International Association
for Cryptologic Research

Sensitive pictures such as passport photos and nudes are commonly shared through mobile chat applications. One popular strategy for the privacy protection of this material is to use ephemeral messaging features, such as the view once snaps in Snapchat. However, design limitations and implementation bugs in messaging apps may allow attackers to bypass the restrictions imposed by those features on the received material. One way by which attackers may accomplish so is by tampering with the software stack on their own devices. In this paper, we propose and test a protection strategy based on a multiplatform system that encrypts and decrypts sensitive pictures on top of messaging apps and performs remote attestation with available app integrity APIs to safeguard its security. Our analysis and experiments show that, compared to previous proposals for image encryption in a middleware, remote attestation offers increased security, adds privacy benefits, simplifies integration, and improves usability by not requiring users to exchange key material a priori. In our experiments, it incurs an added average latency of 3.8 and 4.5 seconds when sending and receiving private pictures, respectively.

The so-called Gaussian template attacks (TA) is one of the optimal Side-Channel Analyses (SCA) when the measurements are captured with normal noise. In the SCA literature, several optimizations of its implementation are introduced, such as coalescence and spectral computation. The coalescence consists of averaging traces corresponding to the same plaintext value, thereby coalescing (synonymous: compacting) the dataset. Spectral computation consists of sharing the computational workload when estimating likelihood across key hypotheses. State-of-the-art coalescence leverages the Law of Large Numbers (LLN) to compute the mean of equivalent traces. This approach comes with a drawback because the LLN is just an asymptotic approximation. So it does not lead to an exact Template Attack, especially for a few number of traces. In this paper, we introduce a way of calculating the TA exactly and with the same computational complexity (using the spectral approach), without using the LLN, regardless of the number of messages. For the experimental validation of this approach, we use the ANSSI SCA Database (ASCAD), with different numbers of messages and different amounts of samples per trace. Recall that this dataset concerns a software implementation of AES-128 bits, running on an ATMEGA-8515 microprocessor.

The growing adoption of secure multi-party computation (MPC) has driven the development of efficient symmetric key primitives tailored for MPC. Recent advancements, such as the alternating moduli paradigm, have shown promise but leave room for cryptographic and practical improvements. In this paper, we analyze a family of weak pseudorandom functions (wPRF) proposed at Crypto 2024, focusing on the One-to-One parameter sets. We demonstrate that these configurations fail to achieve their intended one-to-one mappings and exploit this observation to develop an efficient key recovery attack. The attacks reveal significant vulnerabilities, reducing the complexity of key recovery to O(2^(λ/2) log_2 (λ)) for the Standard One-to-One wPRF and O(2^(0.84λ)) for the Reversed Moduli variant– both substantially below their claimed λ-bit security. We validate our findings through experimental evaluations, confirming alignment between predicted and observed attack complexities.

The impossible differential (ID) attack is crucial for analyzing the strength of block ciphers. The critical aspect of this technique is to identify IDs, and the researchers introduced several methods to detect them. Recently, the researchers extended the mixed-integer linear programming (MILP) approach by partitioning the input and output differences to identify IDs. The researchers proposed techniques to determine the representative set and partition table of a set over any nonlinear function. In this paper, we introduce a deterministic algorithm using the greedy approach~\cite{cormen2022introduction} for finding the representative set and partition table of a set over any nonlinear function. This algorithm iteratively selects the set that covers the maximum uncovered elements of the target set. This performs better than the existing algorithms in terms of time complexity. We use this algorithm to compute the representative sets and partition tables of the two block ciphers, IVLBC and GIFT-64, and identify 6-round IDs for them. Our research contributes a deterministic algorithm to compute the representative set and partition table of a set over any nonlinear function with at most 16-bit input size.

The design of tweakable wide block ciphers has advanced significantly over the past two decades. This evolution began with the approach of designing a wide block cipher by Naor and Reingold. Since then, numerous tweakable wide block ciphers have been proposed, many of which build on existing block ciphers and are secure up to the birthday bound for the total number of blocks queried. Although there has been a slowdown in the development of tweakable wide block cipher modes in last couple of years, the latest NIST proposal for accordion modes has reignited interest and momentum in the design and analysis of these ciphers. Although new designs have emerged, their security often falls short of optimal (i.e., $n$-bit) security, where $n$ is the output size of the primitive. In this direction, designing an efficient tweakable wide block cipher with $n$-bit security seems to be an interesting research problem. An optimally secure tweakable wide block cipher mode can easily be turned into a misuse-resistant RUP secure authenticated encryption scheme with optimal security. This paper proposes $\textsf{HCTR+}$, which turns an $n$-bit tweakable block cipher (TBC) with $n$-bit tweak into a variable input length tweakable block cipher. Unlike tweakable $\textsf{HCTR}$, $\textsf{HCTR+}$ ensures $n$-bit security regardless of tweak repetitions. We also propose two TBC-based almost-xor-universal hash functions, named $\textsf{PHASH+}$ and $\textsf{ZHASH+}$, and use them as the underlying hash functions in the $\textsf{HCTR+}$ construction to create two TBC-based $n$-bit secure tweakable wide block cipher modes, $\textsf{PHCTR+}$ and $\textsf{ZHCTR+}$. Experimental results show that both $\textsf{PHCTR+}$ and $\textsf{ZHCTR+}$ exhibit excellent software performance when their underlying TBC is instantiated with $\textsf{Deoxys-BC-128-128}$.

One of the primary approaches used to construct lattice-based signature schemes is through the “Fiat-Shamir with aborts” methodology. Such a scheme may abort and restart during signing which corresponds to rejection sampling produced signatures to ensure that they follow a distribution that is independent of the secret key. This rejection sampling is only feasible when the output distribution is sufficiently wide, limiting how compact this type of signature schemes can be.

In this work, we develop a new method to construct signatures influenced by the rejection condition. This allows our rejection sampling to target significantly narrower output distributions than previous approaches, allowing much more compact signatures. The combined size of a signature and a verification key for the resulting scheme is less than half of that for ML-DSA and comparable to that of compact hash-and-sign lattice signature schemes, such as Falcon.

Recent work proposed by Bernstein et al. (from EPRINT 2024) identified two timing attacks, KyberSlash1 and KyberSlash2, targeting ML-KEM decryption and encryption algorithms, respectively, enabling efficient recovery of secret keys. To mitigate these vulnerabilities, correctives were promptly applied across implementations. In this paper, we demonstrate a very simple side-channel-assisted power analysis attack on the patched implementations of ML-KEM. Our result showed that original timing leakage can be shifted to power consumption leakage that can be exploited on specific data. We performed a practical validation of this attack on both the standard and a shuffled implementations of ML-KEM on a Cortex-M4 platform, confirming its effectiveness. Our approach enables the recovery of the ML-KEM secret key in just 30 seconds for the standard implementation, and approximately 3 hours for the shuffled implementation, achieving a 100% success rate in both cases.

Multi-input functional encryption is a primitive that allows for the evaluation of an $\ell$-ary function over multiple ciphertexts, without learning any information about the underlying plaintexts. This type of computation is useful in many cases where one has to compute over encrypted data, such as privacy-preserving cloud services, federated learning, or more generally delegation of computation from multiple clients. It has recently been shown by Alborch et al. in PETS '24 to be useful to construct a randomized functional encryption scheme for obtaining differentially private data analysis over an encrypted database supporting linear queries.

In this work we propose the first secret-key multi-input quadratic functional encryption scheme satisfying simulation security. Current constructions supporting quadratic functionalities, proposed by Agrawal et al. in CRYPTO '21 and TCC '22, only reach indistinguishibility-based security. Our proposed construction is generic, and for a concrete instantiation, we propose a new function-hiding inner-product functional encryption scheme proven simulation secure against one challenge ciphertext in the standard model, which is of independent interest. We then use these two results to construct an efficient randomized quadratic functional encryption scheme, from which we obtain differentially private data analysis over an encrypted database supporting quadratic queries. Finally, we give and fully benchmark an implementation of the randomized scheme. This work is an extended version of the paper "Simulation Secure Multi-Input Quadratic Functional Encryption" at SAC '24, where the multi-input quadratic functional encryption scheme and function-hiding inner-product functional encryption schemes were first presented (Section 3 and Seciton 4).

At FSE'15, Mennink introduced two tweakable block ciphers, $\widetilde{F}[1]$ and $\widetilde{F}[2]$, both utilizing an $n$-bit tweak. It was demonstrated that $\widetilde{F}[1]$ is secure for up to $2^{2n/3}$ queries, while $\widetilde{F}[2]$ is secure for up to $2^n$ queries, assuming the underlying block cipher is an ideal cipher with $n$-bit key and $n$-bit data. Later, at ASIACRYPT'16, Wang et al. showed a birthday bound attack on Mennink's design (which was later corrected in the eprint version {\textbf eprint 2015/363}) and proposed 32 new candidates for tweakable block ciphers that are derived from $n$-bit ideal block ciphers. It was shown that all the $32$ constructions are provably secure up to $2^n$ queries. All the proposed designs by both Mennink and Wang et al. admit only $n$-bit tweaks. In FSE'23, Shen and Standaert proposed a tweakable block cipher, $\widetilde{G2}$, which uses $2n$-bit tweaks and is constructed from three $n$-bit block cipher calls, proving its security up to $n$ bits, assuming that the underlying block cipher is an ideal cipher. They have also shown that it is impossible to design a tweakable block cipher with $2n$-bit tweaks using only two $n$-bit block cipher calls while achieving security beyond the birthday bound. In this paper, we advance this research further. We show that any tweakable block cipher design with $3n$-bit tweaks based on only three block cipher calls, where at least one key is tweak-independent, is vulnerable to a birthday bound distinguishing attack. We then propose a tweakable block cipher, $\widetilde{\textsf{G}_3}^*$ that uses three block cipher calls and admits $3n$-bit tweaks, achieves security up to $O(2^{2n/3})$ queries when all three block cipher keys are tweak-dependent. Furthermore, we prove that using four ideal block cipher calls, with at least one key being tweak-dependent, is necessary and sufficient to achieve $n$-bit security for a tweakable block cipher that admits $3n$-bit tweaks. Finally, we propose a tweakable block cipher, $\widetilde{\textsf{G}_r}$, which uses $(r+1)$ block cipher calls and processes $rn$-bit tweaks, achieving security up to $O(2^n)$ queries when at least one block cipher key is tweak-dependent.

Garbled Circuits are essential building blocks in cryptography, and extensive research has explored their construction from both applied and theoretical perspectives. However, a challenge persists: While theoretically designed garbled circuits offer optimal succinctness--remaining constant in size regardless of the underlying circuit’s complexit--and are reusable for multiple evaluations, their concrete computational costs are prohibitively high. On the other hand, practically efficient garbled circuits, inspired by Yao’s garbled circuits, encounter limitations due to substantial communication bottlenecks and a lack of reusability.

To strike a balance, we propose a novel concept: online-offline garbling. This approach leverages instance-independent and (partially) reusable preprocessing during an offline phase, to enable the creation of constant-size garbled circuits in an online phase, while maintaining practical efficiency. Specifically, during the offline stage, the garbler generates and transmits a reference string, independent of the computation to be performed later. Subsequently, in the online stage, the garbler efficiently transforms a circuit into a constant-size garbled circuit. The evaluation process relies on both the reference string and the garbled circuit.

We demonstrate that by leveraging existing tools such as those introduced by Applebaum et al. (Crypto’13) and Chongwon et al. (Crypto’17), online-offline garbling can be achieved under a variety of assumptions, including the hardness of Learning With Errors (LWE), Computational Diffie-Hellman (CDH), and factoring. In contrast, without the help of an offline phase, constant-size garbling is only feasible under the LWE and circular security assumptions, or the existence of indistinguishability obfuscation. However, these schemes are still very inefficient, several orders of magnitude more costly than Yao-style garbled circuits.

To address this, we propose a new online-offline garbling scheme based on Ring LWE. Our scheme offers both asymptotic and concrete efficiency. It serves as a practical alternative to Yao-style garbled circuits, especially in scenarios where online communication is constrained. Furthermore, we estimate the concrete latency using our approach in realistic settings and demonstrate that it is 2-20X faster than using Yao-style garbled circuits. This improvement is estimated without taking into account parallelization of computation, which can lead to further performance improvement using our scheme.

The SPDM (Security Protocol and Data Model) protocol is a standard under development by the DMTF consortium, and supported by major industry players including Broadcom, Cisco, Dell, Google, HP, IBM, Intel, and NVIDIA. SPDM 1.2 is a complex protocol that aims to provide platform security, for example for communicating hardware components or cloud computing scenarios. In this work, we provide the first holistic, formal analysis of SPDM 1.2: we model the full protocol flow of SPDM considering all of its modes – especially the complex interaction between its different key-exchange modes – in the framework of the Tamarin prover, making our resulting model one of the most complex Tamarin models to date. To our surprise, Tamarin finds a cross-protocol attack that allows a network attacker to completely break authentication of the pre-shared key mode. We implemented our attack on the SPDM reference implementation, and reported the issue to the SPDM developers. DMTF registered our attack as a CVE with CVSS rating 9 (critical). We propose a fix and develop the first formal symbolic proof using the Tamarin prover for the fixed SPDM 1.2 protocol as a whole. The resulting model of the main modes and their interactions is highly complex, and we develop supporting lemmas to enable proving properties in the Tamarin prover, including the absence of all cross-protocol attacks. Our fix has been incorporated into both the reference implementation and the newest version of the standard. Our results highlight the need for a holistic analysis of other internet standards and the importance of providing generalized security guarantees across entire protocols.

The CRYSTALS-Dilithium digital signature scheme, selected by NIST as a post-quantum cryptography (PQC) standard under the name ML-DSA, employs a public key compression technique intended for performance optimization. Specifically, the module learning with error instance $({\bf A}, {\bf t})$ is compressed by omitting the low-order bits ${\bf t_0}$ of the vector ${\bf t}$. It was recently shown that knowledge of ${\bf t_0}$ enables more effective side-channel attacks on Dilithium implementations. Another recent work demonstrated a method for reconstructing ${\bf t_0}$ from multiple signatures. In this paper, we build on this method by applying profiled deep learning-assisted side-channel analysis to partially recover the least significant bit of ${\bf t_0}$ from power traces. As a result, the number of signatures required for the reconstruction of ${\bf t_0}$ can be reduced by roughly half. We demonstrate how the new ${\bf t_0}$ reconstruction method enhances the efficiency of recovering the secret key component ${\bf s}_1$, and thus facilitates digital signature forgery, on an ARM Cortex-M4 implementation of Dilithium.

We present the first public and in-depth cryptanalysis of TEA-3, a stream cipher used in TETRA radio networks that was kept secret until recently. While the same also holds for the six other TETRA encryption algorithms, we pick TEA-3 to start with as (i) it is not obviously weakened as TEA-{1,4,7} but (ii) in contrast to TEA-2 it is approved only for extra-European emergency service, and (iii) as already noted by [MBW23] the TEA-3 design surprisingly contains a non-bijective S-box. Most importantly, we show that the 80-bit non-linear feedback shift register operating on the key decomposes into a cascade of two 40-bit registers. Although this hints at an intentional weakness at first glance, we are not able to lift our results to a practical attack. Other than that, we show how the balanced non-linear feedback functions used in the state register of TEA-3 can be constructed.

Local Differential Privacy (LDP) mechanisms consist of (locally) adding controlled noise to data in order to protect the privacy of their owner. In this paper, we introduce a new cryptographic primitive called LDP commitment. Usually, a commitment ensures that the committed value cannot be modified before it is revealed. In the case of an LDP commitment, however, the value is revealed after being perturbed by an LDP mechanism. Opening an LDP commitment therefore requires a proof that the mechanism has been correctly applied to the value, to ensure that the value is still usable for statistical purposes. We also present a security model for this primitive, in which we define the hiding and binding properties. Finally, we present a concrete scheme for an LDP staircase mechanism (generalizing the randomized response technique), based on classical cryptographic tools and standard assumptions. We provide an implementation in Rust that demonstrates its practical efficiency (the generation of a commitment requires just a few milliseconds). On the application side, we show how our primitive can be used to ensure simultaneously privacy, usability and traceability of medical data when it is used for statistical studies in an open science context. We consider a scenario where a hospital provides sensitive patients data signed by doctors to a research center after it has been anonymized, so that the research center can verify both the provenance of the data (i.e. verify the doctors’ signatures even though the data has been noised) and that the data has been correctly anonymized (i.e. is usable even though it has been anonymized).

TU Wien is Austria's largest institution of research and higher education in the fields of technology and natural sciences. The Research Unit of Privacy Enhancing Technologies at TU Wien is offering a position as university assistant post-doc (all genders) limited to expected 6 years for 40 hours/week. Expected start: January 2025 but negotiable.

Topics of interest include (but are not limited to):
-Privacy preserving cryptocurrencies
-Efficient proof systems such as (non-interactive) zero-knowledge, SNARKs, etc.
-Cryptographic protocols
-Functional encryption
-Fully homomorphic encryption
-Information-theoretic approaches such as differential privacy

Your profile:
-Completion of an excellent doctorate in Computer Science or a closely related field
-Strong background in cryptography, privacy-preserving mechanisms, or data security
-In-depth knowledge and experience in at least one subject area: secure computation, differential privacy, anonymous communication systems, privacy-preserving machine learning, cryptocurrencies, cryptographic protocols, identity management, homomorphic encryption, or zero-knowledge proofs
- An outstanding publication record in top conferences, e.g., CCS, Crypto, Eurocrypt, Usenix Security, NDSS, EEE S&P,...

Salary: Entry level salary is determined by the pay grade B1 of the Austrian collective agreement for university staff. This is a minimum of currently EUR 4,932.90/month gross, 14 times/year for 40 hours/week.

Deadline: January 9th, 2025.

Application only via: https://jobs.tuwien.ac.at/Job/245103

Closing date for applications:

Contact: Univ. Prof. Dr. Dominique Schröder

More information: https://jobs.tuwien.ac.at/Job/245103

The University of Klagenfurt is pleased to announce the following open position at the Department of Artificial Intelligence and Cybersecurity, at the Faculty of Technical Sciences.

Assistant Professor (postdoc), non-tenure track (limited to 6 years)

Responsibilities:

• Independent research with the aim of habilitation, with a specific emphasis on research in cybersecurity such as cryptography, side-channel analysis, efficient implementation, high-assurance software
• Independent delivery of courses using established and innovative methods (e.g. digital teaching)
• Participation in the research and teaching projects run by the organisational unit
• Supervision of students
• Participation in organisational and administrative tasks and in quality assurance measures
• Contribution to expanding the international scientific and cultural contacts of the organisational unit
• Participation in public relations activities

Requirements:

• Doctoral degree in computer science or a related field, completed at a domestic or foreign higher education institution.
• Strong background and practical experience in one or more of the following fields: cryptography, side-channel analysis, efficient implementation, high-assurance software
• Proven academic track record via accepted papers in a reputable cybersecurity venue or in venues (journals) of a comparable standing in the areas of cybersecurity
• Solid communication and dissemination skills
• Fluency in English (both written and spoken)

Application deadline: 22nd January 2025

For more information and how to apply, please visit: https://jobs.aau.at/en/job/12-2/

Closing date for applications:

Contact: Chitchanok Chuengsatiansup (chitchanok.chuengsatiansup@aau.at)

More information: https://jobs.aau.at/en/job/12-2/

The University of Klagenfurt invites applications for PhD positions at the Digital Age Research Center and at the Department of Artificial Intelligence and Cybersecurity (Faculty of Technical Sciences).

Responsibilities:

• Autonomous scientific work including the publication of research articles in the field of cybersecurity, with a specific emphasis on cryptography, side-channel analysis, efficient implementation, high-assurance software and related areas
• Independent teaching and assessment
• Contribution to organisational and administrative tasks
• Participation in public relations activities

Requirements:

• Master’s degree at a domestic or foreign higher education institution in computer science or a related field
• Strong background and practical experience in one or more of the following fields: cryptography, side-channel analysis, efficient implementation, high-assurance software
• Solid communication and dissemination skills
• Fluency in English (both written and spoken)

Application deadline: 29th January 2025

For more information and how to apply, please visit: https://jobs.aau.at/job/university-assistant-predoctoral-all-genders-welcome-13/

Closing date for applications:

Contact: Chitchanok Chuengsatiansup (chitchanok.chuengsatiansup@aau.at)

More information: https://jobs.aau.at/job/university-assistant-predoctoral-all-genders-welcome-13/

The Institute of Cybersecurity and Cryptology at the University of Wollongong (Australia) is recruiting two PhD students to carry out cutting-edge research in privacy-preserving post-quantum cryptography. The positions are fully funded for up to 4 years. Applicants are expected to have a strong background in computer science, cryptography and mathematics, as well as proficiency in English. Applications will be processed continuously until the positions are filled.

Closing date for applications:

Contact: Applications (CV, transcripts, contacts for references) can be emailed to Dr Khoa Nguyen (khoa@uow.edu.au).

The Gentry-Peikert-Vaikuntanathan (GPV) framework is utilized for constructing digital signatures, which is proven to be secure in the classical/quantum random-oracle model. Falcon is such a signature scheme, recognized as a compact and efficient signature among NIST-standardized signature schemes. Although a signature scheme based on the GPV framework is theoretically highly secure, it could be vulnerable to side-channel attacks and hence further research on physical attacks is required to make a robust signature scheme. We propose a general secret key recovery attack on GPV signatures using partial information about signatures obtained from side-channel attack. The three main contributions are summarized as follows. First, we introduce, for the first time, a concept of vulnerable partial information of GPV signatures and propose a secret key recovery attack, called OLS attack, which effectively utilizes partial information. In contrast to the approaches of Guerreau et al. (CHES 2022) and Zhang et al. (Eurocrypt 2023), which utilize filtered (or processed) signatures with hidden parallelepiped or learning slice schemes, the OLS attack leverages all the available signatures without filtering. We prove that the secret key recovered by the OLS attack converges to the real secret key in probability as the number of samples increases. Second, we utilize Gaussian leakage as partial information for the OLS attack on Falcon. As a result, the OLS attack shows a significantly higher success rate with fewer samples than the existing attack schemes. Furthermore, by incorporating the DDGR attack, the OLS attack can recover the secret key using much less samples with a success rate close to 100%. Moreover, we propose more efficient OLS attack on Falcon, which reduces the number of required side-channel attacks. Third, we propose an error-tolerant power analysis attack using MAP decoding, which effectively corrects the errors in samples to utilize Gaussian leakage correctly. In conclusion, the OLS attack is expected to strengthen the security of the GPV signatures including Falcon.

In PKC'24, de Saint Guilhem and Pedersen give a pseudorandom function basing on a relaxed group action assumption in the semi-honest setting. Basing on the assumption, they build an oblivious pseudorandom function (OPRF). Later, a recent paper by Levin and Pedersen uses the same function to build a verifiable random function (VRF), using the same assumption.

We give a structural attack on this problem by reducing it to a few group action inverse problems (GAIP/DLog) over small subgroups. This reduction allows us to apply a CRT-based attack to recover the secret key, ultimately lowering the problem’s effective security strength to under 70 classical bits when using CSIDH-512. Hence the strength of their pseudorandom functions is bounded above by the GAIP over the largest prime order subgroup. Clearly, Kuperberg’s subexponential attack can be used to further reduce its quantum security.