International Association
for Cryptologic Research

Blockchain sharding is a promising approach to solving the dilemma between decentralisation and high performance (transaction throughput) for blockchain. The main challenge of Blockchain sharding systems is how to reach a decision on a statement among a sub-group (shard) of people while ensuring the whole population recognises this statement. Namely, the challenge is to prevent an adversary who does not have the majority of nodes globally but have the majority of nodes inside a shard. Most Blockchain sharding approaches can only reach a correct consensus inside a shard with at most $n/3$ evil nodes in a $n$ node system. There is a blockchain sharding approach which can prevent an incorrect decision to be reached when the adversary does not have $n/2$ nodes globally. However, the system can be stopped from reaching consensus (become deadlocked) if the adversary controls a smaller number of nodes.

In this paper, we present an improved Blockchain sharding approach that can withstand $n/2$ adversarial nodes and recover from deadlocks. The recovery is made by dynamically adjusting the number of shards and the shard size. A performance analysis suggests our approach has a high performance (transaction throughput) while requiring little bandwidth for synchronisation.

Multi-valued Byzantine Consensus (BC), in which $n$ processes must reach agreement on a single $L$-bit value, is an essential primitive in the design of distributed cryptographic protocols and fault-tolerant distributed systems. One of the most desirable traits for a multi-valued BC protocol is to be error-free. In other words, have zero probability of producing incorrect results.

The most efficient error-free multi-valued BC protocols are built as extension protocols, which reduce agreement on large values to agreement on small sequences of bits whose lengths are independent of $L$. The best extension protocols achieve $\mathcal{O}(Ln)$ communication complexity, which is optimal, when $L$ is large relative to $n$. Unfortunately, all known error-free and communication-optimal BC extension protocols require each process to broadcast at least $n$ bits with a binary Byzantine Broadcast (BB) protocol. This design limits the scalability of these protocols to many processes, since when $n$ is large, the binary broadcasts significantly inflate the overall number of bits communicated by the extension protocol.

In this paper, we present Byzantine Consensus with Parallel Execution (BCPE), the first error-free and communication-optimal BC extension protocol in which each process only broadcasts a single bit with a binary BB protocol. BCPE is a synchronous and deterministic protocol, and tolerates $f < n/3$ faulty processes (the best resilience possible). Our evaluation shows that BCPE's design makes it significantly more scalable than the best existing protocol by Ganesh and Patra. For 1,000 processes to agree on 2 MB of data, BCPE communicates $10.92\times$ fewer bits. For agreement on 10 MB of data, BCPE communicates $6.97\times$ fewer bits. BCPE also matches the best existing protocol in all other standard efficiency metrics.

We present a domain-specific co-processor to speed up Saber, a post-quantum key encapsulation mechanism competing on the NIST Post-Quantum Cryptography standardization process. Contrary to most lattice-based schemes, Saber doesn’t use NTT-based polynomial multiplication. We follow a hardware-software co-design approach: the execution is performed on an ARM core and only the most computationally expensive operation, i.e., polynomial multiplication, is offloaded to the co-processor to obtain a compact design. We exploit the idea of distributed computing at micro-architectural level together with novel algorithmic optimizations to achieve approximately a 6 times speedup with respect to optimized software at a small area cost.

Protocols for password authenticated key exchange (PAKE) allow two parties who share only a weak password to agree on a cryptographically strong key. We revisit the notion of PAKE in the framework of universal composability, and propose a relaxation of the PAKE functionality of Canetti et al. that we call lazy-extraction PAKE (lePAKE). Roughly, our relaxation allows the ideal-world adversary to postpone its password guess even until after a session is complete. We argue that this relaxed notion still provides meaningful security in the password-only setting. As our main result, we show that several PAKE protocols that were previously only proven secure with respect to a "game-based" definition can in fact be shown to realize the lePAKE functionality in the random-oracle model. These include SPEKE, SPAKE2, and TBPEKE, the most efficient PAKE schemes currently known.

The $k$-nearest neighbors ($k$NN) classifier predicts a class of a query, $q$, by taking the majority class of its $k$ neighbors in an existing (already classified) database, $S$. In secure $k$NN, $q$ and $S$ are owned by two different parties and $q$ is classified without sharing data. In this work we present a classifier based on $k$NN, that is more efficient to implement with homomorphic encryption (HE). The efficiency of our classifier comes from a relaxation we make to consider $\kappa$ nearest neighbors for $\kappa\approx k$ with probability that increases as the statistical distance between Gaussian and the distribution of the distances from $q$ to $S$ decreases. We call our classifier $k$-ish Nearest Neighbors ($k$-ish NN). For the implementation we introduce {\em double-blinded coin-toss} where the bias and output of the toss are encrypted. We use it to approximate the average and variance of the distances from $q$ to $S$ in a scalable circuit whose depth is independent of $|S|$. We believe these to be of independent interest. We implemented our classifier in an open source library based on HElib and tested it on a breast tumor database. Our classifier has accuracy and running time comparable to current state of the art (non-HE) MPC solution that have better running time but worse communication complexity. It also has communication complexity similar to naive HE implementation that have worse running time.

We present a new general framework for constructing compact and adaptively secure attribute-based encryption (ABE) schemes from $k$-Lin in asymmetric bilinear pairing groups. Previously, the only construction [Kowalczyk and Wee, Eurocrypt '19] that simultaneously achieves compactness and adaptive security from static assumptions supports policies represented by Boolean formulae. Our framework enables supporting more expressive policies represented by arithmetic branching programs.

Our framework extends to ABE for policies represented by uniform models of computation such as Turing machines. Such policies enjoy the feature of being applicable to attributes of arbitrary lengths. We obtain the first compact adaptively secure ABE for deterministic and non-deterministic finite automata (DFA and NFA) from $k$-Lin, previously unknown from any static assumptions. Beyond finite automata, we obtain the first ABE for large classes of uniform computation, captured by deterministic and non-deterministic logspace Turing machines (the complexity classes $\mathsf{L}$ and $\mathsf{NL}$) based on $k$-Lin. Our ABE scheme has compact secret keys of size linear in the description size of the Turing machine $M$. The ciphertext size grows linearly in the input length, but also linearly in the time complexity, and exponentially in the space complexity. Irrespective of compactness, we stress that our scheme is the first that supports large classes of Turing machines based solely on standard assumptions. In comparison, previous ABE for general Turing machines all rely on strong primitives related to indistinguishability obfuscation.

Mathematically-secure cryptographic algorithms leak significant side-channel information through their power supplies when implemented on a physical platform. These side-channel leakages can be exploited by an attacker to extract the secret key of an embedded device. The existing state-of-the-art countermeasures mainly focus on the power balancing, gate-level masking, or signal-to-noise (SNR) reduction using noise injection and signature attenuation, all of which suffer either from the limitations of high power/area overheads, performance degradation or are not synthesizable. In this article, we propose a generic low-overhead digital-friendly power SCA countermeasure utilizing physical Time-Varying Transfer Functions (TVTF) by randomly shuffling distributed switched capacitors to significantly obfuscate the traces in the time domain. System-level simulation results of the TVTF-AES implemented in TSMC 65nm CMOS technology show > 4000x MTD improvement over the unprotected implementation with ~ 1.25x power and ~ 1.2x area overheads, and without any performance degradation.

A software watermarking scheme provides a method to prevent unauthorized distribution of software. Specifically, watermarking schemes allow a user to embed an identifier into a piece of code such that the resulting program is nearly functionally-equivalent to the original program, and yet, it is difficult to remove the identifier without destroying the functionality of the program. Existing constructions of watermarking have focused primarily on watermarking pseudorandom functions (PRFs).

In this work, we revisit the definitional foundations of watermarking, and begin by highlighting a major limitation in the existing security notion. Currently, the security requirement in watermarking schemes only requires that the identifier be successfully extracted from programs that preserve the exact input/output behavior of the original program. In the context of PRFs, this means that an adversary that constructs a program which computes a quarter of the output bits of the PRF or that is able to distinguish the outputs of the PRF from random are considered to be outside the threat model. However, in any application (e.g., watermarking a decryption device or an authentication token) that relies on PRF security, an adversary that manages to predict a quarter of the bits or distinguishes the PRF outputs from random would be considered to have defeated the scheme. Thus, existing watermarking schemes provide very little, if any, security guarantee against realistic adversaries. We emphasize that none of the existing constructions of watermarkable PRFs would be able to extract the identifier from a program that only outputs a quarter of the bits of the PRF or one that perfectly distinguishes.

To address the shortcomings in existing watermarkable PRF definitions, we introduce a new primitive called a traceable PRF. Our definitions are inspired by similar definitions from public-key traitor tracing, and aim to capture a very robust set of adversaries: namely, any adversary that produces a useful distinguisher (i.e., a program that can break PRF security), can be traced to a specific identifier. We provide a general framework for constructing traceable PRFs via an intermediate primitive called private linear constrained PRFs. Finally, we show how to construct traceable PRFs from a similar set of assumptions previously used to realize software watermarking. Namely, we obtain a single-key traceable PRF from standard lattice assumptions and a fully collusion-resistant traceable PRF from indistinguishability obfuscation (together with injective one-way functions).

We present a protocol for checking the values of a committed polynomial $f\in \mathbb{F}_{<n}[X]$ over a multiplicative subgroup $H\subset \mathbb{F}$ of size $n$, are contained in the values of a table $t\in \mathbb{F}^d$. Our protocol can be viewed as a simplification of one from Bootle et. al [BCGJM, ASIACRYPT 2018] for a similar problem, with potential efficiency improvements when $d\leq n$. In particular, [BCGJM]'s protocol requires comitting to several auxiliary polynomials of degree $d\cdot \log n$, whereas ours requires three commitments to auxiliary polynomials of degree $n$, which can be much smaller in the case $d\sim n$. One common use case of this primitive in the zk-SNARK setting is a ``batched range proof'', where one wishes to check all of $f$'s values on $H$ are in a range $[0,\ldots,M]$. We present a slightly optimized protocol for this special case, and pose improving it as an open problem.

We have proposed before a multivariate public key cryptosystem (MPKC) that does not rely on the difficulty of prime factorization, and whose modulus is the product of many small prime numbers. In this system, the prime factorization by the attackers is self-trivial, and the structure of the secret key is based on CRT (Chinese Remainder Theorem). In this paper we propose MPKC with security of IND-CPA by adding random numbers to central transformation vectors in the system proposed before.

We show that a slight variant of Protocol $\mathit{SPAKE2}+$, which was presented but not analyzed in Cash, Kiltz, and Shoup (2008) is a secure asymmetric password-authenticated key exchange protocol (PAKE), meaning that the protocol still provides good security guarantees even if a server is compromised and the password file stored on the server is leaked to an adversary. The analysis is done in the UC framework (i.e., a simulation-based security model), under the computational Diffie-Hellman (CDH) assumption, and modeling certain hash functions as random oracles. The main difference between our variant and the original Protocol~$\mathit{SPAKE2}+$ is that our variant includes standard key confirmation flows; also, adding these flows allows some slight simplification to the remainder of the protocol.

Along the way, we also: provide the first proof (under the same assumptions) that a slight variant of Protocol $\mathit{SPAKE2}$ from Abdalla and Pointcheval (2005) is a secure symmetric PAKE in the UC framework (previous security proofs were all in the weaker BPR framework of Bellare, Pointcheval, and Rogaway (2000); provide a proof (under very similar assumptions) that a variant of Protocol $\mathit{SPAKE2}+$ that is currently being standardized is also a secure asymmetric PAKE; repair several problems in earlier UC formulations of secure symmetric and asymmetric PAKE.

Confidential transactions are used in distributed digital assets to demonstrate the balance of values hidden in commitments, while retaining signer ambiguity. Previous work describes a signer-ambiguous proof of knowledge of the opening of commitments to zero at the same index across multiple public commitment sets and the evaluation of a verifiable random function used as a linking tag, and uses this to build a linkable ring signature called Triptych that can be used as a building block for a confidential transaction model. In this work, we extend Triptych to build Triptych-2, a proving system that proves knowledge of openings of multiple commitments to zero within a single set, correct construction of a verifiable random function evaluated at each opening, and value balance across a separate list of commitments within a single proof. While soundness depends on a novel dual discrete-logarithm hardness assumption, we use data from the Monero blockchain to show that Triptych-2 can be used in a confidential transaction model to provide faster total batch verification time than other state-of-the-art constructions without a trusted setup.

Event date: 23 September to 25 September 2020
Submission deadline: 11 May 2020
Notification: 3 July 2020

Dear IACR member,

Recent developments related to the outbreak of the novel coronavirus (COVID-19) and the increasing number of travel restrictions that have been put in place have forced us to make the following decisions regarding the schedule of forthcoming IACR conferences:

• FSE 2020, which was supposed to be held in Athens, Greece, during 22-26 March 2020, has been postponed to 8-12 November 2020;
• PKC 2020, which was supposed to be held in Edinburgh, Scotland, during 4-7 May 2020, has been postponed; and
• EUROCRYPT 2020, which was supposed to be held in Zagreb, Croatia, during 10-14 May 2020, has been postponed.

We are currently considering several rescheduling options for EUROCRYPT 2020 and PKC 2020 and we will be informing the membership and attendees about these changes as soon as possible via the IACR news system and other appropriate communication channels.

The publication schedule of these conferences has not been altered and authors will have the option to record a presentation that would go online on the website of these conferences. Registered attendees have been offered either a full refund or the option to transfer the registration fee to the postponed version.

No changes have been made at this time to the schedule of CRYPTO 2020, CHES 2020, TCC 2020, and ASIACRYPT 2020, but we will continue to closely monitor the situation and will inform members if changes are needed.

The FSE steering committee and the general chairs of FSE 2020, have decided to reschedule FSE 2020 to November 8-12 2020, at the same venue.

News and updates will be posted on the conference website https://fse.iacr.org/2020/

For any questions please contact the General Chairs at fse2020@iacr.org

Many blockchain researches focus on the privacy protection. However, criminals can leverage strong privacy protection of the blockchain to do illegal crimes (such as ransomware) without being punished. These crimes have caused huge losses to society and users. Implementing identity tracing is an important step in dealing with issues arising from privacy protection. In this paper, we propose a blockchain traceable scheme with oversight function (BTSOF). The design of BTSOF builds on SkyEye (Tianjun Ma et al., Cryptology ePrint Archive 2020). In BTSOF, the regulator must obtain the consent of the committee to enable tracing. Moreover, we construct a non-interactive verifiable multi-secret sharing scheme (VMSS scheme) and leverage the VMSS scheme to design a distributed multi-key generation (DMKG) protocol for the Cramer-Shoup public key encryption scheme. The DMKG protocol is used in the design of BTSOF. We provide the security definition and security proof of the VMSS scheme and DMKG protocol.

Recent works in side-channel analysis have been fully relying on training classification models to recover sensitive information from traces. However, the knowledge of an attacker or an evaluator is not taken into account and poorly capturedby solely training a classifier on signals. This paper proposes to inject prior information in preprocessing and classification in order to increase the performance of side-channel attacks (SCA). First wepropose to use the Wavelet Scattering Transform, recently proposed by Mallat, for mapping traces into a time-frequency space which is stable under small translation and diffeomorphism. That way, we address the issues of desynchronization and deformation generally present in signals for SCA. The second part of our paper extends the canonical attacks over byteand Hamming weight by introducing a more general attack. Classifiers are trained on different labelings of the sensitive variable and combined by minimizing a cross-entropy criterion so as to find the best labeling strategy. With these two key ideas, we successfully increase the performance of Template Attacks on artificially desynchronized traces and signals from a jitter-protected implementation.

Spook is one of the 32 candidates that has made it to the second round of the NIST Lightweight Cryptography Standardization process, and is particularly interesting since it proposes differential side channel resistance. In this paper, we present practical distinguishers of the full 6-step version of the underlying permutations of Spook, namely Shadow-512 and Shadow-384, solving challenges proposed by the designers on the permutation. We also propose practical forgeries with 4-step Shadow for the S1P mode of operation in the nonce misuse scenario, which is allowed by the CIML2 security game considered by the authors. All the results presented in this paper have been implemented.

We present our integration of post-quantum cryptography (PQC), more specifically of the post-quantum KEM scheme Kyber for key establishment and the post-quantum signature scheme SPHINCS$^+$, into the embedded TLS library mbed TLS. We measure the performance of these post-quantum primitives on four different embedded platforms with three different ARM processors and an Xtensa LX6 processor. Furthermore, we compare the performance of our experimental PQC cipher suite to a classical TLS variant using elliptic curve cryptography (ECC). Post-quantum key establishment and signature schemes have been either integrated into TLS or ported to embedded devices before. However, to the best of our knowledge, we are the first to combine TLS, post-quantum schemes, and embedded systems and to measure and evaluate the performance of post-quantum TLS on embedded platforms. Our results show that post-quantum key establishment with Kyber performs well in TLS on embedded devices compared to ECC variants. The use of SPHINCS$^+$ signatures comes with certain challenges in terms of signature size and signing time, which mainly affects the use of embedded systems as PQC-TLS server but does not necessarily prevent embedded systems to act as PQC-TLS clients.

We characterize the ANF and the univariate representation of any vectorial function as parts of the ANF and bivariate representation of the Boolean function equal to its graph indicator. We show how this provides, when $F$ is bijective, the expression of $F^{-1}$ and/or allows deriving properties of $F^{-1}$. We illustrate this with examples and with a tight upper bound on the algebraic degree of $F^{-1}$ by means of that of $F$. We characterize by the Fourier-Hadamard transform, by the ANF, and by the bivariate representation, that a given Boolean function is the graph indicator of a vectorial function. We also give characterizations of those Boolean functions that are affine equivalent to graph indicators. We express the graph indicators of the sum, product, composition and concatenation of vectorial functions by means of the graph indicators of the functions. We deduce from these results a characterization of the bijectivity of a generic $(n,n)$-function by the fact that some Boolean function, which appears as a part of the ANF (resp. the bivariate representation) of its graph indicator, is equal to constant function 1. We also address the injectivity of $(n,m)$-functions. Finally, we study the characterization of the almost perfect nonlinearity of vectorial functions by means of their graph indicators.