International Association
for Cryptologic Research

Differential cryptanalysis is an old and powerful attack against block ciphers. While different techniques have been introduced throughout the years to improve the complexity of this attack, the key recovery phase remains a tedious and error-prone procedure. In this work, we propose a new algorithm and its associated tool that permits, given a distinguisher, to output an efficient key guessing strategy. Our tool can be applied to SPN ciphers whose linear layer consists of a bit-permutation and whose key schedule is linear or almost linear. It can be used not only to help cryptanalysts find the best differential attack on a given cipher but also to assist designers in their security analysis. We applied our tool to four targets: RECTANGLE, PRESENT-80, SPEEDY-7-192 and GIFT-64. We extend the previous best attack on RECTANGLE-128 by one round and the previous best differential attack against PRESENT-80 by 2 rounds. We improve a previous key recovery step in an attack against SPEEDY and present more efficient key recovery strategies for RECTANGLE-80 and GIFT. Our tool outputs the results in only a second for most targets.

Physical attacks pose a substantial threat to the secure implementation of cryptographic algorithms. While considerable research efforts are dedicated to protecting against passive physical attacks (e.g., side-channel analysis (SCA)), the landscape of protection against other types of physical attacks remains a challenge. Fault attacks (FA), though attracting growing attention in research, still lack the prevalence of provably secure designs when compared to SCA. The realm of combined attacks, which leverage the capabilities of both SCA and FA adversaries, introduces powerful adversarial models, rendering protection against them challenging. This challenge has consequently led to a relatively unexplored area of research, resulting in a notable gap in understanding and efficiently protecting against combined attacks. The CAPA countermeasure, published at CRYPTO 2018, addresses this challenge with a robust adversarial model that goes beyond conventional SCA and FA adversarial models. Drawing inspiration from the principles of Multiparty Computation (MPC), CAPA claims security against higher-order SCA, higher-order fault attacks, and their combination. In this work, we present a combined attack that breaks CAPA within the constraints of its assumed adversarial model. In response, we propose potential fixes to the design of CAPA that increase the complexity of the proposed attack, although not provably thwarting it. With this presented combined attack, we highlight the difficulty of effectively protecting against combined attacks.

We present a novel technique within the MPC-in-the-Head framework, aiming to design efficient zero-knowledge protocols and digital signature schemes. The technique allows for the simultaneous use of additive and multiplicative sharings of secret information, enabling efficient proofs of linear and multiplicative relations. The applications of our technique are manifold. It is first applied to construct zero-knowledge arguments of knowledge for Double Discrete Logarithms (DDLP). The resulting protocol achieves improved communication complexity without compromising efficiency. We also propose a new zero-knowledge argument of knowledge for the Permuted Kernel Problem. Eventually, we suggest a short (candidate) post-quantum digital signature scheme constructed from a new one-way function based on simple polynomials known as fewnomials. This scheme offers simplicity and ease of implementation. Finally, we present two additional results inspired by this work but using alternative approaches. We propose a zero-knowledge argument of knowledge of an RSA plaintext for a small public exponent that significantly improves the state-of-the-art communication complexity. We also detail a more efficient forward-backward construction for the DDLP.

Randomized Partial Checking (RPC} was proposed by Jakobsson, Juels, and Rivest and attracted attention as an efficient method of verifying the correctness of the mixing process in numerous applied scenarios. In fact, RPC is a building block for many electronic voting schemes, including Prêt à Voter, Civitas, Scantegrity II as well as voting-systems used in real-world elections (e.g., in Australia). Mixing is also used in anonymous transfers of cryptocurrencies.

It turned out, however, that a series of works showed, however, subtle issues with analyses behind RPC. First, that the actual security level of the RPC protocol is way off the claimed bounds. The probability of successful manipulation of $k$ votes is $(\frac{3}{4})^k$ instead of the claimed $\frac{1}{2^k}$ (this difference, in turn, negatively affects actual implementations of the notion within existing election systems. This is so since concrete implemented procedures of a given length were directly based on this parameter).

Further, privacy guarantees that a constant number of mix-servers is enough turned out to also not be correct. We can conclude from the above that these analyses of the processes of mixing are not trivial.

In this paper, we review the relevant attacks, and we present Mirrored-RPC -- a fix to RPC based on ``mirrored commitment'' which makes it optimally secure; namely, having a probability of successful manipulation of $k$ votes $\frac{1}{2^k}$.

Then, we present an analysis of the privacy level of both RPC and mRPC. We show that for $n$ messages, the number of mix-servers (rounds) needed to be $\varepsilon$-close to the uniform distribution in total variation distance is lower bounded by: \[ r(n, \varepsilon) \geq \log_{2}{n \choose 2}/\varepsilon. \]

This proof of privacy, in turn, gives insights into the anonymity of various cryptocurrencies (e.g., Zerocash) using anonymizing pools. If a random fraction $q$ of $n$ existing coins is mixed (in each block), then to achieve full anonymity, the number of blocks one needs to run the protocol for, is: \[ rb(n, q, \varepsilon) \geq - \frac{\log n + \log (n-1) - \log (2\varepsilon)}{ {\log({1-q^2}})}. \]

Statistical Fault Attacks (SFA), introduced by Fuhr et al., exploit the statistical bias resulting from injected faults. Unlike prior fault analysis attacks, which require both faulty and correct ciphertexts under the same key, SFA leverages only faulty ciphertexts. In CHES 2018, more powerful attacks called Statistical Ineffective Fault Attacks (SIFA) have been proposed. In contrast to the previous fault attacks that utilize faulty ciphertexts, SIFA exploits the distribution of the intermediate values leading to fault-free ciphertexts. As a result, the SIFA attacks were shown to be effective even in the presence of widely used fault injection countermeasures based on detection and infection. In this work, we build upon the core idea of SIFA, and provide two main practical improvements over the previously proposed analysis methods. Firstly, we show how to perform SIFA from the input side, which in contrast to the original SIFA, requires injecting faults in the earlier rounds of an encryption or decryption operation. If we consider the start of the operation as the trigger for fault injection, the cumulative jitter in the first few rounds of a cipher is much lower than the last rounds. Hence, performing the attack in the first or second round requires a narrower parameter range for fault injection and hence less fault injection attempts to recover the secret key. Secondly, in comparison to the straightforward SIFA approach of guessing 32-bits at a time, we propose a chosen input approach that reduces the guessing effort to 16-bits at a time. This decreases the key search space for full key recovery of an AES-128 implementation from $2^{34}$ to $2^{19}$.

A recent work by Ball, Li, Lin, and Liu [Eurocrypt'23] presented a new instantiation of the arithmetic garbling paradigm introduced by Applebaum, Ishai, and Kushilevitz [FOCS'11]. In particular, Ball et al.'s garbling scheme is the first constant-rate garbled circuit over large enough bounded integer computations, inferring the first constant-round constant-rate secure two-party computation (2PC) over bounded integer computations in the presence of semi-honest adversaries.

The main source of difficulty in lifting the security of garbling schemes-based protocols to the malicious setting lies in proving the correctness of the underlying garbling scheme. In this work, we analyze the security of Ball et al.'s scheme in the presence of malicious attacks.

- We demonstrate an overflow attack, which is inevitable in this computational model, even if the garbled circuit is fully correct. Our attack follows by defining an adversary, corrupting either the garbler or the evaluator, that chooses a bad input and causes the computation to overflow, thus leaking information about the honest party's input. By utilizing overflow attacks, we show that $1$-bit leakage is necessary for achieving security against a malicious garbler, discarding the possibility of achieving full malicious security in this model. We further demonstrate a wider range of overflow attacks against a malicious evaluator with more than $1$ bit of leakage.

- We boost the security level of Ball et al.'s scheme by utilizing two variants of Vector Oblivious Linear Evaluation, denoted by VOLEc and aVOLE. We present the first constant-round constant-rate 2PC protocol over bounded integer computations, in the presence of a malicious garbler with $1$-bit leakage and a semi-honest evaluator, in the {VOLEc,aVOLE}-hybrid model and being black-box in the underlying group and ring. Compared to the semi-honest variant, our protocol incurs only a constant factor overhead, both in computation and communication. The constant-round and constant-rate properties hold even in the plain model.

Since its introduction by Wagner (CRYPTO `02), the $k$-list algorithm has found significant utility in cryptanalysis. One important application thereof is in computing forgeries on several interactive signature schemes that implicitly rely on the hardness of the ROS problem formulated by Schnorr (ICICS `01). The current best attack strategy for these schemes relies the conjectured runtime of the $k$-list algorithm over $\mathbb{Z}_p$. The tightest known analysis of Wagner's algorithm over $\mathbb{Z}_p$ is due to Shallue (ANTS `08). However, it hides large polynomial factors and leaves a gap with respect to desirable concrete parameters for the attack. In this work, we develop a degraded version of the $k$-list algorithm which provably enforces the heuristic invariants in Wagner's original. In the process, we devise and analyze a new list merge procedure that we dub the interval merge. We give a thorough analysis of the runtime and success probability of our degraded algorithm, and show that it beats the projected runtime of the analysis by Shallue for parameters relevant to the generalized ROS attack of Benhamouda et al. (EUROCRYPT `21). For a $256$-bit prime $p$, and $k = 8$, our degraded $k$-list algorithm runs in time $\approx 2^{70.4}$, while Shallue's analysis states that the Wagner's original algorithm runs in time $\approx 2^{98.3}$.

Polynomial commitment scheme allows a prover to commit to a polynomial $f \in \mathcal{R}[X]$ of degree $L$, and later prove that the committed function was correctly evaluated at a specified point $x$; in other words $f(x)=u$ for public $x,u \in\mathcal{R}$. Most applications of polynomial commitments, e.g. succinct non-interactive arguments of knowledge (SNARKs), require that (i) both the commitment and evaluation proof are succinct (i.e., polylogarithmic in the degree $L$) - with the latter being efficiently verifiable, and (ii) no pre-processing step is allowed.

Surprisingly, as far as plausibly quantum-safe polynomial commitments are concerned, the currently most efficient constructions only rely on weak cryptographic assumptions, such as security of hash functions. Indeed, despite making use of the underlying algebraic structure, prior lattice-based polynomial commitments still seem to be much behind the hash-based ones. Moreover, security of the aforementioned lattice constructions against quantum adversaries was never formally discussed.

In this work, we bridge the gap and propose the first (asymptotically and concretely) efficient lattice-based polynomial commitment with transparent setup and post-quantum security. Our interactive variant relies on the standard (Module-)SIS problem, and can be made non-interactive in the random oracle model using Fiat-Shamir transformation. In addition, we equip the scheme with a knowledge soundness proof against quantum adversaries which can be of independent interest. In terms of concrete efficiency, for $L=2^{20}$ our scheme yields proofs of size $2$X smaller than the hash-based \textsf{FRI} commitment (Block et al., Asiacrypt 2023), and $70$X smaller than the very recent lattice-based construction by Albrecht et al. (Eurocrypt 2024).

Threshold variants of the Schnorr signature scheme have recently been at the center of attention due to their applications to Bitcoin, Ethereum, and other cryptocurrencies. However, existing constructions for threshold Schnorr signatures among a set of $n$ parties with corruption threshold $t_c$ suffer from at least one of the following drawbacks: (i) security only against static (i.e., non-adaptive) adversaries, (ii) cubic or higher communication cost to generate a single signature, (iii) strong synchrony assumptions on the network, or (iv) $t_c+1$ are sufficient to generate a signature, i.e., the corruption threshold of the scheme equals its reconstruction threshold. Especially (iv) turns out to be a severe limitation for many asynchronous real-world applications where $t_c < n/3$ is necessary to maintain liveness, but a higher signing threshold of $n-t_c$ is needed. A recent scheme, ROAST, proposed by Ruffing et al. (ACM CCS `22) addresses (iii) and (iv), but still falls short of obtaining subcubic complexity and adaptive security.

In this work, we present HARTS, the first threshold Schnorr signature scheme to incorporate all these desiderata. More concretely:

- HARTS is adaptively secure and remains fully secure and operational even under asynchronous network conditions in the presence of up to $t_c < n/3$ malicious parties. This is optimal.

- HARTS outputs a Schnorr signature of size $\lambda$ with a near-optimal amortized communication cost of $O(\lambda n^2 \log{n})$ bits and $O(1)$ rounds per signature.

- HARTS is a high-threshold scheme: no fewer than $t_r+1$ signature shares can be combined to yield a full signature, where $t_r\geq 2n/3 > 2t_c$. This is optimal.

We prove our result in a modular fashion in the algebraic group model. At the core of our construction, we design a new simple, and adaptively secure high-threshold AVSS scheme which may be of independent interest.

In this paper, we present an efficient attack against ${\tt PROV}$, a recent variant of the popular Unbalanced Oil and Vinegar (${\tt UOV}$) multivariate signature scheme, that has been submitted to the ongoing ${\tt NIST}$ standardization process for additional post-quantum signature schemes. A notable feature of ${\tt PROV}$ is its proof of security, namely, existential unforgeability under a chosen-message attack (${\tt EUF-CMA}$), assuming the hardness of solving the system formed by the public-key non-linear equations. We present a polynomial-time key-recovery attack against the first specification of ${\tt PROV}$ (v$1.0$). To do so, we remark that a small fraction of the ${\tt PROV}$ secret-key is leaked during the signature process. Adapting and extending previous works on basic ${\tt UOV}$, we show that the entire secret-key can be then recovered from such a small fraction in polynomial-time. This leads to an efficient attack against ${\tt PROV}$ that we validated in practice. For all the security parameters suggested in by the authors of ${\tt PROV}$, our attack recovers the secret-key in at most $8$ seconds. We conclude the paper by discussing the apparent mismatch between such a practical attack and the theoretical security claimed by ${\tt PROV}$ designers. Our attack is not structural but exploits that the current specification of ${\tt PROV}$ differs from the required security model. A simple countermeasure makes ${\tt PROV}$ immune against the attack presented here and led the designers to update the specification of ${\tt PROV}$ (v$1.1$).

Event date: 13 August to 14 August 2024
Submission deadline: 15 April 2024
Notification: 14 June 2024

Event date: 9 September to 20 December 2024
Submission deadline: 15 March 2024

Post Doc positions in the Mathematics Department of IISER Pune. Positions are not only for cryptography but for the broader field of Mathematics. However cryptography is included in that.

Closing date for applications:

Contact: math.postdocapplications@iiserpune.ac.in

More information: https://www.iiserpune.ac.in/announcements/10/postdoctoral-positions-in-mathematics

Blanqet is a new company that is focused on shaping the future of cryptography in a quantum world. Current members include faculty from Computer Science, Mathematics, Physics and Law at the University of Chicago and Penn State University. For more information see our website, www.blanqet.net.

We are looking to hire several researchers to join our Chicago based team for periods of one to three years with the potential for longer employment. Our focus is on imaginative individuals who are devoted to both research and its practical realization. Relevant areas of interest include, but are not limited to, cryptography, quantum and post-quantum cryptography, computer security, computational algebra and number theory.

Successful candidates will have the opportunity to work alongside other researchers at Blanqet and at the nearby University of Chicago. Joint academic affiliations with the University of Chicago are possible when appropriate.

Applicants are expected to have (or expect to soon have) a Ph.D. in computer science, mathematics, physics or a related area. To apply, submit a curriculum vitae (including a list of publications), and a brief description of your research interests (description of research interests not to exceed two pages, more or less, and arrange for three letters of reference. Applications and letters should be sent via email to contact@blanqet.net. We will make offers on a rolling basis with flexibility as to the start date.

Closing date for applications:

Contact: contact@blanqet.net

Our chair performs research and teaching in the area of IT Security with a strong focus on Network Security and Online Privacy. Our goal is to advance the state of the art in research and to educate qualified computer scientists in the area of IT Security who are able to meet the challenges of the growing demand on securing IT Systems and provide data protection in various areas of our life and society. More information about us can be found at https://www.b-tu.de/en/fg-it-sicherheit.

Tasks:

• Active research in the area of intrusion detection systems (IDS) for critical infrastructures, secure cyber-physical systems, and artificial intelligence / machine learning for traffic analysis
• Implementation and evaluation of new algorithms and methods
• Cooperation and knowledge transfer with industrial partners
• Publication of scientific results
• Assistance with teaching

Requirements:

• Master’s degree (or equivalent) and PhD degree (only for PostDocs) in Computer Science or related disciplines
• Strong interest in IT security and/or networking and distributed systems
• Knowledge of at least one programming language (C++, Java, etc.) and one scripting language (Perl, Python, etc.) or strong willingness to quickly learn new programming languages
• Linux/Unix skills
• Knowledge of data mining, machine learning, statistics and result visualization concepts is of advantage
• Excellent working knowledge of English; German is of advantage
• Excellent communication skills

Applications containing the following documents:

• A detailed Curriculum Vitae
• Transcript of records from your Master studies
• An electronic version of your Master thesis, if possible should be sent in a single PDF file as soon as possible, but not later than 15.03.2024 at itsec-jobs.informatik@lists.b-tu.de. Applications sent to email addresses other than that will be automatically discarded.

Closing date for applications:

Contact:

For more information about the vacant position please contact Prof. A. Panchenko (E-Mail: itsec-jobs.informatik@lists.b-tu.de).

More information: https://www.informatik.tu-cottbus.de/~andriy/phd-ad-btu_en.pdf

Traditional STARKs require a cyclic group of a smooth order in the field. This allows efficient interpolation of points using the FFT algorithm, and writing constraints that involve neighboring rows. The Elliptic Curve FFT (ECFFT, Part I and II) introduced a way to make efficient STARKs for any finite field, by using a cyclic group of an elliptic curve. We show a simpler construction in the lines of ECFFT over the circle curve $x^2 + y^2 = 1$. When $p + 1$ is divisible by a large power of $2$, this construction is as efficient as traditional STARKs and ECFFT. Applied to the Mersenne prime $p = 2^{31} − 1$, which has been recently advertised in the IACR eprint 2023:824, our preliminary benchmarks indicate a speed-up by a factor of $1.4$ compared to a traditional STARK using the Babybear prime $p = 2^{31} − 2^{27} + 1$.

Multivariate cryptography is one of the main candidates for creating post-quantum public key cryptosystems. Especially in the area of digital signatures, there exist many practical and secure multivariate schemes. The signature schemes UOV and Rainbow are two of the most promising and best studied multivariate schemes which have proven secure for more than a decade. However, so far the security of multivariate signature schemes towards physical attacks has not been appropriately assessed. Towards a better understanding of the physical security of multivariate signature schemes, this paper presents fault attacks against SingleField schemes, especially UOV and Rainbow. Our analysis shows that although promising attack vectors exist, multivariate signature schemes inherently offer a good protection against fault attacks.

The learning parity with noise (LPN) problem has been widely utilized in classical cryptography to construct cryptographic primitives. Various variants of LPN have been proposed, including LPN over large fields and LPN with regular noise, depending on the underlying space and the noise regularity. These LPN variants have proven to be useful in constructing cryptographic primitives.

We propose an improvement to the Gaussian elimination attack, which is also known as Prange's information set decoding algorithm, for solving the LPN problem. Contrary to prevailing knowledge, we find that the Gaussian elimination attack is highly competitive and currently the best method for solving LPN over large fields. Our improvement involves applying partial Gaussian elimination repeatedly, rather than the whole Gaussian algorithm, which we have named the ``Reduce and Prange's algorithm".

Moreover, we provide two applications of Reduce and Prange algorithms: One is the hybrid algorithm of ours and Berstein, Lange and Peters's algorithm at PQCrypto'08, and the other one is Reduce and Prange algorithm for LPN with regular noise.

Last, we provide a concrete estimation of the bit-security of LPN variants using our Reduce and Prange's frameworks. Our results show that the bit-security of LPN over $\mathbb{F}_q$ is reduced by 5-11 bits when $\log q = 128$ compared to previous analysis by Liu et al. (will appear at Eurocrypt'24). Furthermore, we show that our algorithm outperforms recent work by Briaud and Øygard (Eurocrypt'23) and Liu et al. for certain parameters. It reduces the bit-security of LPN with regular noise by 5-28 bits.

Multi-user (mu) security considers large-scale attackers that, given access to a number of cryptosystem in stances, attempt to compromise at least one of them. We initiate the study of mu security of the so-called GGMtree that stems from the PRG-to-PRF transformation of Goldreich, Goldwasser, and Micali, with a goal to provide references for its recently popularized use in applied cryptography. We propose a generalized model for GGM trees and analyze its mu prefix-constrained PRF security in the random oracle model. Our model allows to derive concrete bounds and improvements for various protocols, and we showcase on the Bitcoin-Improvement-Proposal standard Bip32 hierarchical wallets and function secret sharing (FSS) proto cols. In both scenarios, we propose improvements with better performance and concrete security bounds at the same time. Compared with the state-of-the-art designs, our SHACAL3- and KeccaK-?-based Bip32 vari ants reduce the communication cost of MPC-based implementations by 73.3%∼93.8%, while our AES-based FSS substantially improves mu security while reducing computations by 50%.

We present a new method for efficient look-up table (LUT) evaluation in homomorphic encryption (HE), based on Ring-LWE-based HE schemes, including both integer-message schemes such as Brakerski-Gentry-Vaikuntanathan (BGV) and Brakerski/Fan-Vercauteren (BFV), and complex-number-message schemes like the Cheon-Kim-Kim-Song (CKKS) scheme. Our approach encodes bit streams into codewords and translates LUTs into low-degree multivariate polynomials, allowing for the simultaneous evaluation of multiple independent LUTs with minimal overhead. To mitigate noise accumulation in the CKKS scheme, we propose a novel noise-reduction technique, accompanied by proof demonstrating its effectiveness in asymptotically decreasing noise levels. We demonstrate our algorithm's effectiveness through a proof-of-concept implementation, showcasing significant efficiency gains, including a 0.029ms per slot evaluation for 8-input, 8-output LUTs and a 280ms amortized decryption time for AES-128 using CKKS on a single GPU. This work not only advances LUT evaluation in HE but also introduces a transciphering method for the CKKS scheme utilizing standard symmetric-key encryption, bridging the gap between discrete bit strings and numerical data.