International Association
for Cryptologic Research

In 1993, Benaloh and De Mare introduced cryptographic accumulator, a primitive that allows the representation of a set of values by a short object (the accumulator) and offers the possibility to prove that some input values are in the accumulator. For this purpose, so-called asymmetric accumulators require the creation of an additional cryptographic object, called a witness. Through the years, several instantiations of accumulators were proposed either based on number theoretic assumptions, hash functions, bilinear pairings or more recently lattices. In this work, we present the first instantiation of an asymmetric cryptographic accumulator that allows private computation of the accumulator but public witness creation. This is obtained thanks to our unique combination of the pairing based accumulator of Nguyen with dual pairing vector spaces. We moreover introduce the new concept of dually computable cryptographic accumulators, in which we offer two ways to compute the representation of a set: either privately (using a dedicated secret key) or publicly (using only the scheme's public key), while there is a unique witness creation for both cases. All our constructions of accumulators have constant size accumulated value and witness, and satisfy the accumulator security property of collision resistance, meaning that it is not possible to forge a witness for an element that is not in the accumulated set. As a second contribution, we show how our new concept of dually computable cryptographic accumulator can be used to build a Ciphertext Policy Attribute Based Encryption (CP-ABE). Our resulting scheme permits policies expressed as disjunctions of conjunctions (without ``NO'' gates), and is adaptively secure in the standard model. This is the first CP-ABE scheme having both constant-size user secret keys and ciphertexts (i.e. independent of the number of attributes in the scheme, or the policy size). For the first time, we provide a way to use cryptographic accumulators for both key management and encryption process.

We initiate the study of witness authenticating NIZK proof systems (waNIZKs), in which one can use a witness $w$ of a statement $x$ to identify whether a valid proof for $x$ is indeed generated using $w$. Such a new identification functionality enables more diverse applications, and it also puts new requirements on soundness that: (1) no adversary can generate a valid proof that will not be identified by any witness; (2) or forge a proof using some valid witness to frame others. To work around the obvious obstacle towards conventional zero-knowledgeness, we define entropic zero-knowledgeness that requires the proof to leak no partial information, if the witness has sufficient computational entropy. We give a formal treatment of this new primitive. The modeling turns out to be quite involved and multiple subtle points arise and particular cares are required. We present general constructions from standard assumptions. We also demonstrate three applications in non-malleable (perfectly one-way) hash, group signatures with verifier-local revocations and plaintext-checkable public-key encryption. Our waNIZK provides a new tool to advance the state of the art in all these applications.

The Fast IDentity Online (FIDO) Alliance develops open standards to replace password-based authentication methods by token-based solutions. The latest protocol suite FIDO2 provides such a promising alternative which many key players have already adopted or are willing to. The central authentication mechanism WebAuthn uses cryptographic keys stored on the device to authenticate clients to a relying party via a challenge-response protocol. Yet, this approach leaves several open issues about post-quantum secure instantiations and methods for recovery of credentials. Recently Frymann et al. (CCS 2020, ACNS 2023, EuroS&P 2023) made significant progress to advance the security of FIDO2 sys- tems. Following a suggestion by device manufacturer Yubico, they considered a WebAuthn-compliant mechanism to store recovery information at the relying party. If required, the client can recover essential data with the help of a backup authenticator device. They analyzed the Diffie-Hellman based scheme, showing that it provides basic authentication and privacy features. One of their solutions also provides a post-quantum secure variant, but only for a weaker version of authentication security. Our starting point is to note that the security definitions of Fry- mann et al., especially the privacy notion, do not seem to capture real threats appropriately. We thus strengthen the notions. De- spite this strengthening, we show a generic construction based on (anonymous) KEMs and signature schemes. It follows that, us- ing post-quantum secure instances, like Kyber and Dilitihium, one immediately obtains a post-quantum and strongly secure solution.

Attribute-based encryption (ABE) is a popular type of public-key encryption that enforces access control cryptographically, and has spurred the proposal of many use cases. To satisfy the requirements of the setting, tailor-made schemes are often introduced. However, designing secure schemes---as well as verifying that they are secure---is notoriously hard. Several of these schemes have turned out to be broken, making them dangerous to deploy in practice.

To overcome these shortcomings, we introduce ACABELLA. ACABELLA simplifies generating and verifying security proofs for pairing-based ABE schemes. It consists of a framework for security proofs that are easy to verify manually and an automated tool that efficiently generates these security proofs. Creating such security proofs generally takes no more than a few seconds. The output is easy to understand, and the proofs can be verified manually. In particular, the verification of a security proof generated by ACABELLA boils down to performing simple linear algebra.

The ACABELLA tool is open source and also available via a web interface. With its help, experts can simplify their proof process by verifying or refuting the security claims of their schemes and practitioners can get an assurance that the ABE scheme of their choice is secure.

We study the problem of committee selection in the context of proof-of-stake consensus mechanisms or distributed ledgers. These settings determine a family of participating parties---each of which has been assigned a non-negative "stake"---and are subject to an adversary that may corrupt a subset of the parties. The challenge is to select a committee of participants that accurately reflects the proportion of corrupt and honest parties, as measured by stake, in the full population. The trade-off between committee size and the probability of selecting a committee that over-represents the corrupt parties is a fundamental factor in both security and efficiency of proof-of-stake consensus, as well as committee-run layer-two protocols.

We propose and analyze several new committee selection schemes that improve upon existing techniques by adopting low-variance assignment of certain committee members that hold significant stake. These schemes provide notable improvements to the size--security trade-off arising from the stake distributions of many deployed ledgers.

Liskov, Rivest and Wagner laid the theoretical foundations for tweakable block ciphers (TBC). In a seminal paper, they proposed two (up to) birthday-bound secure design strategies --- LRW1 and LRW2 --- to convert any block cipher into a TBC. Several of the follow-up works consider cascading of LRW-type TBCs to construct beyond-the-birthday bound (BBB) secure TBCs. Landecker et al. demonstrated that just two-round cascading of LRW2 can already give a BBB security. Bao et al. undertook a similar exercise in context of LRW1 with TNT --- a three-round cascading of LRW1 --- that has been shown to achieve BBB security as well. In this paper, we present a CCA distinguisher on TNT that achieves a non-negligible advantage with $ O(2^{n/2}) $ queries, directly contradicting the security claims made by the designers. We provide a rigorous and complete advantage calculation coupled with experimental verifications that further support our claim. Next, we provide new and simple proofs of birthday-bound CCA security for both TNT and its single-key variant, which confirm the tightness of our attack. Furthering on to a more positive note, we show that adding just one more block cipher call, referred as 4-LRW1, does not just reestablish the BBB security, but also amplifies it up to $ 2^{3n/4} $ queries. As a side-effect of this endeavour, we propose a new abstraction of the cascaded LRW-design philosophy, referred to as the LRW+ paradigm, comprising two block cipher calls sandwiched between a pair of tweakable universal hashes. This helps us to provide a modular proof approach covering all cascaded LRW constructions with at least $ 2 $ rounds, including 4-LRW1, and its more established relative, the well-known CLRW2, or more aptly, 2-LRW2.

In the past decade, blockchains have seen various financial and technological innovations, with cryptocurrencies reaching a market cap of over 1 trillion dollars. However, scalability is one of the key issues hindering the deployment of blockchains in many applications. To improve the throughput of the transactions, zkRollups and zkEVM techniques using the cryptographic primitive of zero-knowledge proofs (ZKPs) have been proposed and many companies are adopting these technologies in the layer-2 solutions. However, in these technologies, the proof generation of the ZKP is the bottleneck and the companies have to deploy powerful machines with TBs of memory to batch a large number of transactions in a ZKP.

In this work, we improve the scalability of these techniques by proposing new schemes of fully distributed ZKPs. Our schemes can improve the efficiency and the scalability of ZKPs using multiple machines, while the communication among the machines is minimal. With our schemes, the ZKP generation can be distributed to multiple participants in a model similar to the mining pools. Our protocols are based on Plonk, an efficient zero-knowledge proof system with a universal trusted setup. The first protocol is for data-parallel circuits. For a computation of $M$ sub-circuits of size $T$ each, using $M$ machines, the prover time is $O(T\log T + M \log M)$, while the prover time of the original Plonk on a single machine is $O(MT\log (MT))$. Our protocol incurs only $O(1)$ communication per machine, and the proof size and verifier time are both $O(1)$, the same as the original Plonk. Moreover, we show that with minor modifications, our second protocol can support general circuits with arbitrary connections while preserving the same proving, verifying, and communication complexity. The technique is general and may be of independent interest for other applications of ZKP.

We implement Pianist (Plonk vIA uNlimited dISTribution), a fully distributed ZKP system using our protocols. Pianist can generate the proof for 8192 transactions in 313 seconds on 64 machines. This improves the scalability of the Plonk scheme by 64$\times$. The communication per machine is only 2.1 KB, regardless of the number of machines and the size of the circuit. The proof size is 2.2 KB and the verifier time is 3.5 ms. We further show that Pianist has similar improvements for general circuits. On a randomly generated circuit with $2^{25}$ gates, it only takes 5s to generate the proof using 32 machines, 24.2$\times$ faster than Plonk on a single machine.

A wiretap coding scheme for a pair of noisy channels $(\mathsf{ChB},\mathsf{ChE})$ enables Alice to reliably communicate a message to Bob by sending its encoding over $\mathsf{ChB}$, while hiding the message from an adversary Eve who obtains the same encoding over $\mathsf{ChE}$.

A necessary condition for the feasibility of wiretap coding is that $\mathsf{ChB}$ is not a degradation of $\mathsf{ChE}$, namely Eve cannot simulate Bob’s view. While insufficient in the information-theoretic setting, a recent work of Ishai, Korb, Lou, and Sahai (Crypto 2022) showed that the non-degradation condition is sufficient in the computational setting, assuming idealized flavors of obfuscation. The question of basing a similar feasibility result on standard cryptographic assumptions was left open, even in simple special cases.

In this work, we settle the question for all discrete memoryless channels where the (common) input alphabet of $\mathsf{ChB}$ and $\mathsf{ChE}$ is binary, and with arbitrary finite output alphabet, under standard (sub-exponential) hardness assumptions: namely those assumptions that imply indistinguishability obfuscation (Jain-Lin-Sahai 2021, 2022), and injective PRGs. In particular, this establishes the feasibility of computational wiretap coding when $\mathsf{ChB}$ is a binary symmetric channel with crossover probability $p$ and $\mathsf{ChE}$ is a binary erasure channel with erasure probability $e$, where $e>2p$.

On the information-theoretic side, our result builds on a new polytope characterization of channel degradation for pairs of binary-input channels, which may be of independent interest.

Secure 2-party computation (2PC) enables secure inference that offers protection for both proprietary machine learning (ML) models and sensitive inputs to them. However, the existing secure inference solutions suffer from high latency and communication overheads, particularly for transformers. Function secret sharing (FSS) is a recent paradigm for obtaining efficient 2PC protocols with a preprocessing phase. We provide SIGMA, the first end-to-end system for secure transformer inference based on FSS. By constructing new FSS-based protocols for complex machine learning functionalities, such as Softmax and GeLU, and also accelerating their computation on GPUs, SIGMA improves the latency of secure inference of transformers by $11-19\times$ over the state-of-the-art that uses preprocessing and GPUs. We present the first secure inference of generative pre-trained transformer (GPT) models. In particular, SIGMA executes GPT-Neo with 1.3 billion parameters in 7.4s and HuggingFace's GPT2 in 1.6s.

Orientations of supersingular elliptic curves encode the information of an endomorphism of the curve. Computing the full endomorphism ring is a known hard problem, so one might consider how hard it is to find one such orientation. We prove that access to an oracle which tells if an elliptic curve is $\mathfrak{O}$-orientable for a fixed imaginary quadratic order $\mathfrak{O}$ provides non-trivial information towards computing an endomorphism corresponding to the $\mathfrak{O}$-orientation. We provide explicit algorithms and in-depth complexity analysis.

We also consider the question in terms of quaternion algebras. We provide algorithms which compute an embedding of a fixed imaginary quadratic order into a maximal order of the quaternion algebra ramified at $p$ and $\infty$. We provide code implementations in Sagemath which is efficient for finding embeddings of imaginary quadratic orders of discriminants up to $O(p)$, even for cryptographically sized $p$.

We present a hardware implementation for the MAYO post-quantum digital signature scheme, which is submitted to the American National Institute of Standards and Technology’s call for diversification of quantum-resistant public key cryptographic standards. The scheme is based on the Unbalanced Oil and Vinegar signature scheme, which operates on the fact that solving systems of multivariate polynomial equations is NP-complete. MAYO utilizes a unique whipping technique in combination with emulsifier maps to offer a significant reduction in key size compared to other Unbalanced Oil and Vinegar signature schemes. In this paper, we demonstrate how to design a hardware architecture for the MAYO post-quantum signature scheme. We also provide a comprehensive analysis and propose multiple optimization techniques to reduce resource utilization and accelerate computation on hardware platforms.

\ascon is the final winner of the lightweight cryptography standardization competition $(2018-2023)$. In this paper, we focus on preimage attacks against round-reduced \ascon. The preimage attack framework, utilizing the linear structure with the allocating model, was initially proposed by Guo \textit{et al.} at ASIACRYPT 2016 and subsequently improved by Li \textit{et al.} at EUROCRYPT 2019, demonstrating high effectiveness in breaking the preimage resistance of \keccak. In this paper, we extend this preimage attack framework to \ascon from two aspects. Firstly, we propose a linearize-and-guess approach by analyzing the algebraic properties of the \ascon permutation. As a result, the complexity of finding a preimage for 2-round \ascon-\xof with a 64-bit hash value can be significantly reduced from $2^{39}$ guesses to $2^{27.56}$ guesses. To support the effectiveness of our approach, we find an actual preimage of all ‘0’ hash in practical time. Secondly, we develop a SAT-based automatic preimage attack framework using the linearize-and-guess approach, which is efficient to search for the optimal structures exhaustively. Consequently, we present the best theoretical preimage attacks on 3-round and 4-round \ascon-\xof so far.

In the Random Oracle Model (ROM) all parties have oracle access to a common random function, and the parties are limited in the number of queries they can make to the oracle. The Merkle’s Puzzles protocol, introduced by Merkle [CACM ’78], is a key-agreement protocol in the ROM with a quadratic gap between the query complexity of the honest parties and the eavesdropper. This quadratic gap is known to be optimal, by the works of Impagliazzo and Rudich [STOC ’89] and Barak and Mahmoody [Crypto ’09].

When the oracle function is injective or a permutation, Merkle’s Puzzles has perfect completeness. That is, it is certain that the protocol results in agreement between the parties. However, without such an assumption on the random function, there is a small error probability, and the parties may end up holding different keys. This fact raises the question: Is there a key-agreement protocol with perfect completeness and super-linear security in the ROM?

In this paper we give a positive answer to the above question, showing that changes to the query distribution of the parties in Merkle’s Puzzles, yield a protocol with perfect completeness and roughly the same security.

We slightly generalize Plonk's ([GWC19]) permutation argument by replacing permutations with (possibly non-injective) self-maps of an interval. We then use this succinct argument to obtain a protocol for weighted sums on committed vectors, which, in turn, allows us to eliminate the intermediate gates arising from high fan-in additions in Plonkish circuits.

We use the KZG10 polynomial commitment scheme, which allows for a universal updateable CRS linear in the circuit size. In keeping with our recent work ([Th23]), we have used the monomial basis since it is compatible with any sufficiently large prime scalar field. In settings where the scalar field has a suitable smooth order subgroup, the techniques can be efficiently ported to a Lagrange basis.

The proof size is constant, as is the verification time which is dominated by a single pairing check. For committed vectors of length $n$, the proof generation is $O(n\cdot \log(n))$ and is dominated by the $\mathbb{G}_1$-MSMs and a single sum of a few polynomial products over the prime scalar field via multimodular FFTs.

Wave is a code-based digital signature scheme. Its hardness relies on the unforgeability of signature and the indistinguishability of its public key, a parity check matrix of a ternary $(U, U+V)$-code. The best known attacks involve solving the Decoding Problem using the Information Set Decoding algorithm (ISD) to defeat these two problems. Our main contribution is the description of a quantum smoothed Wagner's algorithm within the ISD, which improves the forgery attack on Wave in the quantum model. We also recap the best known key and forgery attacks against Wave in the classical and quantum models. For each one, we explicitly express their time complexity in the function of Wave parameters and deduce the claimed security of Wave.

We present Phoenixx, a round and leader based Byzantine fault tolerant consensus protocol, that operates in the partial synchrony network communications model. Phoenixx combines the three phase approach from HotStuff, with a novel \textit{Endorser Sampling}, that selects a subset of nodes, called \textit{endorsers}, to ``compress'' the opinion of the network.

Unlike traditional sampling approaches that select a subset of the network to run consensus on behalf of the network and disseminate the outcome, Phoenixx still requires participation of the whole network. The endorsers, however, assume a special role as they confirm that at least $2f+1$ validators are in agreement and issue a compressed certificate, attesting the network reached a decision. Phoenixx achieves linear communication complexity, while maintaining safety, liveness, and optimistic responsiveness, without using threshold signatures.

We generalize the Bernstein-Yang (BY) algorithm for constant-time modular inversion to compute the Kronecker symbol, of which the Jacobi and Legendre symbols are special cases. We start by developing a basic and easy-to-implement divstep version of the algorithm defined in terms of full-precision division steps. We then describe an optimized version due to Hamburg over word-sized inputs, similar to the jumpdivstep version of the BY algorithm, and formally verify its correctness. Along the way, we introduce a number of optimizations for implementing both versions in constant time and at high-speed. The resulting algorithms are particularly suitable for the special case of computing the Legendre symbol with dense prime $p$, where no efficient addition chain is known for the conventional approach by exponentiation to $\frac{p-1}{2}$. This is often the case for the base field of popular pairing-friendly elliptic curves. Our high-speed implementation for a range of parameters shows that the new algorithm is up to 40 times faster than the conventional exponentiation approach, and up to 25.7\% faster than the previous state of the art. We illustrate the performance of the algorithm with an application for hashing to elliptic curves, where the observed savings amount to 14.7\% -- 48.1\% when used for testing quadratic residuosity within the SwiftEC hashing algorithm. We also apply our techniques to the CTIDH isogeny-based key exchange, with savings of 3.5--13.5\%.

The Cryptography and Privacy Engineering Group (ENCRYPTO) @CS Department @Technical University of Darmstadt offers a fully funded position as Doctoral Researcher (Research Assistant/PhD Student) in Cryptography and Privacy Engineering to be filled as soon as possible and initially for 3 years with the possibility of extension.

Job description:

You'll work in the collaborative research center CROSSING funded by the German Research Foundation (DFG). In our project E4 Compiler for Privacy-Preserving Protocols, we build compilers to automatically generate optimized MPC protocols for privacy-preserving applications. See https://encrypto.de/CROSSING for details. As PhD@ENCRYPTO, you primarily focus on your research aiming to publish&present the results at top venues.

We offer:

We demonstrate that privacy is efficiently protectable in real-world applications via cryptographic protocols. Our open and international working environment facilitates excellent research in a sociable team. TU Darmstadt is a top research university for IT security, cryptography and CS in Europe. Darmstadt is a very international, livable and well-connected city in the Rhine-Main area around Frankfurt.

Your profile:

• Completed Master's degree (or equivalent) at a top university with excellent grades in IT security, computer science, or a similar area.
• Extensive knowledge in applied cryptography/IT security and very good software development skills. Knowledge in cryptographic protocols (ideally MPC) is a plus.
• Experience and interest to engage in teaching.
• Self-motivated, reliable, creative, can work independently, and striving to do excellent research.
• Our working language is English: Able to discuss/write/present scientific results in English. German is beneficial but not required.

Application deadline: Sep 30, 2023. Later applications are considered.

Closing date for applications:

Contact: Thomas Schneider (application@encrypto.cs.tu-darmstadt.de)

More information: https://encrypto.de/2023-CROSSING

The Cryptography and Privacy Engineering Group (ENCRYPTO) @Department of Computer Science @TU Darmstadt offers a fully funded position for a Postdoctoral Researcher, to be filled asap and initially til January 31, 2025 with the potential of extension.

Our mission is to demonstrate that privacy can be efficiently protected in real-world applications via cryptographic protocols.

TU Darmstadt is located in the center of Germany and is a top research university for IT security, cryptography, and computer science. No German language skills are necessary and we established a hybrid working mode flexibly combining mobile work and in-presence time in office depending on individual preferences.

Job description:

As postdoc @ENCRYPTO, your primary focus is on collaborations with our PhDs and external international collaborators for cutting-edge research in applied cryptography as well as the publication and presentation of the results at top-tier security and cryptography conferences/journals. In our ERC-funded project PSOTI, we develop protocols for privately processing data among untrusted service providers using MPC. Examples are privacy-preserving alternatives for common applications such as email, file sharing, and forms. Also, the active research field of PPML is of high relevance for our group.

Your profile

• Completed PhD degree (or equivalent) at a top university in IT security, computer science, applied mathematics, electrical engineering, or a similar area
• Publications at top venues (CORE rank A*/A) for IT security/applied cryptography (e.g., EUROCRYPT, S&P, CCS, NDSS, USENIX SEC), ideally on cryptographic protocols and secure computation
• Experience in software development, project management and supervising students
• Self-motivated, reliable, creative, team-minded, and want to do excellent research on challenging scientific problems with practical relevance
• The working language at ENCRYPTO is English, so you must be able to discuss/write/present scientific results in English, whereas German is not required.

Closing date for applications:

Contact: Thomas Schneider (application@encrypto.cs.tu-darmstadt.de)

More information: https://encrypto.de/POSTDOC

The Research Institute CODE (https://www.unibw.de/code), established in 2017, with currently 15 professorships and over 130 researchers, is being expanded to one of the largest European research institutes for cyber security.

For a newly established professorship in Cryptography, Daniel Slamanig is seeking multiple PhD and Post-Doc researchers. Relevant topics include:

• Public-key cryptographic primitives
• Malleable and updatable cryptography
• Foundations and applications of privacy-preserving cryptography
• Post-quantum cryptography
• (Non-interactive) Zero-knowledge proofs and zk-SNARKs
• Real-world cryptography

Candidates are expected to do cutting edge research in cryptography. We offer the opportunity to engage with research projects and international partners from academia and industry. Candidates will also gain experience with supporting teaching activities.

Requirements:

• Master's degree (or equivalent) or PhD in Mathematics, Computer Science, Information Security, or a similar discipline.
• PostDoc candidates must have a strong track record (ideally with publications at IACR conferences and/or the top 4 security conferences) and good academic writing and presentation skills.
• High motivation for research work and ability to work independently.
• Good organisation and communication skills.
• Eager to disseminate research results through publications and presentations at top-tier conferences.
• Fluency in written and spoken English (German desirable but not required).

All positions are available for start from November 2023 (flexible) and are fully funded at federal salary levels TV-ÖD E13/14 (~50k to 65k EUR p.a. depending on qualifications and experience).

How to apply? Send a mail to Daniel Slamanig with subject line "Application UniBWM" including your cover/motivation letter, CV, transcripts of grades, and references.

Closing date for applications: Applications will be reviewed until the positions are filled.

Closing date for applications:

Contact: Daniel Slamanig (daniel.slamanig [AT] gmail.com)

More information: https://danielslamanig.info/