show how to recover both the session key and the long-term secret key from 16 traces by deep learning-based power analysis without explicitly extracting the random mask at each execution. Since the presented method is not dependent on the mask, we can improve success probability by combining score vectors of multiple traces captured for the same ciphertext. This is...

Public-key encryption (PKE) schemes or key-encapsulation mechanisms (KEMs) are fundamental cryptographic building blocks to realize secure communication protocols. There are several known transformations that generically turn weakly secure schemes into strongly (i.e., IND-CCA) secure ones. While most of these transformations require the weakly secure scheme to provide perfect correctness, Hofheinz, Hövelmanns, and Kiltz (HHK) (TCC 2017) have recently shown that variants of the...

We study in this work a particular class of QC-MDPC codes for which the decoding failure rate is significantly larger than for typical QC-MDPC codes of same parameters. Our purpose is to figure out whether the existence of such weak codes impacts the security of cryptographic schemes using QC-MDPC codes as secret keys. A class of weak keys was exhibited in [DGK19]. We generalize it and show that, though their Decoding Failure Rate (DFR) is higher than normal, the set is not large enough to...

In the implementation of post-quantum primitives, it is well known that all computations that handle secret information need to be implemented to run in constant time. Using the Fujisaki-Okamoto transformation or any of its different variants, a CPA-secure primitive can be converted into an IND-CCA secure KEM. In this paper we show that although the transformation does not handle secret information apart from calls to the CPA-secure primitive, it has to be implemented in constant time....

We present KEMTLS, an alternative to the TLS 1.3 handshake that uses key-encapsulation mechanisms (KEMs) instead of signatures for server authentication. Among existing post-quantum candidates, signature schemes generally have larger public key/signature sizes compared to the public key/ciphertext sizes of KEMs: by using an IND-CCA-secure KEM for server authentication in post-quantum TLS, we obtain multiple benefits. A size-optimized post-quantum instantiation of KEMTLS requires less than...

The QC-MDPC code-based KEM BIKE is one of the Round-2 candidates of the NIST PQC standardization project. Its specification document describes a version that is claimed to have IND-CCA security. The security proof uses the Fujisaki-Okamoto transformation and a de-coder that targeted a Decoding Failure Rate (DFR) of 2^{-128} (for Level-1 security). However, there are several aspects that need to be amended in order for the IND-CCA proof to hold. The main issue is that using a decoder with DFR...

The US National Institute of Standards and Technology (NIST) recently announced the public-key cryptosystems (PKC) that have passed to the second round of the post-quantum standardization process. Most of these PKC come in two flavours: a weak IND-CPA version and a strongly secure IND-CCA construction. For the weaker scheme, no level of security is claimed in the plaintext-checking attack (PCA) model. However, previous works showed that, for several NIST candidates, only a few PCA queries...

Mersenne number schemes are a new strain of potentially quantum-safe cryptosystems that use sparse integer arithmetic modulo a Mersenne prime to encrypt messages. Two Mersenne number based schemes were submitted to the NIST post-quantum standardization process: Ramstake and Mersenne-756839. Typically, these schemes admit a low but non-zero probability that ciphertexts fail to decrypt correctly. In this work we show that the information leaked from failing ciphertexts can be used to gain...

NTS-KEM is one of the 17 post-quantum public-key encryption (PKE) and key establishment schemes remaining in contention for standardization by NIST. It is a code-based cryptosystem that starts with a combination of the (weakly secure) McEliece and Niederreiter PKE schemes and applies a variant of the Fujisaki-Okamoto (Journal of Cryptology 2013) or Dent (IMACC 2003) transforms to build an IND-CCA secure key encapsulation mechanism (KEM) in the classical random oracle model (ROM). Such...

In functional encryption (FE) a sender, Alice, encrypts plaintexts that a receiver, Bob, can obtain functional evaluations of, while Charlie is responsible for initializing the encryption keys and issuing the decryption keys. Standard notions of security for FE deal with a malicious Bob and how the confidentiality of Alice's messages can be maintained taking into account the leakage that occurs due to the functional keys that are revealed to the adversary via various forms of...

The efficiency of a black-box reduction is an important goal of modern cryptography. Traditionally, the time complexity and the success probability were considered as the main aspects of efficiency measurements. In CRYPTO 2017, Auerbach et al. introduced the notion of memory-tightness in cryptographic reductions and showed a memory-tight reduction of the existential unforgeability of the RSA-FDH signature scheme. Unfortunately, their techniques do not extend directly to the reductions...

McEliece-like code-based key exchange mechanisms using QC-MDPC codes can reach IND-CPA security under hardness assumptions from coding theory, namely quasi-cyclic syndrome decoding and quasi-cyclic codeword finding. To reach higher security requirements, like IND-CCA security, it is necessary in addition to prove that the decoding failure rate (DFR) is negligible, for some decoding algorithm and a proper choice of parameters. Getting a formal proof of a low DFR is a difficult task. Instead,...

Code-based public key cryptosystems have been found to be an interesting option in the area of Post-Quantum Cryptography. In this work, we present a key encapsulation mechanism (KEM) using a parity check matrix of the Generalized Srivastava code as the public key matrix. Generalized Srivastava codes are privileged with the decoding technique of Alternant codes as they belong to the family of Alternant codes. We exploit the dyadic structure of the parity check matrix to reduce the storage of...

We present Séta, a new family of public-key encryption schemes with post-quantum security based on isogenies of supersingular elliptic curves. It is constructed from a new family of trapdoor one-way functions, where the inversion algorithm uses Petit's so called torsion attacks on SIDH to compute an isogeny between supersingular elliptic curves given an endomorphism of the starting curve and images of torsion points. We prove the OW-CPA security of Séta and present an IND-CCA variant using...

The QC-MDPC code-based KEM Bit Flipping Key Encapsulation (BIKE) is one of the Round-2 candidates of the NIST PQC standardization project. It has a variant that is proved to be IND-CCA secure. The proof models the KEM with some black-box ("ideal") primitives. Specifically, the decapsulation invokes an ideal primitive called "decoder", required to deliver its output with a negligible Decoding Failure Rate (DFR). The concrete instantiation of BIKE substitutes this ideal primitive with a new...

The use of rank instead of Hamming metric has been proposed to address the main drawback of code-based cryptography: large key sizes. There exist several Key Encapsulation Mechanisms (KEM) and Public Key Encryption (PKE) schemes using rank metric including some submissions to the NIST call for standardization of Post-Quantum Cryptography. In this work, we present an IND-CCA PKE scheme based on the McEliece adaptation to rank metric proposed by Loidreau at PQC 2017. This IND-CCA PKE scheme...

This work envisions a new encryption primitive for many-to-many paradigms such as group messaging systems. Previously, puncturable encryption (PE) was introduced to provide forward security for asynchronous messaging services. However, existing PE schemes were proposed only for one-to-one communication, and causes a significant overhead for a group messaging system. In fact, the group communication over PE can only be achieved by encrypting a message multiple times for each receiver by the...

We propose two efficient public key encryption (PKE) schemes satisfying key dependent message security against chosen ciphertext attacks (KDM-CCA security). The first one is KDM-CCA secure with respect to affine functions. The other one is KDM-CCA secure with respect to polynomial functions. Both of our schemes are based on the KDM-CPA secure PKE schemes proposed by Malkin, Teranishi, and Yung (EUROCRYPT 2011). Although our schemes satisfy KDM-CCA security, their efficiency overheads...

In this work, we demonstrate generic and practical side-channel assisted chosen ciphertext attacks on multiple LWE/LWR-based Public Key Encryption (PKE) and Key Encapsulation Mechanism (KEM) secure in the chosen ciphertext model (IND-CCA security). Firstly, we identified EM-based side-channel vulnerabilities in the error correcting codes (ECC) used in LWE/LWR-based schemes that enable to distinguish the value/validity of the codewords output from the decryption operation. We also identified...

In this work, we present a fast parallel architecture to perform supersingular isogeny key encapsulation (SIKE). We propose and implement a fast isogeny accelerator architecture that uses fast and parallelized isogeny formulas. On top of our isogeny accelerator, we build a novel architecture for the SIKE primitive, which provides both quantum and IND-CCA security. Since SIKE can support static keys, we propose and implement additional differential power analysis countermeasures. We...

[Modified slightly because MathJax doesn't render $U^{notbot}$ correctly] We revisit the construction of IND-CCA secure key encapsulation mechanisms (KEM) from public-key encryption schemes (PKE). We give new, tighter security reductions for several constructions. Our main result is a tight reduction for the security of the $U^{notbot}$-transform of Hofheinz, Hövelmanns, and Kiltz (TCC'17) which turns OW-CPA secure deterministic PKEs into IND-CCA secure KEMs. This result is enabled by a new...

Lattice-based cryptosystems are less efficient than their number-theoretic counterparts (based on RSA, discrete logarithm, etc.) in terms of key and ciphertext (signature) sizes. For adequate security the former typically needs thousands of bytes while in contrast the latter only requires at most hundreds of bytes. This significant difference has become one of the main concerns in replacing currently deployed public-key cryptosystems with lattice-based ones. Observing the inherent...

Key encapsulation mechanism (KEM) variants of the Fujisaki-Okamoto (FO) transformation (TCC 2017) that turn a weakly-secure public-key encryption (PKE) into an IND-CCA-secure KEM, were widely used among the KEM submissions to the NIST Post-Quantum Cryptography Standardization Project. Under the standard CPA security assumptions, i.e., OW-CPA and IND-CPA, the security of these variants in the quantum random oracle model (QROM) has been proved by black-box reductions, e.g., Jiang et al....

While error correcting codes (ECC) have the potential to significantly reduce the failure probability of post-quantum schemes, they add an extra ECC decoding step to the algorithm. Even though this additional step does not compute directly on the secret key, it is susceptible to side-channel attacks. We show that if no precaution is taken, it is possible to use timing information to distinguish between ciphertexts that result in an error before decoding and ciphertexts that do not contain...

We study the relationship among public-key encryption (PKE) satisfying indistinguishability against chosen plaintext attacks (IND-CPA security), that against chosen ciphertext attacks (IND-CCA security), and trapdoor functions (TDF). Specifically, we aim at finding a unified approach and some additional requirement to realize IND-CCA secure PKE and TDF based on IND-CPA secure PKE, and show the following two main results. As the first main result, we show how to achieve IND-CCA security via...

We present the ring-based configuration of the NIST submission Round5, a Ring Learning with Rounding (RLWR)- based IND-CPA secure public-key encryption scheme. It combines elements of the NIST candidates Round2 (use of RLWR as underlying problem, having $1+x+\ldots +x^n$ with $n+1$ prime as reduction polynomial, allowing for a large design space) and HILA5 (the constant-time error-correction code XEf). Round5 performs part of encryption, and decryption via multiplication in...

The recent post-quantum cryptography standardization project launched by NIST increased the interest in generic key encapsulation mechanism (KEM) constructions in the quantum random oracle (QROM). Based on a OW-CPA-secure public-key encryption (PKE), Hofheinz, Hövelmanns and Kiltz (TCC 2017) first presented two generic constructions of an IND-CCA-secure KEM with quartic security loss in the QROM, one with implicit rejection (a pseudorandom key is return for an invalid ciphertext) and the...

OCB2 is a widely standardized mode of operation of a blockcipher that aims at providing authenticated encryption. A recent report by Inoue and Minematsu (IACR EPRINT report 2018/1040) indicates that OCB2 does not meet this goal. Concretely, by describing simple forging attacks the authors evidence that the (sub)goal of authenticity is not reached. The report does not question the confidentiality offered by OCB2. In this note we show how the attacks of Inoue and Minematsu can be extended to...

For years, researchers have been engaged in finding new cryptography schemes with high security and efficiency that can resist against the attacking from quantum computers. Lattice-based cryptography scheme is believed as a promising candidate. But to achieve both high efficiency and high security is not easy. Until recently, some Lattice-based schemes with enough efficiency have been proposed and submitted to the post-quantum cryptography standardization project that initiated by NIST....

A multitude of post-quantum key encapsulation mechanisms (KEMs) and public key encryption (PKE) schemes implicitly rely on a protocol by which Alice and Bob exchange public messages and converge on secret values that are identical up to some small noise. By our count, 24 out of 49 KEM or PKE submissions to the NIST Post-Quantum Cryptography Standardization project follow this strategy. Yet the notion of a noisy key agreement (NKA) protocol lacks a formal definition as a primitive in its own...

In this paper, we introduce a new framework for constructing public-key encryption (PKE) schemes resilient to joint post-challenge/after-the-fact leakage and tampering attacks in the bounded leakage and tampering (BLT) model, introduced by Damgård et al. (Asiacrypt 2013). All the prior formulations of PKE schemes considered leakage and tampering attacks only before the challenge ciphertext is made available to the adversary. However, this restriction seems necessary, since achieving security...

This paper studies indistinguishability against quantum chosen-ciphertext attacks (IND-qCCA security) of key-encapsulation mechanisms (KEMs) in quantum random oracle model (QROM). We show that the SXY conversion proposed by Saito, Yamakawa, and Xagawa (EUROCRYPT 2018) and the HU conversion proposed by Jiang, Zhang, and Ma (PKC 2019) turn a weakly-secure deterministic public-key encryption scheme into an IND-qCCA-secure KEM scheme in the QROM. The proofs are very similar to those for the...

Dual receiver encryption (DRE), proposed by Diament et al. at ACM CCS 2004, is a special extension notion of public-key encryption, which enables two independent receivers to decrypt a ciphertext into a same plaintext. This primitive is quite useful in designing combined public key cryptosystems and denial of service attack-resilient protocols. Up till now, a series of DRE schemes are constructed from bilinear pairing groups and lattices. In this work, we introduce a construction of...

We analyze the structure of commutative ring homomorphic encryption schemes and show that they are not quantum IND-CCA secure.

We propose definitions and constructions of authenticated encryption (AE) schemes that offer security guarantees even in the presence of nonce misuse and side-channel leakages. This is part of an important ongoing effort to make AE more robust, while preserving appealing efficiency properties. Our definitions consider an adversary enhanced with the leakages of all the computations of an AE scheme, together with the possibility to misuse nonces, be it during all queries (in the spirit of...

In this paper, we introduce Saber, a package of cryptographic primitives whose security relies on the hardness of the Module Learning With Rounding problem (Mod-LWR). We first describe a secure Diffie-Hellman type key exchange protocol, which is then transformed into an IND-CPA encryption scheme and finally into an IND-CCA secure key encapsulation mechanism using a post-quantum version of the Fujisaki-Okamoto transform. The design goals of this package were simplicity, efficiency and...

The security for authenticated encryption schemes is often captured by demanding CCA security (IND-CCA) and integrity of plaintexts (INT-PTXT). In this short note, we prove that this implies in particular integrity of ciphertexts, i.e., INT-CTXT. Hence, the two sets of requirements mentioned in the title are equivalent.

We show that the NISTPQC submission HILA5 is not secure against chosen-ciphertext attacks. Specifically, we demonstrate a key-recovery attack on HILA5 using an active attack on reused keys. The attack works around the error correction in HILA5. The attack applies to the HILA5 key-encapsulation mechanism (KEM), and also to the public-key encryption mechanism (PKE) obtained by NIST's procedure for combining the KEM with authenticated encryption. This contradicts the most natural interpretation...

With the gradual progress of NIST's post-quantum cryptography standardization, the Round-1 KEM proposals have been posted for public to discuss and evaluate. Among the IND-CCA-secure KEM constructions, mostly, an IND-CPA-secure (or OW-CPA-secure) public-key encryption (PKE) scheme is first introduced, then some generic transformations are applied to it. All these generic transformations are constructed in the random oracle model (ROM). To fully assess the post-quantum security, security...

Code-based Cryptography is one of the main areas of interest for the Post-Quantum Cryptography Standardization call. In this paper, we introduce DAGS, a Key Encapsulation Mechanism (KEM) based on Quasi-Dyadic Generalized Srivastava codes. The scheme is proved to be IND-CCA secure in both Random Oracle Model and Quantum Random Oracle Model. We believe that DAGS will offer competitive performance, especially when compared with other existing code-based schemes, and represent a valid candidate...

Key-encapsulation mechanisms secure against chosen ciphertext attacks (IND-CCA-secure KEMs) in the quantum random oracle model have been proposed by Boneh, Dagdelen, Fischlin, Lehmann, Schafner, and Zhandry (CRYPTO 2012), Targhi and Unruh (TCC 2016-B), and Hofheinz, Hövelmanns, and Kiltz (TCC 2017). However, all are non-tight and, in particular, security levels of the schemes obtained by these constructions are less than half of original security levels of their building blocks. In this...

We present a generic, yet simple and efficient transformation to obtain a forward secure authenticated key exchange protocol from a two-move passively secure unauthenticated key agreement scheme (such as standard Diffie--Hellman or Frodo or NewHope). Our construction requires only an IND-CCA public key encryption scheme (such as RSA-OAEP or a method based on ring-LWE), and a message authentication code. Particularly relevant in the context of the state-of-the-art of postquantum secu re...

The Fujisaki-Okamoto (FO) transformation (CRYPTO 1999 and Journal of Cryptology 2013) turns any weakly secure public-key encryption scheme into a strongly (i.e., IND-CCA) secure one in the random oracle model. Unfortunately, the FO analysis suffers from several drawbacks, such as a non-tight security reduction, and the need for a perfectly correct scheme. While several alternatives to the FO transformation have been proposed, they have stronger requirements, or do not obtain all desired...

Hedged PKE schemes are designed to provide useful security when the per-message randomness fails to be uniform, say, due to faulty implementations or adversarial actions. A simple and elegant theoretical approach to building such schemes works like this: Synthesize fresh random bits by hashing all of the encryption inputs, and use the resulting hash output as randomness for an underlying PKE scheme. The idea actually goes back to the Fujisaki-Okamoto transform for turning CPA-secure...

We present a new approach to construct several leakage-resilient cryptographic primitives, including leakage-resilient public-key encryption (PKE) schemes, authenticated key exchange (AKE) protocols and low-latency key exchange (LLKE) protocols. To this end, we introduce a new primitive called leakage-resilient non-interactive key exchange (LR-NIKE) protocol. We introduce a generic security model for LR-NIKE protocols, which can be instantiated in both the bounded and continuous-memory...

We provide a tight security proof for an IND-CCA Ring-LWE based Key Encapsulation Mechanism that is derived from a generic construction of Dent (IMA Cryptography and Coding, 2003). Such a tight reduction is not known for the generic construction. The resulting scheme has shorter ciphertexts than can be achieved with other generic constructions of Dent or by using the well-known Fujisaki-Okamoto constructions (PKC 1999, Crypto 1999). Our tight security proof is obtained by reducing to the...

ID based generalized signcryption can adaptively work as a signature scheme, an encryption scheme or a signcryption scheme and avoid weighty and complicated certificate management like Public Key Infrastructure. It has application in emerging paradigm big data security. Recently,Wei et al proposed a new ID based generalized signcryption scheme to obtain con…dentiality or/and authenticity in big data, and claimed that their scheme is provably secure in standard model. Unfortunately, by...

The LWE problem has been widely used in many constructions for post-quantum cryptography due to its strong security reduction from the worst-case of lattice hard problems and its lightweight operations. The PKE schemes based on the LWE problem have a simple and fast decryption, but the encryption phase is rather slow due to large parameter size for the leftover hash lemma or expensive Gaussian samplings. In this paper, we propose a novel PKE scheme, called Lizard, without relying on either...

The Learning with Errors (LWE) problem has been widely used as a hardness assumption to construct public-key primitives. In this paper, we propose an efficient instantiation of a PKE scheme based on LWE with a sparse secret, named as spLWE. We first construct an IND-CPA PKE and convert it to an IND-CCA scheme in the quantum random oracle model by applying a modified Fujisaki-Okamoto conversion of Unruh. In order to guarantee the security of our base problem suggested in this paper, we...

Algorithms for secure encryption in a post-quantum world are currently receiving a lot of attention in the research community, including several larger projects and a standardization effort from NIST. One of the most promising algorithms is the code-based scheme called QC-MDPC, which has excellent performance and a small public key size. In this work we present a very efficient key recovery attack on the QC-MDPC scheme using the fact that decryption uses an iterative decoding step and this...

We show that two well-known and widely employed public-key encryption schemes -- RSA Optimal Asymmetric Encryption Padding (RSA-OAEP) and Diffie-Hellman Integrated Encryption Scheme (DHIES), instantiated with a one-time pad, -- are secure under (the strong, simulation-based security notion of) selective opening security against chosen-ciphertext attacks in the random oracle model. Both schemes are obtained via known generic transformations that transform relatively weak primitives (with...

Proxy Re-Encryption (PRE) is a type of Public-Key Encryption (PKE) that provides an additional re-encryption functionality. Although PRE is inherently more complex than PKE, attack models for PRE have not been developed further than those inherited from PKE. In this paper we address this gap and define a parametric family of attack models for PRE, based on the availability of both the decryption and re-encryption oracles during the security game. This family enables the definition of...