International Association
for Cryptologic Research

Asynchronous verifiable secret sharing (AVSS) protocols protect a secret that is distributed among N parties. Dual-threshold AVSS protocols guarantee consensus in the presence of T Byzantine failures and privacy if fewer than P parties attempt to reconstruct the secret. In this work, we construct a dual-threshold AVSS protocol that is optimal along several dimensions. First, it is a high-threshold AVSS scheme, meaning that it is a dual-threshold AVSS with optimal parameters T < N/3 and P < 2N/3. Second, it has O(N^2) message complexity, and for large secrets it achieves the optimal O(N) communication overhead, without the need for a public key infrastructure or trusted setup. While these properties have been achieved individually before, to our knowledge this is the first protocol that is achieves all of the above simultaneously. The core component of our construction is a high-threshold AVSS scheme for small secrets based on polynomial commitments that achieves O(N^2 log(N)) communication overhead, as compared to prior schemes that require O(N^3) overhead with T<N/4 Byzantine failures or O(N^4) overhead for the recent high-threshold protocol of Kokoris-Kogias et al (CCS 2020). Using standard amortization techniques based on erasure coding, we can reduce the communication complexity to O(N*|F|) for a large secret F.

In this paper, we introduce FPPW, a new payment channel with watchtower scheme for Bitcoin. This new scheme provides fairness w.r.t. all channel participants including both channel parties and the watchtower. It means that the funds of any honest channel participant are safe even assuming that other two channel participants are corrupted and/or collude with each other. Furthermore, the watchtower in FPPW learns no information about the off-chain transactions and hence the channel balance privacy is preserved. As a byproduct, we also define the coverage of a watchtower scheme, that is the total capacity of channels that a watchtower can cover on a scale of 0 to 1, and show that FPPW's coverage is higher than those of PISA and Cerberus. The scheme can be implemented without any update in Bitcoin script.

We offer a public key exchange protocol based on a semidirect product of two cyclic (semi)groups of matrices over Z_p. One of the (semi)groups is additive, the other one multiplicative. This allows us to take advantage of both operations on matrices to diffuse information. We note that in our protocol, no power of any matrix or of any element of Z_p is ever exposed, so all standard attacks on Diffie-Hellman-like protocols (including Shor's quantum algorithm attack) are not applicable.

The IMDEA Software Institute offers a postdoc position in the area of cryptography. Topics of particular interest include (but are not limited to): secure computation (multiparty computation, homomorphic/functional encryption), zero knowledge proofs, and verifiable computation. The postdoc will work under the supervision of Dario Fiore and Ignacio Cascudo.

Who should apply?
Applicants should have (or be about to complete) a PhD in cryptography or a related topic.

Working at IMDEA Software
The position is based in Madrid, Spain where the IMDEA Software Institute is situated. Salaries are internationally competitive and include attractive conditions such as access to an excellent public healthcare system. The working language at the institute is English. Knowledge of Spanish is not required.

Dates
The position has guaranteed funding for at least 2 years. The starting date is flexible with a preference in mid 2021.

How to apply?
Applicants interested in the position should submit their application at https://careers.software.imdea.org/ using reference code 2021-02-postdoc-cryptoprimitives.
Deadline for applications is February 28th, 2021.
We encourage early applications and review of applications will begin immediately.

Closing date for applications:

Contact: Dario Fiore (dario.fiore (at) imdea.org) and Ignacio Cascudo (ignacio.cascudo (at) imdea.org)

More information: https://careers.software.imdea.org/postdoc/2021-02-postdoc-cryptoprimitives/

Event date: 13 December to 15 December 2021
Submission deadline: 1 June 2021
Notification: 1 October 2021

Event date: 5 December to 9 December 2021

Abstract The Supersingular Isogeny Key Encapsulation mechanism (SIKE) is the only post-quantum key encapsulation mechanism based on supersingular elliptic curves and isogenies between them. Despite the security of the protocol, unlike the rest of the NIST post-quantum algorithms, SIKE requires more number of clock cycles and hence does not provide competitive timing, energy and power consumption results. However, it is more attractive offering smallest public key sizes as well as ciphertext sizes, which taking into account the impact of the communication costs and storage of the keys could become as good fit for resource-constrained devices. In this work, we present the fastest practical implementation of SIKE, targeting the platform Cortex-M4 based on the ARMv7-M architecture. We performed our measurements on NIST recommended device based on STM32F407 microcontroller, for benchmarking the clock cycles, and on the target board Nucleo-F411RE, attached to X-NUCLEO-LPM01A (Power Shield), for measuring the power and energy consumption. The lower level finite field arithmetic and extension field operations play main role determining the efficiency of SIKE. Therefore, we mainly focus on those improvements and apply them to all NIST required security levels. Our SIKEp434 implementations for NIST security level 1 take about 850ms which is about 22.3% faster than the counterparts appeared in previous work. Moreover, our implementations are 21.9%, 19.7% and 19.5% faster for SIKEp503, SIKEp610 and SIKEp751 in comparison to the previously reported work for other NIST recommended security levels. Finally, we benchmark power and energy consumption and report the results for comparison.

In response to standardization requests regarding password-authenticated key exchange (PAKE) protocols, the IRTF working group CFRG has setup a PAKE selection process in 2019, which led to the selection of the CPace protocol in the balanced setting, in which parties share a common password.

In this paper, we provide a security analysis of CPace in the universal composability framework for implementations on elliptic-curve groups. When doing so, we restrict the use of random oracles to hash functions only and refrain from modeling CPace's MapToPoint function that maps field elements to curve points as an idealized function. As a result, CPace can be proven secure under standard complexity assumptions in the random-oracle model.

Finally, in order to extend our proofs to different CPace variants optimized for specific environments, we employ a new approach, which represents the assumptions required by the proof as libraries which a simulator can access. By allowing for the modular replacement of assumptions used in the proof, this new approach avoids a repeated analysis of unchanged protocol parts and lets us efficiently analyze the security guarantees of all the different CPace variants.

Secure multi-party computation (MPC) allows a set of n servers to jointly compute an arbitrary function of their inputs, without revealing these inputs to each other. A (k,n) threshold secret sharing is a protocol in which a single secret is divided into n shares and the secret can be recovered from a threshold k shares. Typically, multiplication of (k,n) secret sharing will result in increase of polynomial degree from k-1 to 2k-2, thus increasing the number of shares required from k to 2k-1. Since each server typically hold only one share, the number of servers required in MPC will also increase from k to 2k-1. Therefore, a set of n servers can compute multiplication securely if the adversary corrupts at most k-1<n/2 of the servers. In this paper, we differentiate the number of servers N required and parameter n of (k,n) secret sharing scheme, and propose a method of computing (k-1) sharing of multiplication ab by using only N=k servers. By allowing each server to hold two shares, we realize MPC of multiplication with the setting of N=k,n&#8805;2k-1. We also show that our proposed method is information theoretic secure against a semi-honest adversary.

Multi-Party Non-Interactive Key Exchange (MP-NIKE) is a fundamental cryptographic primitive in which users register into a key generation centre and receive a public/private key pair each. After that, any subset of these users can compute a shared key without any interaction. Nowadays, IoT devices suffer from a high number and large size of messages exchanged in the Key Management Protocol (KMP). To overcome this, an MP-NIKE scheme can eliminate the airtime and latency of messages transferred between IoT devices. MP-NIKE schemes can be realized by using multilinear maps. There are several attempts for constructing multilinear maps based on indistinguishable obfuscation, lattices and the Chinese Remainder Theorem (CRT). Nevertheless, these schemes are inefficient in terms of computation cost and memory overhead. Besides, several attacks have been recently reported against CRT-based and lattice-based multilinear maps. There is only one modular exponentiation-based MP-NIKE scheme in the literature which has been claimed to be both secure and efficient. In this article, we present an attack on this scheme based on the Euclidean algorithm, in which two colluding users can obtain the shared key of any arbitrary subgroup of users. We also propose an efficient and secure MP-NIKE scheme. We show how our proposal is secure in the random oracle model assuming the hardness of the root extraction modulo a composite number.

Even though the currently used encryption and signature schemes are well tested and secure in a classical computational setting, they are not quantum-resistant as Shor's work proves. Taking this into account, alternatives based on hard mathematical problems that cannot be solved using quantum methods are needed, and lattice-based cryptography offers such solutions. The well-known GGH and NTRUEncrypt encryption schemes are proven secure, but their corresponding signature schemes are flawed in their design approach. Once introducing the computationally hard problems like Ring-LWE, elegant and efficient cryptographic primitives could be built.

Security of cryptographic primitives is usually proved by assuming ``ideal'' probability distributions. We need to replace them with approximated ``real'' distributions in the real-world systems without losing the security level. We demonstrate that the Hellinger distance is useful for this problem, while the statistical distance is mainly used in the cryptographic literature. First, we show that for preserving $\lambda$-bit security of a given security game, the closeness of $2^{-\lambda/2}$ to the ideal distribution is sufficient for the Hellinger distance, whereas $2^{-\lambda}$ is generally required for the statistical distance. The result can be applied to both search and decision primitives through the bit security framework of Micciancio and Walter (Eurocrypt 2018). We also show that the Hellinger distance gives a tighter evaluation of closeness than the max-log distance when the distance is small. Finally, we show that the leftover hash lemma can be strengthened to the Hellinger distance. Namely, a universal family of hash functions gives a strong randomness extractor with optimal entropy loss for the Hellinger distance. Based on the results, a $\lambda$-bit entropy loss in randomness extractors is sufficient for preserving $\lambda$-bit security. The current understanding based on the statistical distance is that a $2\lambda$-bit entropy loss is necessary.

Due to high IC design costs and emergence of countless untrusted foundries, logic encryption has been taken into consideration more than ever. In state-of-the-art logic encryption works, a lot of performance is sold to guarantee security against both the SAT-based and the removal attacks. However, the SAT-based attack cannot decrypt the sequential circuits if the scan chain is protected or if the unreachable states encryption is adopted. Instead, these security schemes can be defeated by the model checking attack that searches iteratively for different input sequences to put the activated IC to the desired reachable state. In this paper, we propose a practical logic encryption approach to defend against the model checking attack on sequential circuits. The robustness of the proposed approach is demonstrated by experiments on around fifty benchmarks.

In July 2020, the lattice-based CRYSTALS-Dilithium digital signature scheme has been chosen as one of the three third-round finalists in the post-quantum cryptography standardization process by the National Institute of Standards and Technology (NIST). In this work, we present the first Very High Speed Integrated Circuit Hardware Description Language (VHDL) implementation of the CRYSTALS-Dilithium signature scheme for Field-Programmable Gate Arrays (FPGAs). Due to our parallelization-based design requiring only low numbers of cycles, running at high frequency and using reasonable amount of hardware resources on FPGA, our implementation is able to sign 15832 messages per second and verify 10524 signatures per second. In particular, the signing algorithm requires 68461 Look-Up Tables (LUTs), 86295 Flip-Flops (FFs), and the verification algorithm takes 61738 LUTs and 34963 FFs on Virtex 7 UltraScale+ FPGAs. In this article, experimental results for each Dilithium security level are provided and our VHDL-based implementation is compared with related High-Level Synthesis (HLS)-based implementations. Our solution is ca 114 times faster (in the signing algorithm) and requires less hardware resources.

Gun violence results in a significant number of deaths in the United States. Starting in the 1960’s, the US Congress passed a series of gun control laws to regulate the sale and use of firearms. One of the most important but politically fraught gun control measures is a national gun registry. A US Senate office is currently drafting legislation that proposes the creation of a voluntary national gun registration system. At a high level, the bill envisions a decentralized system where local county officials would control and manage the registration data of their constituents. These local databases could then be queried by other officials and law enforcement to trace guns. Due to the sensitive nature of this data, however, these databases should guarantee the confidentiality of the data.

In this work, we translate the high-level vision of the proposed legislation into technical requirements and design a cryptographic protocol that meets them. Roughly speaking, the protocol can be viewed as a decentralized system of locally-managed end-to-end encrypted databases. Our design relies on various cryptographic building blocks including structured encryption, secure multi-party computation and secret sharing. We propose a formal security definition and prove that our design meets it. We implemented our protocol and evaluated its performance empirically at the scale it would have to run if it were deployed in the United States. Our results show that a decentralized and end-to-end encrypted national gun registry is not only possible in theory but feasible in practice.

Event date: 28 June to 1 July 2021
Submission deadline: 18 March 2021
Notification: 29 April 2021

The Services and Cybersecurity (SCS) group at the University of Twente invites applications for a 4-years PhD position on the topic of 'cryptographic protocols for privacy-preserving machine learning'.

We are looking for candidates with a strong background in (applied) cryptography.

More information:
https://www.utwente.nl/en/organisation/careers/!/2021-218/phd-position-on-cryptographic-protocols-for-privacy-preserving-machine-learning

Deadline for applications: 11 February 2021, 23:59 CET

Closing date for applications:

Contact: Prof. Dr. Andreas Peter (a.peter@utwente.nl)

More information: https://www.utwente.nl/en/organisation/careers/!/2021-218/phd-position-on-cryptographic-protocols-for-privacy-preserving-machine-learning

The IACR Test-of-Time Award is given annually for each one of the three IACR General Conferences (Eurocrypt, Crypto, and Asiacrypt). An award will be given at a conference for a paper which has had a lasting impact on the field and was published 15 years prior.

We welcome nominations for the 2021 award (for papers published in 2006) until Feb 20, 2021. The proceedings of these conferences can be found here:

• Asiacrypt 2006
• Crypto 2006
• Eurocrypt 2006

To submit your nomination please send an email to testoftime@iacr.org

More information about the IACR Test-of-Time awards can be found in iacr.org/testoftime/

The 2021 Selection Committee:

• Ueli Maurer (chair)
• Nigel Smart
• Francois-Xavier Standaert (Eurocrypt 2021 program co-chair)
• Chris Peikert (Crypto 2021 program co-chair)
• Mehdi Tibouchi (Asiacrypt 2021 program co-chair)

For security token adoption by financial institutions and industry players on the blockchain, there is a need for a secure asset management protocol that enables condential asset issuance and transfers by concealing from the public the transfer amounts and asset types, while on a public blockchain. Flexibly supporting arbitrary restrictions on financial transactions, only some of which need to be supported by zero-knowledge proofs. This paper proposes leveraging a hybrid design approach, by using zero-knowledge proofs, supported by restrictions enforced by trusted mediators. As part of our protocol, we also describe a novel transaction ordering mechanism that can support a flexible transaction workflow without putting any timing constraints on when the transactions should be generated by the users or processed by the network validators. This technique is likely to be of independent interest.

So far, most of the Identity-Based Encryption (IBE) schemes have been realized by employing bilinear pairings, lattices, trapdoor discrete logarithm, or based on the quadratic residue problem. Among the IBE schemes, only pairing-based methods seem to be practical. Previously published non-pairing-based schemes are generally inefficient in encryption, decryption, key generation, ciphertext size or key size. In this paper, we propose an IBE scheme based on a hybrid of Diffie-Hellman and RSA-like hardness assumption. The computational cost of the proposed scheme is lower than the previous schemes and the ciphertext size for an $l$-bit plaintext is only $2l$ bits. The proposed scheme is similar to the well-known ElGamal encryption algorithm; therefore it might be used in applications such as oblivious computation.