International Association
for Cryptologic Research

In this short note, we present a simplified (but slower) version Clapoti of Clapotis, whose full description will appear later. Let ?/?_? be an elliptic curve with an effective primitive orientation by a quadratic imaginary order ? ⊂ End(?). Let ? be an invertible ideal in ?. Clapoti is a randomized polynomial time algorithm in ? ((log Δ_? + log ?)^?(1) ) operations to compute the class group action ? ↦ ?_? ≃ ?/?[?].

The Perebor (Russian for “brute-force search”) conjectures, which date back to the 1950s and 1960s are some of the oldest conjectures in complexity theory. The conjectures are a stronger form of the NP ̸ = P conjecture (which they predate) and state that for “meta-complexity” problems, such as the Time-bounded Kolmogorov complexity Problem, and the Minimum Circuit Size Problem, there are no better algorithms than brute force search.

In this paper, we disprove the non-uniform version of the Perebor conjecture for the Time-Bounded Kolmogorov complexity problem. We demonstrate that for every polynomial t(·), there exists of a circuit of size $2^{4n/5+o(n)}$ that solves the t(·)-bounded Kolmogorov complexity problem on every instance.

Our algorithm is black-box in the description of the Universal Turing Machine employed in the definition of Kolmogorov Complexity, and leverages the characterization of one-way functions through the hardness of the time-bounded Kolmogorov complexity problem of Liu and Pass (FOCS’20), and the time-space trade-off for one-way functions of Fiat and Naor (STOC’91). We additionally demonstrate that no such black-box algorithm can have sub-exponential circuit size.

Along the way (and of independent interest), we extend the result of Fiat and Naor and demonstrate that any efficiently computable function can be inverted (with probability 1) by a circuit of size 2^{4n/5+o(n)}; as far as we know, this yields the first formal proof that a non-trivial circuit can invert any efficient function.

How to achieve distributed differential privacy (DP) without a trusted central party is of great interest in both theory and practice. Recently, the shuffle model has attracted much attention. Unlike the local DP model in which the users send randomized data directly to the data collector/analyzer, in the shuffle model an intermediate untrusted shuffler is introduced to randomly permute the data, which have already been randomized by the users, before they reach the analyzer. The most appealing aspect is that while shuffling does not explicitly add more noise to the data, it can make privacy better. The privacy amplification effect in consequence means the users need to add less noise to the data than in the local DP model, but can achieve the same level of differential privacy. Thus, protocols in the shuffle model can provide better accuracy than those in the local DP model. What looks interesting to us is that the architecture of the shuffle model is similar to private aggregation, which has been studied for more than a decade. In private aggregation, locally randomized user data are aggregated by an intermediate untrusted aggregator. Thus, our question is whether aggregation also exhibits some sort of privacy amplification effect? And if so, how good is this ``aggregation model'' in comparison with the shuffle model. We conducted the first comparative study between the two, covering privacy amplification, functionalities, protocol accuracy, and practicality. The results as yet suggest that the new shuffle model does not have obvious advantages over the old aggregation model. On the contrary, protocols in the aggregation model outperform those in the shuffle model, sometimes significantly, in many aspects.

We present a three-party protocol that can protect both Transformer parameters and user data during the inference phase. For each feedforward inference process, our protocol only introduces permutation computation of input and output data on the user side. Our protocol, Secure Transformer Inference Protocol (STIP), can be applied to real-world services like ChatGPT.

Verification of program safety is often reducible to proving the unsatisfiability (i.e., validity) of a formula in Satisfiability Modulo Theories (SMT): Boolean logic combined with theories that formalize arbitrary first-order fragments. Zero-knowledge (ZK) proofs allow SMT formulas to be validated without revealing the underlying formulas or their proofs to other parties, which is a crucial building block for proving the safety of proprietary programs. Recently, Luo et al. (CCS 2022) studied the simpler problem of proving the unsatisfiability of pure Boolean formulas, but it does not support safety proofs generated by SMT solvers. This work presents ZKSMT, a novel framework for proving the validity of SMT formulas in ZK. We design a virtual machine (VM) tailored to efficiently represent the verification process of SMT validity proofs in ZK. Our VM can support the vast majority of popular theories when proving program safety while being complete and sound. To demonstrate this, we instantiate the commonly used theories of equality and linear integer arithmetic in our VM with theory-specific optimizations for proving them in ZK. ZKSMT achieves high practicality even when running on realistic SMT formulas generated by Boogie, a common tool for software verification. It achieves a three-order-of-magnitude improvement compared to a baseline that executes the proof verification code in a general ZK system.

Post-compromise security (PCS) has been a core goal of end-to-end encrypted messaging applications for many years, both in one-to-one continuous key agreement (CKA) and for groups (CGKA). At its essence, PCS relies on a compromised party to perform a key update in order to `self-heal'. However, due to bandwidth constraints, receive-only mode, and various other environmental demands of the growing number of use cases for such CGKA protocols, a group member may not be able to issue such updates. In this work, we address the issue of devices functioning in limited mode through the introduction of guardianship, where a designated guardian can perform key updates on the behalf of its paired edge device. We introduce a Guardianship PCS (GPCS) security, and provide an associated security experiment. We investigate various architectural designs in the pursuit of GPCS, provide constructions and security analyses, and describe trade-offs.

This paper describes Biscuit, a new multivariate-based signature scheme derived using the MPCitH approach. The security of Biscuit is related to the problem of solving a set of quadratic structured systems of algebraic equations. These equations are highly compact and can be evaluated using very few multiplications. The core of Biscuit is a rather simple MPC protocol which consists of the parallel execution of a few secure multiplications using standard optimized multiplicative triples. This paper also includes several improvements with respect to Biscuit submission to the last NIST PQC standardization process for additional signature schemes. Notably, we introduce a new hypercube variant of Biscuit, refine the security analysis with recent third-party attacks, and present a new avx2 implementation of Biscuit.

In this paper, we consider to generalize NIZK by empowering a prover to share a witness in a fine-grained manner with verifiers. Roughly, the prover is able to authorize a verifier to obtain extra information of witness, i.e., besides verifying the truth of the statement, the verifier can additionally obtain certain function of the witness from the accepting proof using a secret functional key provided by the prover.

To fulfill these requirements, we introduce a new primitive called \emph{non-interactive zero-knowledge functional proofs (fNIZKs)}, and formalize its security notions. We provide a generic construction of fNIZK for any $\textsf{NP}$ relation $\mathcal{R}$, which enables the prover to share any function of the witness with a verifier. For a widely-used relation about set membership proof (implying range proof), we construct a concrete and efficient fNIZK, through new building blocks (set membership encryption and dual inner-product encryption), which might be of independent interest.

Widespread efforts to subvert acccess to strong cryptography has renewed interest in steganography, the practice of embedding sensitive messages in mundane cover messages. Recent efforts at provably secure steganography have only focused on text-based generative models and cannot support other types of models, such as diffusion models, which are used for high-quality image synthesis. In this work, we initiate the study of securely embedding steganographic messages into the output of image diffusion models. We identify that the use of variance noise during image generation provides a suitable steganographic channel. We develop our construction, Pulsar, by building optimizations to make this channel practical for communication. Our implementation of Pulsar is capable of embedding $\approx 275$-$542$ bytes (on average) into a single image without altering the distribution of the generated image, all in the span of $\approx 3$ seconds of online time on a laptop. In addition, we discuss how the results of Pulsar can inform future research into diffusion models. Pulsar shows that diffusion models are a promising medium for steganography and censorship resistance.

We consider the mainstream model in secure computation known as the bare PKI setup, also as the {bulletin-board PKI}. It assumes that players can broadcast once and non-interactively before they receive their inputs and start the execution. We consider both the problems of consensus with strong unanimity, also known as ``Byzantine agreement'' (BA), and of ``Validated Byzantine agreement'' [CKPS, Crypto'01] (VBA, also known as MVBA). Most works on BA use a bulletin-board PKI setup only for the purpose of publishing verification keys. This implements the {messages-authentication model}, i.e., when $P$ is forwarded a message issued by $R$, it is convinced that $R$ is the author. Without messages-authentication, it is known since [Lamport et al, 82] that BA under honest majority is impossible, let alone secure computation. Thus, since the bare PKI setup and the messages-authentication model seem close, this raizes the question whether there is a separation between the two. In the bare PKI setup, the most communication-efficient synchronous BA is the one of [Boyle-Cohen-Goel, Podc'21 \& J. Cryptol.'24], which has $O(n.\text{polylog}(n))$ bit complexity, $f
{- Optimality.} We show that resilience up to a tight honest majority $f
{- Separation.} We show impossibility of subquadratic multicast-based BA in the messages-authentication model. Our model for this lower bound is even stronger, since it onboards other assumptions at least as strong as all popular implications of a bulletin-board PKI setup: {secure channels}, {a (possibly structured) random string}, {NIZK}.

{- Partial synchrony.} We then show that the separation also holds under partial synchrony. On the one hand, our upper-bound also holds, with $f
{- Extension to VBA.} We extend to VBA the logarithmic latency lower bound. This is the first communication lower bound for adaptively secure VBA to our knowledge. It shows that the separation under partial synchrony also holds for VBA. Along the way, we close the categorization of [Civit et al, Podc'23] of validity conditions in authenticated consensus, by apparently new results on VBA: both BA and VBA are infeasible under partial synchrony beyond $f

Quantum copy protection, introduced by Aaronson, enables giving out a quantum program-description that cannot be meaningfully duplicated. Despite over a decade of study, copy protection is only known to be possible for a very limited class of programs.

As our first contribution, we show how to achieve "best-possible" copy protection for all programs. We do this by introducing quantum state indistinguishability obfuscation (qsiO), a notion of obfuscation for quantum descriptions of classical programs. We show that applying qsiO to a program immediately achieves best-possible copy protection.

Our second contribution is to show that, assuming injective one-way functions exist, qsiO is concrete copy protection for a large family of puncturable programs --- significantly expanding the class of copy-protectable programs. A key tool in our proof is a new variant of unclonable encryption (UE) that we call coupled unclonable encryption (cUE). While constructing UE in the standard model remains an important open problem, we are able to build cUE from one-way functions. If we additionally assume the existence of UE, then we can further expand the class of puncturable programs for which qsiO is copy protection.

Finally, we construct qsiO relative to an efficient quantum oracle.

Regular access to unpredictable and bias-resistant randomness is important for applications such as blockchains, voting, and secure distributed computing. Distributed random beacon protocols address this need by distributing trust across multiple nodes, with the majority of them assumed to be honest. These protocols have found applications in blockchain technology, leading to the proposal of several distributed random beacon protocols, with some already implemented. However, many current random beacon systems rely on threshold cryptographic setups or exhibit high computational costs, while others assume partial or bounded synchronous networks. To overcome these limitations, we propose HashRand, a computation and communication-efficient asynchronous random beacon protocol that uses a secure Hash function to generate beacons and pairwise secure channels. HashRand has a per-node communication complexity of $\mathcal{O}(\lambda n \log(n))$ bits per beacon. The computational efficiency of HashRand is attributed to the two orders of magnitude lower time of a one-way Hash computation compared to discrete log exponentiation. Interestingly, besides reduced overhead, HashRand achieves Post-Quantum security by leveraging the secure Hash function against quantum adversaries, setting it apart from other random beacon protocols that use discrete log cryptography. In a geo-distributed testbed of $n=160$ nodes, HashRand produces 1 beacon every second, which is at least 4x higher than Spurt. We also demonstrate the practical utility of HashRand by implementing a Post-Quantum secure Asynchronous SMR protocol, which has a response rate of over 122k txns per second over a WAN at $n=40$ nodes.

The Snowden's revelations kick-started a community-wide effort to develop cryptographic tools against mass surveillance. In this work, we propose to add another primitive to that toolbox: Fail-Stop Signatures (FSS) [EC'89]. FSS are digital signatures enhanced with a forgery-detection mechanism that can protect a PPT signer from more powerful attackers. Despite the fascinating concept, research in this area stalled after the '90s. However, the ongoing transition to post-quantum cryptography, with its hiccups due to the novelty of underlying assumptions, has become the perfect use case for FSS. This paper aims to reboot research on FSS with practical use in mind: Our framework for FSS includes ``fine-grained'' security definitions (that assume a powerful, but bounded adversary e.g: can break $128$-bit of security, but not $256$-bit). As an application, we show new FSS constructions for the post-quantum setting. We show that FSS are equivalent to standard, provably secure digital signatures that do not require rewinding or programming random oracles, and that this implies lattice-based FSS. Our main construction is an FSS version of SPHINCS, which required building FSS versions of all its building blocks: WOTS, XMSS, and FORS. In the process, we identify and provide generic solutions for two fundamental issues arising when deriving a large number of private keys from a single seed, and when building FSS for Hash-and-Sign-based signatures.

The Internet Key Exchange version 2 (IKEv2) (RFC 7296) is a component of IPsec used to authenticate two parties (the initiator and responder) to each other and to establish a set of security parameters for the communications. The security parameters include secret keys to encrypt and authenticate data as well as the negotiation of a set of cryptographic algorithms. The core documentation uses exclusively Diffie-Hellman exchanges to agree the security information. However, this is not a quantum-secure option due to the ability of Shor's algorithm to break the security assumption underlying the Diffie-Hellman. A post-quantum solution is to include a preshared key in the exchange, as proposed by the extension RFC 8784; assuming that this preshared key has sufficient entropy, the keys created in the IKEv2 exchange will be resistant to a quantum computer. In this paper, we investigate the security claims of RFC 8784 using formal verification methods. We find that keys created using the preshared key are secret from an adversary. However, certain authentication properties of the protocol that are weakened under the assumption that Diffie-Hellman is insecure are not recovered using the preshared key.

This paper introduces a novel approach to enhancing cryp- tographic security. It proposes the use of one-time message sharing com- bined with Physically Unclonable Functions (PUF) to securely exchange keys and generate an S-subbyte-box for encryption. This innovative tech- nique aims to elevate the security standards of cryptographic applica- tions.

In this paper, we describe an algorithm to compute chains of $(2,2)$-isogenies between products of elliptic curves in the theta model. The description of the algorithm is split into various subroutines to allow for a precise field operation counting.

We present a constant time implementation of our algorithm in Rust and an alternative implementation in SageMath. Our work in SageMath runs ten times faster than a comparable implementation of an isogeny chain using the Richelot correspondence. The Rust implementation runs up to forty times faster than the equivalent isogeny in SageMath and has been designed to be portable for future research in higher-dimensional isogeny-based cryptography.

With the Institute for Photonic Quantum Systems (PhoQS), Paderborn University wants to establish an international research centre in the field of photonic quantum technologies. The aim is to develop new technologies for photon-based quantum applications as well as new theoretical and experimental concepts and research approaches. Ultimately, the focus is on understanding and controlling photonic quantum simulators and quantum computers. In the Faculty of Electrical Engineering, Computer Science and Mathematics, the Institute of Computer Science - Department "Codes and Cryptography" - has a vacancy for a

Postdoc (m/f/d)
(salary is according to 13 TV-L)

position with 100% of the regular working hours within the framework of the MKW (Ministry of Culture and Science of the State of North Rhine-Westphalia) NRW project "Photonic Quantum Computing (PhoQC)". The employment is initially limited to three years and is based on the legal regulations of the WissZeitVG.
Your duties and responsibilities:
- Complexity-theoretical analysis of boson sampling-like experiments
- Development of quantum algorithms using boson sampling for practical applications
- Development and analysis of theoretical frameworks for universal quantum computing based on quantum photonics
Hiring requirements:
- Completed PhD in computer science, mathematics or physics or comparable qualification

It is expected that the applicant has experience in at least one of the following areas:

- Quantum algorithms
- Quantum complexity
- Theoretical quantum photonics
- Quantum information theory

Closing date for applications:

Contact: Please send your application including a CV (preferably in a single pdf file) using the Ref. No. 6207 by bloemer@upb.de. Information regarding the processing of your person data can be located at: https://www.uni-paderborn.de/en/zv/personaldatenschutz.

More information: https://cs.uni-paderborn.de/en/cuk/?cHash=8829d72369c94558f8571cc307314a2c

LIST is looking for a highly motivated candidate with proven skills in federated data processing such as applying simple statistics or advanced machine learning tools, to improve the utility of the obtained information. For example, collaborative cyberthreat intelligence systems that use advanced analytics solutions can offer significant advantages over the local systems by detecting cyberattacks early and promptly responding to them. While recent developments in cryptography and the rise of homomorphic encryption and secure multi-party computation allow operations without revealing the underlying data in cleartext, these technologies still suffer from a noticeable overhead in terms of computation and communication. The main mission of the candidate, together with local and international collaborators, is to continue research on these technologies to address scalability and performance requirements.

Closing date for applications:

Contact: Dr. Qiang Tang (qiang.tang@list.lu)

Event date: 5 March to 8 March 2024

As NIST is putting the final touches on the standardization of PQC (Post Quantum Cryptography) public key algorithms, it is a racing certainty that peskier cryptographic attacks undeterred by those new PQC algorithms will surface. Such a trend in turn will prompt more follow-up studies of attacks and countermeasures. As things stand, from the attackers’ perspective, one viable form of attack that can be implemented thereupon is the so-called “side-channel attack”. Two best-known countermeasures heralded to be durable against side-channel attacks are: “masking” and “hiding”. In that dichotomous picture, of particular note are successful single-trace attacks on some of the NIST’s PQC then-candidates, which worked to the detriment of the former: “masking”. In this paper, we cast an eye over the latter: “hiding”. Hiding proves to be durable against both side-channel attacks and another equally robust type of attacks called “fault injection attacks”, and hence is deemed an auspicious countermeasure to be implemented. Mathematically, the hiding method is fundamentally based on random permutations. There has been a cornucopia of studies on generating random permutations. However, those are not tied to implementation of the hiding method. In this paper, we propose a reliable and efficient verification of permutation implementation, through employing Fisher–Yates’ shuffling method. We introduce the concept of an ?-th order permutation and explain how it can be used to verify that our implementation is more efficient than its previous-gen counterparts for hiding countermeasures.