International Association
for Cryptologic Research

Koç University College of Engineering invites applications for Full-time Faculty positions in Computer Science and Engineering starting in Fall 2024. Outstanding applicants with strong theoretical research contributions in all areas of computer science and engineering are invited to apply for the position. The ideal candidates are expected to have a visionary research agenda with an exceptional track record in research and publication, demonstrating a deep commitment to academic excellence and innovation; together with a keen commitment to teaching and learning.

Faculty members are expected to teach undergraduate and graduate courses in addition to maintaining a vigorous research program, collaborating across multiple disciplines, and leveraging the research infrastructure of Koç University such as Koç University Is Bank Artificial Intelligence Research Center (KUIS AI) and Koç University Translational Medicine Research Center (KUTTAM).

Koç University is a private, nonprofit institution located on a state-of-the-art campus in Istanbul, Turkey. The medium of instruction is English. Koç University hosts the highest number of European Research Council (ERC) Grant recipients and continues to receive the largest total amount of research funding from Horizon 2020 in the nation.

We are looking for outstanding individuals who are able to build strong research and teaching programs and who can develop into intellectual leaders. It is also important that the candidates interact closely with colleagues across different disciplines and contribute positively to the successful advancement of the College. We offer a competitive salary and benefit package (e.g., housing support, private insurance, K12 package, research startup support).

Applicants should submit their application online at Academic Jobs Online: CV, a statement of teaching interests, a description of the proposed research program, and the names and addresses of at least three references. The evaluation of applications will commence in mid-January and will continue until the positions are filled. All applications will be considered and treated confidentially.

Closing date for applications:

Contact: Questions regarding the position can be directed to Asst. Prof. Gözde Gül Şahin (gosahin{at}ku.edu.tr) and Assoc. Prof. Aykut Erdem (aerdem{at}ku.edu.tr) chair of this faculty search committee.

More information: https://academicjobsonline.org/ajo/jobs/26651

Threshold Implementation (TI) is a well-known Boolean masking technique that provides provable security against side-channel attacks. In the presence of glitches, the probing model was replaced by the so-called glitch-extended probing model which specifies a broader security framework. In CHES 2021, Shahmirzadi et al. introduced a general search method for finding first-order 2-share TI schemes without fresh randomness (under the presence of glitches) for a given encryption algorithm. Although it handles well single-output Boolean functions, this method has to store output shares in registers when extended to vector Boolean functions, which results in more chip area and increased latency. Therefore, the design of TI schemes that have low implementation cost under the glitch-extended probing model appears to be an important research challenge. In this paper, we propose an approach to design the first-order glitch-extended probing secure TI schemes when quadratic functions are employed in the substitution layer. This method only requires a small amount of fresh random bits and a single clock cycle for its implementation. In particular, the random bits in our approach are reusable and compatible with the changing of the guards technique. Our dedicated TI scheme for the AES cipher gives 20.23% smaller implementation area and 4.2% faster encryption compared to the TI scheme of AES (without using fresh randomness) proposed in CHES 2021. Additionally, we propose a parallel implementation of two S-boxes that further reduces latency (about 39.83%) at the expense of increasing the chip area by 9%. We have positively confirmed the security of AES under the glitch-extended probing model using the verification tool - SILVER and the side-channel leakage assessment method - TVLA.

Decentralized Finance, mushrooming in permissionless blockchains, has attracted a recent surge in popularity. Due to the transparency of permissionless blockchains, opportunistic traders can compete to earn revenue by extracting Miner Extractable Value (MEV), which undermines both the consensus security and efficiency of blockchain systems. The Flashbots bundle mechanism further aggravates the MEV competition because it empowers opportunistic traders with the capability of designing more sophisticated MEV extraction. In this paper, we conduct the first systematic study on DeFi MEV activities in Flashbots bundle by developing ActLifter, a novel automated tool for accurately identifying DeFi actions in transactions of each bundle, and ActCluster, a new approach that leverages iterative clustering to facilitate us to discover known/unknown DeFi MEV activities. Extensive experimental results show that ActLifter can achieve nearly 100% precision and recall in DeFi action identification, significantly outperforming state-of-the-art techniques. Moreover, with the help of ActCluster, we obtain many new observations and discover 17 new kinds of DeFi MEV activities, which occur in 53.12% of bundles but have not been reported in existing studies.

The general quantum approximate optimization algorithm (QAOA) produces approximate solutions for combinatorial optimization problems. The algorithm depends on a positive integer $p$ and the quality of approximation improves as $p$ is increased. In this note, we put some questions about the general QAOA. We also find the recursive QAOA for MaxCut problem is flawed because all quantum gates involved in the algorithm are single qubit gates. No any entangling gate is used, which results in that the quantum computing power cannot be certified for the problem.

This report analyzes the 16 submissions to the Korean post-quantum cryptography (KpqC) competition.

The security of code-based cryptography relies primarily on the hardness of decoding generic linear codes. Until very recently, all the best algorithms for solving the decoding problem were information set decoders ($\mathsf{ISD}$). However, recently a new algorithm called RLPN-decoding which relies on a completely different approach was introduced and it has been shown that RLPN outperforms significantly $\mathsf{ISD}$ decoders for a rather large range of rates. This RLPN decoder relies on two ingredients, first reducing decoding to some underlying LPN problem, and then computing efficiently many parity-checks of small weight when restricted to some positions. We revisit RLPN-decoding by noticing that, in this algorithm, decoding is in fact reduced to a sparse-LPN problem, namely with a secret whose Hamming weight is small. Our new approach consists this time in making an additional reduction from sparse-LPN to plain-LPN with a coding approach inspired by $\mathsf{coded}$-$\mathsf{BKW}$. It outperforms significantly the $\mathsf{ISD}$'s and RLPN for code rates smaller than $0.42$. This algorithm can be viewed as the code-based cryptography cousin of recent dual attacks in lattice-based cryptography. We depart completely from the traditional analysis of this kind of algorithm which uses a certain number of independence assumptions that have been strongly questioned recently in the latter domain. We give instead a formula for the LPN noise relying on duality which allows to analyze the behavior of the algorithm by relying only on the analysis of a certain weight distribution. By using only a minimal assumption whose validity has been verified experimentally we are able to justify the correctness of our algorithm. This key tool, namely the duality formula, can be readily adapted to the lattice setting and is shown to give a simple explanation for some phenomena observed on dual attacks in lattices in [DP23].

The existence of a quantum computer is one of the most significant threats cryptography has ever faced. However, it seems that real world protocols received little attention so far with respect to their future security. Indeed merely relying upon post-quantum primitives may not suffice in order for a security protocol to be resistant in a full quantum world. In this paper, we consider the fundamental UMTS key agreement used in 3G but also in 4G (LTE), and in the (recently deployed) 5G technology. We analyze the protocol in a quantum setting, with quantum communications (allowing superposition queries by the involved parties), and where quantum computation is granted to the adversary. We prove that, assuming the underlying symmetric-key primitive is quantum-secure, the UMTS key agreement is also quantum-secure. We also give a quantum security analysis of the underlying primitives, namely Milenage and TUAK. To the best of our knowledge this paper provides the first rigorous proof of the UMTS key agreement in a strong quantum setting. Our result shows that in the quantum world to come, the UMTS technology remains a valid scheme in order to secure the communications of billions of users.

The Dual-Sieve Attack on Learning with Errors (LWE), or more generally Bounded Distance Decoding (BDD), has seen many improvements in the recent years, and ultimately led to claims that it outperforms the primal attack against certain lattice-based schemes in the PQC standardization process organised by NIST. However, the work of Ducas--Pulles (Crypto '23) revealed that the so-called "Independence Heuristic", which all recent dual attacks used, leads to wrong predictions in a contradictory regime, which is relevant for the security of cryptoschemes. More specifically, the stated distributions of scores for the actual solution and for incorrect candidates were both incorrect.

In this work, we propose to use the weaker heuristic that the output vectors of a lattice sieve are uniformly distributed in a ball. Under this heuristic, we give an analysis of the score distribution in the case of an error of fixed length. Integrating over this length, we extend this analysis to any radially distributed error, in particular the gaussian as a fix for the score distribution of the actual solution. This approach also provides a prediction for the score of incorrect candidates, using a ball as an approximation of the Voronoi cell of a lattice.

We compare the predicted score distributions to extensive experiments, and observe them to be qualitatively and quantitatively quite accurate. This constitutes a first step towards fixing the analysis of the dual-sieve attack: we can now accurately estimate false-positives and false-negatives. Now that the analysis is fixed, one may consider how to fix the attack itself, namely exploring the opportunities to mitigate a large number of false-positives.

Driven by the open problem raised by Hofheinz and Kiltz (Journal of Cryptology, 2012), we study the formalization of lattice-based programmable hash function (PHF), and give three types of concrete constructions by using several techniques such as a novel combination of cover-free sets and lattice trapdoors. Under the Inhomogeneous Small Integer Solution (ISIS) assumption, we show that any (non-trivial) lattice-based PHF is a collision-resistant hash function, which gives a direct application of this new primitive.

We further demonstrate the power of lattice-based PHF by giving generic constructions of signature and identity-based encryption (IBE) in the standard model, which not only provide a way to unify several previous lattice-based schemes using the partitioning proof techniques, but also allow us to obtain new short signature schemes and IBE schemes from (ideal) lattices. Specifically, by instantiating the generic constructions with our Type-II and Type-III PHF constructions, we immediately obtain two short signatures and two IBE schemes with asymptotically much shorter keys. A major downside which inherits from our Type-II and Type-III PHF constructions is that we can only prove the security of the new signatures and IBEs in the bounded security model that the number Q of the adversary’s queries is required to be known in advance. Another downside is that the computational time of our new signatures and IBEs is a linear function of Q, which is large for typical parameters.

To overcome the above limitations, we also give a refined way of using Type-II and Type-III PHFs to construct lattice-based short signatures with short verification keys in the full security model. In particular, our methods depart from the confined guessing technique of B¨ohl et al. (Eurocrypt’13) that was used to construct previous standard model short signature schemes with short verification keys by Ducas and Micciancio (Crypto’14) and by Alperin-Sheriff (PKC’15), and allow us to achieve much tighter security from weaker hardness assumptions.

Credential tweaking attacks use breached passwords to generate semantically similar passwords and gain access to victims' services. These attacks sidestep the first generation of compromised credential checking (C3) services. The second generation of compromised credential checking services, called "Might I Get Pwned" (MIGP), is a privacy-preserving protocol that defends against credential tweaking attacks by allowing clients to query whether a password or a semantically similar variation is present in the server's compromised credentials dataset. The desired privacy requirements include not revealing the user's entered password to the server and ensuring that no compromised credentials are disclosed to the client.

In this work, we formalize the cryptographic leakage of the MIGP protocol and perform a security analysis to assess its impact on the credentials held by the server. We focus on how this leakage aids breach extraction attacks, where an honest-but-curious client interacts with the server to extract information about the stored credentials. Furthermore, we discover additional leakage that arises from the implementation of Cloudflare's deployment of MIGP. We evaluate how the discovered leakage affects the guessing capability of an attacker in relation to breach extraction attacks. Finally, we propose MIGP 2.0, a new iteration of the MIGP protocol designed to minimize data leakage and prevent the introduced attacks.

Nonlinear feedback shift registers (NFSRs) are used in many stream ciphers as their main building blocks. One security criterion for the design of a stream cipher is to assure its keystream has a long period. To meet this criterion, the NFSR used in a stream cipher must have a long state cycle. Further, to simultaneously avoid equivalent keys, the keystream's period is not compressed compared to the NFSR's state cycle length, which can be guaranteed if the NFSR is observable in the sense that any two distinct initial states are distinguishable from their resulting output sequences. The cycle structure of a general NFSR remains an open hard problem. Constructing Fibonacci NFSRs with maximum state cycles has therefore attracted much attention, but so far such Fibonacci NFSRs with known feedback functions have been found only for their stage numbers no greater than 33.

Considering that Galois NFSRs may decrease the area and increase the throughput compared to Fibonacci NFSRs, this paper studies two types of $n$-stage Galois NFSRs, whose state transition matrices are circulant matrices with only one nonzero element of 1 in each column. The cycle structure and observability of both types are disclosed using the semi-tensor product based Boolean network approach. In the first type, each Galois NFSR has the state transition matrix, in which the position of the element 1 in the first column is even. It has the maximum state cycle with an arbitrary stage number and an explicit feedback functions. It is observable if and only if its output function is dependent on the first state bit. In the second type, each Galois NFSR has the state transition matrix, in which the position of the element 1 in the first column is $2^m+1$ with positive integer $m\leq n-1$ for the NFSR's stage number $n$. It has $2^m$ cycles of length $2^{n-m}$, and it is observable if its output function is dependent on all the state bits whose indices are no smaller than $n-m+1$.

Common block ciphers like AES specified by the NIST or KASUMI (A5/3) of GSM are extensively utilized by billions of individuals globally to protect their privacy and maintain confidentiality in daily communications. However, these ciphers lack comprehensive security proofs against the vast majority of known attacks. Currently, security proofs are limited to differential and linear attacks for both AES and KASUMI. For instance, the consensus on the security of AES is not based on formal mathematical proofs but on intensive cryptanalysis over its reduced rounds spanning several decades. In this work, we introduce new security proofs for AES against another attack method: impossible differential (ID) attacks. We classify ID attacks as reciprocal and nonreciprocal ID attacks. We show that sharp and generic lower bounds can be imposed on the data complexities of reciprocal ID attacks on substitution permutation networks. We prove that the minimum data required for a reciprocal ID attack on AES using a conventional ID characteristic is $2^{66}$ chosen plaintexts whereas a nonreciprocal ID attack involves at least $2^{88}$ computational steps. We mount a nonreciprocal ID attack on 6-round AES for 192-bit and 256-bit keys, which requires only $2^{18}$ chosen plaintexts and outperforms the data complexity of any attack. Given its marginal time complexity, this attack does not pose a substantial threat to the security of AES. However, we have made enhancements to the integral attack on 6-round AES, thereby surpassing the longstanding record for the most efficient attack after a period of 23 years.

Anonymous credentials are cryptographic mechanisms enabling users to authenticate themselves with a fine-grained control on the information they leak in the process. They have been the topic of countless papers which have improved the performance of such mechanisms or proposed new schemes able to prove ever-more complex statements about the attributes certified by those credentials. However, whereas these papers have studied in depth the problem of the information leaked by the credential and/or the attributes, almost all of them have surprisingly overlooked the information one may infer from the knowledge of the credential issuer.

In this paper we address this problem by showing how one can efficiently hide the actual issuer of a credential within a set of potential issuers. The novelty of our work is that we do not resort to zero-knowledge proofs but instead we show how one can tweak Pointcheval-Sanders signatures to achieve this issuer-hiding property at a very low cost. This results in an efficient anonymous credential system that indeed provide a complete control of the information leaked in the authentication process. Our construction is moreover modular and can then fit a wide spectrum of applications, notably for Self-Sovereign Identity (SSI) systems.

We show the following unconditional results on quantum commitments in two related yet different models:

1. We revisit the notion of quantum auxiliary-input commitments introduced by Chailloux, Kerenidis, and Rosgen (Comput. Complex. 2016) where both the committer and receiver take the same quantum state, which is determined by the security parameter, as quantum auxiliary inputs. We show that computationally-hiding and statistically-binding quantum auxiliary-input commitments exist unconditionally, i.e., without relying on any unproven assumption, while Chailloux et al. assumed a complexity-theoretic assumption, ${\bf QIP}\not\subseteq{\bf QMA}$. On the other hand, we observe that achieving both statistical hiding and statistical binding at the same time is impossible even in the quantum auxiliary-input setting. To the best of our knowledge, this is the first example of unconditionally proving computational security of any form of (classical or quantum) commitments for which statistical security is impossible. As intermediate steps toward our construction, we introduce and unconditionally construct post-quantum sparse pseudorandom distributions and quantum auxiliary-input EFI pairs which may be of independent interest.

2. We introduce a new model which we call the common reference quantum state (CRQS) model where both the committer and receiver take the same quantum state that is randomly sampled by an efficient setup algorithm. We unconditionally prove that there exist statistically hiding and statistically binding commitments in the CRQS model, circumventing the impossibility in the plain model.

We also discuss their applications to zero-knowledge proofs, oblivious transfers, and multi-party computations.

This study focuses on spotting and stopping new types of online threats by improving the UGRansome dataset to detect unusual activity in real-time. By blending different machine learning methods, like naïve tree-based ensemble learning and recursive feature elimination (RFE), the research achieves a high accuracy rate of 97%. Naïve Bayes (NB) stands out as the most effective classifier. The suggested setup, combining gradient boosting (GB) and random forest (RF) with NB, effectively identifies and prevents unknown vulnerabilities in computer systems. UGRansome successfully blocks over 100 kilobits per second (kbps) of harmful online traffic by using details pinpointed by the RFE method, specifically uniform resource locators (URLs). This outperforms existing Intrusion Detection System (IDS) datasets. It's particularly good at stopping secure shell attacks, proving the dataset's usefulness in making networks safer. This research marks significant progress in detecting intrusions. The NB model excels in accuracy, precision, and remembering patterns, especially in identifying new threats. Moreover, the suggested naïve tree-based ensemble model shows outstanding accuracy, standing out as the best-performing technique among all models studied. Applying the UGRansome properties-based rule noticeably changes how traffic is sorted, decreasing unknown traffic while increasing unclassified traffic, which requires more investigation.

Lido, the leading Liquidity Staking Derivative (LSD) provider on Ethereum, allows users to stake an arbitrary amount of ETH to receive stETH, which can be integrated with Decentralized Finance (DeFi) protocols such as Aave. The composability between Lido and Aave enables a novel strategy called “leverage staking”, where users stake ETH on Lido to acquire stETH, utilize stETH as collateral on Aave to borrow ETH, and then restake the borrowed ETH on Lido. Users can iteratively execute this process to optimize potential returns based on their risk profile.

This paper systematically studies the opportunities and risks associated with leverage staking. We are the first to formalize the stETH-ETH leverage staking strategy within the Lido-Aave ecosystem. Our empirical study identifies 262 leverage staking positions on Ethereum, with an aggregated staking amount of 295,243 ETH (482M USD). We discover that 90.13% of leverage staking positions have achieved higher returns than conventional staking. Furthermore, we perform stress tests to evaluate the risk introduced by leverage staking under extreme conditions. We find that leverage staking significantly amplifies the risk of cascading liquidations. We hope this paper can inform and encourage the development of robust risk management approaches to protect the Lido-Aave LSD ecosystem.

Quantum no-cloning theorem gives rise to the intriguing possibility of quantum copy protection where we encode a program in a quantum state such that a user in possession of $k$ such states cannot create $k+1$ working copies. Introduced by Aaronson (CCC'09) over a decade ago, copy protection has proven to be notoriously hard to achieve.

In this work, we construct public-key encryption and functional encryption schemes whose secret keys are copy-protected against unbounded collusions in the plain model (i.e. without any idealized oracles), assuming (post-quantum) subexponentially secure $\mathcal{iO}$, one-way functions and LWE. This resolves a long-standing open question of constructing fully collusion-resistant copy-protected functionalities raised by multiple previous works.

Prior to our work, copy-protected functionalities were known only in restricted collusion models where either an a-priori bound on the collusion size was needed, in the plain model with the same assumptions as ours (Liu, Liu, Qian, Zhandry [TCC'22]), or adversary was only prevented from doubling their number of working programs, in a structured quantum oracle model (Aaronson [CCC'09]).

We obtain our results through a novel technique which uses identity-based encryption to construct unbounded collusion resistant copy-protection schemes from $1\to2$ secure schemes. This is analogous to the technique of using digital signatures to construct full-fledged quantum money from single banknote schemes (Lutomirski et al. [ICS'09], Farhi et al. [ITCS'12], Aaronson and Christiano [STOC'12]). We believe our technique is of independent interest.

Along the way, we also construct a puncturable functional encryption scheme whose master secret key can be punctured at all functions $f$ such that $f(m_0) \neq f(m_1)$. This might also be of independent interest.

We demonstrate how to build computationally secure commitment schemes with the aid of quantum auxiliary inputs without unproven complexity assumptions. Furthermore, the quantum auxiliary input can be prepared either (1) efficiently through a trusted setup similar to the classical common random string model, or (2) strictly between the two involved parties in uniform exponential time. Classically this remains impossible without first proving $\mathsf{P} \neq \mathsf{NP}$.

We extend the known pseudorandomness of Ring-LWE to be based on lattices that do not correspond to any ideal of any order in the underlying number field. In earlier works of Lyubashevsky et al (EUROCRYPT 2010) and Peikert et al (STOC 2017), the hardness of RLWE was based on ideal lattices of ring of integers of number fields, which are known to be Dedekind domains. While these works extended Regev's (STOC 2005) quantum polynomial-time reduction for LWE, thus allowing more efficient and more structured cryptosystems, the additional algebraic structure of ideals of Dedekind domains leaves open the possibility that such ideal lattices are not as hard as general lattices.

In this work we show that hardness of $q$-Ring-LWE can be based on worst-case hardness of ideal lattices in arbitrary orders $O$, as long as the order $O$ satisfies the property that $\frac{1}{m}\cdot O$ contains the ring of integers, for some $m$ co-prime to $q$. Further, the hard lattice problems need not be given the order $O$ itself as input. The reduction requires that the noise be a factor $m$ more than the original Ring-LWE reduction. We also show that for the power-of-two cyclotomic number fields, there exist orders with $m=4$ such that non-trivial ideals of the order, which are not contained in the conductor, are non-invertible.

Another reduction shows that hardness of $q$-Ring-LWE can be based on worst-case hardness of lattices that correspond to sum of ideal-lattices in arbitrary and different orders in the number field, as long as the (set of) orders $\{O_i\}$ satisfy the property that $\frac{1}{m}\cdot O_i$ contains the ring of integers, for some $m$ co-prime to $q$. We also show that for the power-of-two cyclotomic number fields, there exist orders $O_1, O_2$ with $m=8$ such that there are ideals $I_1, I_2$ of $O_1, O_2$ resp. with $I_1+ I_2$ not an ideal of any order in the number field.

There appears to be a widespread belief that some processes of selecting cryptosystems are less risky than other processes. As a case study of quantifying the difference in risks, this paper compares the currently-known-failure rates of three large groups of cryptosystems: (1) the round-1 submissions to the NIST Post-Quantum Cryptography Standardization Project, (2) the round-1 submissions not broken by the end of round 1, and (3) the round-1 submissions selected by NIST for round 2 of the same project. These groups of cryptosystems turn out to have currently-known-failure rates that are strikingly high, and that include statistically significant differences across the groups, not matching the pattern of differences that one might expect. Readers are cautioned that the actual failure rates could be much higher than the currently-known-failure rates.