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#### 10 July 2024

###### Paul Grandamme, Pierre-Antoine Tissot, Lilian Bossuet, Jean-Max Dutertre, Brice Colombier, Vincent Grosso

ePrint Report
Physical attacks, and among them fault injection attacks, are a significant threat to the security of embedded systems. Among the means of fault injection, laser has the significant advantage of being extremely spatially accurate. Numerous state-of-the-art studies have investigated the use of lasers to inject faults into a target at run-time. However, the high precision of laser fault injection comes with requirements on the knowledge of the implementation and exact execution time of the victim code. The main contribution of this work is the demonstration on experimental basis that it is also possible to perform laser fault injection on an unpowered device. Specifically, we targeted the Flash non-volatile memory of a 32-bit microcontroller. The advantage of this new attack path is that it does not require any synchronisation between the victim and the attacker. We provide an experimental characterization of this phenomenon with a description of the fault model from the physical level up to the software level. Finally, we applied these results to carry out a persistent fault analysis on a 128-bit AES with a particularly realistic attacker model which reinforces the interest of the PFA.

###### Giacomo Fenzi, Jan Gilcher, Fernando Virdia

ePrint Report
In 2018, Mouha et al. (IEEE Trans. Reliability, 2018) performed a post-mortem investigation of the correctness of reference implementations submitted to the SHA3 competition run by NIST, finding previously unidentified bugs in a significant portion of them, including two of the five finalists. Their innovative approach allowed them to identify the presence of such bugs in a black-box manner, by searching for counterexamples to expected cryptographic properties of the implementations under test. In this work, we extend their approach to key encapsulation mechanisms (KEMs) and digital signature schemes (DSSs). We perform our tests on multiple versions of the LibOQS collection of post-quantum schemes, to capture implementations at different points of the recent Post-Quantum Cryptography Standardization Process run by NIST. We identify multiple bugs, ranging from software bugs (segmentation faults, memory overflows) to cryptographic bugs, such as ciphertext malleability in KEMs claiming IND-CCA security. We also observe various features of KEMs and DSS that do not contradict any security guarantees, but could appear counter-intuitive.

###### Onur İşler

ePrint Report
The use of Internet of Things (IoT) devices in embedded systems has become increasingly popular with advancing technologies. These devices become vulnerable to cyber attacks as they gain popularity. The cryptographic operations performed for the purpose of protection against cyber attacks are crucial to yield fast results in open networks and not slow down network traffic. Therefore, to enhance communication security, studies have been conducted in the literature on using asymmetric encryption and symmetric encryption together in IoT devices for activities such as key sharing, encryption, decryption, data signing, and verifying signed data. In this study, we first propose a cryptographic system engaging of IoT devices operated from a server. Then we do performance analysis of our proposal. In particular, we evaluate the elliptic curve Diffie-Hellman key exchange and elliptic curve digital signature algorithms on the Secp256r1 elliptic curve and AES symmetric encryption via the Micro uECC library conducted with the 32-bit STM32F410RB Nucleo development board microprocessor running at 48 MHz.

###### Jing Tian, Bo Wu, Lang Feng, Haochen Zhang, Zhongfeng Wang

ePrint Report
This paper proposes a fast and efficient FPGA-based hardware-software co-design for the supersingular isogeny key encapsulation (SIKE) protocol controlled by a custom RISC-V processor. Firstly, we highly optimize the core unit, the polynomial-based field arithmetic logic unit (FALU), with the proposed fast convolution-like multiplier (FCM) to significantly reduce the resource consumption while still maintaining low latency and constant time for all the four SIKE parameters. Secondly, we pack the small isogeny and point operations in hardware, devise a coarse-grained reconfigurable hardware architecture (CGRHA) based on FALU as the co-processor, and apply it to the RISC-V core with customized instructions, effectively avoiding extra time consumption for the data exchange with the software side and meanwhile increasing flexibility. Finally, we code the hardware in SystemVerilog language and the software in C language and run experiments on FPGAs. In the co-processor implementation, the experiment results show that our design for the four SIKE parameters achieves 2.6-4.4x speedup and obtains comparable or better area-time product to or than the state-of-the-art. In the hardware-software co-design experiments, we still have the superiority in speed and only <10\% of extra time is introduced by mutual communication.

###### Dario Catalano, Emanuele Giunta, Francesco Migliaro

ePrint Report
The notion of Anamorphic Encryption (Persiano et al. Eurocrypt 2022) aims at establishing private communication against an adversary who can access secret decryption keys and influence the chosen messages. Persiano et al. gave a simple, black-box, rejection sampling-based technique to send anamorphic bits using any IND-CPA secure scheme as underlying PKE.

In this paper however we provide evidence that their solution is not as general as claimed: indeed there exists a (contrived yet secure) PKE which lead to insecure anamorphic instantiations. Actually, our result implies that such stateless black-box realizations of AE are impossible to achieve, unless weaker notions are targeted or extra assumptions are made on the PKE. Even worse, this holds true even if one resorts to powerful non-black-box techniques, such as NIZKs, $ i\mathcal{O} $ or garbling.

From a constructive perspective, we shed light those required assumptions. Specifically, we show that one could bypass (to some extent) our impossibility by either considering a weaker (but meaningful) notion of AE or by assuming the underlying PKE to (always) produce high min-entropy ciphertexts.

Finally, we prove that, for the case of Fully-Asymmetric AE, $ i\mathcal{O}$ can actually be used to overcome existing impossibility barriers. We show how to use $ i\mathcal{O} $ to build Fully-Asymmetric AE (with small anamorphic message space) generically from any IND-CPA secure PKE with sufficiently high min-entropy ciphertexts. Put together our results provide a clearer picture of what black-box constructions can and cannot achieve.

In this paper however we provide evidence that their solution is not as general as claimed: indeed there exists a (contrived yet secure) PKE which lead to insecure anamorphic instantiations. Actually, our result implies that such stateless black-box realizations of AE are impossible to achieve, unless weaker notions are targeted or extra assumptions are made on the PKE. Even worse, this holds true even if one resorts to powerful non-black-box techniques, such as NIZKs, $ i\mathcal{O} $ or garbling.

From a constructive perspective, we shed light those required assumptions. Specifically, we show that one could bypass (to some extent) our impossibility by either considering a weaker (but meaningful) notion of AE or by assuming the underlying PKE to (always) produce high min-entropy ciphertexts.

Finally, we prove that, for the case of Fully-Asymmetric AE, $ i\mathcal{O}$ can actually be used to overcome existing impossibility barriers. We show how to use $ i\mathcal{O} $ to build Fully-Asymmetric AE (with small anamorphic message space) generically from any IND-CPA secure PKE with sufficiently high min-entropy ciphertexts. Put together our results provide a clearer picture of what black-box constructions can and cannot achieve.

###### Poulami Das, Andreas Erwig, Sebastian Faust

ePrint Report
Cryptographic wallets are an essential tool in Blockchain networks to ensure the secure storage and maintenance of an user's cryptographic keys. Broadly, wallets can be divided into three categories, namely custodial, non-custodial, and shared-custodial wallets. The first two are centralized solutions, i.e., the wallet is operated by a single entity, which inherently introduces a single point of failure. Shared-custodial wallets, on the other hand, are maintained by two independent parties, e.g., the wallet user and a service provider, and hence avoid the single point of failure centralized solutions. Unfortunately, current shared-custodial wallets suffer from significant privacy issues.

In our work, we introduce password-authenticated deterministic wallets (PADW), a novel and efficient shared-custodial wallet solution, which exhibits strong security and privacy guarantees. In a nutshell, in a PADW scheme, the secret key of the user is shared between the user and the server. In order to generate a signature, the user first authenticates itself to the server by providing a password and afterwards engages in an interactive signing protocol with the server. Security is guaranteed as long as at most one of the two parties is corrupted. Privacy, on the other hand, guarantees that a corrupted server cannot link a transaction to a particular user. We formally model the notion of PADW schemes and we give an instantiation from blind Schnorr signatures. Our construction allows for deterministic key derivation, a feature that is widely used in practice by existing wallet schemes, and it does not rely on any heavy cryptographic primitives. We prove our scheme secure against adaptive adversaries in the random oracle model and under standard assumptions. That is, our security proof only relies on the assumption that the Schnorr signature scheme is unforgeable and that a public key encryption scheme is CCA-secure.

In our work, we introduce password-authenticated deterministic wallets (PADW), a novel and efficient shared-custodial wallet solution, which exhibits strong security and privacy guarantees. In a nutshell, in a PADW scheme, the secret key of the user is shared between the user and the server. In order to generate a signature, the user first authenticates itself to the server by providing a password and afterwards engages in an interactive signing protocol with the server. Security is guaranteed as long as at most one of the two parties is corrupted. Privacy, on the other hand, guarantees that a corrupted server cannot link a transaction to a particular user. We formally model the notion of PADW schemes and we give an instantiation from blind Schnorr signatures. Our construction allows for deterministic key derivation, a feature that is widely used in practice by existing wallet schemes, and it does not rely on any heavy cryptographic primitives. We prove our scheme secure against adaptive adversaries in the random oracle model and under standard assumptions. That is, our security proof only relies on the assumption that the Schnorr signature scheme is unforgeable and that a public key encryption scheme is CCA-secure.

###### Ke Zhong, Sebastian Angel

ePrint Report
This paper proposes Oryx, a system for efficiently detecting cycles in federated graphs where parts of the graph are held by different parties and are private. Cycle detection is an important building block in designing fraud detection algorithms that operate on confidential transaction data held by different financial institutions. Oryx allows detecting cycles of various length while keeping the topology of the graphs secret, and it does so efficiently; Oryx achieves quasilinear computational complexity and scales well with more machines thanks to a parallel design. Our implementation of Oryx running on a single 32-core AWS machine (for each party) can detect cycles of up to length 6 in under 5 hours in a financial transaction graph that consists of tens of millions of nodes and edges. While the costs are high, adding more machines further reduces the completion time. Furthermore, Oryx is, to our knowledge, the first and only system that can handle this task.

###### Shen Dong, Hongrui Cui, Kaiyi Zhang, Kang Yang, Yu Yu

ePrint Report
Oblivious transfer (OT) is a fundamental cryptographic protocol that plays a crucial role in secure multi-party computation (MPC). Most practical OT protocols by, e.g., Naor and Pinkas (SODA'01) or Chou and Orlandi (Latincrypt'15), are based on Diffie-Hellman (DH)-like assumptions and not post-quantum secure. In contrast, many other components of MPC protocols, including garbled circuits and secret sharings, are post-quantum secure. The reliance on non-post-quantum OT protocols presents a significant security bottleneck with the advent of quantum computing.
In this paper, we address this issue by constructing a simple, efficient OT protocol based on Saber, a Mod-LWR-based key exchange protocol. We implemented our OT protocol and conducted experiments to evaluate its performance. Our results show that our OT protocol significantly outperforms the state-of-the-art Kyber-based post-quantum OT protocol by Masny and Rindal (CCS'19) in terms of both computation and communication costs. Furthermore, the computation speed of our OT protocol is faster than the best-known DH-based OT protocol by Chou and Orlandi (Latincrypt'15), making it competitive to replace DH-based OT in the high-bandwidth network setting.

###### Bilel Zaghdoudi, Maria Potop Butucaru

ePrint Report
This paper reports the experimental results related to lineage event storage via smart contracts deployed on private and public blockchain. In our experiments we measure the following three metrics: the cost to deploy the storage smart contract on the blockchain, which measures the initial expenditure, typically in gas units, required to deploy the smart contract that facilitates lineage event storage, then the time and gas costs needed to store a lineage event. We investigated both single and multi-clients scenarios. We considered the following public blockchains: Hedera, Fantom, Harmony Shard0, Polygon Amoy, Ethereum Sepolia, Optimism Sepolia, Klaytn Baobab and Arbitrum Sepolia. Furthermore, we investigate the performances of Hyperledger Besu with different consensus algorithms as private blockchains.

###### Shintaro Narisada, Hiroki Okada, Kazuhide Fukushima, Takashi Nishide

ePrint Report
We propose time-memory trade-off algorithms for evaluating look-up table (LUT) in both the leveled homomorphic encryption (LHE) and fully homomorphic encryption (FHE) modes in TFHE. For an arbitrary $n$-bit Boolean function, we reduce evaluation time by a factor of $O(n)$ at the expense of an additional memory of "only" $O(2^n)$ as a trade-off: The total asymptotic memory is also $O(2^n)$, which is the same as that of prior works. Our empirical results demonstrate that a $7.8 \times$ speedup in runtime is obtained with a $3.8 \times$ increase in memory usage for 16-bit Boolean functions in the LHE mode. Additionally, in the FHE mode, we achieve reductions in both runtime and memory usage by factors of $17.9 \times$ and $2.5 \times $, respectively, for 8-bit Boolean functions. The core idea is to decompose the function $f$ into sufficiently small subfunctions and leverage the precomputed results for these subfunctions, thereby achieving significant performance improvements at the cost of additional memory.

###### Cecilia Boschini, Darya Kaviani, Russell W. F. Lai, Giulio Malavolta, Akira Takahashi, Mehdi Tibouchi

ePrint Report
A threshold signature scheme splits the signing key among $\ell$ parties, such that any $t$-subset of parties can jointly generate signatures on a given message. Designing concretely efficient post-quantum threshold signatures is a pressing question, as evidenced by NIST's recent call.

In this work, we propose, implement, and evaluate a lattice-based threshold signature scheme, Ringtail, which is the first to achieve a combination of desirable properties: (i) The signing protocol consists of only two rounds, where the first round is message-independent and can thus be preprocessed offline. (ii) The scheme is concretely efficient and scalable to $t \leq 1024$ parties. For $128$-bit security and $t = 1024$ parties, we achieve $13.4$ KB signature size and $10.5$ KB of online communication. (iii) The security is based on the standard learning with errors (LWE) assumption in the random oracle model. This improves upon the state-of-the-art (with comparable efficiency) which either has a three-round signing protocol [Eurocrypt'24] or relies on a new non-standard assumption [Crypto'24].

To substantiate the practicality of our scheme, we conduct the first WAN experiment deploying a lattice-based threshold signature, across 8 countries in 5 continents. We observe that an overwhelming majority of the end-to-end latency is consumed by network latency, underscoring the need for round-optimized schemes.

In this work, we propose, implement, and evaluate a lattice-based threshold signature scheme, Ringtail, which is the first to achieve a combination of desirable properties: (i) The signing protocol consists of only two rounds, where the first round is message-independent and can thus be preprocessed offline. (ii) The scheme is concretely efficient and scalable to $t \leq 1024$ parties. For $128$-bit security and $t = 1024$ parties, we achieve $13.4$ KB signature size and $10.5$ KB of online communication. (iii) The security is based on the standard learning with errors (LWE) assumption in the random oracle model. This improves upon the state-of-the-art (with comparable efficiency) which either has a three-round signing protocol [Eurocrypt'24] or relies on a new non-standard assumption [Crypto'24].

To substantiate the practicality of our scheme, we conduct the first WAN experiment deploying a lattice-based threshold signature, across 8 countries in 5 continents. We observe that an overwhelming majority of the end-to-end latency is consumed by network latency, underscoring the need for round-optimized schemes.

###### Luke Harmon, Gaetan Delavignette, Hanes Oliveira

ePrint Report
In this work we present $\mathsf{HERatio}$, a homomorphic encryption scheme that builds on the scheme of Brakerski, and Fan and Vercauteren. Our scheme naturally accepts Laurent polynomials as inputs, allowing it to work with rationals via their bounded base-$b$ expansions. This eliminates the need for a specialized encoder and streamlines encryption, while maintaining comparable efficiency to BFV. To achieve this, we introduce a new variant of the Polynomial Learning With Errors (PLWE) problem which employs Laurent polynomials instead of the usual ``classic'' polynomials, and provide a reduction to the PLWE problem.

###### John Preuß Mattsson

ePrint Report
Advanced Encryption Standard Galois/Counter Mode (AES-GCM) is the most widely used Authenticated Encryption with Associated Data (AEAD) algorithm in the world. In this paper, we analyze the use of GCM with all the Initialization Vector (IV) constructions and lengths approved by NIST SP 800-38D when encrypting multiple plaintexts with the same key. We derive attack complexities in both ciphertext-only and known-plaintext models, with or without nonce hiding, for collision attacks compromising integrity and confidentiality. Our analysis shows that GCM with random IVs provides less than 128 bits of security. When 96-bit IVs are used, as recommended by NIST, the security drops to less than 97 bits. Therefore, we strongly recommend NIST to forbid the use of GCM with 96-bit random nonces.

###### Falko Strenzke, Johannes Roth

ePrint Report
This work describes vulnerabilities in the specification of the AEAD packets as introduced in the novel LibrePGP specification that is implemented by the widely used GnuPG application and the AES-based AEAD schemes as well as the Key Wrap
Algorithm specified in the Cryptographic Message Syntax (CMS).
These new attacks exploit the possibility to downgrade AEAD or AES Key Wrap ciphertexts to valid legacy CFB- or CBC-encrypted related ciphertexts and require that the attacker learns the content of the legacy decryption result.
This can happen either due to the human recipient returning the decryption output, which has entirely pseudo-random appearance, to the attacker or due to a programmatic decryption oracle in the receiving system.
The attacks effect the decryption of low-entropy plaintext blocks in AEAD ciphertexts and, in the case of LibrePGP, also the manipulation of existing AEAD ciphertexts.
For AES Key Wrap in CMS, full key decryption is possible.
Some of the attacks require multiple successful oracle queries.
The attacks thus demonstrate that CCA2 security is not achieved by the LibrePGP and CMS AEAD or Key Wrap encryption in the presence of a legacy cipher mode decryption oracle.
The proper countermeasure to thwart the attacks is a key derivation that ensures the use of unrelated block cipher keys for the different encryption modes.

###### Banashri Karmakar, Shyam Murthy, Arpita Patra, Protik Paul

ePrint Report
Online ride-sharing services (RSS) have become very popular owing to increased awareness of environmental concerns and as a response to increased traffic congestion. To request a ride, users submit their locations and route information for ride matching to a service provider (SP), leading to possible privacy concerns caused by leakage of users' location data. We propose QuickPool, an efficient SP-aided RSS solution that can obliviously match multiple riders and drivers simultaneously, without involving any other auxiliary server. End-users, namely, riders and drivers share their route information with SP as encryptions of the ordered set of points-of-interest (PoI) of their route from their start to end locations. SP performs a zone based oblivious matching of drivers and riders, based on partial route overlap as well as proximity of start and end points. QuickPool is in the semi-honest setting, and makes use of secure multi-party computation. We provide security proof of our protocol, perform extensive testing of our implementation and show that our protocol simultaneously matches multiple drivers and riders very efficiently. We compare the performance of QuickPool with state-of-the-art works and observe a speed up of 1.6 - 2$\times$.

#### 08 July 2024

###### Matthieu Rambaud

ePrint Report
Blockchain consensus, a.k.a. BFT SMR, are protocols enabling $n$ processes to decide on an ever-growing chain. The fastest known asynchronous one is called 2-chain VABA (PODC'21 and FC'22), and is used as fallback chain in Abraxas* (CCS'23). It has a claimed $9.5\delta$ expected latency when used for a single shot instance, a.k.a. an MVBA.
We exhibit attacks breaking it. Hence, the title of the fastest asynchronous MVBA with quadratic messages complexity goes to sMVBA (CCS'22), with $10\delta$ expected latency.
Our positive contributions are two new and complementary designs.

$\bullet$ 2PAC (2-phase asynchronous consensus). It has a simpler and lighter chaining than in previous approaches. Instantiated with either quadratic or cubic phases of voting, it yields:

2PAC$^\text{lean}$: $+90\%$ throughput and $9.5\delta$ expected latency, with quadratic ($O(n^2)$) messages complexity. In both 2-chain VABA and sMVBA (as if chained, with pipelining), the quorum-certified transactions which were produced in the worst-case 1/3 of views with a slow leader were dumped, so the work was lost. The simpler design of 2PAC inserts such blocks in straight-line in the chain. Thus, contrary to naive uncle-referencing, this comes with no computational overhead, yielding a net $+50\%$ throughput gain over chained sMVBA. Both the remaining throughput and latency ($-0.5\delta$) gains, come from the lighter interactive construction of proofs of consistency appended to proposed blocks, compared to sMVBA.

2PAC$^\text{BIG}$: the fastest asynchronous blockchain consensus with cubic ($O(n^3)$) messages complexity. Fault-free single shot MVBA runs decide in just $4\delta$, as soon as no message is delivered more than twice faster than others: GradedDAG (SRDS'23) required furthermore no messages reordering.

$\bullet$ Super Fast Pipelined Blocks. This is an upgrade of previous approaches for pipelining: in 2-chain VABA, Cordial Miners (DISC'23) and GradedDAG, a block pipelined by a leader in the middle of the view had almost twice larger latency than the non-pipelined block. Our design provides a fast path deciding the pipelined block with even smaller latency than the non-pipelined block. The fast delay is guaranteed in all executions with a fair scheduler, but remarkably, whatever the behaviors of faulty processes. Consistency is preserved by a lightweight mechanism, of one threshold signature appended per proposal. Instantiated with the previous protocols, it yields: s2PAC$^\text{lean}$, with fast decision of pipelined blocks in $4\delta$; s2PAC$^\text{BIG}$, in $3\delta$; and sGradedDAG, in $3\delta$.

$\bullet$ 2PAC (2-phase asynchronous consensus). It has a simpler and lighter chaining than in previous approaches. Instantiated with either quadratic or cubic phases of voting, it yields:

2PAC$^\text{lean}$: $+90\%$ throughput and $9.5\delta$ expected latency, with quadratic ($O(n^2)$) messages complexity. In both 2-chain VABA and sMVBA (as if chained, with pipelining), the quorum-certified transactions which were produced in the worst-case 1/3 of views with a slow leader were dumped, so the work was lost. The simpler design of 2PAC inserts such blocks in straight-line in the chain. Thus, contrary to naive uncle-referencing, this comes with no computational overhead, yielding a net $+50\%$ throughput gain over chained sMVBA. Both the remaining throughput and latency ($-0.5\delta$) gains, come from the lighter interactive construction of proofs of consistency appended to proposed blocks, compared to sMVBA.

2PAC$^\text{BIG}$: the fastest asynchronous blockchain consensus with cubic ($O(n^3)$) messages complexity. Fault-free single shot MVBA runs decide in just $4\delta$, as soon as no message is delivered more than twice faster than others: GradedDAG (SRDS'23) required furthermore no messages reordering.

$\bullet$ Super Fast Pipelined Blocks. This is an upgrade of previous approaches for pipelining: in 2-chain VABA, Cordial Miners (DISC'23) and GradedDAG, a block pipelined by a leader in the middle of the view had almost twice larger latency than the non-pipelined block. Our design provides a fast path deciding the pipelined block with even smaller latency than the non-pipelined block. The fast delay is guaranteed in all executions with a fair scheduler, but remarkably, whatever the behaviors of faulty processes. Consistency is preserved by a lightweight mechanism, of one threshold signature appended per proposal. Instantiated with the previous protocols, it yields: s2PAC$^\text{lean}$, with fast decision of pipelined blocks in $4\delta$; s2PAC$^\text{BIG}$, in $3\delta$; and sGradedDAG, in $3\delta$.

###### Colin O'Flynn

ePrint Report
Measuring the fluctuations of the clock phase of a target was identified as a leakage source on early electromagnetic side-channel investigations. Despite this, only recently was directly measuring the clock phase (or jitter) of digital signals from a target connected to being a source of exploitable leakage. As the phase of a clock output will be related to signal propagation delay through the target, and this propagation delay is related to voltage, this means that most digital devices perform an unintended phase modulation (PM) of their internal voltage onto clock output phases.

This paper first demonstrates an unprofiled CPA attack against a Cortex-M microcontroller using the phase of a clock output, observing the signal on both optically isolated and capacitively isolated paths. The unprofiled attack takes only 2-4x more traces than an attack using a classic shunt-resistor measurement.

It is then demonstrated how the JTAG bypass mode can be used to force a clock through a digital device. This forced clock signal can then be used as a highly effective oscilloscope that is located on the target device. As the attack does not require modifications to the device (such as capacitor removal or heat spreader removal) it is difficult to detect using existing countermeasures. The example attack over JTAG uses an unprofiled CPA attack, requiring only about 5x more traces than an ideal shunt-resistor based measurement. In addition, a version of this attack using a fault correlation analysis attack is also demonstrated.

Countermeasures are discussed, and a simple resampling countermeasure is tested. All tools both offensive and defensive presented in the paper have been released under open-source licenses.

This paper first demonstrates an unprofiled CPA attack against a Cortex-M microcontroller using the phase of a clock output, observing the signal on both optically isolated and capacitively isolated paths. The unprofiled attack takes only 2-4x more traces than an attack using a classic shunt-resistor measurement.

It is then demonstrated how the JTAG bypass mode can be used to force a clock through a digital device. This forced clock signal can then be used as a highly effective oscilloscope that is located on the target device. As the attack does not require modifications to the device (such as capacitor removal or heat spreader removal) it is difficult to detect using existing countermeasures. The example attack over JTAG uses an unprofiled CPA attack, requiring only about 5x more traces than an ideal shunt-resistor based measurement. In addition, a version of this attack using a fault correlation analysis attack is also demonstrated.

Countermeasures are discussed, and a simple resampling countermeasure is tested. All tools both offensive and defensive presented in the paper have been released under open-source licenses.

###### Maxime Spyropoulos, David Vigilant, Fabrice Perion, Renaud Pacalet, Laurent Sauvage

ePrint Report
Anticipating the advent of large quantum computers, NIST started a worldwide competition in 2016 aiming to define the next cryptographic standards. HQC is one of these post-quantum schemes still in contention, with four others already in the process of being standardized. In 2022, Guo et al. introduced a timing attack that exploited an inconsistency in HQC rejection sampling function to recover its secret key in 866,000 calls to an oracle. The authors of HQC updated its specification by applying an algorithm to sample vectors in constant time. A masked implementation of this function was then proposed for BIKE but it is not directly applicable to HQC. In this paper we propose a masked specification-compliant version of HQC vector sampling function which relies, to our knowledge, on the first masked implementation of the Barrett reduction.

###### Anil Kumar Pradhan

ePrint Report
We have proposed a novel FHE scheme that uniquely encodes the plaintext with noise in a way that prevents the increasing noise from overflowing and corrupting the plaintext. This allows users to perform computations on encrypted data smoothly. The scheme is constructed using the Chinese Remainder Theorem (CRT), supporting a predefined number of modular operations on encrypted plaintext without the need for bootstrapping.
Although FHE recently became popular after Gentry's work and various developments have occurred in the last decade, the idea of "Fully Homomorphic Encryption (FHE)" scheme was first introduced in the 1970s by Rivest. The Chinese Remainder Theorem is one of the most suitable tools for developing a FHE Scheme because it forms a ring homomorphism \( Z_{p_1} \times Z_{p_2} \times \ldots \times Z_{p_k} \cong Z_{p_1 p_2 \ldots p_k} \).
Various attempts have been made to develop a FHE using CRT, but most of them were unsuccessful, mainly due to the chosen plaintext attack (CPA).
The proposed scheme overcomes the chosen plaintext attack. The scheme also adds random errors to the message during encryption. However, these errors are added in such a way that, when homomorphic operations are performed over encrypted data, the increasing values of errors never overwrite the values of the messages, as happens in LWE-based homomorphic schemes. Therefore, one can perform a predefined number of homomorphic operations (both addition and multiplication) without worrying about the increasing values of errors.

###### Amos Beimel, Tal Malkin, Noam Mazor

ePrint Report
We address the black-box complexity of constructing pseudorandom functions (PRF) from pseudorandom generators (PRG). The celebrated GGM construction of Goldreich, Goldwasser, and Micali (Crypto 1984) provides such a construction, which (even when combined with Levin's domain-extension trick) has super-logarithmic depth. Despite many years and much effort, this remains essentially the best construction we have to date. On the negative side, one step is provided by the work of Miles and Viola (TCC 2011), which shows that a black-box construction which just calls the PRG once and outputs one of its output bits, cannot be a PRF.

In this work, we make significant further progress: we rule out black-box constructions of PRF from PRG that follow certain structural constraints, but may call the PRG adaptively polynomially many times. In particular, we define ``tree constructions" which generalize the GGM structure: they apply the PRG $G$ along a tree path, but allow for different choices of functions to compute the children of a node on the tree and to compute the next node on the computation path down the tree. We prove that a tree construction of logarithmic depth cannot be a PRF (while GGM is a tree construction of super-logarithmic depth). We also show several other results and discuss the special case of one-call constructions.

Our main results in fact rule out even weak PRF constructions with one output bit. We use the oracle separation methodology introduced by Gertner, Malkin, and Reingold (FOCS 2001), and show that for any candidate black-box construction $F^G$ from $G$, there exists an oracle relative to which $G$ is a PRG, but $F^G$ is not a PRF.

In this work, we make significant further progress: we rule out black-box constructions of PRF from PRG that follow certain structural constraints, but may call the PRG adaptively polynomially many times. In particular, we define ``tree constructions" which generalize the GGM structure: they apply the PRG $G$ along a tree path, but allow for different choices of functions to compute the children of a node on the tree and to compute the next node on the computation path down the tree. We prove that a tree construction of logarithmic depth cannot be a PRF (while GGM is a tree construction of super-logarithmic depth). We also show several other results and discuss the special case of one-call constructions.

Our main results in fact rule out even weak PRF constructions with one output bit. We use the oracle separation methodology introduced by Gertner, Malkin, and Reingold (FOCS 2001), and show that for any candidate black-box construction $F^G$ from $G$, there exists an oracle relative to which $G$ is a PRG, but $F^G$ is not a PRF.