## CryptoDB

### Papers from ASIACRYPT 2020

**Year**

**Venue**

**Title**

2020

ASIACRYPT

Twisted-PHS: Using the Product Formula to Solve Approx-SVP in Ideal Lattices
📺 Abstract

Approx-SVP is a well-known hard problem on lattices, which asks to find short vectors on a given lattice, but its variant restricted to ideal lattices (which correspond to ideals of the ring of integers $\mathcal{O}_{K}$ of a number field $K$) is still not fully understood. For a long time, the best known algorithm to solve this problem on ideal lattices was the same as for arbitrary lattice. But recently, a series of works tends to show that solving this problem could be easier in ideal lattices than in arbitrary ones, in particular in the quantum setting.
Our main contribution is to propose a new ``twisted'' version of the PHS (by Pellet-Mary, Hanrot and Stehlé 2019) algorithm, that we call Twisted-PHS. As a minor contribution, we also propose several improvements of the PHS algorithm. On the theoretical side, we prove that our twisted-PHS algorithm performs at least as well as the original PHS algorithm. On the practical side though, we provide a full implementation of our algorithm which suggest that much better approximation factors are achieved, and that the given lattice bases are a lot more orthogonal than the ones used in PHS. This is the first time to our knowledge that this type of algorithm is completely implemented and tested for fields of degrees up to 60.

2020

ASIACRYPT

Succinct Diophantine-Satisfiability Arguments
📺 Abstract

A Diophantine equation is a multi-variate polynomial equation with integer coefficients and it is satisfiable if it has a solution with all unknowns taking integer values. Davis, Putnam, Robinson and Matiyasevich showed that the general Diophantine satisfiability problem is undecidable (giving a negative answer to Hilbert's tenth problem) but it is nevertheless possible to argue in zero-knowledge the knowledge of a solution, if a solution is known to a prover.
We provide the first succinct honest-verifier zero-knowledge argument for the satisfiability of Diophantine equations with a communication complexity and a round complexity that grows logarithmically in the size of the polynomial equation. The security of our argument relies on standard assumptions on hidden-order groups. As the argument requires to commit to integers, we introduce a new integer-commitment scheme that has much smaller parameters than Damgard and Fujisaki's scheme. We finally show how to succinctly argue knowledge of solutions to several NP-complete problems and cryptographic problems by encoding them as Diophantine equations.

2020

ASIACRYPT

On the Adaptive Security of MACs and PRFs
📺 Abstract

We consider the security of two of the most commonly used cryptographic primitives--message authentication codes (MACs) and pseudorandom functions (PRFs)--in a multi-user setting with adaptive corruption. Whereas is it well known that any secure MAC or PRF is also multi-user secure under adaptive corruption, the trivial reduction induces a security loss that is linear in the number of users.
Our main result shows that black-box reductions from "standard" assumptions cannot be used to provide a tight, or even a linear-preserving, security reduction for adaptive multi-user secure deterministic stateless MACs and thus also PRFs. In other words, a security loss that grows with the number of users is necessary for any such black-box reduction.

2020

ASIACRYPT

Individual Simulations
📺 Abstract

We develop an individual simulation technique that explicitly makes use of particular properties/structures of a given adversary's functionality. Using this simulation technique, we obtain the following results.
1. We construct the first protocols that break previous black-box barriers under the standard hardness of factoring, both of which are polynomial time simulatable against all a-priori bounded polynomial size distinguishers:
a)Two-round selective opening secure commitment scheme.
b)Three-round concurrent zero knowledge and concurrent witness hiding
argument for NP in the bare public-key model.
2. We present a simpler two-round weak zero knowledge and witness hiding argument for NP in the plain model under the sub-exponential hardness of factoring. Our technique also yields a significantly simpler proof that existing distinguisher-dependent simulatable zero knowledge protocols are also polynomial time simulatable against all distinguishers of a-priori bounded polynomial size.
The core conceptual idea underlying our individual simulation technique is an observation of the existence of nearly optimal extractors for all hard distributions: For any NP-instance(s) sampling algorithm, there exists a polynomial-size witness extractor (depending on the sampler's functionality) that almost outperforms any circuit of a-priori bounded polynomial size in terms of the success probability.

2020

ASIACRYPT

Circuit Amortization Friendly Encodings and their Application to Statistically Secure Multiparty Computation
📺 Abstract

At CRYPTO 2018, Cascudo et al. introduced Reverse Multiplication Friendly Embeddings (RMFEs). These are a mechanism to compute $\delta$ parallel evaluations of the same arithmetic circuit over a field $\mathbb{F}_q$ at the cost of a single evaluation of that circuit in $\mathbb{F}_{q^d}$, where $\delta < d$. Due to this inequality, RMFEs are a useful tool when protocols require to work over $\mathbb{F}_{q^d}$ but one is only interested in computing over $\mathbb{F}_q$. In this work we introduce Circuit Amortization Friendly Encodings (CAFEs), which generalize RMFEs while having concrete efficiency in mind. For a Galois Ring $R = GR(2^k,d)$, CAFEs allow to compute certain circuits over $\mathbb{Z}_{2^k}}$ at the cost of a single secure multiplication in $R$.
We present three CAFE instantiations, which we apply to the protocol for MPC over $\mathbb{Z}_{2^k}}$ via Galois Rings by Abspoel et al. (TCC 2019).
Our protocols allow for efficient switching between the different CAFEs, as well as between computation over $GR(2^k,d)$ and $\mathbb{F}_{2^{d}}$ in a way that preserves the CAFE in both rings. This adaptability leads to efficiency gains for e.g. Machine Learning applications, which can be represented as highly parallel circuits over $\mathbb{Z}_{2^k}}$ followed by bit-wise operations. From an implementation of our techniques, we estimate that an SVM can be evaluated on 250 images in parallel up to $\times 7$ as efficient using our techniques, compared to using the protocols from Abspoel et al. (TCC 2019).

2020

ASIACRYPT

Unbounded Dynamic Predicate Compositions in ABE from Standard Assumptions
📺 Abstract

At Eurocrypt'19, Attrapadung presented several transformations that dynamically compose a set of attribute-based encryption (ABE) schemes for simpler predicates into a new ABE scheme for more expressive predicates. Due to the powerful unbounded and modular nature of his compositions, many new ABE schemes can be obtained in a systematic manner (including those that resolved some open problems at the time). However, his approach heavily relies on so-called $q$-type assumptions, which are not standard. Devising such powerful compositions from standard assumptions was left as an important open problem.
In this paper, we present a new framework for constructing ABE schemes that allow unbounded and dynamic predicate compositions among them, and show that the adaptive security of these composed ABE will be preserved by relying only on the standard matrix Diffie-Hellman (MDDH) assumption. This thus resolves the open problem posed by Attrapadung.
As for applications, we obtain various ABEs that are the first such instantiations of their kinds from standard assumptions. These include the following adaptively secure \emph{large-universe} ABEs for Boolean formulae under MDDH:
- The first completely unbounded monotone key-policy (KP)/ciphertext-policy (CP) ABE. Previously, such ABE has been only recently proposed, but only for the KP and \emph{small-universe} flavor (Kowalczyk and Wee, Eurocrypt'19).
- The first completely unbounded non-monotone KP/CP-ABE. Especially, our ABEs support a new type of non-monotonicity that subsumes previous two types of non-monotonicity, namely, by Ostrovsky et al. (CCS'07) and by Okamoto and Takashima (CRYPTO'10).
- The first non-monotone KP and CP-ABE with constant-size ciphertexts and secret keys, respectively.
- The first monotone KP and CP-ABE with constant-size secret keys and ciphertexts, respectively.
At the core of our framework lies a new \emph{partially symmetric} design of the core 1-key 1-ciphertext oracle component called Key Encoding Indistinguishability, which exploits the symmetry so as to obtain compositions.

2020

ASIACRYPT

Simpler Statistically Sender Private Oblivious Transfer from Ideals of Cyclotomic Integers
📺 Abstract

We present a two-message oblivious transfer protocol achieving statistical sender privacy and computational receiver privacy based on the RLWE assumption for cyclotomic number fields. This work improves upon prior lattice-based statistically sender-private oblivious transfer protocols by reducing the total communication between parties by a factor O(nlogq) for transfer of length O(n) messages.
Prior work of Brakerski and Dottling uses transference theorems to show that either a lattice or its dual must have short vectors, the existence of which guarantees lossy encryption for encodings with respect to that lattice, and therefore statistical sender privacy. In the case of ideal lattices from embeddings of cyclotomic integers, the existence of one short vector implies the existence of many, and therefore encryption with respect to either a lattice or its dual is guaranteed to ``lose" more information about the message than can be ensured in the case of general lattices. This additional structure of ideals of cyclotomic integers allows for efficiency improvements beyond those that are typical when moving from the generic to ideal lattice setting, resulting in smaller message sizes for sender and receiver, as well as a protocol that is simpler to describe and analyze.

2020

ASIACRYPT

Efficient and Round-Optimal Oblivious Transfer and Commitment with Adaptive Security
📺 Abstract

We construct the most efficient two-round adaptively secure bit-OT in the Common Random String (CRS) model. The scheme is UC secure under the Decisional Diffie-Hellman (DDH) assumption. It incurs O(1) exponentiations and sends O(1) group elements, whereas the state of the art requires O(k^2) exponentiations and communicates poly(k) bits, where k is the computational security parameter. Along the way, we obtain several other efficient UC-secure OT protocols under DDH :
- The most efficient yet two-round adaptive string-OT protocol assuming global programmable random oracle. Furthermore, the protocol can be made non-interactive in the simultaneous message setting, assuming random inputs for the sender.
- The first two-round string-OT with amortized constant exponentiations and communication overhead which is secure in the global observable random oracle model.
- The first two-round receiver equivocal string-OT in the CRS model that incurs constant computation and communication overhead.
We also obtain the first non-interactive adaptive string UC-commitment in the CRS model which incurs a sublinear communication overhead in the security parameter. Specically, we commit to polylog(k) bits while communicating O(k) bits. Moreover, it is additively homomorphic.
We can also extend our results to the single CRS model where multiple
sessions share the same CRS. As a corollary, we obtain a two-round
adaptively secure MPC protocol in this model.

2020

ASIACRYPT

Two-Pass Authenticated Key Exchange with Explicit Authentication and Tight Security
📺 Abstract

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.

2020

ASIACRYPT

Succinct and Adaptively Secure ABE for Arithmetic Branching Programs from k-Lin
📺 Abstract

We present succinct and adaptively secure attribute-based encryption (ABE)
schemes for arithmetic branching programs, based on k-Lin in pairing groups.
Our key-policy ABE scheme have ciphertexts of constant size, independent of
the length of the attributes, and our ciphertext-policy ABE scheme have
secret keys of constant size. Our schemes improve upon the recent succinct
ABE schemes in [Tomida and Attrapadung, ePrint '20], which only handles
Boolean formulae. All other prior succinct ABE schemes either achieve
only selective security or rely on q-type assumptions.
Our schemes are obtained through a general and modular approach that combines
a public-key inner product functional encryption satisfying a new security
notion called gradual simulation security and an information-theoretic
randomized encoding scheme called arithmetic key garbling scheme.

2020

ASIACRYPT

CCA Updatable Encryption Against Malicious Re-Encryption Attacks
📺 Abstract

Updatable encryption (UE) is an attractive primitive, which allows the secret key of the outsourced encrypted data to be updated to a fresh one periodically. Several elegant works exist studying various security properties. We notice several major issues in existing security models of (ciphertext dependent) updatable encryption, in particular, integrity and CCA security. The adversary in the models is only allowed to request the server to re-encrypt {\em honestly} generated ciphertext, while in practice, an attacker could try to inject arbitrary ciphertexts into the server as she wishes. Those malformed ciphertext could be updated and leveraged by the adversary and cause serious security issues.
In this paper, we fill the gap and strengthen the security definitions in multiple aspects: most importantly our integrity and CCA security models remove the restriction in previous models and achieve standard notions of integrity and CCA security in the setting of updatable encryption. Along the way, we refine the security model to capture post-compromise security and enhance the re-encryption indistinguishability to the CCA style.
Guided by the new models, we provide a novel construction \recrypt, which satisfies our strengthened security definitions. The technical building block of homomorphic hash from a group may be of independent interests. We also study the relations among security notions; and a bit surprisingly, the folklore result in authenticated encryption that IND-CPA plus ciphertext integrity imply IND-CCA security does {\em not} hold for ciphertext dependent updatable encryption.

2020

ASIACRYPT

Efficient Fully Secure Computation via Distributed Zero-Knowledge Proofs
📺 Abstract

Secure computation protocols enable mutually distrusting parties to compute a function of their private inputs while revealing nothing but the output.
Protocols with {\em full security} (also known as {\em guaranteed output delivery}) in particular protect against denial-of-service attacks, guaranteeing that honest parties receive a correct output. This feature can be realized in the presence of an honest majority, and significant research effort has gone toward attaining full security with good asymptotic and concrete efficiency.
We present a fully secure protocol for {\em any constant} number of parties $n$ and $t<n/2$ corruptions that achieves full security with the {\em same amortized communication} as for semi-honest security: $\frac{3t}{2t+1}|C| + o(|C|)$ $R$-elements per party ($\approx 1.5$ $R$-elements), for a circuit with $|C|$ multiplication gates over either a finite field $R=\FF$ or over the ring $R=\Z_{2^k}$.
Our techniques include new methods for utilizing the distributed zero-knowledge proofs of Boneh {\em et al.} (CRYPTO 2019) for both distributed verifiers {\em and} provers. As a secondary contribution, we show that similar techniques can be used to compile the best known honest-majority protocols for an arbitrary (super-constant) number of semi-honest parties into ones that achieve {\em security with abort} against malicious parties, with sublinear additive cost.
We present an efficient protocol for {\em any constant} number of parties $n$, with full security against $t<n/2$ corrupted parties, that makes a black-box use of a pseudorandom generator. Our protocol evaluates an arithmetic circuit $C$ over a finite ring $R$ (either a finite field or $R=\Z_{2^k}$) with communication complexity of $\frac{3t}{2t+1}S + o(S)$ $R$-elements per party, where $S$ is the number of multiplication gates in $C$ (namely, $<1.5$ elements per party per gate). This matches the best known protocols for the semi-honest model up to the sublinear additive term. For a small number of parties $n$, this improves over a recent protocol of Goyal {\em et al.} (Crypto 2020) by a constant factor for circuits over large fields, and by at least an $\Omega(\log n)$ factor for Boolean circuits or circuits over rings.
Our protocol provides new methods for applying the distributed zero-knowledge proofs of Boneh {\em et al.}~(Crypto 2019), which only require logarithmic communication, for compiling semi-honest protocols into fully secure ones in the more challenging case of $t>1$ corrupted parties.
%Similarly to the recent fully secure 3-party protocol of Boyle {\em et al.} (CCS 2019), our protocol builds on the sublinear-communication distributed zero-knowledge proofs of Boneh {\em et al.} (Crypto 2019) to compile any ``natural'' semi-honest protocol into a fully secure protocol. However, applying this tool with $t>1$ corrupted parties introduces several nontrivial challenges that we overcome in this work.
Our protocol relies on {\em replicated secret sharing} to minimize communication and simplify the mechanism for achieving full security. This results in computational cost that scales exponentially with $n$.
Our main protocol builds on a new honest-majority protocol for verifying the correctness of multiplication triples by making a {\em general} use of distributed zero-knowledge proofs. While the protocol only achieves the weaker notion of {\em security with abort}, it applies to any linear secret-sharing scheme and provides a conceptually simpler, more general, and more efficient alternative to previous protocols from the literature. In particular, it can be combined with the Fiat-Shamir heuristic to simultaneously achieve logarithmic communication complexity and constant round complexity.

2020

ASIACRYPT

Practical Exact Proofs from Lattices: New Techniques to Exploit Fully-Splitting Rings
📺 Abstract

We propose a lattice-based zero-knowledge proof system for exactly proving knowledge of a ternary solution $\vec{s} \in \{-1,0,1\}^n$ to a linear equation $A\vec{s}=\vec{u}$ over $\mathbb{Z}_q$, which improves upon the protocol by Bootle, Lyubashevsky and Seiler (CRYPTO 2019) by producing proofs that are shorter by a factor of $7.5$.
At the core lies a technique that utilizes the module-homomorphic BDLOP commitment scheme (SCN 2018) over the fully splitting cyclotomic ring $\mathbb{Z}_q[X]/(X^d + 1)$ to prove scalar products with the NTT vector of a secret polynomial.

2020

ASIACRYPT

MOTIF: (Almost) Free Branching in GMW via Vector-Scalar Multiplication
📺 Abstract

MPC functionalities are increasingly specified in high-level languages, where control-flow constructions such as conditional statements are extensively used. Today, concretely efficient MPC protocols are circuit-based and must evaluate all conditional branches at high cost to hide the taken branch.
The Goldreich-Micali-Wigderson, or GMW, protocol is a foundational circuit-based technique that realizes MPC for p players and is secure against up to p-1 semi-honest corruptions. While GMW requires communication rounds proportional to the computed circuit’s depth, it is effective in many natural settings.
Our main contribution is MOTIF (Minimizing OTs for IFs), a novel GMW extension that evaluates conditional branches almost for free by amortizing Oblivious Transfers (OTs) across branches. That is, we simultaneously evaluate multiple independent AND gates, one gate from each mutually exclusive branch, by representing them as a single cheap vector-scalar multiplication (VS) gate.
For 2PC with b branches, we simultaneously evaluate up to b AND gates using only two 1-out-of-2 OTs of b-bit secrets. This is a factor ~b improvement over the state-of-the-art 2b 1-out-of-2 OTs of 1-bit secrets. Our factor b improvement generalizes to the multiparty setting as well: b AND gates consume only p(p ? 1) 1-out-of-2 OTs of b-bit secrets.
We implemented our approach and report its performance. For 2PC and a circuit with 16 branches, each comparing two length-65000 bitstrings, MOTIF outperforms standard GMW in terms of communication by ~9.4x. Total wall-clock time is improved by 4.1 - 9.2x depending on network settings.
Our work is in the semi-honest model, tolerating all-but-one corruptions.

2020

ASIACRYPT

Towards Classical Hardness of Module-LWE: The Linear Rank Case
📺 Abstract

We prove that the module learning with errors (M-LWE) problem with arbitrary polynomial-sized modulus $p$ is \emph{classically} at least as hard as standard worst-case lattice problems, as long as the module rank $d$ is
not smaller than the ring dimension $n$.
Previous publications only showed the hardness under quantum reductions.
We achieve this result in an analogous manner as in the case of the learning with errors (LWE) problem.
First, we show the classical hardness of M-LWE with an exponential-sized modulus.
In a second step, we prove the hardness of M-LWE using a binary secret.
And finally, we provide a modulus reduction technique.
The complete result applies to the class of power-of-two cyclotomic fields.
However, several tools hold for more general classes of number fields and may be of independent interest.

2020

ASIACRYPT

Maliciously Secure Matrix Multiplication with Applications to Private Deep Learning
📺 Abstract

Computing on data in a manner that preserve the privacy is of growing importance. Multi-Party Computation (MPC) and Homomorphic Encryption (HE) are two cryptographic techniques for privacy-preserving computations. In this work, we have developed efficient UC-secure multiparty protocols for matrix multiplications and two-dimensional convolutions. We built upon the SPDZ framework and integrated the state-of-the-art HE algorithms for matrix multiplication. Our protocol achieved communication cost linear only in the input and output dimensions and not on the number of multiplication operations. We eliminate the ''triple sacrifice'' step of SPDZ to improve efficiency and simplify the zero-knowledge proofs. We implemented our protocols and benchmarked them against the SPDZ LowGear variant (Keller et al. Eurocrypt'18). For multiplying two square matrices of size 128, we reduced the communication cost from 1.54 GB to 12.46 MB, an improvement of over two orders of magnitude that only improves with larger matrix sizes. For evaluating all convolution layers of the ResNet-50 neural network, the communication reduces cost from 5 TB to 41 GB.

2020

ASIACRYPT

Finding Collisions in a Quantum World: Quantum Black-Box Separation of Collision-Resistance and One-Wayness
📺 Abstract

★ Best Paper Award

Since the celebrated work of Impagliazzo and Rudich (STOC 1989), a number of black-box impossibility results have been established. However, these works only ruled out classical black-box reductions among cryptographic primitives.
Therefore it may be possible to overcome these impossibility results by using quantum reductions.
To exclude such a possibility, we have to extend these impossibility results to the quantum setting.
In this paper, we study black-box impossibility in the quantum setting.
We first formalize a quantum counterpart of fully-black-box reduction following the formalization by Reingold, Trevisan and Vadhan (TCC 2004).
Then we prove that there is no quantum fully-black-box reduction from collision-resistant hash functions to one-way permutations (or even trapdoor permutations).
We take both of classical and quantum implementations of primitives into account.
This is an extension to the quantum setting of the work of Simon (Eurocrypt 1998) who showed a similar result in the classical setting.

2020

ASIACRYPT

Estimating quantum speedups for lattice sieves
📺 Abstract

Quantum variants of lattice sieve algorithms are routinely used to assess the security of lattice based cryptographic constructions. In this work we provide a heuristic, non-asymptotic, analysis of the cost of several algorithms for near neighbour search on high dimensional spheres. These algorithms are key components of lattice sieves. We design quantum circuits for near neighbour search algorithms and provide software that numerically optimises algorithm parameters according to various cost metrics. Using this software we estimate the cost of classical and quantum near neighbour search on spheres. For the most performant near neighbour search algorithm that we analyse we find a small quantum speedup in dimensions of cryptanalytic interest. Achieving this speedup requires several optimistic physical and algorithmic assumptions.

2020

ASIACRYPT

B-SIDH: supersingular isogeny Diffie-Hellman using twisted torsion
📺 Abstract

This paper explores a new way of instantiating isogeny-based cryptography in which parties can work in both the (p+1)-torsion of a set of supersingular curves and in the (p-1)-torsion corresponding to the set of their quadratic twists. Although the isomorphism between a given supersingular curve and its quadratic twist is not defined over GF(p^2) in general, restricting operations to the x-lines of both sets of twists allows all arithmetic to be carried out over GF(p^2) as usual. Furthermore, since supersingular twists always have the same GF(p^2)-rational j-invariant, the SIDH protocol remains unchanged when Alice and Bob are free to work in both sets of twists.
This framework lifts the restrictions on the shapes of the underlying prime fields originally imposed by Jao and De Feo, and allows a range of new options for instantiating isogeny-based public key cryptography. These include alternatives that exploit Mersenne and Montgomery-friendly primes, as well as the possibility of halving the size of the primes in the Jao-De Feo construction at no known loss of asymptotic security. For a given target security level, the resulting public keys are smaller than the public keys of all of the key encapsulation schemes currently under consideration in the NIST post-quantum standardisation effort.
The best known attacks against the instantiations proposed in this paper are the classical path finding algorithm due to Delfs and Galbraith and its quantum adapation due to Biasse, Jao and Sankar; these run in respective time O(p^(1/2)) and O(p^(1/4)), and are essentially memory-free. The upshot is that removing the big-O's and obtaining concrete security estimates is a matter of costing the circuits needed to implement the corresponding isogeny. In contrast to other post-quantum proposals, this makes the security analysis of B-SIDH rather straightforward.
Searches for friendly parameters are used to find several primes that range from 237 to 256 bits, the conjectured security of which are comparable to the 434-bit prime used to target NIST level 1 security in the SIKE proposal. One noteworthy example is a 247-bit prime for which Alice's secret isogeny is 7901-smooth and Bob's secret isogeny is 7621-smooth.

2020

ASIACRYPT

Mind the Propagation of States New Automatic Search Tool for Impossible Differentials and Impossible Polytopic Transitions
📺 Abstract

Impossible differentials cryptanalysis and impossible polytopic cryptanalysis are the most effective approaches to estimate the security of block ciphers. However, the previous automatic search methods of their distinguishers, impossible differentials and impossible polytopic transitions, neither consider the impact of key schedule in the single-key setting and the differential property of large S-boxes, nor apply to the block ciphers with variable rotations.
Thus, unlike previous methods which focus on the propagation of the difference or s-difference, we redefine the impossible differentials and impossible (s + 1)-polytopic transitions according to the propagation of state, which allow us to break through those limitations of the previous methods. Theoretically, we prove that traditional impossible differentials and impossible (s+1)-polytopic transitions are equivalent to part of our redefinitions, which have advantages from broader view. Technically, we renew the automatic search model and design an SAT-based tool to evaluate our redefined impossible differentials and impossible (s + 1)-polytopic transitions efficiently.
As a result, for GIFT64, we get the 6-round impossible differentials which cannot be detected by all previous tools. For PRINTcipher, we propose the first modeling method for the key-dependent permutation and key-dependent S-box. For MISTY1, we derive 902 4-round impossible differentials by exploiting the differential property of S-boxes. For RC5, we present the first modeling method for the variable rotation and get 2.5-round impossible differentials for each version of it. More remarkable, our tool can be used to evaluate the security of given cipher against the impossible differentials, and we prove that there exists no 5-round 1 input active word and 1 output active word impossible differentials for AES-128 even consider the relations of 3-round keys. Besides, we also get the impossible (s + 1)-polytopic transitions for PRINTcipher, GIFT64, PRESENT, and RC5, all of which can cover more rounds than their corresponding impossible differentials as far as we know.

2020

ASIACRYPT

Minimizing the Two-Round Tweakable Even-Mansour Cipher
📺 Abstract

In CRYPTO 2015, Cogliati et al. have proposed one-round tweakable Even-Mansour (\textsf{1-TEM}) cipher constructed out of a single $n$-bit public permutation $\pi$ and a uniform and almost XOR-universal hash function \textsf{H} as $(k, t, x) \mapsto \textsf{H}_k(t) \oplus \pi(\textsf{H}_k(t) \oplus x)$, where $t$ is the tweak, and $x$ is the $n$-bit message. Authors have shown that its two-round extension, which we refer to as \textsf{2-TEM}, obtained by cascading $2$-independent instances of the construction gives $2n/3$-bit security and $r$-round cascading gives $rn/r+2$-bit security. In ASIACRYPT 2015, Cogliati and Seurin have shown that four-round tweakable Even-Mansour cipher, which we refer to as \textsf{4-TEM}, constructed out of four independent $n$-bit permutations $\pi_1, \pi_2, \pi_3, \pi_4$ and two independent $n$-bit keys $k_1, k_2$, defined as $$k_1 \oplus t \oplus \pi_4(k_2 \oplus t \oplus \pi_3(k_1 \oplus t \oplus \pi_2(k_2 \oplus t \oplus \pi_1(k_1 \oplus t \oplus x)))), $$ is secure upto $2^{2n/3}$ adversarial queries. In this paper, we have shown that if we replace two independent permutations of \textsf{2-TEM} (Cogliati et al., CRYPTO 2015) with a single $n$-bit public permutation, then the resultant construction still guarrantees security upto $2^{2n/3}$ adversarial queries. Using the results derived therein, we also show that replacing the permutation $(\pi_4, \pi_3)$ with $(\pi_1, \pi_2)$ in the above equation preserves security upto $2^{2n/3}$ adversarial queries.

2020

ASIACRYPT

Determining the Core Primitive for Optimally Secure Ratcheting
📺 Abstract

After ratcheting attracted attention mostly due to practical real-world protocols, recently a line of work studied ratcheting as a primitive from a theoretic point of view. Literature in this line, pursuing the strongest security of ratcheting one can hope for, utilized for constructions strong, yet inefficient key-updatable primitives – based on hierarchical identity based encryption (HIBE). As none of these works formally justified utilizing these building blocks, we answer the yet open question under which conditions their use is actually necessary.
We revisit these strong notions of ratcheted key exchange (RKE), and propose a more realistic (slightly stronger) security definition. In this security definition, both exposure of participants' local secrets and attacks against executions' randomness are considered. While these two attacks were partially considered in previous work, we are the first to unify them cleanly in a natural game based notion.
Our definitions are based on the systematic RKE notion by Poettering and Rösler (CRYPTO 2018). Due to slight (but meaningful) changes to regard attacks against randomness, we are ultimately able to show that, in order to fulfill strong security for RKE, public key cryptography with (independently) updatable key pairs is a necessary building block. Surprisingly, this implication already holds for the simplest RKE variant.
Hence, (1) we model optimally secure RKE under randomness manipulation to cover realistic attacks, (2) we (provably) extract the core primitive that is necessary to realize strongly secure RKE, and (3) our results indicate which relaxations in security allow for constructions that only rely on standard public key cryptography.

2020

ASIACRYPT

Beyond Birthday Bound Secure Fresh Rekeying: Application to Authenticated Encryption
📺 Abstract

Fresh rekeying is a well-established method to protect a primitive or mode against side-channel attacks: an easy to protect but cryptographically not so involved function generates a subkey from the master key, and this subkey is then used for the block encryption of a single or a few messages. It is an efficient way to achieve side-channel protection, but current solutions only achieve birthday bound security in the block size of the cipher and thus halve its security (except if more involved primitives are employed). We present generalized solutions to parallel block cipher rekeying that, for the first time, achieve security beyond the birthday bound in the block size $n$. The first solution involves, next to the subkey generation, one multiplication and the core block cipher call and achieves $2^{2n/3}$ security. The second solution makes two block cipher calls, and achieves optimal $2^n$ security. Our third solution uses a slightly larger subkey generation function but requires no adaptations to the core encryption and also achieves optimal security. The construction seamlessly generalizes to permutation based fresh rekeying. Central to our schemes is the observation that fresh rekeying and generic tweakable block cipher design are two very related topics, and we can take lessons from the advanced results in the latter to improve our understanding and development of the former. We subsequently use these rekeying schemes in a constructive manner to deliver three authenticated encryption modes that achieve beyond birthday bound security and are easy to protect against side-channel attacks.

2020

ASIACRYPT

The Direction of Updatable Encryption does not Matter Much
📺 Abstract

Updatable encryption schemes allow for key rotation on ciphertexts. A client outsourcing storage of encrypted data to a cloud server can change its encryption key. The cloud server can update the stored ciphertexts to the new key using only a token provided by the client.
This paper solves two open problems in updatable encryption, that of uni-directional vs. bi-directional updates, and post-quantum security.
The main result in this paper is to analyze the security notions based on uni- and bi-directional updates. Surprisingly, we prove that uni- and bi-directional variants of each security notion are equivalent.
The second result in this paper is to provide a new and efficient updatable encryption scheme based on the Decisional Learning with Error assumption. This gives us post-quantum security. Our scheme is bi-directional, but because of our main result, this is sufficient.

2020

ASIACRYPT

Incrementally Aggregatable Vector Commitments and Applications to Verifiable Decentralized Storage
📺 Abstract

Vector commitments with subvector openings (SVC) [Lai-Malavolta, Boneh-Bunz-Fisch; CRYPTO'19] allow one to open a committed vector at a set of positions with an opening of size independent of both the vector's length and the number of opened positions.
We continue the study of SVC with two goals in mind: improving their efficiency and making them more suitable to decentralized settings.
We address both problems by proposing a new notion for VC that we call \emph{incremental aggregation} and that allows
one to merge openings in a succinct way an \emph{unbounded} number of times.
We show two applications of this property. The first one is immediate and is a method to generate openings in a distributed way. The second application is an algorithm for faster generation of openings via preprocessing.
We then proceed to realize SVC with incremental aggregation. We provide two constructions in groups of unknown order that, similarly to that of Boneh et al. (which supports aggregating only once), have constant-size public parameters, commitments and openings. As an additional feature, for the first construction we propose efficient arguments of knowledge of subvector openings which immediately yields a keyless proof of storage with compact proofs.
Finally, we address a problem closely related to that of SVC: storing a file efficiently in completely decentralized networks. We introduce and construct \emph{verifiable decentralized storage} (VDS), a cryptographic primitive that allows to check the integrity of a file stored by a network of nodes in a distributed and decentralized way. Our VDS constructions rely on our new vector commitment techniques.

2020

ASIACRYPT

Lattice-Based E-Cash, Revisited
📺 Abstract

Electronic cash (e-cash) was introduced 40 years ago as the digital analogue of traditional cash. It allows users to withdraw electronic coins that can be spent anonymously with merchants. As advocated by Camenisch et al. (Eurocrypt 2005), it should be possible to store the withdrawn coins compactly (i.e., with logarithmic cost in the total number of coins), which has led to the notion of compact e-cash. Many solutions were proposed for this problem but the security proofs of most of them were invalidated by a very recent paper by Bourse et al. (Asiacrypt 2019). The same paper describes a generic way of fixing existing constructions/proofs but concrete instantiations of this patch are currently unknown in some settings. In particular, compact e-cash is no longer known to exist under quantum-safe assumptions.
In this work, we resolve this problem by proposing the first secure compact e-cash system based on lattices following the result from Bourse et al. Contrarily to the latter work, our construction is not only generic, but we describe two concrete instantiations. We depart from previous frameworks of e-cash systems by leveraging lossy trapdoor functions to construct our coins. The indistinguishability of lossy and injective keys allows us to avoid the very strong requirements on the involved pseudo-random functions that were necessary to instantiate the generic patch proposed by Bourse et al.

2020

ASIACRYPT

Privacy-Preserving Pattern Matching on Encrypted Data
📺 Abstract

Pattern matching is one of the most fundamental and important paradigms in several application domains such as digital forensics, cyber threat intelligence, or genomic and medical data analysis. While it is a straightforward operation when performed on plaintext data, it becomes a challenging task when the privacy of both the analyzed data and the analysis patterns must be preserved. In this paper, we propose new provably correct, secure, and relatively efficient (compared to similar existing schemes) public and private key based constructions that allow arbitrary pattern matching over encrypted data while protecting both the data to be analyzed and the patterns to be matched. That is, except the pattern provider (resp. the data owner), all other involved parties in the proposed constructions will learn nothing about the patterns to be searched (resp. the data to be inspected). Compared to existing solutions, the constructions we propose has some interesting properties: (1) the size of the ciphertext is linear to the size of plaintext and independent of the sizes and the number of the analysis patterns; (2) the sizes of the issued trapdoors are constant on the size of the data to be analyzed; and (3) the search complexity is linear on the size of the data to be inspected and is constant on the sizes of the analysis patterns. The conducted evaluations show that our constructions drastically improve the performance of the most efficient state of the art solution.

2020

ASIACRYPT

SiGamal: A supersingular isogeny-based PKE and its application to a PRF
📺 Abstract

We propose two new supersingular isogeny-based public key encryptions: SiGamal and C-SiGamal. They were developed by giving an additional point of the order $2^r$ to CSIDH. SiGamal is similar to ElGamal encryption, while C-SiGamal is a compressed version of SiGamal. We prove that SiGamal and C-SiGamal are IND-CPA secure without using hash functions under a new assumption: the P-CSSDDH assumption. This assumption comes from the expectation that no efficient algorithm can distinguish between a random point and a point that is the image of a public point under a hidden isogeny.
Next, we propose a Naor-Reingold type pseudo random function (PRF) based on SiGamal. If the P-CSSDDH assumption and the CSSDDH$^*$ assumption, which guarantees the security of CSIDH that uses a prime $p$ in the setting of SiGamal, hold, then our proposed function is a pseudo random function. Moreover, we estimate that the computational costs of group actions to compute our proposed PRF are about $\sqrt{\frac{8T}{3\pi}}$ times that of the group actions in CSIDH, where $T$ is the Hamming weight of the input of the PRF.
Finally, we experimented with group actions in SiGamal and C-SiGamal. The computational costs of group actions in SiGamal-512 with a $256$-bit plaintext message space were about $2.62$ times that of a group action in CSIDH-512.

2020

ASIACRYPT

Improving Speed and Security in Updatable Encryption Schemes
📺 Abstract

Periodic key rotation is a common practice designed to limit the long-term power of cryptographic keys. Key rotation refers to the process of re-encrypting encrypted content under a fresh key, and overwriting the old ciphertext with the new one. When encrypted data is stored in the cloud, key rotation can be very costly: it may require downloading the entire encrypted content from the cloud, re-encrypting it on the client's machine, and uploading the new ciphertext back to the cloud.
An updatable encryption scheme is a symmetric-key encryption scheme designed to support efficient key rotation in the cloud. The data owner sends a short update token to the cloud. This update token lets the cloud rotate the ciphertext from the old key to the new key, without learning any information about the plaintext. Recent work on updatable encryption has led to several security definitions and proposed constructions. However, existing constructions are not yet efficient enough for practical adoption, and the existing security definitions can be strengthened.
In this work we make three contributions. First, we introduce stronger security definitions for updatable encryption (in the ciphertext-dependent setting) that capture desirable security properties not covered in prior work. Second, we construct two new updatable encryption schemes. The first construction relies only on symmetric cryptographic primitives, but only supports a bounded number of key rotations. The second construction supports a (nearly) unbounded number of updates, and is built from the Ring Learning with Errors (RLWE) assumption. Due to complexities of using RLWE, this scheme achieves a slightly weaker notion of integrity compared to the first. Finally, we implement both constructions and compare their performance to prior work. Our RLWE-based construction is 200x faster than a prior proposal for an updatable encryption scheme based on the hardness of elliptic curve DDH. Our first construction, based entirely on symmetric primitives, has the highest encryption throughput, approaching the performance of AES, and the highest decryption throughput on ciphertexts that were re-encrypted fewer than fifty times. For ciphertexts re-encrypted over fifty times, the RLWE construction dominates it in decryption speed.

2020

ASIACRYPT

Non-Committing Encryption with Constant Ciphertext Expansion from Standard Assumptions
📺 Abstract

Non-committing encryption (NCE) introduced by Canetti et al. (STOC '96) is a central tool to achieve multi-party computation protocols secure in the adaptive setting. Recently, Yoshida et al. (ASIACRYPT '19) proposed an NCE scheme based on the hardness of the DDH problem, which has ciphertext expansion $\mathcal{O}(\log\lambda)$ and public-key expansion $\mathcal{O}(\lambda^2)$.
In this work, we improve their result and propose a methodology to construct an NCE scheme that achieves \emph{constant} ciphertext expansion. Our methodology can be instantiated from the DDH assumption and the LWE assumption. When instantiated from the LWE assumption, the public-key expansion is $\lambda\cdot\mathsf{poly}(\log\lambda)$. They are the first NCE schemes satisfying constant ciphertext expansion without using iO or common reference strings.
Along the way, we define a weak notion of NCE, which satisfies only weak forms of correctness and security. We show how to amplify such a weak NCE scheme into a full-fledged one using wiretap codes with a new security property.

2020

ASIACRYPT

Efficient Homomorphic Comparison Methods with Optimal Complexity
📺 Abstract

Comparison of two numbers is one of the most frequently used operations, but it has been a challenging task to efficiently compute the comparison function in homomorphic encryption~(HE) which basically support addition and multiplication.
Recently, Cheon et al.~(Asiacrypt~2019) introduced a new approximate representation of the comparison function with a rational function, and showed that this rational function can be evaluated by an iterative algorithm.
Due to this iterative feature, their method achieves a logarithmic computational complexity compared to previous polynomial approximation methods;
however, the computational complexity is still not optimal, and the algorithm is quite slow for large-bit inputs in HE implementation.
In this work, we propose new comparison methods with \emph{optimal} asymptotic complexity based on \emph{composite polynomial} approximation.
The main idea is to systematically design a constant-degree polynomial $f$ by identifying the \emph{core properties} to make a composite polynomial $f\circ f \circ \cdots \circ f$ get close to the sign function (equivalent to the comparison function) as the number of compositions increases.
We additionally introduce an acceleration method applying a mixed polynomial composition $f\circ \cdots \circ f\circ g \circ \cdots \circ g$ for some other polynomial $g$ with different properties instead of $f\circ f \circ \cdots \circ f$.
Utilizing the devised polynomials $f$ and $g$, our new comparison algorithms only require $\Theta(\log(1/\epsilon)) + \Theta(\log\alpha)$ computational complexity to obtain an approximate comparison result of $a,b\in[0,1]$ satisfying $|a-b|\ge \epsilon$ within $2^{-\alpha}$ error.
The asymptotic optimality results in substantial performance enhancement:
our comparison algorithm on $16$-bit encrypted integers for $\alpha = 16$ takes $1.22$ milliseconds in amortized running time based on an approximate HE scheme HEAAN, which is $18$ times faster than the previous work.

2020

ASIACRYPT

Fuzzy Asymmetric Password-Authenticated Key Exchange
📺 Abstract

Password-Authenticated Key Exchange (PAKE) lets users with passwords exchange a cryptographic key. There have been two variants of PAKE which make it more applicable to real-world scenarios:
* Asymmetric PAKE (aPAKE), which aims at protecting a client's password even if the authentication server is untrusted, and
* Fuzzy PAKE (fPAKE), which enables key agreement even if passwords of users are noisy, but "close enough".
Supporting fuzzy password matches eases the use of higher entropy passwords and enables using biometrics and environmental readings (both of which are naturally noisy).
Until now, both variants of PAKE have been considered only in separation. In this paper, we consider both of them simultaneously. We introduce the notion of Fuzzy Asymmetric PAKE (fuzzy aPAKE), which protects against untrusted servers and supports noisy passwords. We formulate our new notion in the Universal Composability framework of Canetti (FOCS'01), which is the preferred model for password-based primitives. We then show that fuzzy aPAKE can be obtained from oblivious transfer and some variant of robust secret sharing (Cramer et al, EC'15). We achieve security against malicious parties while avoiding expensive tools such as non-interactive zero-knowledge proofs.
Our construction is round-optimal, with message and password file sizes that are independent of the schemes error tolerance.

2020

ASIACRYPT

Public-Key Generation with Verifiable Randomness
📺 Abstract

We revisit the problem of proving that a user algorithm selected and correctly used a truly random seed in the generation of her cryptographic key. A first approach was proposed in 2002 by Juels and Guajardo for the validation of RSA secret keys. We present a new security model and general tools to efficiently prove that a private key was generated at random according to a prescribed process, without revealing any further information about the private key.
We give a generic protocol for all key-generation algorithms based on probabilistic circuits and prove its security. We also propose a new protocol for factoring-based cryptography that we prove secure in the aforementioned model. This latter relies on a new efficient zero-knowledge argument for the double discrete logarithm problem that achieves an exponential improvement in communication complexity compared to the state of the art, and is of independent interest.

2020

ASIACRYPT

Side Channel Information Set Decoding using Iterative Chunking
📺 Abstract

This paper presents an attack based on side-channel information and information set decoding (ISD) on the code-based Niederreiter cryptosystem and an evaluation of the practicality of the attack using an electromagnetic side channel. We start by directly adapting the timing side-channel plaintext-recovery attack by Shoufan et al. from 2010 to the constant-time implementation of the Niederreiter cryptosystem as used in the official FPGA-implementation of the NIST finalist “Classic McEliece”. We then enhance our attack using ISD and a new technique that we call iterative chunking to further significantly reduce the number of required side-channel measurements. We theoretically show that our attack improvements have a significant impact on reducing the number of required side-channel measurements. For example, for the 256-bit security parameter set kem/mceliece6960119 of “Classic McEliece”, we improve the basic attack that requires 5415 measurements to less than 562 measurements on average to mount a successful plaintext-recovery attack. Further reductions can be achieved at the price of increasing the cost of the ISD computations. We confirm our findings by practically mounting the attack on the official FPGA-implementation of “Classic McEliece” for all proposed parameter sets.

2020

ASIACRYPT

On the Exact Round Complexity of Best-of-both-Worlds Multi-party Computation
📺 Abstract

The two traditional streams of multiparty computation (MPC) protocols consist of-- (a) protocols achieving guaranteed output delivery (\god) or fairness (\fair) in the honest-majority setting and (b) protocols achieving unanimous or selective abort (\uab, \sab) in the dishonest-majority setting. The favorable presence of honest majority amongst the participants is necessary to achieve the stronger notions of \god~or \fair. While the constructions of each type are abound in the literature, one class of protocols does not seem to withstand the threat model of the other. For instance, the honest-majority protocols do not guarantee privacy of the inputs of the honest parties in the face of dishonest majority and likewise the dishonest-majority protocols cannot achieve $\god$ and $\fair$, tolerating even a single corruption, let alone dishonest minority. The promise of the unconventional yet much sought-after species of MPC, termed as `Best-of-Both-Worlds' (BoBW), is to offer the best possible security depending on the actual corruption scenario.
This work nearly settles the exact round complexity of two classes of BoBW protocols differing on the security achieved in the honest-majority setting, namely $\god$ and $\fair$ respectively, under the assumption of no setup (plain model), public setup (CRS) and private setup (CRS + PKI or simply PKI). The former class necessarily requires the number of parties to be strictly more than the sum of the bounds of corruptions in the honest-majority and dishonest-majority setting, for a feasible solution to exist. Demoting the goal to the second-best attainable security in the honest-majority setting, the latter class needs no such restriction.
Assuming a network with pair-wise private channels and a broadcast channel, we show that 5 and 3 rounds are necessary and sufficient for the class of BoBW MPC with $\fair$ under the assumption of `no setup' and `public and private setup' respectively. For the class of BoBW MPC with $\god$, we show necessity and sufficiency of 3 rounds for the public setup case and 2 rounds for the private setup case. In the no setup setting, we show the sufficiency of 5 rounds, while the known lower bound is 4. All our upper bounds are based on polynomial-time assumptions and assume black-box simulation. With distinct feasibility conditions, the classes differ in terms of the round requirement. The bounds are in some cases different and on a positive note at most one more, compared to the maximum of the needs of the honest-majority and dishonest-majority setting. Our results remain unaffected when security with abort and fairness are upgraded to their identifiable counterparts.

2020

ASIACRYPT

Improved Security Analysis for Nonce-based Enhanced Hash-then-Mask MACs
📺 Abstract

In this paper, we prove that the nonce-based enhanced hash-then-mask MAC (nEHtM) is secure up to 2^{3n/4} MAC queries and 2^n verification queries (ignoring logarithmic factors) as long as the number of faulty queries \mu is below 2^{3n/8}, significantly improving the previous bound by Dutta et al. Even when \mu goes beyond 2^{3n/8}, nEHtM enjoys graceful degradation of security.
The second result is to prove the security of PRF-based nEHtM; when nEHtM is based on an n-to-s bit random function for a fixed size s such that 1 <= s <= n, it is proved to be secure up to any number of MAC queries and 2^s verification queries, if (1) s = n and \mu < 2^{n/2} or (2) n/2 < s < 2^{n-s} and \mu < max{2^{s/2}, 2^{n-s}}, or (3) s <= n/2 and \mu < 2^{n/2}. This result leads to the security proof of truncated nEHtM that returns only s bits of the original tag since a truncated permutation can be seen as a pseudorandom function. In particular, when s <= 2n/3, the truncated nEHtM is secure up to 2^{n - s/2} MAC queries and 2^s verification queries as long as \mu < min{2^{n/2}, 2^{n-s}}. For example, when s = n/2 (resp. s = n/4), the truncated nEHtM is secure up to 2^{3n/4} (resp. 2^{7n/8}) MAC queries. So truncation might provide better provable security than the original nEHtM with respect to the number of MAC queries.

2020

ASIACRYPT

Simulation-Sound Arguments for LWE and Applications to KDM-CCA2 Security
📺 Abstract

The Naor-Yung paradigm is a well-known technique that constructs IND-CCA2-secure encryption schemes by means of non-interactive zero-knowledge proofs satisfying a notion of simulation-soundness. Until recently, it was an open problem to instantiate it under the sole Learning-With-Errors (LWE) assumption without relying on random oracles. While the recent results of Canetti et al. (STOC'19) and Peikert-Shiehian (Crypto'19) provide a solution to this problem by applying the Fiat-Shamir transform in the standard model, the resulting constructions are extremely inefficient as they proceed via a reduction to an NP-complete problem. In this paper, we give a direct, non-generic method for instantiating Naor-Yung under the LWE assumption outside the random oracle model. Specifically, we give a direct construction of an unbounded simulation-sound NIZK argument system which, for carefully chosen parameters, makes it possible to express the equality of plaintexts encrypted under different keys in Regev's cryptosystem. We also give a variant of our argument that provides tight security. As an application, we obtain an LWE-based public-key encryption scheme for which we can prove (tight) key-dependent message security under chosen-ciphertext attacks in the standard model.

2020

ASIACRYPT

MPC with Synchronous Security and Asynchronous Responsiveness
📺 Abstract

Two paradigms for secure MPC are synchronous and asynchronous
protocols. While synchronous protocols tolerate more corruptions and allow every party to give its input, they are very slow because the speed depends on the conservatively assumed worst-case delay $\Delta$ of the network. In contrast, asynchronous protocols allow parties to obtain output as fast as the actual network allows, a property called \emph{responsiveness}, but unavoidably have lower resilience and parties with slow network connections cannot give input.
It is natural to wonder whether it is possible to leverage synchronous MPC protocols to achieve responsiveness, hence obtaining the advantages of both paradigms: full security with responsiveness up to t corruptions, and 'extended' security (full security or security with unanimous abort) with no responsiveness up to a larger threshold T of corruptions. We settle the question by providing matching feasibility and impossibility results:
-For the case of unanimous abort as extended security, there is an MPC protocol if and only if T + 2t < n.
-For the case of full security as extended security, there is an MPC protocol if and only if T < n/2 and T + 2t < n. In particular, setting t = n/4 allows to achieve a fully secure MPC for honest majority, which in addition benefits from having substantial responsiveness.

2020

ASIACRYPT

A New Decryption Failure Attack against HQC
📺 Abstract

HQC is an IND-CCA2 KEM running for standardization in NIST's post-quantum cryptography project and has advanced to the second round. It is a code-based scheme in the class of public key encryptions, with given sets of parameters spanning NIST security strength 1, 3 and 5, corresponding to 128, 192 and 256 bits of classic security.
In this paper we present an attack recovering the secret key of an HQC instance named hqc-256-1. The attack requires a single precomputation performed once and then never again. The online attack on an HQC instance then submits about $2^{64}$ special ciphertexts for decryption (obtained from the precomputation) and a phase of analysis studies the subset of ciphertexts that are not correctly decrypted. In this phase, the secret key of the HQC instance is determined.
The overall complexity is estimated to be \(2^{246}\) if the attacker balances the costs of precomputation and post-processing, thereby claiming a successful attack on hqc-256-1 in the NIST setting. If we allow the precomputation cost to be $2^{254}$, which is below exhaustive key search on a 256 bit secret key,
the computational complexity of the later parts can be no more than $2^{64}$. This is a setting relevant to practical security since the large precomputation needs to be done only once.
Also, we note that the complexity of the precomputation can be lower if the online attack is allowed to submit more than $2^{64}$ ciphertexts for decryption.

2020

ASIACRYPT

Possibility and Impossibility Results for Receiver Selective Opening Secure PKE in the Multi-Challenge Setting
📺 Abstract

Public key encryption (PKE) schemes are usually deployed in an open system with numerous users. In practice, it is common that some users are corrupted. A PKE scheme is said to be receiver selective opening (RSO) secure if it can still protect messages transmitted to uncorrupted receivers after the adversary corrupts some receivers and learns their secret keys. This is usually defined by requiring the existence of a simulator that can simulate the view of the adversary given only the opened messages. Existing works construct RSO secure PKE schemes in a single-challenge setting, where the adversary can only obtain one challenge ciphertext for each public key. However, in practice, it is preferable to have a PKE scheme with RSO security in the multi-challenge setting, where public keys can be used to encrypt multiple messages.
In this work, we explore the possibility for achieving PKE schemes with receiver selective opening security in the multi-challenge setting. Our contributions are threefold. First, we demonstrate that PKE schemes with RSO security in the single-challenge setting are not necessarily RSO secure in the multi-challenge setting. Then, we show that it is impossible to achieve RSO security for PKE schemes if the number of challenge ciphertexts under each public key is a priori unbounded. In particular, we prove that no PKE scheme can be RSO secure in the $k$-challenge setting (i.e., the adversary can obtain $k$ challenge ciphertexts for each public key) if its secret key contains less than $k$ bits. On the positive side, we give a concrete construction of PKE scheme with RSO security in the $k$-challenge setting, where the ratio of the secret key length to $k$ approaches the lower bound 1.

2020

ASIACRYPT

Cryptography from One-Way Communication: On Completeness of Finite Channels
📺 Abstract

Garg et al. (Crypto 2015) initiated the study of cryptographic protocols over noisy channels in the non-interactive setting, namely when only one party speaks. A major question left open by this work is the completeness of {\em finite} channels, whose input and output alphabets do not grow with the desired level of security. In this work, we address this question by obtaining the following results:
Completeness of Bit-ROT with Inverse Polynomial Error: We show that bit-ROT (i.e., Randomized Oblivious Transfer channel, where each of the two messages is a single bit) can be used to realize general randomized functionalities with inverse polynomial error. Towards this, we provide a construction of string-ROT from bit-ROT with inverse polynomial error.
No Finite Channel is Complete with Negligible Error: To complement the above, we show that {\it no} finite channel can be used to realize string-ROT with negligible error, implying that the inverse polynomial error in the completeness of bit-ROT is inherent. This holds even with semi-honest parties and for computational security, and is contrasted with the (negligible-error) completeness of string-ROT shown by Garg et al.
Characterization of Finite Channels Enabling Zero-Knowledge Proofs: An important instance of secure computation is zero-knowledge proofs.
Noisy channels can potentially be used to realize truly non-interactive zero-knowledge proofs, without trusted common randomness, and with non-transferability and deniability features that cannot be realized in the plain model. Garg et al. obtain such zero-knowledge proofs from the binary erasure channel (BEC) and the binary symmetric channel (BSC). We complete the picture by showing that in fact any non-trivial channel suffices.

2020

ASIACRYPT

Tight Security Analysis of 3-Round Key-Alternating Cipher with A Single Permutation
📺 Abstract

The tight security bound of the KAC (Key-Alternating Cipher) construction whose round permutations are independent from each other has been well studied. Then a natural question is how the security bound will change when we use fewer permutations in a KAC construction. In CRYPTO 2014, Chen et al. proved that 2-round KAC with a single permutation (2KACSP) has the same security level as the classic one (i.e., 2-round KAC). But we still know little about the security bound of incompletely-independent KAC constructions with more than 2 rounds. In this paper,we will show that a similar result also holds for 3-round case. More concretely, we prove that 3-round KAC with a single permutation (3KACSP) is secure up to $\varTheta(2^{\frac{3n}{4}})$ queries, which also caps the security of 3-round KAC. To avoid the cumbersome graphical illustration used in Chen et al.'s work, a new representation is introduced to characterize the underlying combinatorial problem. Benefited from it, we can handle the knotty dependence in a modular way, and also show a plausible way to study the security of $r$KACSP. Technically, we abstract a type of problems capturing the intrinsic randomness of $r$KACSP construction, and then propose a high-level framework to handle such problems. Furthermore, our proof techniques show some evidence that for any $r$, $r$KACSP has the same security level as the classic $r$-round KAC in random permutation model.

2020

ASIACRYPT

Quantum Collision Attacks on AES-like Hashing with Low Quantum Random Access Memories
📺 Abstract

At EUROCRYPT 2020, Hosoyamada and Sasaki proposed the first dedicated quantum attack on hash functions -- a quantum version of the rebound attack exploiting differentials whose probabilities are too low to be useful in the classical setting. This work opens up a new perspective toward the security of hash functions against quantum attacks. In particular, it tells us that the search for differentials should not stop at the classical birthday bound. Despite these interesting and promising implications, the concrete attacks described by Hosoyamada and Sasaki make use of large quantum random access memories (qRAMs), a resource whose availability in the foreseeable future is controversial even in the quantum computation community. Without large qRAMs, these attacks incur significant increases in time complexities. In this work, we reduce or even avoid the use of qRAMs by performing a quantum rebound attack based on differentials with non-full-active super S-boxes. Along the way, an MILP-based method is proposed to systematically explore the search space of useful truncated differentials with respect to rebound attacks. As a result, we obtain improved attacks on \aes-\texttt{MMO}, \aes-\texttt{MP}, and the first classical collision attacks on 4- and 5-round \grostl-\texttt{512}.
Interestingly, the use of non-full-active super S-box differentials in the analysis of \aes-\texttt{MMO} gives rise to new difficulties in collecting enough starting points. To overcome this issue, we consider attacks involving two message blocks to gain more degrees of freedom, and we successfully compress the qRAM demand of the collision attacks on \texttt{AES}-\texttt{MMO} and \texttt{AES}-\texttt{MP} (EUROCRYPT 2020) from $2^{48}$ to a range from $2^{16}$ to $0$, while still maintaining a comparable time complexity.
To the best of our knowledge, these are the first dedicated quantum attacks on hash functions that slightly outperform Chailloux, Naya-Plasencia, and Schrottenloher's generic quantum collision attack (ASIACRYPT 2017) in a model where large qRAMs are not available. This work demonstrates again how a clever combination of classical cryptanalytic technique and quantum computation leads to improved attacks, and shows that the direction pointed out by Hosoyamada and Sasaki deserves further investigation.

2020

ASIACRYPT

An Algebraic Formulation of the Division Property: Revisiting Degree Evaluations, Cube Attacks, and Key-Independent Sums
📺 Abstract

Since it was proposed in 2015 as a generalization
of integral properties, the division property has
evolved into a powerful tool for probing the
structures of Boolean functions whose
algebraic normal forms are not available.
We capture the most essential elements for the detection of division properties
from a pure algebraic perspective, proposing a technique named as {\it monomial prediction}, which
can be employed to determine the presence or absence of a
monomial in the product of the coordinate functions of a vectorial
Boolean function $\bs f$ by counting the number of the so-called {\it monomial trails}
across a sequence of simpler functions whose composition is $\bs f$.
Under the framework of the monomial prediction, we formally prove that
most algorithms for detecting division properties in previous literature
raise no false alarms but may miss.
We also establish the equivalence between the monomial prediction and
the three-subset bit-based division property without unknown subset presented at EUROCRYPT 2020,
and show that these two techniques are perfectly accurate.
This algebraic formulation gives more insights into division properties
and inspires new search strategies. With the monomial prediction,
we obtain the {\it exact} algebraic degrees of \TRIVIUM up
to 834 rounds for the first time. In the context of cube attacks,
we are able to explore a larger search space in limited time and
recover the exact algebraic normal forms of complex superpolies
with the help of a divide-and-conquer strategy. As a result,
we identify more cubes with smaller dimensions, leading
to improvements of some near-optimal attacks against 840-, 841-
and 842-round \TRIVIUM.

2020

ASIACRYPT

CCA-Secure (Puncturable) KEMs from Encryption With Non-Negligible Decryption Errors
📺 Abstract

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 Fujisaki-Okamoto (FO) transform can work with schemes that have negligible correctness error in the (quantum) random oracle model (QROM). Many recent schemes in the NIST post-quantum competition (PQC) use variants of these transformations. Some of their CPA-secure versions even have a non-negligible correctness error and so the techniques of HHK cannot be applied.
In this work, we study the setting of generically transforming PKE schemes with potentially large, i.e., non-negligible, correctness error to ones having negligible correctness error. While there have been previous treatments in an asymptotic setting by Dwork et al. (EUROCRYPT 2004), our goal is to come up with practically efficient compilers in a concrete setting and apply them in two different contexts: firstly, we show how to generically transform weakly secure deterministic or randomized PKEs into CCA-secure KEMs in the (Q)ROM using variants of HHK. This applies to essentially all candidates to the NIST PQC based on lattices and codes with non-negligible error, for which we provide an extensive analysis. We thereby show that it improves some of the code-based candidates. Secondly, we study puncturable KEMs in terms of the Bloom Filter KEM (BFKEM) proposed by Derler et al. (EUROCRYPT 2018) which inherently have a non-negligible correctness error. BFKEMs are a building block to construct fully forward-secret zero round-trip time (0-RTT) key-exchange protocols. In particular, we show how to achieve the first post-quantum secure BFKEM generically from lattices and codes by applying our techniques to identity-based encryption (IBE) schemes with (non-)negligible correctness error.

2020

ASIACRYPT

A Bit-Vector Differential Model for the Modular Addition by a Constant
📺 Abstract

ARX algorithms are a class of symmetric-key algorithms constructed by Addition, Rotation, and XOR, which achieve the best software performances in low-end microcontrollers. To evaluate the resistance of an ARX cipher against differential cryptanalysis and its variants, the recent automated methods employ constraint satisfaction solvers, such as SMT solvers, to search for optimal characteristics. The main difficulty to formulate this search as a constraint satisfaction problem is obtaining the differential models of the non-linear operations, that is, the constraints describing the differential probability of each non-linear operation of the cipher. While an efficient bit-vector differential model was obtained for the modular addition with two variable inputs, no differential model for the modular addition by a constant has been proposed so far, preventing ARX ciphers including this operation from being evaluated with automated methods.
In this paper, we present the first bit-vector differential model for the n-bit modular addition by a constant input. Our model contains O(log2(n)) basic bit-vector constraints and describes the binary logarithm of the differential probability. We also represent an SMT-based automated method to look for differential characteristics of ARX, including constant additions, and we provide an open-source tool ArxPy to find ARX differential characteristics in a fully automated way. To provide some examples, we have searched for related-key differential characteristics of TEA, XTEA, HIGHT, and LEA, obtaining better results than previous works. Our differential model and our automated tool allow cipher designers to select the best constant inputs for modular additions and cryptanalysts to evaluate the resistance of ARX ciphers against differential attacks.

2020

ASIACRYPT

MoniPoly---An Expressive $q$-SDH-Based Anonymous Attribute-Based Credential System
📺 Abstract

Modern attribute-based anonymous credential (ABC) systems benefit from special encodings that yield expressive and highly efficient show proofs on logical statements. The technique was first proposed by Camenisch and Gro{\ss}, who constructed an SRSA-based ABC system with prime-encoded attributes that offers efficient \textsf{AND}, \textsf{OR} and \textsf{NOT} proofs. While other ABC frameworks have adopted constructions in the same vein, the Camenisch-Gro{\ss} ABC has been the most expressive and asymptotically most efficient proof system to date, even if it was constrained by the requirement of a trusted message-space setup and an inherent restriction to finite-set attributes encoded as primes.
In this paper, combining a new set commitment scheme and an SDH-based signature scheme, we present a provably secure ABC system that supports show proofs for complex statements. This construction is not only more expressive than existing approaches, but it is also highly efficient under unrestricted attribute space due to its ECC protocols only requiring a constant number of bilinear pairings by the verifier; none by the prover.
Furthermore, we introduce strong security models for impersonation and unlinkability under adaptive active and concurrent attacks to allow for the expressiveness of our ABC as well as for a systematic comparison to existing schemes.
Given this foundation, we are the first to comprehensively formally prove the security of an ABC with expressive show proofs.
Specifically, we prove the security against impersonation under the $q$-(co-)SDH assumption with a tight reduction.
Besides the set commitment scheme, which may be of independent interest, our security models can serve as a foundation for the design of future ABC systems.

2020

ASIACRYPT

Collusion Resistant Trace-and-Revoke for Arbitrary Identities from Standard Assumptions
📺 Abstract

A traitor tracing scheme is a multi-user public-key encryption scheme where each user in the system holds a decryption key that is associated with the user's identity. Using the public key, a content distributor can encrypt a message to all of the users in the system. At the same time, if a malicious group of users combine their respective decryption keys to build a "pirate decoder," there is an efficient tracing algorithm that the content distributor can use to identify at least one of the keys used to construct the decoder. A trace-and-revoke scheme is an extension of a standard traitor tracing scheme where there is an additional key-revocation mechanism that the content distributor can use to disable the decryption capabilities of compromised keys. Namely, during encryption, the content distributor can encrypt a message with respect to a list of revoked users such that only non-revoked users can decrypt the resulting ciphertext.
Trace-and-revoke schemes are challenging to construct. Existing constructions from standard assumptions can only tolerate bounded collusions (i.e., there is an a priori bound on the number of keys an adversary obtains), have system parameters that scale exponentially in the bit-length of the identities, or satisfy weaker notions of traceability that are vulnerable to certain types of "pirate evolution" attacks. In this work, we provide the first construction of a trace-and-revoke scheme that is fully collusion resistant and capable of supporting arbitrary identities (i.e., the identities can be drawn from an exponential-size space). Our scheme supports public encryption and secret tracing, and can be based on the sub-exponential hardness of the LWE problem (with a super-polynomial modulus-to-noise ratio). The ciphertext size in our construction scales logarithmically in the size of the identity space and linearly in the size of the revocation list. Our scheme leverages techniques from both combinatorial and algebraic constructions for traitor tracing.

2020

ASIACRYPT

An Algebraic Attack on Ciphers with Low-Degree Round Functions: Application to Full MiMC
📺 Abstract

Algebraically simple PRFs, ciphers, or cryptographic hash functions are becoming increasingly popular, for example due to their attractive properties for MPC and new proof systems (SNARKs, STARKs, among many others).
In this paper, we focus on the algebraically simple construction MiMC, which became an attractive cryptanalytic target due to its simplicity, but also due to its use as a baseline in a competition for more recent algorithms exploring this design space.
For the first time, we are able to describe key-recovery attacks on all full-round versions of MiMC over GF(2^n), requiring half the code book. In the chosen-ciphertext scenario, recovering the key from this data for the n-bit full version of MiMC takes the equivalent of less than 2^(n - log_2(n) + 1) calls to MiMC and negligible amounts of memory.
The attack procedure is a generalization of higher-order differential cryptanalysis, and it is based on two main ingredients. First, we present a higher-order distinguisher which exploits the fact that the algebraic degree of MiMC grows significantly slower than originally believed. Secondly, we describe an approach to turn this distinguisher into a key-recovery attack without guessing the full subkey. Finally, we show that approximately ceil(log_3(2 * R)) more rounds (where R = ceil(n * log_3(2)) is the current number of rounds of MiMC-n/n) can be necessary and sufficient to restore the security against the key-recovery attack presented here.
The attack has been practically verified on toy versions of MiMC. Note that our attack does not affect the security of MiMC over prime fields.

2020

ASIACRYPT

Adaptively Secure Inner Product Encryption from LWE
📺 Abstract

Attribute-based encryption (ABE) is an advanced form of encryption scheme allowing for access policies to be embedded within the secret keys and ciphertexts. By now, we have ABEs supporting numerous types of policies based on hardness assumptions over bilinear maps and lattices. However, one of the distinguishing differences between ABEs based on these two breeds of assumptions is that the former can achieve adaptive security for quite expressible policies (e.g., inner-products, boolean formula) while the latter can not. Recently, two adaptively secure lattice-based ABEs have appeared and changed the state of affairs: a non-zero inner-product (NIPE) encryption by Katsumata and Yamada (PKC'19) and an ABE for t-CNF policies by Tsabary (CRYPTO'19). However, the policies supported by these ABEs are still quite limited and do not embrace the more interesting policies that have been studied in the literature. Notably, constructing an adaptively secure inner-product encryption (IPE) based on lattices still remains open.
In this work, we propose the first adaptively secure IPE based on the learning with errors (LWE) assumption with sub-exponential modulus size (without resorting to complexity leveraging). Concretely, our IPE supports inner-products over the integers Z with polynomial sized entries and
satisfies adaptively weakly-attribute-hiding security.
We also show how to convert such an IPE to an IPE supporting inner-products over Z_p for a polynomial-sized p and a fuzzy identity-based encryption (FIBE) for small and large universes. Our result builds on the ideas presented in Tsabary (CRYPTO'19), which uses constrained pseudorandom functions (CPRF) in a semi-generic way to achieve adaptively secure ABEs, and the recent lattice-based adaptively secure CPRF for inner-products by Davidson et al. (CRYPTO'20). Our main observation is realizing how to weaken the conforming CPRF property introduced in Tsabary (CRYPTO'19) by taking advantage of the specific linearity property enjoyed by the lattice evaluation algorithms by Boneh et al. (EUROCRYPT'14).

2020

ASIACRYPT

Scalable Ciphertext Compression Techniques for Post-Quantum KEMs and their Applications
📺 Abstract

A multi-recipient key encapsulation mechanism, or mKEM, provides a scalable solution to securely communicating to a large group, and offers savings in both bandwidth and computational cost compared to the trivial solution of communicating with each member individually. All prior works on mKEM are only limited to classical assumptions and, although some generic constructions are known, they all require specific properties that are not shared by most post-quantum schemes.
In this work, we first provide a simple and efficient generic construction of mKEM that can be instantiated from versatile assumptions, including post-quantum ones. We then study these mKEM instantiations at a practical level using 8 post-quantum KEMs (which are lattice and isogeny-based NIST candidates), and CSIDH, and show that compared to the trivial solution, our mKEM offers savings of at least one order of magnitude in the bandwidth, and make encryption time shorter by a factor ranging from 1.92 to 35. Additionally, we show that by combining mKEM with the TreeKEM protocol used by MLS – an IETF draft for secure group messaging – we obtain significant bandwidth savings.

2020

ASIACRYPT

SILVER - Statistical Independence and Leakage Verification
📺 Abstract

Implementing cryptographic functions securely in the presence of physical adversaries is still a challenge although a lion's share of research in the physical security domain has been put in development of countermeasures. Among several protection schemes, masking has absorbed the most attention of research in both academic and industrial communities, due to its theoretical foundation allowing to provide proofs or model the achieved security level. In return, masking schemes are difdicult to implement as the implementation process often is manual, complex, and error-prone. This motivated the need for formal verification tools that allow the designers and engineers to analyze and verify the designs before manufacturing.
In this work, we present a new framework to analyze and verify masked implementations against various security notions using different security models as reference. In particular, our framework { which directly processes the resulting gate-level netlist of a hardware synthesis { particularly relies on Reduced Ordered Binary Decision Diagrams (ROBDDs) and the concept of statistical independence of probability distributions. Compared to existing tools, our framework captivates due to its simplicity, accuracy, and functionality while still having a reasonable efficiency for many applications and common use-cases.

2020

ASIACRYPT

Lower Bounds on the Degree of Block Ciphers
📺 Abstract

Only the method to estimate the upper bound of the algebraic degree on block ciphers is known so far, but it is not useful for the designer to guarantee the security. In this paper we provide meaningful lower bounds on the algebraic degree of modern block ciphers.

2020

ASIACRYPT

Improvements of Algebraic Attacks for solving the Rank Decoding and MinRank problems
📺 Abstract

In this paper, we show how to significantly improve algebraic techniques for solving the MinRank problem, which is ubiquitous in multivariate and rank metric code based cryptography. In the case of the structured MinRank instances arising in the latter, we build upon a recent breakthrough in Bardet et al. (EUROCRYPT 2020) showing that algebraic attacks outperform the combinatorial ones that were considered state of the art up until now. Through a slight modification of this approach, we completely avoid Gr\¨obner bases computations for certain parameters and are left only with solving linear systems. This does not only substantially improve the complexity, but also gives a convincing argument as to why algebraic techniques work in this case. When used against the second round NIST-PQC candidates ROLLO-I-128/192/256, our new attack has bit complexity respectively 71, 87, and 151, to be compared to 117, 144, and 197 as obtained in Bardet et al. (EUROCRYPT 2020). The linear systems arise from the nullity of the maximal minors of a certain matrix associated to the algebraic modeling. We also use a similar approach to improve the algebraic MinRank solvers for the usual MinRank problem. When applied against the second round NIST-PQC candidates GeMSS and Rainbow, our attack has a complexity that is very close to or even slightly better than those of the best known attacks so far. Note that these latter attacks did not rely on MinRank techniques since the MinRank approach used to give complexities that were far away from classical security levels.

2020

ASIACRYPT

Succinct Functional Commitment for a Large Class of Arithmetic Circuits
📺 Abstract

A succinct functional commitment (SFC) scheme for a circuit class $\mathbf{CC}$ enables, for any circuit $\mathcal{C}\in \mathbf{CC}$, the committer to first succinctly commit to a vector $\vec{\alpha}$, and later succinctly open the commitment to $\mathcal{C} (\vec{\alpha}, \vec{\beta})$, where the verifier chooses $\vec{\beta}$ at the time of opening. Unfortunately, SFC commitment schemes are known only for severely limited function classes like the class of inner products. By making non-black-box use of SNARK-construction techniques, we propose a SFC scheme for the large class of semi-sparse polynomials. The new SFC scheme can be used to, say, efficiently (1) implement sparse polynomials, and (2) aggregate various interesting SFC (e.g., vector commitment and polynomial commitment) schemes. The new scheme is evaluation-binding under a new instantiation of the computational uber-assumption. We provide a thorough analysis of the new assumption.

2020

ASIACRYPT

Secure MPC: Laziness Leads to GOD
📺 Abstract

Motivated by what we call "honest but lazy” parties in the context of secure multi party computation, we revisit the notion of multi-key FHE schemes (MFHE). In MFHE, any message encrypted using a public key pk_i can be "expanded" so that the resulting ciphertext is encrypted with respect to a set of public keys (pk_1,..,pk_n). Such expanded ciphertexts can be homomorphically evaluated with respect to any circuit to generate a ciphertext ct. Then, this ciphertext ct can be partially decrypted using a secret key sk_i (corresponding to the public key pk_i) to produce a partial decryption p_i. Finally, these partial decryptions {p_{i}}_{i in [n]} can be combined to recover the output. However, this definition of MFHE works only for n-out-of-n access structures and, thus, each node in the system is a point of failure. In the context of "honest but lazy” parties, it is necessary to be able to decrypt even when only given a subset of partial decryptions (say t out of n). In order to solve this problem, we introduce a new notion of multi-key FHE designed to handle arbitrary access patterns that can reconstruct the output. We call it a threshold multi-key FHE scheme (TMFHE).
Our main contributions are the following:
* We formally define and construct TMFHE for any access structure given by a monotone boolean formula, assuming LWE.
* We construct the first simulation-extractable multi-string NIZK from polynomially hard LWE.
* We use TMFHE and our multi-string NIZK to obtain the first round-optimal (three round) MPC protocol in the plain model with guaranteed output delivery secure against malicious adversaries or, more generally, mixed adversaries (which supports "honest but lazy” parties), assuming LWE.
* Our MPC protocols simultaneously achieve security against the maximum number of corruptions under which guaranteed output delivery is achievable, depth-proportional communication complexity, and reusability.

2020

ASIACRYPT

Cryptographic Group Actions and Applications
📺 Abstract

Isogeny-based assumptions have emerged as a viable option for quantum-secure cryptography. Recent works have shown how to build efficient (public-key) primitives from isogeny-based assumptions such as CSIDH and CSI-FiSh. However, in its present form, the landscape of isogenies does not seem very amenable to realizing new cryptographic applications. Isogeny-based assumptions often have unique efficiency and security properties, which makes building new cryptographic applications from them a potentially tedious and time-consuming task.
In this work, we propose a new framework based on group actions that enables the easy usage of a variety of isogeny-based assumptions. Our framework generalizes the works of Brassard and Yung (Crypto'90) and Couveignes (Eprint'06). We provide new definitions for group actions endowed with natural hardness assumptions that model isogeny-based constructions amenable to group actions such as CSIDH and CSI-FiSh.
We demonstrate the utility of our new framework by leveraging it to construct several primitives that were not previously known from isogeny-based assumptions. These include smooth projective hashing, dual-mode PKE, two-message statistically sender-private OT, and Naor-Reingold style PRF. These primitives are useful building blocks for a wide range of cryptographic applications.
We introduce a new assumption over group actions called Linear Hidden Shift (LHS) assumption. We then present some discussions on the security of the LHS assumption and we show that it implies symmetric KDM-secure encryption, which in turn enables many other primitives that were not previously known from isogeny-based assumptions.

2020

ASIACRYPT

Security Reductions for White-Box Key-Storage in Mobile Payments
📺 Abstract

The goal of white-box cryptography is to provide security even when the cryptographic implementation is executed in adversarially controlled environments. White-box implementations nowadays appear in commercial products such as mobile payment applications, e.g., those certified by Mastercard. Interestingly, there, white-box cryptography is championed as a tool for secure storage of payment tokens, and importantly, the white-boxed storage functionality is bound to a hardware functionality to prevent code-lifting attacks.
In this paper, we show that the approach of using hardware-binding and obfuscation for secure storage is conceptually sound. Following security specifications by Mastercard and also EMVCo, we first define security for a white-box key derivation functions (WKDF) that is bound to a hardware functionality. WKDFs with hardware-binding model a secure storage functionality, as the WKDFs in turn can be used to derive encryption keys for secure storage. We then provide a proof-of-concept construction of WKDFs based on pseudorandom functions (PRF) and obfuscation. To show that our use of cryptographic primitives is sound, we perform a cryptographic analysis and reduce the security of our WKDF to the cryptographic assumptions of indistinguishability obfuscation and PRF-security. The hardware-functionality that our WKDF is bound to is a PRF-like functionality. Obfuscation helps us to hide the secret key used for the verification, essentially emulating a signature functionality as is provided by the Android key store.
We rigorously define the required security properties of a hardware-bound white-box payment application (WPAY) for generating and encrypting valid payment requests. We construct a WPAY, which uses a WKDF as a secure building block. We thereby show that a WKDF can be securely combined with any secure symmetric encryption scheme, including those based on standard ciphers such as AES.

2020

ASIACRYPT

Cryptanalysis of Masked Ciphers: A not so Random Idea
📺 Abstract

A new approach to the security analysis of hardware-oriented masked ciphers against second-order side-channel attacks is developed. By relying on techniques from symmetric-key cryptanalysis, concrete security bounds are obtained in a variant of the probing model that allows the adversary to make only a bounded, but possibly very large, number of measurements. Specifically, it is formally shown how a bounded-query variant of robust probing security can be reduced to the linear cryptanalysis of masked ciphers.
As a result, the compositional issues of higher-order threshold implementations can be overcome without relying on fresh randomness. From a practical point of view, the aforementioned approach makes it possible to transfer many of the desirable properties of first-order threshold implementations, such as their low randomness usage, to the second-order setting. For example, a straightforward application to the block cipher LED results in a masking using less than 700 random bits including the initial sharing. In addition, the cryptanalytic approach introduced in this paper provides additional insight into the design of masked ciphers and allows for a quantifiable trade-off between security and performance.

2020

ASIACRYPT

A Combinatorial Approach to Quantum Random Functions
📺 Abstract

Quantum pseudorandom functions (QPRFs) extend the classical security of a PRF by allowing the adversary to issue queries on input superpositions. Zhandry [Zhandry, FOCS 2012] showed a separation between the two notions and proved that common construction paradigms are also quantum secure, albeit with a new ad-hoc analysis. In this work, we revisit the question of constructing QPRFs and propose a new method starting from small-domain (classical) PRFs: At the heart of our approach is a new domain-extension technique based on bipartite expanders. Interestingly, our analysis is almost entirely classical.
As a corollary of our main theorem, we obtain the first (approximate) key-homomorphic quantum PRF based on the quantum intractability of the learning with errors problem.

2020

ASIACRYPT

Calamari and Falafl: Logarithmic (Linkable) Ring Signatures from Isogenies and Lattices
📺 Abstract

We construct efficient ring signatures (RS) from isogeny and
lattice assumptions. Our ring signatures are based on a logarithmic OR
proof for group actions. We instantiate this group action by either the
CSIDH group action or an MLWE-based group action to obtain our
isogeny-based or lattice-based RS scheme, respectively. Even though the
OR proof has a binary challenge space and therefore requires a number of
repetitions which is linear in the security parameter, the sizes of our ring
signatures are small and scale better with the ring size N than previously
known post-quantum ring signatures. We also construct linkable ring
signatures (LRS) that are almost as efficient as the non-linkable variants.
The isogeny-based scheme produces signatures whose size is an order of
magnitude smaller than all previously known logarithmic post-quantum
ring signatures, but it is relatively slow (e.g. 5.5 KB signatures and 79
s signing time for rings with 8 members). In comparison, the lattice-based
construction is much faster, but has larger signatures (e.g. 30 KB
signatures and 90 ms signing time for the same ring size). For small ring
sizes our lattice-based ring signatures are slightly larger than state-of-the-
art schemes, but they are smaller for ring sizes larger than N approximately 1024.

2020

ASIACRYPT

Crowd Verifiable Zero-Knowledge and End-to-end Verifiable Multiparty Computation
📺 Abstract

Auditing a secure multiparty computation (MPC) protocol
entails the validation of the protocol transcript
by a third party that is otherwise untrusted.
In this work we introduce the concept of end-to-end verifiable
MPC (VMPC), that requires the validation to provide a correctness
guarantee even in the setting that all servers, trusted setup
primitives and all the client systems utilized by the input-providing
users of the MPC protocol are subverted by an adversary.
To instantiate VMPC, we introduce a new concept in the setting of
zero-knowlegde protocols that we term crowd verifiable zero-knowledge
(CVZK). A CVZK protocol enables a prover to convince a set of verifiers
about a certain statement, even though each one individually contributes
a small amount of entropy for verification and some of them are adversarially
controlled. Given CVZK, we present a VMPC protocol that
is based on discrete-logarithm related assumptions.
At the high level of adversity that VMPC is meant to withstand,
it is infeasible to ensure perfect correctness,
thus we investigate the classes of functions and
verifiability relations that are feasible in our framework, and
present a number of possible applications the underlying
functions of which can be implemented via VMPC.

2020

ASIACRYPT

Towards Closing The Security Gap of Tweak-aNd-Tweak (TNT)
📺 Abstract

Tweakable block ciphers (TBCs) have been established as a valuable replacement for many applications of classical block ciphers. While several dedicated TBCs have been proposed in the previous years, generic constructions that build a TBC from a classical block cipher are still highly useful, for example, to reuse an existing implementation. However, most generic constructions need an additional call to either the block cipher or a universal hash function to process the tweak, which limited their efficiency.
To address this deficit, Bao et al. proposed Tweak-aNd-Tweak (TNT) at EUROCRYPT'20. Their construction chains three calls to independent keyed permutations and adds the unmodified tweak to the state in between the calls. They further suggested an efficient instantiation TNT-AES that was based on round-reduced AES for each of the permutations. Their work could prove 2n/3-bit security for their construction, where n is the block size in bits. Though, in the absence of an upper bound, their analysis had to consider all possible attack vectors with up to 2^n time, data, and memory. Still, closing the gap between both bounds remained a highly interesting research question.
In this work, we show that a variant of Mennink's distinguisher on CLRW2 with O(sqrt{n} 2^{3n/4}) data and O(2^{3n/2}) time from TCC'18 also applies to TNT. We reduce its time complexity to O(sqrt{n} 2^{3n/4}), show the existence of a second similar distinguisher, and demonstrate how to transform the distinguisher to a key-recovery attack on TNT-AES[5,*,*] from an impossible differential. From a constructive point of view, we adapt the rigorous STPRP analysis of CLRW2 by Jha and Nandi to show O(2^{3n/4}) TPRP security for TNT. Thus, we move towards closing the gap between the previous proof and attacks for TNT as well as its proposed instance.

2020

ASIACRYPT

Improved Classical and Quantum Algorithms for Subset-Sum
📺 Abstract

We present new classical and quantum algorithms for solving random subset-sum instances. First, we improve over the Becker-Coron-Joux algorithm (EUROCRYPT 2011) from $\widetilde{O}(2^{0.291 n})$ down to $\widetilde{O}(2^{0.283 n})$, using more general representations with values in $\{0,1,-1,2\}$.
Next, we improve the state of the art of quantum algorithms for this problem in several directions. By combining the Howgrave-Graham-Joux algorithm (EUROCRYPT 2010) and quantum search, we devise an algorithm with asymptotic cost $\widetilde{O}(2^{0.236 n})$, lower than the cost of the quantum walk based on the same classical algorithm proposed by Bernstein, Jeffery, Lange and Meurer (PQCRYPTO 2013). This algorithm has the advantage of using \emph{classical} memory with quantum random access, while the previously known algorithms used the quantum walk framework, and required \emph{quantum} memory with quantum random access.
We also propose new quantum walks for subset-sum, performing better than the previous best time complexity of $\widetilde{O}(2^{0.226 n})$ given by Helm and May (TQC 2018). We combine our new techniques to reach a time $\widetilde{O}(2^{0.216 n})$. This time is dependent on a heuristic on quantum walk updates, formalized by Helm and May, that is also required by the previous algorithms. We show how to partially overcome this heuristic, and we obtain an algorithm with quantum time $\widetilde{O}(2^{0.218 n})$ requiring only the standard classical subset-sum heuristics.

2020

ASIACRYPT

KVaC: Key-Value Commitments for Blockchains and Beyond
📺 Abstract

As blockchains grow in size, validating new transactions becomes more and more resource intensive. To deal with this, there is a need to discover compact encodings of the (effective) state of a blockchain --- an encoding that allows for efficient proofs of membership and updates. In the case of account-based cryptocurrencies, the state can be represented by a key-value map, where keys are the account addresses and values consist of account balance, nonce, etc.
We propose a new commitment scheme for key-value maps whose size does not grow with the number of keys, yet proofs of membership are of constant-size. In fact, both the encoding and the proofs consist of just two and three group elements respectively (in groups of unknown order like class groups). Verifying and updating proofs involves just a few group exponentiations. Additive updates to key values enjoy the same level of efficiency too.
Key-value commitments can be used to build dynamic accumulators and vector commitments, which find applications in group signatures, anonymous credentials, verifiable databases, interactive oracle proofs, etc. Using our new key-value commitment, we provide the most efficient constructions of (sub)vector commitments to date.

2020

ASIACRYPT

Radical Isogenies
📺 Abstract

This paper introduces a new approach to computing isogenies called ``radical isogenies'' and a corresponding method to compute chains of $N$-isogenies that is very efficient for small $N$. The method is fully deterministic and completely avoids generating $N$-torsion points. It is based on explicit formulae for the coordinates of an $N$-torsion point $P'$ on the codomain of a cyclic $N$-isogeny $\varphi : E \to E'$, such that composing $\varphi$ with $E' \to E' / \langle P' \rangle$ yields a cyclic $N^2$-isogeny. These formulae are simple algebraic expressions in the coefficients of $E$, the coordinates of a generator $P$ of $\ker \varphi$, and an $N$th root $\sqrtN{\rho}$, where the radicand $\rho$ itself is given by an easily computable algebraic expression in the coefficients of $E$ and the coordinates of $P$. The formulae can be iterated and are particularly useful when computing chains of $N$-isogenies over a finite field $\F_q$ with $\gcd(q-1, N) = 1$, where taking an $N$th root is a simple exponentiation. Compared to the state-of-the-art, our method results in an order of magnitude speed-up for $N \leq 13$; for larger $N$, the advantage disappears due to the increasing complexity of the formulae. When applied to CSIDH, we obtain a speed-up of about $19 \%$ over the implementation by Bernstein, De Feo, Leroux and Smith for the CSURF-512 parameters.

2020

ASIACRYPT

Subvert KEM to Break DEM: Practical Algorithm-Substitution Attacks on Public-Key Encryption
📺 Abstract

Motivated by the widespread concern about mass surveillance of encrypted communications, Bellare \textit{et al.} introduced at CRYPTO 2014 the notion of Algorithm-Substitution Attack (ASA) where the legitimate encryption algorithm is replaced by a subverted one that aims to undetectably exfiltrate the secret key via ciphertexts. Practically implementable ASAs on various cryptographic primitives (Bellare \textit{et al.}, CRYPTO'14 \& CCS'15; Ateniese \textit{et al.}, CCS'15; Berndt and Li\'{s}kiewicz, CCS'17) have been constructed and analyzed, leaking the secret key successfully. Nevertheless, in spite of much current attention, the practical impact of ASAs (formulated originally for symmetric key cryptography) on public-key (PKE) encryption operations remains unclear, primarily since the encryption operation of PKE does not involve the secret key and previously known ASAs become relatively inefficient for leaking the plaintext due to the logarithmic upper bound of exfiltration rate (Berndt and Li\'{s}kiewicz, CCS'17).
In this work, we formulate a practical ASA on PKE encryption algorithm which, perhaps surprisingly, turns out to be much more efficient and robust than existing ones, showing that ASAs on PKE schemes are far more dangerous than previously believed. We mainly target PKE of hybrid encryption which is the most prevalent way to employ PKE in the literature and in practical systems. The main strategy of our ASA is to subvert the underlying key encapsulation mechanism (KEM) so that the session key encapsulated could be efficiently extracted, which, in turn, breaks the data encapsulation mechanism (DEM) enabling us to learn the plaintext itself. Concretely, our non-black-box attack enables recovering the plaintext from only two successive ciphertexts and minimally depends on a short state of previous internal randomness. A widely used class of KEMs is shown to be subvertible by our powerful attack.
Our attack relies on a novel identification and formalization of specific non-black-box yet general enough properties that yield practical ASAs on KEMs. More broadly, this may shed some light on exploring the structural weakness of other composed cryptographic primitives, which may make them susceptible to more dangerous ASAs that surpass the logarithmic upper bound of exfiltration rate on universal ASAs.

2020

ASIACRYPT

Inner-Product Functional Encryption with Fine-Grained Access Control
📺 Abstract

We construct new functional encryption schemes that combine the access control functionality of attribute-based encryption with the possibility of performing linear operations on the encrypted data. While such a primitive could be easily realized from fully fledged functional encryption schemes, what makes our result interesting is the fact that our schemes simultaneously achieve all the following properties. They are public-key, efficient and can be proved secure under standard and well established assumptions (such as LWE or pairings). Furthermore, security is guaranteed in the setting where adversaries are allowed to get functional keys that decrypt the challenge ciphertext. Our first results are two functional encryption schemes for the family of functions that allow users to embed policies (expressed by monotone span programs) in the encrypted data, so that one can generate functional keys to compute weighted sums on the latter. Both schemes are pairing-based and quite generic: they combine the ALS functional encryption scheme for inner products from Crypto 2016 with any attribute-based encryption schemes relying on the dual-system encryption methodology. As an additional bonus, they yield simple and elegant multi-input extensions essentially for free, thereby broadening the set of applications for such schemes. Multi-input is a particularly desirable feature in our setting, since it gives a finer access control over the encrypted data, by allowing users to associate different access policies to different parts of the encrypted data. Our second result builds identity-based functional encryption for inner products from lattices. This is achieved by carefully combining existing IBE schemes from lattices with adapted, LWE-based, variants of ALS. We point out to intrinsic technical bottlenecks to obtain richer forms of access control from lattices. From a conceptual point of view, all our results can be seen as further evidence that more expressive forms of functional encryption can be realized under standard assumptions and with little computational overhead.

2020

ASIACRYPT

Unbounded HIBE with Tight Security
📺 Abstract

We construct the first unbounded hierarchical identity-based encryption (HIBE) scheme with tight security reductions based on standard assumptions. Our main technical contribution is a novel proof strategy that allows us to tightly randomize user secret keys for identities with arbitrary hierarchy depths using low entropy hidden in a small and hierarchy-independent master public key.
The notion of unbounded HIBE is proposed by Lewko and Waters (Eurocrypt 2011). In contrast to most HIBE schemes, an unbounded scheme does not require any maximum depth to be specified in the setup phase, and user secret keys or ciphertexts can be generated for identities of arbitrary depths with hierarchy-independent system parameters.
While all the previous unbounded HIBE schemes have security loss that grows at least linearly in the number of user secret key queries, the security loss of our scheme is only dependent on the security parameter, even in the multi-challenge setting, where an adversary can ask for multiple challenge ciphertexts. We prove the adaptive security of our scheme based on the Matrix Decisional Diffie-Hellman assumption in prime-order pairing groups, which generalizes a family of standard Diffie-Hellman assumptions such as k-Linear.

2020

ASIACRYPT

Asymptotically Good Multiplicative LSSS over Galois Rings and Applications to MPC over Z/p^k Z
📺 Abstract

We study information-theoretic multiparty computation (MPC) protocols over rings Z/p^k Z that have good asymptotic communication complexity for a large number of players. An important ingredient for such protocols is arithmetic secret sharing, i.e., linear secret-sharing schemes with multiplicative properties. The standard way to obtain these over fields is with a family of linear codes C, such that C, $C^\perp$ and C^2 are asymptotically good (strongly multiplicative). For our purposes here it suffices if the square code C^2 is not the whole space, i.e., has codimension at least 1 (multiplicative).
Our approach is to lift such a family of codes defined over a finite field F to a Galois ring, which is a local ring that has F as its residue field and that contains Z/p^k Z as a subring, and thus enables arithmetic that is compatible with both structures. Although arbitrary lifts preserve the distance and dual distance of a code, as we demonstrate with a counterexample, the multiplicative property is not preserved. We work around this issue by showing a dedicated lift that preserves \emph{self-orthogonality} (as well as distance and dual distance), for p > 2. Self-orthogonal codes are multiplicative, therefore we can use existing results of asymptotically good self-dual codes over fields to obtain arithmetic secret sharing over Galois rings. For p = 2 we obtain multiplicativity by using existing techniques of secret-sharing using both C and $C^\perp$, incurring a constant overhead. As a result, we obtain asymptotically good arithmetic secret-sharing schemes over Galois rings.
With these schemes in hand, we extend existing field-based MPC protocols to obtain MPC over Z/p^k Z, in the setting of a submaximal adversary corrupting less than a fraction 1/2 - \varepsilon of the players, where \varepsilon > 0 is arbitrarily small. We consider 3 different corruption models, and obtain O(n) bits communicated per multiplication for both passive security and active security with abort. For full security with guaranteed output delivery we use a preprocessing model and get O(n) bits per multiplication in the online phase and O(n log n) bits per multiplication in the offline phase.
Thus, we obtain true linear bit complexities, without the common assumption that the ring size depends on the number of players.

2020

ASIACRYPT

New results on Gimli: full-permutation distinguishers and improved collisions
📺 Abstract

★ Best Paper Award

Gimli is a family of cryptographic primitives (both a hash function and an AEAD scheme) that has been selected for the second round of the NIST competition for standardizing new lightweight designs. The candidate Gimli is based on the permutation Gimli, which was presented at CHES 2017. In this paper, we study the security of both the permutation and the constructions that are based on it. We exploit the slow diffusion in Gimli and its internal symmetries to build, for the first time, a distinguisher on the full permutation of complexity $2^{64}$. We also provide a practical distinguisher on 23 out of the full 24 rounds of Gimli that has been implemented.
Next, we give (full state) collision and semi-free-start collision attacks on Gimli-Hash, reaching respectively up to 12 and 18 rounds. On the practical side, we compute a collision on 8-round Gimli-Hash. In the quantum setting, these attacks reach 2 more rounds. Finally, we perform the first study of linear trails in the permutation, and we propose differential-linear cryptanalysis that reach up to 17 rounds of Gimli.

2020

ASIACRYPT

How to Build Optimally Secure PRFs Using Block Ciphers
📺 Abstract

In EUROCRYPT '96, Aiello and Venkatesan proposed two candidates for $ 2n $-bit to $ 2n $-bit pseudorandom functions (PRFs), called Benes and modified Benes (or mBenes), based on $ n $-bit to $ n $-bit PRFs. While Benes is known to be secure up to $ 2^n $ queries (Patarin, AFRICACRYPT '08), the security of mBenes has only been proved up to $ 2^{n(1-\epsilon)} $ queries for all $ \epsilon > 0 $ by Patarin and Montreuil in ICISC '05. In this work, we show that the composition of a $ 2n $-bit hash function with mBenes is a secure variable input length (VIL) PRF up to $ 2^{n-2} $ queries (given appropriate hash function bounds). We extend our analysis with block ciphers as the underlying primitive and obtain two optimally secure VIL PRFs using block ciphers. The first of these candidates requires $ 6 $ calls to the block cipher. The second candidate requires just $ 4 $ calls to the block cipher, but here the proof is based on Patarin's mirror theory. Further, we instantiate the hash function with a PMAC+/LightMAC+ like hash, to get six candidates for deterministic message authentication codes with optimal security.

2020

ASIACRYPT

Security Limitations of Classical-Client Delegated Quantum Computing
📺 Abstract

Secure delegated quantum computing allows a computationally weak client to outsource an arbitrary quantum computation to an untrusted quantum server in a privacy-preserving manner. One of the promising candidates to achieve classical delegation of quantum computation is classical-client remote state preparation ($\sf{RSP}_{CC}$), where a client remotely prepares a quantum state using a classical channel. However, the privacy loss incurred by employing $\sf{RSP}_{CC}$ as a sub-module is unclear.
In this work, we investigate this question using the Constructive Cryptography framework by Maurer and Renner (ICS'11). We first identify the goal of $\sf{RSP}_{CC}$ as the construction of ideal \RSP resources from classical channels and then reveal the security limitations of using $\sf{RSP}_{CC}$. First, we uncover a fundamental relationship between constructing ideal \RSP resources (from classical channels) and the task of cloning quantum states. Any classically constructed ideal \RSP resource must leak to the server the full classical description (possibly in an encoded form) of the generated quantum state, even if we target computational security only. As a consequence, we find that the realization of common \RSP resources, without weakening their guarantees drastically, is impossible due to the no-cloning theorem. Second, the above result does not rule out that a specific $\sf{RSP}_{CC}$ protocol can replace the quantum channel at least in some contexts, such as the Universal Blind Quantum Computing ($\sf{UBQC}$) protocol of Broadbent et al. (FOCS '09). However, we show that the resulting UBQC protocol cannot maintain its proven composable security as soon as $\sf{RSP}_{CC}$ is used as a subroutine. Third, we show that replacing the quantum channel of the above $\sf{UBQC}$ protocol by the $\sf{RSP}_{CC}$ protocol QFactory of Cojocaru et al. (Asiacrypt '19) preserves the weaker, game-based, security of $\sf{UBQC}$.

2020

ASIACRYPT

Packed Multiplication: How to Amortize the Cost of Side-channel Masking?
📺 Abstract

Higher-order masking countermeasures provide strong provable security against side-channel attacks at the cost of incurring significant overheads, which largely hinders its applicability. Previous works towards remedying cost mostly concentrated on ``local'' calculations, i.e., optimizing the cost of computation units such as a single AND gate or a field multiplication. This paper explores a complementary ``global'' approach, i.e., considering multiple operations in the masked domain as a batch and reducing randomness and computational cost via amortization. In particular, we focus on the amortization of $\ell$ parallel field multiplications for appropriate integer $\ell > 1$, and design a kit named {\it packed multiplication} for implementing such a batch.
Higher-order masking countermeasures provide strong provable security against side-channel attacks at the cost of incurring significant overheads, which largely hinders its applicability. Previous works towards remedying cost mostly concentrated on ``local'' calculations, i.e., optimizing the cost of computation units such as a single AND gate or a field multiplication. This paper explores a complementary ``global'' approach, i.e., considering multiple operations in the masked domain as a batch and reducing randomness and computational cost via amortization.
In particular, we focus on the amortization of $\ell$ parallel field multiplications for appropriate integer $\ell > 1$, and design a kit named {\it packed multiplication} for implementing such a batch.
For $\ell+d\leq2^m$, when $\ell$ parallel multiplications over $\mathbb{F}_{2^{m}}$ with $d$-th order probing security are implemented, packed multiplication consumes $d^2+2\ell d + \ell$ bilinear multiplications and $2d^2 + d(d+1)/2$ random field variables, outperforming the state-of-the-art results with $O(\ell d^2)$ multiplications and $\ell \left \lfloor d^2/4\right \rfloor + \ell d$ randomness. To prove $d$-probing security for packed multiplications, we introduce some weaker security notions for multiple-inputs-multiple-outputs gadgets and use them as intermediate steps, which may be of independent interest.
As parallel field multiplications exist almost everywhere in symmetric cryptography, lifting optimizations from ``local'' to ``global'' substantially enlarges the space of improvements. To demonstrate, we showcase the method on the AES Subbytes step, GCM and TET (a popular disk encryption). Notably, when $d=8$, our implementation of AES Subbytes in ARM Cortex M architecture achieves a gain of up to $33\%$ in total speeds and saves up to $68\%$ random bits than the state-of-the-art bitsliced implementation reported at ASIACRYPT~2018.

2020

ASIACRYPT

Catalic: Delegated PSI Cardinality with Applications to Contact Tracing
📺 Abstract

Private Set Intersection Cardinality (PSI-CA) allows two parties, each holding a set of items, to learn the size of the intersection of those sets without
revealing any additional information. To the best of our knowledge, this work
presents the first protocol that allows one of the parties to delegate PSI-CA
computation to untrusted servers. At the heart of our delegated PSI-CA protocol
is a new oblivious distributed key PRF (Odk-PRF) abstraction, which may be
of independent interest.
We explore in detail how to use our delegated PSI-CA protocol to perform
privacy-preserving contact tracing. It has been estimated that a significant
percentage of a given population would need to use a contact tracing app to stop a disease’s spread. Prior privacy-preserving contact tracing systems, however, impose heavy bandwidth or computational demands on client devices. These demands present an economic disincentive to participate for end users who may be billed per MB by their mobile data plan or for users who want to save battery life. We propose Catalic (ContAct TrAcing for LIghtweight Clients), a new contact tracing system that minimizes bandwidth cost and computation workload on client devices. By applying our new delegated PSI-CA protocol, Catalic shifts most of the client-side computation of contact tracing to untrusted servers, and potentially saves each user hundreds of megabytes of mobile data per day while preserving privacy.

2020

ASIACRYPT

Secret-Shared Shuffle
📺 Abstract

Generating additive secret shares of a shuffled dataset - such that neither party knows the order in which it is permuted - is a fundamental building block in many protocols, such as secure collaborative filtering, oblivious sorting, and secure function evaluation on set intersection. Traditional approaches to this problem either involve expensive public-key based crypto or using symmetric crypto on permutation networks. While public-key-based solutions are bandwidth efficient, they are computation-heavy. On the other hand, constructions based on permutation networks are communication-bound, especially when the dataset contains large elements, for e.g., feature vectors in an ML context.
We design a new 2-party protocol for this task of computing secret shares of shuffled data, which we refer to as secret-shared shuffle. Our protocol is secure against a static semi-honest adversary.
At the heart of our approach is a new primitive we define (which we call ``Share Translation'') that generates two sets of pseudorandom values ``correlated via the permutation''. This allows us to reduce the problem of shuffling the dataset to the problem of shuffling pseudorandom values, which enables optimizations both in computation and communication. We then design a Share Translation protocol based on oblivious transfer and puncturable PRFs.
Our final protocol for secret-shared shuffle uses lightweight operations like XOR and PRGs, and in particular doesn't use public-key operations besides the base OTs. As a result, our protocol is concretely more efficient than the existing solutions. In particular, we are two-three orders of magnitude faster than public-key-based approach and one order of magnitude faster compared to the best known symmetric-key approach when the elements are moderately large.

2020

ASIACRYPT

Multi-Client Oblivious RAM with Poly-Logarithmic Communication
📺 Abstract

Oblivious RAM enables oblivious access to memory in the single-client setting, which may not be the best fit in the network setting. Multi-client oblivious RAM (MCORAM) considers a collaborative but untrusted environment, where a database owner selectively grants read access and write access to different entries of a confidential database to multiple clients. Their access pattern must remain oblivious not only to the server but also to fellow clients. This upgrade rules out many techniques for constructing ORAM, forcing us to pursue new techniques.
MCORAM not only provides an alternative solution to private anonymous data access (Eurocrypt 2019) but also serves as a promising building block for equipping oblivious file systems with access control and extending other advanced cryptosystems to the multi-client setting.
Despite being a powerful object, the current state-of-the-art is unsatisfactory: The only existing scheme requires $O(\sqrt n)$ communication and client computation for a database of size $n$. Whether it is possible to reduce these complexities to $\mathsf{polylog}(n)$, thereby matching the upper bounds for ORAM, is an open problem, i.e., can we enjoy access control and client-obliviousness under the same bounds?
Our first result answers the above question affirmatively by giving a construction from fully homomorphic encryption (FHE). Our main technical innovation is a new technique for cross-key trial evaluation of ciphertexts.
We also consider the same question in the setting with $N$ non-colluding servers, out of which at most $t$ of them can be corrupt. We build multi-server MCORAM from distributed point functions (DPF), and propose new constructions of DPF via a virtualization technique with bootstrapping, assuming the existence of homomorphic secret sharing and pseudorandom generators in NC0, which are not known to imply FHE.

2020

ASIACRYPT

SQISign: Compact Post-Quantum signatures from Quaternions and Isogenies
📺 Abstract

★ Best Paper Award

We introduce a new signature scheme, \emph{SQISign}, (for \emph{Short Quaternion and Isogeny Signature}) from isogeny graphs of supersingular elliptic curves. The signature scheme is derived from a new one-round, high soundness, interactive identification protocol. Targeting the post-quantum NIST-1 level of security, our implementation results in signatures of $204$ bytes, secret keys of $16$ bytes and public keys of $64$ bytes. In particular, the signature and public key sizes combined are an order of magnitude smaller than all other post-quantum signature schemes. On a modern workstation, our implementation in C takes 0.6s for key generation, 2.5s for signing, and 50ms for verification.
While the soundness of the identification protocol follows from classical assumptions, the zero-knowledge property relies on the second main contribution of this paper.
We introduce a new algorithm to find an isogeny path connecting two given supersingular elliptic curves of known endomorphism rings.
A previous algorithm to solve this problem, due to Kohel, Lauter, Petit and Tignol, systematically reveals paths from the input curves to a `special' curve. This leakage would break the zero-knowledge property of the protocol. Our algorithm does not directly reveal such a path, and subject to a new computational assumption, we prove that the resulting identification protocol is zero-knowledge.

2020

ASIACRYPT

Post-Quantum Verification of Fujisaki-Okamoto
📺 Abstract

We present a computer-verified formalization of the post-quantum
security proof of the Fujisaki-Okamoto transform (as analyzed by
Hövelmanns, Kiltz, Schäge, and Unruh, PKC 2020). The formalization is
done in quantum relational Hoare logic and checked in the qrhl-tool
(Unruh, POPL 2019).

2020

ASIACRYPT

Circular Security Is Complete for KDM Security
📺 Abstract

Circular security is the most elementary form of key-dependent message (KDM) security, which allows us to securely encrypt only a copy of secret key bits. In this work, we show that circular security is complete for KDM security in the sense that an encryption scheme satisfying this security notion can be transformed into one satisfying KDM security with respect to all functions computable by a-priori bounded-size circuits (bounded-KDM security). This result holds in the presence of any number of keys and in any of secret-key/public-key and CPA/CCA settings. Such a completeness result was previously shown by Applebaum (EUROCRYPT 2011) for KDM security with respect to projection functions (projection-KDM security) that allows us to securely encrypt both a copy and a negation of secret key bits.
Besides amplifying the strength of KDM security, our transformation in fact can start from an encryption scheme satisfying circular security against CPA attacks and results in one satisfying bounded-KDM security against CCA attacks. This result improves the recent result by Kitagawa and Matsuda (TCC 2019) showing a CPA-to-CCA transformation for KDM secure public-key encryption schemes.

2020

ASIACRYPT

Towards Efficiency-Preserving Round Compression in MPC: Do fewer rounds mean more computation?
📺 Abstract

Reducing the rounds of interaction in secure multiparty computation (MPC) protocols has been the topic of study of many works. One popular approach to reduce rounds is to construct {\em round compression compilers}. A round compression compiler is one that takes a highly interactive protocol and transforms it into a protocol with far fewer rounds. The design of round compression compilers has traditionally focused on preserving the security properties of the underlying protocol and in particular, not much attention has been given towards preserving their computational and communication efficiency. Indeed, the recent round compression compilers that yield round-optimal MPC protocols incur large computational and communication overhead.
In this work, we initiate the study of {\em efficiency-preserving} round compression compilers, i.e. compilers that translate the efficiency benefits of the underlying highly interactive protocols to the fewer round setting. Focusing on the honest majority setting (with near-optimal corruption threshold $\frac{1}{2} - \varepsilon$, for any $\varepsilon > 0$), we devise a new compiler that yields two round (i.e., round optimal) semi-honest MPC with similar communication efficiency as the underlying (arbitrary round) protocol. By applying our compiler on the most efficient known MPC protocols, we obtain a two-round semi-honest protocol based on one-way functions, with total communication (and per-party computation) cost $\widetilde{O}(s+n^4)$ -- a significant improvement over prior two-round protocols with cost $\widetilde{O}(n^\tau s+n^{\tau+1}d)$, where $\tau\geq 2$, $s$ is the size of the circuit computing the function and $d$ the corresponding depth. Our result can also be extended to handle malicious adversaries, either using stronger assumptions in the public key infrastructure (PKI) model, or in the plain model using an extra round.
An artifact of our approach is that the resultant protocol is ``unbalanced'' in the amount of computation performed by different parties. We give evidence that this is {\em necessary} in our setting. Our impossibility result makes novel use of the ``MPC-in-the-head" paradigm which has typically been used to demonstrate feasibility results.

2020

ASIACRYPT

Quantum Circuit Implementations of AES with Fewer Qubits
📺 Abstract

We propose some quantum circuit implementations of AES with the following improvements. Firstly, we propose some quantum circuits of the AES S-box and S-box$^{-1}$,which require fewer qubits than prior work. Secondly, we reduce the
number of qubits in the zig-zag method by introducing the S-box$^{-1}$ operation
in our quantum circuits of AES. Thirdly, we present a method to reduce the number of qubits in the key schedule of AES. While the previous quantum circuits of AES-128, AES-192, and AES-256 need at least 864, 896, and 1232 qubits respectively,our quantum circuit implementations of AES-128, AES-192, and AES-256 only require 512, 640, and 768 qubits respectively, where the number of qubits is reduced by more than 30\%.

2020

ASIACRYPT

ALBATROSS: publicly AttestabLe BATched Randomness based On Secret Sharing
📺 Abstract

In this paper we present ALBATROSS, a family of multiparty randomness generation protocols with guaranteed output delivery and public verification that allows to trade off corruption tolerance for a much improved amortized computational complexity. Our basic stand alone protocol is based on publicly verifiable secret sharing (PVSS) and is secure under in the random oracle model under the decisional Diffie-Hellman (DDH) hardness assumption. We also address the important issue of constructing Universally Composable randomness beacons, showing two UC versions of Albatross: one based on simple UC NIZKs and another one based on novel efficient ``designated verifier'' homomorphic commitments. Interestingly this latter version can be instantiated from a global random oracle under the weaker Computational Diffie-Hellman (CDH) assumption. An execution of ALBATROSS with $n$ parties, out of which up to $t=(1/2-\epsilon)\cdot n$ are corrupt for a constant $\epsilon>0$, generates $\Theta(n^2)$ uniformly random values, requiring in the worst case an amortized cost per party of $\Theta(\log n)$ exponentiations per random value. We significantly improve on the SCRAPE protocol (Cascudo and David, ACNS 17), which required $\Theta(n^2)$ exponentiations per party to generate one uniformly random value. This is mainly achieved via two techniques: first, the use of packed Shamir secret sharing for the PVSS; second, the use of linear $t$-resilient functions (computed via a Fast Fourier Transform-based algorithm) to improve the randomness extraction.

2020

ASIACRYPT

Oblivious Pseudorandom Functions from Isogenies
📺 Abstract

An oblivious PRF, or OPRF, is a protocol between a client and a server, where the server has a key $k$ for a secure pseudorandom function $F$, and the client has an input $x$ for the function. At the end of the protocol the client learns $F(k,x)$, and nothing else, and the server learns nothing. An OPRF is verifiable if the client is convinced that the server has evaluated the PRF correctly with respect to a prior commitment to $k$. OPRFs and verifiable OPRFs have numerous applications, such as private-set-intersection protocols, password-based key-exchange protocols, and defense against denial-of-service attacks. Existing OPRF constructions use RSA-, Diffie-Hellman-, and lattice-type assumptions. The first two are not post-quantum secure.
In this paper we construct OPRFs and verifiable OPRFs from isogenies. Our main construction uses isogenies of supersingular elliptic curves over $\Fpp$ and tries to adapt the Diffie-Hellman OPRF to that setting. However, a recent attack on supersingular-isogeny
systems due to Galbraith~et~al.~[ASIACRYPT 2016] makes this approach difficult to secure. To overcome this attack, and to validate the server's response, we develop two new zero-knowledge protocols that convince each party that its peer has sent valid messages. With these protocols in place, we obtain an OPRF in the SIDH setting and prove its security in the UC framework.
Our second construction is an adaptation of the Naor-Reingold PRF to commutative group actions. Combining it with recent constructions of oblivious transfer from isogenies, we obtain an OPRF in the CSIDH setting.

2020

ASIACRYPT

Non-Interactive Composition of Sigma-Protocols via Share-then-Hash
📺 Abstract

Proofs of partial knowledge demonstrate the possession of certain subsets of witnesses for a given collection of statements x_1,\dots,x_n.
Cramer, Damg{\aa}rd, and Schoenmakers (CDS), built proofs of partial knowledge, given "atomic" protocols for individual statements x_i, by having the prover randomly secret share the verifier's challenge and using the shares as challenges for the atomic protocols. This simple and highly-influential transformation has been used in numerous applications, ranging from anonymous credentials to ring signatures.
We consider what happens if, instead of using the shares directly as challenges, the prover first hashes them. We show that this elementary enhancement can result in significant benefits:
- the proof contains a {\em single} atomic transcript per statement x_i,
- it suffices that the atomic protocols are k-special sound for k \geq 2,
- when compiled using the Fiat-Shamir heuristic, the protocol retains its soundness in the {\em non-programmable} random oracle model.
None of the above features is satisfied by the CDS transformation.