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Transactions on Cryptographic Hardware and Embedded Systems
Number of Followers: 1  

  This is an Open Access Journal Open Access journal
ISSN (Online) 2569-2925
Published by Ruhr-Universit├Ąt Bochum Homepage  [4 journals]
  • When Bad News Become Good News

    • Authors: Davide Bellizia, Clément Hoffmann, Dina Kamel, Pierrick Méaux, François-Xavier Standaert
      Pages: 1 - 24
      Abstract: Hard physical learning problems have been introduced as an alternative option to implement cryptosystems based on hard learning problems. Their high-level idea is to use inexact computing to generate erroneous computations directly, rather than to first compute correctly and add errors afterwards. Previous works focused on the applicability of this idea to the Learning Parity with Noise (LPN) problem as a first step, and formalized it as Learning Parity with Physical Noise (LPPN). In this work, we generalize it to the Learning With Errors (LWE) problem, formalized as Learning With Physical Errors (LWPE). We first show that the direct application of the design ideas used for LPPN prototypes leads to a new source of (mathematical) data dependencies in the error distributions that can reduce the security of the underlying problem. We then show that design tweaks can be used to avoid this issue, making LWPE samples natively robust against such data dependencies. We additionally put forward that these ideas open a quite wide design space that could make hard physical learning problems relevant in various applications. And we conclude by presenting a first prototype FPGA design confirming our claims.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.1-24
  • Attacks Against White-Box ECDSA and Discussion of Countermeasures

    • Authors: Sven Bauer, Hermann Drexler, Max Gebhardt, Dominik Klein, Friederike Laus, Johannes Mittmann
      Pages: 25 - 55
      Abstract: This paper deals with white-box implementations of the Elliptic Curve Digital Signature Algorithm (ECDSA): First, we consider attack paths to break such implementations. In particular, we provide a systematic overview of various fault attacks, to which ECDSA white-box implementations are especially susceptible. Then, we propose different mathematical countermeasures, mainly based on masking/blinding of sensitive variables, in order to prevent or at least make such attacks more difficult. We also briefly mention some typical implementational countermeasures and their challenges in the ECDSA white-box scenario. Our work has been initiated by the CHES challenge WhibOx Contest 2021, which consisted of designing and breaking white-box ECDSA implementations, so called challenges. We illustrate our results and findings by means of the submitted challenges and provide a comprehensive overview which challenge could be solved in which way. Furthermore, we analyze selected challenges in more details.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.25-55
  • SYNFI: Pre-Silicon Fault Analysis of an Open-Source Secure Element

    • Authors: Pascal Nasahl, Miguel Osorio, Pirmin Vogel, Michael Schaffner, Timothy Trippel, Dominic Rizzo, Stefan Mangard
      Pages: 56 - 87
      Abstract: Fault attacks are active, physical attacks that an adversary can leverage to alter the control-flow of embedded devices to gain access to sensitive information or bypass protection mechanisms. Due to the severity of these attacks, manufacturers deploy hardware-based fault defenses into security-critical systems, such as secure elements. The development of these countermeasures is a challenging task due to the complex interplay of circuit components and because contemporary design automation tools tend to optimize inserted structures away, thereby defeating their purpose. Hence, it is critical that such countermeasures are rigorously verified post-synthesis. Since classical functional verification techniques fall short of assessing the effectiveness of countermeasures (due to the circuit being analyzed when no faults are present), developers have to resort to methods capable of injecting faults in a simulation testbench or into a physical chip sample. However, developing test sequences to inject faults in simulation is an error-prone task and performing fault attacks on a chip requires specialized equipment and is incredibly time-consuming. Moreover, identifying the fault-vulnerable circuit is hard in both approaches, and fixing potential design flaws post-silicon is usually infeasible since that would require another tape-out. To that end, this paper introduces SYNFI, a formal pre-silicon fault verification framework that operates on synthesized netlists. SYNFI can be used to analyze the general effect of faults on the input-output relationship in a circuit and its fault countermeasures, and thus enables hardware designers to assess and verify the effectiveness of embedded countermeasures in a systematic and semi-automatic way. The framework automatically extracts sensitive parts of the circuit, induces faults into the extracted subcircuit, and analyzes the faults’ effects using formal methods. To demonstrate that SYNFI is capable of handling unmodified, industry-grade netlists synthesized with commercial and open tools, we analyze OpenTitan, the first opensource secure element. In our analysis, we identified critical security weaknesses in the unprotected AES block, developed targeted countermeasures, reassessed their security, and contributed these countermeasures back to the OpenTitan project. For other fault-hardened IP, such as the life cycle controller, we used SYNFI to confirm that existing countermeasures provide adequate protection.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.56-87
  • An energy and area efficient, all digital entropy source compatible with
           modern standards based on jitter pipelining

    • Authors: Adriaan Peetermans, Ingrid Verbauwhede
      Pages: 88 - 109
      Abstract: This paper proposes an energy and area efficient entropy source, suitable for true random number generation, accompanied with a stochastic model in a 28nm CMOS technology. The design uses a jitter pipelining architecture together with an increased timing resolution to achieve a maximal throughput of 298 Mbit/s and a best energy efficiency of 1.46 pJ/bit at a supply of 0.8V. The generated random bits pass the NIST SP 800-90B IID tests with a min entropy rate of 0.933 bit/bit, which is more than required by the AIS-31 standard. The all digital design allows for effortless transfer to other technology nodes, taking advantage of all benefits related to further technology scaling.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.88-109
  • Faster Constant-Time Decoder for MDPC Codes and Applications to BIKE KEM

    • Authors: Thales B. Paiva, Routo Terada
      Pages: 110 - 134
      Abstract: BIKE is a code-based key encapsulation mechanism (KEM) that was recently selected as an alternate candidate by the NIST’s standardization process on post-quantum cryptography. This KEM is based on the Niederreiter scheme instantiated with QC-MDPC codes, and it uses the BGF decoder for key decapsulation. We discovered important limitations of BGF that we describe in detail, and then we propose a new decoding algorithm for QC-MDPC codes called PickyFix. Our decoder uses two auxiliary iterations that are significantly different from previous approaches and we show how they can be implemented efficiently. We analyze our decoder with respect to both its error correction capacity and its performance in practice. When compared to BGF, our constant-time implementation of PickyFix achieves speedups of 1.18, 1.29, and 1.47 for the security levels 128, 192 and 256, respectively.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.110-134
  • Triplex: an Efficient and One-Pass Leakage-Resistant Mode of Operation

    • Authors: Yaobin Shen, Thomas Peters, François-Xavier Standaert, Gaëtan Cassiers, Corentin Verhamme
      Pages: 135 - 162
      Abstract: This paper introduces and analyzes Triplex, a leakage-resistant mode of operation based on Tweakable Block Ciphers (TBCs) with 2n-bit tweaks. Triplex enjoys beyond-birthday ciphertext integrity in the presence of encryption and decryption leakage in a liberal model where all intermediate computations are leaked in full and only two TBC calls operating a long-term secret are protected with implementationlevel countermeasures. It provides beyond-birthday confidentiality guarantees without leakage, and standard confidentiality guarantees with leakage for a single-pass mode embedding a re-keying process for the bulk of its computations (i.e., birthday confidentiality with encryption leakage under a bounded leakage assumption). Triplex improves leakage-resistant modes of operation relying on TBCs with n-bit tweaks when instantiated with large-tweak TBCs like Deoxys-TBC (a CAESAR competition laureate) or Skinny (used by the Romulus finalist of the NIST lightweight crypto competition). Its security guarantees are maintained in the multi-user setting.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.135-162
  • A Fast Large-Integer Extended GCD Algorithm and Hardware Design for
           Verifiable Delay Functions and Modular Inversion

    • Authors: Kavya Sreedhar, Mark Horowitz, Christopher Torng
      Pages: 163 - 187
      Abstract: The extended GCD (XGCD) calculation, which computes Bézout coefficients ba, bb such that ba ∗ a0 + bb ∗ b0 = GCD(a0, b0), is a critical operation in many cryptographic applications. In particular, large-integer XGCD is computationally dominant for two applications of increasing interest: verifiable delay functions that square binary quadratic forms within a class group and constant-time modular inversion for elliptic curve cryptography. Most prior work has focused on fast software implementations. The few works investigating hardware acceleration build on variants of Euclid’s division-based algorithm, following the approach used in optimized software. We show that adopting variants of Stein’s subtraction-based algorithm instead leads to significantly faster hardware. We quantify this advantage by performing a large-integer XGCD accelerator design space exploration comparing Euclid- and Stein-based algorithms for various application requirements. This exploration leads us to an XGCD hardware accelerator that is flexible and efficient, supports fast average and constant-time evaluation, and is easily extensible for polynomial GCD. Our 16nm ASIC design calculates 1024-bit XGCD in 294ns (8x faster than the state-of-the-art ASIC) and constant-time 255-bit XGCD for inverses in the field of integers modulo the prime 2255−19 in 85ns (31× faster than state-of-the-art software). We believe our design is the first high-performance ASIC for the XGCD computation that is also capable of constant-time evaluation. Our work is publicly available at https://github.com/kavyasreedhar/sreedhar-xgcd-hardware-ches2022.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.163-187
  • Randomness Optimization for Gadget Compositions in Higher-Order Masking

    • Authors: Jakob Feldtkeller, David Knichel, Pascal Sasdrich, Amir Moradi, Tim Güneysu
      Pages: 188 - 227
      Abstract: Physical characteristics of electronic devices, leaking secret and sensitive information to an adversary with physical access, pose a long-known threat to cryptographic hardware implementations. Among a variety of proposed countermeasures against such Side-Channel Analysis attacks, masking has emerged as a promising, but often costly, candidate. Furthermore, the manual realization of masked implementations has proven error-prone and often introduces flaws, possibly resulting in insecure circuits. In the context of automatic masking, a new line of research emerged, aiming to replace each physical gate with a secure gadget that fulfills well-defined properties, guaranteeing security when interconnected to a large circuit. Unfortunately, those gadgets introduce a significant amount of additional overhead into the design, in terms of area, latency, and randomness requirements.
      In this work, we present a novel approach to reduce the demands for randomness in such gadget-composed circuits by reusing randomness across gadgets while maintaining security in the probing adversary model. To this end, we embedded the corresponding optimization passes into an Electronic Design Automation toolchain, able to construct, optimize, and implement masked circuits, starting from an unprotected design. As such, our security-aware optimization offers an additional building block for existing or new Electronic Design Automation frameworks, where security is considered a first-class design constraint.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.188-227
  • Perceived Information Revisited

    • Authors: Akira Ito, Rei Ueno, Naofumi Homma
      Pages: 228 - 254
      Abstract: In this study, we present new analytical metrics for evaluating the performance of side-channel attacks (SCAs) by revisiting the perceived information (PI), which is defined using cross-entropy (CE). PI represents the amount of information utilized by a probability distribution that determines a distinguishing rule in SCA. Our analysis partially solves an important open problem in the performance evaluation of deep-learning based SCAs (DL-SCAs) that the relationship between neural network (NN) model evaluation metrics (such as accuracy, loss, and recall) and guessing entropy (GE)/success rate (SR) is unclear. We first theoretically show that the conventional CE/PI is non-calibrated and insufficient for evaluating the SCA performance, as it contains uncertainty in terms of SR. More precisely, we show that an infinite number of probability distributions with different CE/PI can achieve an identical SR. With the above analysis result, we present a modification of CE/PI, named effective CE/PI (ECE/EPI), to eliminate the above uncertainty. The ECE/EPI can be easily calculated for a given probability distribution and dataset, which would be suitable for DL-SCA. Using the ECE/EPI, we can accurately evaluate the SR hrough the validation loss in the training phase, and can measure the generalization of the NN model in terms of SR in the attack phase. We then analyze and discuss the proposed metrics regarding their relationship to SR, conditions of successful attacks for a distinguishing rule with a probability distribution, a statistic/asymptotic aspect, and the order of key ranks in SCA. Finally, we validate the proposed metrics through experimental attacks on masked AES implementations using DL-SCA.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.228-254
  • VERICA - Verification of Combined Attacks

    • Authors: Jan Richter-Brockmann, Jakob Feldtkeller, Pascal Sasdrich, Tim Güneysu
      Pages: 255 - 284
      Abstract: Physical attacks, including passive Side-Channel Analysis and active Fault Injection Analysis, are considered among the most powerful threats against physical cryptographic implementations. These attacks are well known and research provides many specialized countermeasures to protect cryptographic implementations against them. Still, only a limited number of combined countermeasures, i.e., countermeasures that protect implementations against multiple attacks simultaneously, were proposed in the past. Due to increasing complexity and reciprocal effects, design of efficient and reliable combined countermeasures requires longstanding expertise in hardware design and security. With the help of formal security specifications and adversary models, automated verification can streamline development cycles, increase quality, and facilitate development of robust cryptographic implementations.
      In this work, we revise and refine formal security notions for combined protection mechanisms and specifically embed them in the context of hardware implementations. Based on this, we present the first automated verification framework that can verify physical security properties of hardware circuits with respect to combined physical attacks. To this end, we conduct several case studies to demonstrate the capabilities and advantages of our framework, analyzing secure building blocks (gadgets), S-boxes build from Toffoli gates, and the ParTI scheme. For the first time, we reveal security flaws in analyzed structures due to reciprocal effects, highlighting the importance of continuously integrating security verification into modern design and development cycles.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.255-284
  • Single-Trace Side-Channel Attacks on the Toom-Cook: The Case Study of

    • Authors: Yanbin Li, Jiajie Zhu, Yuxin Huang, Zhe Liu, Ming Tang
      Pages: 285 - 310
      Abstract: The Toom-Cook method is a well-known strategy for building algorithms to multiply polynomials efficiently. Along with NTT-based polynomial multiplication, Toom-Cook-based or Karatsuba-based polynomial multiplication algorithms still have regained attention since the start of the NIST’s post-quantum standardization procedure. Compared to the comprehensive analysis done for NTT, the leakage characteristics of Toom-Cook have not been discussed. We analyze the vulnerabilities of Toom-Cook in the reference implementation of Saber, a third round finalist of NIST’s post-quantum standardization process. In this work, we present the first single-trace attack based on the soft-analytical side-channel attack (SASCA) targeting the Toom-Cook. The deep learning-based power analysis is combined with SASCA to decrease the number of templates since there are a large number of similar operations in the Toom-Cook. Moreover, we describe the optimized factor graph and improved belief propagation to make the attack more practical. The feasibility of the attack is verified by evaluation experiments. We also discuss the possible countermeasures to prevent the attack.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.285-310

    • Authors: Nicolai Müller, Amir Moradi
      Pages: 311 - 348
      Abstract: Even today, Side-Channel Analysis attacks pose a serious threat to the security of cryptographic implementations fabricated with low-power and nanoscale feature technologies. Fortunately, the masking countermeasures offer reliable protection against such attacks based on simple security assumptions. However, the practical application of masking to a cryptographic algorithm is not trivial, and the designer may overlook possible security flaws, especially when masking a complex circuit. Moreover, abstract models like probing security allow formal verification tools to evaluate masked implementations. However, this is computationally too expensive when dealing with circuits that are not based on composable gadgets. Unfortunately, using composable gadgets comes at some area overhead. As a result, such tools can only evaluate subcircuits, not their compositions, which can become the Achilles’ heel of such masked implementations.
      In this work, we apply logic simulations to evaluate the security of masked implementations which are not necessarily based on composable gadgets. We developed PROLEAD, an automated tool analyzing the statistical independence of simulated intermediates probed by a robust probing adversary. Compared to the state of the art, our approach (1) does not require any power model as only the state of a gate-level netlist is simulated, (2) can handle masked full cipher implementations, and (3) can detect flaws related to the combined occurrence of glitches and transitions as well as higher-order multivariate leakages. With PROLEAD, we can evaluate masked mplementations that are too complex for existing formal verification tools while being in line with the robust probing model. Through PROLEAD, we have detected security flaws in several publicly-available masked implementations, which have been claimed to be robust probing secure.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.311-348
  • Multi-Parameter Support with NTTs for NTRU and NTRU Prime on Cortex-M4

    • Authors: Erdem Alkim, Vincent Hwang, Bo-Yin Yang
      Pages: 349 - 371
      Abstract: We propose NTT implementations with each supporting at least one parameter of NTRU and one parameter of NTRU Prime. Our implementations are based on size-1440, size-1536, and size-1728 convolutions without algebraic assumptions on the target polynomial rings. We also propose several improvements for the NTT computation. Firstly, we introduce dedicated radix-(2, 3) butterflies combining Good–Thomas FFT and vector-radix FFT. In general, there are six dedicated radix-(2, 3) butterflies and they together support implicit permutations. Secondly, for odd prime radices, we show that the multiplications for one output can be replaced with additions/subtractions. We demonstrate the idea for radix-3 and show how to extend it to any odd prime. Our improvement also applies to radix-(2, 3) butterflies. Thirdly, we implement an incomplete version of Good–Thomas FFT for addressing potential code size issues. For NTRU, our polynomial multiplications outperform the state-of-the-art by 2.8%−10.3%. For NTRU Prime, our polynomial multiplications are slower than the state-of-the-art. However, the SotA exploits the specific structure of coefficient rings or polynomial moduli, while our NTT-based multiplications exploit neither and apply across different schemes. This reduces the engineering effort, including testing and verification.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.349-371
  • Post-Quantum Authenticated Encryption against Chosen-Ciphertext
           Side-Channel Attacks

    • Authors: Melissa Azouaoui, Yulia Kuzovkova, Tobias Schneider, Christine van Vredendaal
      Pages: 372 - 396
      Abstract: Over the last years, the side-channel analysis of Post-Quantum Cryptography (PQC) candidates in the NIST standardization initiative has received increased attention. In particular, it has been shown that some post-quantum Key Encapsulation Mechanisms (KEMs) are vulnerable to Chosen-Ciphertext Side-Channel Attacks (CC-SCA). These powerful attacks target the re-encryption step in the Fujisaki-Okamoto (FO) transform, which is commonly used to achieve CCA security in such schemes. To sufficiently protect PQC KEMs on embedded devices against such a powerful CC-SCA, masking at increasingly higher order is required, which induces a considerable overhead. In this work, we propose to use a conceptually simple construction, the ΕtS KEM, that alleviates the impact of CC-SCA. It uses the Encrypt-then-Sign (EtS) paradigm introduced by Zheng at ISW ’97 and further analyzed by An, Dodis and Rabin at EUROCRYPT ’02, and instantiates a postquantum authenticated KEM in the outsider-security model. While the construction is generic, we apply it to the CRYSTALS-Kyber KEM, relying on the CRYSTALSDilithium and Falcon signature schemes. We show that a CC-SCA-protected EtS KEM version of CRYSTALS-Kyber requires less than 10% of the cycles required for the CC-SCA-protected FO-based KEM, at the cost of additional data/communication overhead. We additionally show that the cost of protecting the EtS KEM against fault injection attacks, necessarily due to the added signature verification, remains negligible compared to the large cost of masking the FO transform at higher orders. Lastly, we discuss relevant embedded use cases for our EtS KEM construction.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.372-396
  • Breaking Masked Implementations of the Clyde-Cipher by Means of
           Side-Channel Analysis

    • Authors: Aron Gohr, Friederike Laus, Werner Schindler
      Pages: 397 - 437
      Abstract: In this paper we present our solution to the CHES Challenge 2020, the task of which it was to break masked hardware respective software implementations of the lightweight cipher Clyde by means of side-channel analysis. We target the secret cipher state after processing of the first S-box layer. Using the provided trace data we obtain a strongly biased posterior distribution for the secret-shared cipher state at the targeted point; this enables us to see exploitable biases even before the secret sharing based masking. These biases on the unshared state can be evaluated one S-box at a time and combined across traces, which enables us to recover likely key hypotheses S-box by S-box.
      In order to see the shared cipher state, we employ a deep neural network similar to the one used by Gohr, Jacob and Schindler to solve the CHES 2018 AES challenge. We modify their architecture to predict the exact bit sequence of the secret-shared cipher state. We find that convergence of training on this task is unsatisfying with the standard encoding of the shared cipher state and therefore introduce a different encoding of the prediction target, which we call the scattershot encoding. In order to further investigate how exactly the scattershot encoding helps to solve the task at hand, we construct a simple synthetic task where convergence problems very similar to those we observed in our side-channel task appear with the naive target data encoding but disappear with the scattershot encoding.
      We complete our analysis by showing results that we obtained with a “classical” method (as opposed to an AI-based method), namely the stochastic approach, that
      we generalize for this purpose first to the setting of shared keys. We show that the neural network draws on a much broader set of features, which may partially explain why the neural-network based approach massively outperforms the stochastic approach. On the other hand, the stochastic approach provides insights into properties of the implementation, in particular the observation that the S-boxes behave very different regarding the easiness respective hardness of their prediction.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.397-437
  • PreMSat: Preventing Magnetic Saturation Attack on Hall Sensors

    • Authors: Anomadarshi Barua, Mohammad Abdullah Al Faruque
      Pages: 438 - 462
      Abstract: Spoofing a passive Hall sensor with fake magnetic fields can inject false data into the downstream of connected systems. Several works have tried to provide a defense against the intentional spoofing to different sensors over the last six years. However, they either only work on active sensors or against externally injected unwanted weak signals (e.g., EMIs, acoustics, ultrasound, etc.), which can only spoof sensor output in its linear region. However, they do not work against a strong magnetic spoofing attack that can drive the passive Hall sensor output in its saturation region. We name this as the saturation attack. In the saturation region, the output gets flattened, and no information can be retrieved, resulting in a denial-of-service attack on the sensor.
      Our work begins to fill this gap by providing a defense named PreMSat against the saturation attack on passive Hall sensors. The core idea behind PreMSat is that it cangenerate an internal magnetic field having the same strength but in opposite polarity to external magnetic fields injected by an attacker. Therefore, the generated internal magnetic field by PreMSat can nullify the injected external field while preventing: (i) intentional spoofing in the sensor’s linear region, and (ii) saturation attack in the saturation region. PreMSat integrates a low-resistance magnetic path to collect the injected external magnetic fields and utilizes a finely tuned PID controller to nullify the external fields in real-time. PreMSat can prevent the magnetic saturation attack having a strength up to ~4200 A-t within a frequency range of 0 Hz–30 kHz with low cost (~$14), whereas the existing works cannot prevent saturation attacks with any strength. Moreover, it works against saturation attacks originating from any type, such as constant, sinusoidal, and pulsating magnetic fields. We did over 300 experiments on ten different industry-used Hall sensors from four different manufacturers to prove the efficacy of PreMSat and found that the correlation coefficient between the signals before the attack and after the attack is greater than 0.94 in every test case. Moreover, we create a prototype of PreMSat and evaluate its performance in a practical system — a grid-tied solar inverter. We find that PreMSat can satisfactorily prevent the saturation attack on passive Hall sensors in real-time.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.438-462
  • Redshift: Manipulating Signal Propagation Delay via Continuous-Wave Lasers

    • Authors: Kohei Yamashita, Benjamin Cyr, Kevin Fu, Wayne Burleson, Takeshi Sugawara
      Pages: 463 - 489
      Abstract: We propose a new laser injection attack Redshift that manipulates signal propagation delay, allowing for precise control of oscillator frequencies and other behaviors in delay-sensitive circuits. The target circuits have a significant sensitivity to light, and a low-power continuous-wave laser, similar to a laser pointer, is sufficient for the attack. This is in contrast to previous fault injection attacks that use highpowered laser pulses to flip digital bits. This significantly reduces the cost of the attack and extends the range of possible attackers. Moreover, the attack potentially evades sensor-based countermeasures configured for conventional pulse lasers. To demonstrate Redshift, we target ring-oscillator and arbiter PUFs that are used in cryptographic applications. By precisely controlling signal propagation delays within these circuits, an attacker can control the output of a PUF to perform a state-recovery attack and reveal a secret key. We finally discuss the physical causality of the attack and potential countermeasures.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.463-489
  • BreakMi: Reversing, Exploiting and Fixing Xiaomi Fitness Tracking

    • Authors: Marco Casagrande, Eleonora Losiouk, Mauro Conti, Mathias Payer, Daniele Antonioli
      Pages: 490 - 526
      Abstract: Xiaomi is the leading company in the fitness tracking industry. Successful attacks on its fitness tracking ecosystem would result in severe consequences, including the loss of sensitive health and personal data. Despite these relevant risks, we know very little about the security mechanisms adopted by Xiaomi. In this work, we uncover them and show that they are insecure. In particular, Xiaomi protects its fitness tracking ecosystem with custom application-layer protocols spoken over insecure Bluetooth Low-Energy (BLE) connections (ignoring standard BLE security mechanisms already supported by their devices) and TLS connections. We identify severe vulnerabilities affecting such proprietary protocols, including unilateral and replayable authentication. Those issues are critical as they affect all Xiaomi trackers released since 2016 and up-to-date Xiaomi companion apps for Android and iOS. We show in practice how to exploit the identified vulnerabilities by presenting six impactful attacks. Four attacks enable to wirelessly impersonate any Xiaomi fitness tracker and companion app, man-in-the-middle (MitM) them, and eavesdrop on their communication. The other two attacks leverage a malicious Android application to remotely eavesdrop on data from a tracker and impersonate a Xiaomi fitness app. Overall, the attacks have a high impact as they can be used to exfiltrate and inject sensitive data from any Xiaomi tracker and compatible app. We propose five practical and low-overhead countermeasures to mitigate the presented vulnerabilities. Moreover, we present breakmi, a modular toolkit that we developed to automate our reverse-engineering process and attacks. breakmi understands Xiaomi application-layer proprietary protocols, reimplements Xiaomi security mechanisms, and automatically performs our attacks. We demonstrate that our toolkit can be generalized by extending it to be compatible with the Fitbit ecosystem. We will open-source breakmi.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.490-526
  • ECDSA White-Box Implementations: Attacks and Designs from CHES 2021

    • Authors: Guillaume Barbu, Ward Beullens, Emmanuelle Dottax, Christophe Giraud, Agathe Houzelot, Chaoyun Li, Mohammad Mahzoun, Adrián Ranea, Jianrui Xie
      Pages: 527 - 552
      Abstract: Despite the growing demand for software implementations of ECDSA secure against attackers with full control of the execution environment, scientific literature on ECDSA white-box design is scarce. The CHES 2021 WhibOx contest was thus held to assess the state-of-the-art and encourage relevant practical research, inviting developers to submit ECDSA white-box implementations and attackers to break the corresponding submissions.
      In this work, attackers (team TheRealIdefix) and designers (team zerokey) join to describe several attack techniques and designs used during this contest. We explain the methods used by the team TheRealIdefix, which broke the most challenges, and we show the efficiency of each of these methods against all the submitted implementations. Moreover, we describe the designs of the two winning challenges submitted by the team zerokey; these designs represent the ECDSA signature algorithm by a sequence of systems of low-degree equations, which are obfuscated with affine encodings and extra random variables and equations.
      The WhibOx contest has shown that securing ECDSA in the white-box model is an open and challenging problem, as no implementation survived more than two days. In this context, our designs provide a starting methodology for further research, and our attacks highlight the weak points future work should address.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.527-552
  • Bitslicing Arithmetic/Boolean Masking Conversions for Fun and Profit

    • Authors: Olivier Bronchain, Gaëtan Cassiers
      Pages: 553 - 588
      Abstract: The performance of higher-order masked implementations of lattice-based based key encapsulation mechanisms (KEM) is currently limited by the costly conversions between arithmetic and Boolean masking. While bitslicing has been shown to strongly speed up masked implementations of symmetric primitives, its use in arithmetic-to-Boolean and Boolean-to-arithmetic masking conversion gadgets has never been thoroughly investigated. In this paper, we first show that bitslicing can indeed accelerate existing conversion gadgets. We then optimize these gadgets, exploiting the degrees of freedom offered by bitsliced implementations. As a result, we introduce new arbitrary-order Boolean masked addition, arithmetic-to-Boolean and Boolean-to-arithmetic masking conversion gadgets, each in two variants: modulo 2k and modulo p (for any integers k and p). Practically, our new gadgets achieve a speedup of up to 25x over the state of the art. Turning to the KEM application, we develop the first open-source embedded (Cortex-M4) implementations of Kyber768 and Saber masked at arbitrary order. The implementations based on the new bitsliced gadgets achieve a speedup of 1.8x for Kyber and 3x for Saber, compared to the implementation based on state-of-the-art gadgets. The bottleneck of the bitslice implementations is the masked Keccak-f[1600] permutation.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.553-588
  • A Power to Pulse Width Modulation Sensor for Remote Power Analysis Attacks

    • Authors: Brian Udugama, Darshana Jayasinghe, Hassaan Saadat, Aleksandar Ignjatovic, Sri Parameswaran
      Pages: 589 - 613
      Abstract: Field-programmable gate arrays (FPGAs) deployed on commercial cloud services are increasingly gaining popularity due to the cost and compute benefits offered by them. Recent studies have discovered security threats than can be launched remotely on FPGAs that share the logic fabric between trusted and untrusted parties, posing a danger to designs deployed on cloud FPGAs. With remote power analysis (RPA) attacks, an attacker aims to deduce secret information present on a remote FPGA by deploying an on-chip sensor on the FPGA logic fabric. Information captured with the on-chip sensor is transferred off the chip for analysis and existing on-chip sensors demand a significant amount of bandwidth for this task as a result of their wider output bit width. However, attackers are often left with the only option of using a covert communication channel and the bandwidth of such channels is generally limited. This paper proposes a novel area-efficient on-chip power sensor named PPWM that integrates a logic design outputting a pulse whose width is modulated by the power consumption of the FPGA. This pulse is used to clear a flip-flop selectively and asynchronously, and the single-bit output of the flip-flop is used to perform an RPA attack. This paper demonstrates the possibility of successfully recovering a 128-bit Advanced Encryption Standard (AES) key within 16,000 power traces while consuming just 25% of the bandwidth when compared to the state of the art. Moreover, this paper assesses the threat posed by the proposed PPWM to remote FPGAs including those that are deployed on cloud services.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.589-613
  • Improved Plantard Arithmetic for Lattice-based Cryptography

    • Authors: Junhao Huang, Jipeng Zhang, Haosong Zhao, Zhe Liu, Ray C. C. Cheung, Çetin Kaya Koç, Donglong Chen
      Pages: 614 - 636
      Abstract: This paper presents an improved Plantard’s modular arithmetic (Plantard arithmetic) tailored for Lattice-Based Cryptography (LBC). Based on the improved Plantard arithmetic, we present faster implementations of two LBC schemes, Kyber and NTTRU, running on Cortex-M4. The intrinsic advantage of Plantard arithmetic is that one multiplication can be saved from the modular multiplication of a constant. However, the original Plantard arithmetic is not very practical in LBC schemes because of the limitation on the unsigned input range. In this paper, we improve the Plantard arithmetic and customize it for the existing LBC schemes with theoretical proof. The improved Plantard arithmetic not only inherits its aforementioned advantage but also accepts signed inputs, produces signed output, and enlarges its input range compared with the original design. Moreover, compared with the state-of-the-art Montgomery arithmetic, the improved Plantard arithmetic has a larger input range and smaller output range, which allows better lazy reduction strategies during the NTT/INTT implementation in current LBC schemes. All these merits make it possible to replace the Montgomery arithmetic with the improved Plantard arithmetic in LBC schemes on some platforms. After applying this novel method to Kyber and NTTRU schemes using 16-bit NTT on Cortex-M4 devices, we show that the proposed design outperforms the known fastest implementation that uses Montgomery and Barrett arithmetic. Specifically, compared with the state-of-the-art Kyber implementation, applying the improved Plantard arithmetic in Kyber results in a speedup of 25.02% and 18.56% for NTT and INTT, respectively. Compared with the reference implementation of NTTRU, our NTT and INTT achieve speedup by 83.21% and 78.64%, respectively. As for the LBC KEM schemes, we set new speed records for Kyber and NTTRU running on Cortex-M4.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.614-636
  • Roulette: A Diverse Family of Feasible Fault Attacks on Masked Kyber

    • Authors: Jeroen Delvaux
      Pages: 637 - 660
      Abstract: At Indocrypt 2021, Hermelink, Pessl, and Pöppelmann presented a fault attack against Kyber in which a system of linear inequalities over the private key is generated and solved. The attack requires a laser and is, understandably, demonstrated with simulations—not actual equipment. We facilitate and diversify the attack in four ways, thereby admitting cheaper and more forgiving fault-injection setups. Firstly, the attack surface is enlarged: originally, the two input operands of the ciphertext comparison are covered, and we additionally cover re-encryption modules such as binomial sampling and butterflies in the last layer of the inverse numbertheoretic transform (INTT). This extra surface also allows an attacker to bypass the custom countermeasure that was proposed in the Indocrypt paper. Secondly, the fault model is relaxed: originally, precise bit flips are required, and we additionally support set-to-0 faults, random faults, arbitrary bit flips, and instruction skips. Thirdly, masking and blinding methods that randomize intermediate variables kindly help our attack, whereas the IndoCrypt attack is like most other fault attacks either hindered or unaltered by countermeasures against passive side-channel analysis (SCA). Randomization helps because we randomly fault intermediate prime-field elements until a desired set of values is hit. If these prime-field elements are represented on a circle, which is a common visualization, our attack is analogous to spinning a roulette wheel until the ball lands in a desired set of pockets. Hence, the nickname. Fourthly, we accelerate and improve the error tolerance of solving the system of linear inequalities: run times of roughly 100 minutes are reduced to roughly one minute, and inequality error rates of roughly 1% are relaxed to roughly 25%. Benefiting from the four advances above, we use a reasonably priced ChipWhisperer® board to break a masked implementation of Kyber running on an ARM Cortex-M4 through clock glitching.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.637-660
  • SoK: Fully Homomorphic Encryption over the [Discretized] Torus

    • Authors: Marc Joye
      Pages: 661 - 692
      Abstract: First posed as a challenge in 1978 by Rivest et al., fully homomorphic encryption—the ability to evaluate any function over encrypted data—was only solved in 2009 in a breakthrough result by Gentry (Commun. ACM, 2010). After a decade of intense research, practical solutions have emerged and are being pushed for standardization.
      This paper explains the inner-workings of TFHE, a torus-based fully homomorphic encryption scheme. More exactly, it describes its implementation on a discretized version of the torus. It also explains in detail the technique of the programmable bootstrapping. Numerous examples are provided to illustrate the various concepts and definitions.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.661-692
  • Riding the Waves Towards Generic Single-Cycle Masking in Hardware

    • Authors: Rishub Nagpal, Barbara Gigerl, Robert Primas, Stefan Mangard
      Pages: 693 - 717
      Abstract: Research on the design of masked cryptographic hardware circuits in the past has mostly focused on reducing area and randomness requirements. However, many embedded devices like smart cards and IoT nodes also need to meet certain performance criteria, which is why the latency of masked hardware circuits also represents an important metric for many practical applications.
      The root cause of latency in masked hardware circuits is the need for additional register stages that synchronize the propagation of shares. Otherwise, glitches would violate the basic assumptions of the used masking scheme. This issue can be addressed to some extent, e.g., by using lightweight cryptographic algorithms with low-degree Sboxes, however, many applications still require the usage of schemes with higher-degree S-boxes like AES. Several recent works have already proposed solutions that help reduce this latency yet they either come with noticeably increased area/randomness requirements, limitations on masking orders, or specific assumptions on the general architecture of the crypto core.
      In this work, we introduce a generic and efficient method for designing single-cycle glitch-resistant (higher-order) masked hardware of cryptographic S-boxes. We refer to this technique as (generic) Self-Synchronized Masking (“SESYM”). The main idea of our approach is to replace register stages with a partial dual-rail encoding of masked signals that ensures synchronization within the circuit. More concretely, we show that WDDL gates and Muller C-elements can be used in combination with standard masking schemes to design single-cycle S-box circuits that, especially in case of higher-degree S-boxes, have noticeably lower requirements in terms of area and online randomness. We apply our method to DOM-based S-boxes of Ascon and AES and compare the resulting circuits to existing latency optimized circuits based on TI, GLM, and LMDPL. The latency of all three designs is reduced to single-cycle operation and are dth-order secure. Compared to GLM-masked Ascon, our approach comes with a 6.4 times reduction in online randomness for all protection orders. Compared to 1st-order LMDPL-masked AES, our approach achieves comparable results, while it is more generic, amongst others, by also supporting higher-order designs. We also underline the practical protection of our constructions against power analysis attacks via empirical and formal verification approaches.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.693-717
  • Verified NTT Multiplications for NISTPQC KEM Lattice Finalists: Kyber,
           SABER, and NTRU

    • Authors: Vincent Hwang, Jiaxiang Liu, Gregor Seiler, Xiaomu Shi, Ming-Hsien Tsai, Bow-Yaw Wang, Bo-Yin Yang
      Pages: 718 - 750
      Abstract: Postquantum cryptography requires a different set of arithmetic routines from traditional public-key cryptography such as elliptic curves. In particular, in each of the lattice-based NISTPQC Key Establishment finalists, every state-ofthe-art optimized implementation for lattice-based schemes still in the NISTPQC round 3 currently uses a different complex multiplication based on the Number Theoretic Transform. We verify the NTT-based multiplications used in NTRU, Kyber, and SABER for both the AVX2 implementation for Intel CPUs and for the pqm4 implementation for the ARM Cortex M4 using the tool CryptoLine. e extended CryptoLine and as a result are able to verify that in six instances multiplications are correct including range properties.
      We demonstrate the feasibility for a programmer to verify his or her high-speed assembly code for PQC, as well as to verify someone else’s high-speed PQC software in assembly code, with some cooperation from the programmer.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.718-750
  • SoC Root Canal!

    • Authors: Pantea Kiaei, Patrick Schaumont
      Pages: 751 - 773
      Abstract: Finding the root cause of power-based side-channel leakage becomes harder when multiple layers of design abstraction are involved. While side-channel leakage originates in processor hardware, the dangerous consequences may only become apparent in the cryptographic software that runs on the processor. This contribution presents RootCanal, a methodology to explain the origin of side-channel leakage in a software program in terms of the underlying micro-architecture and system architecture. We simulate the hardware power consumption at the gate level and perform a non-specific test to identify the logic gates that contribute most sidechannel leakage. Then, we back-annotate those findings to the related activities in the software. The resulting analysis can automatically point out non-trivial causes of side-channel leakages. To illustrate RootCanal’s capabilities, we discuss a collection of case studies.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.751-773
  • Side-Channel Expectation-Maximization Attacks

    • Authors: Julien Béguinot, Wei Cheng, Sylvain Guilley, Olivier Rioul
      Pages: 774 - 799
      Abstract: Block ciphers are protected against side-channel attacks by masking. On one hand, when the leakage model is unknown, second-order correlation attacks are typically used. On the other hand, when the leakage model can be profiled, template attacks are prescribed. But what if the profiled model does not exactly match that of the attacked device'
      One solution consists in regressing on-the-fly the scaling parameters from the model. In this paper, we leverage an Expectation-Maximization (EM) algorithm to implement such an attack. The resulting unprofiled EM attack, termed U-EM, is shown to be both efficient (in terms of number of traces) and effective (computationally speaking). Based on synthetic and real traces, we introduce variants of our U-EM attack to optimize its performance, depending on trade-offs between model complexity and epistemic noise. We show that the approach is flexible, in that it can easily be adapted to refinements such as different points of interest and number of parameters in the leakage model.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.774-799
  • A Key-Recovery Side-Channel Attack on Classic McEliece Implementations

    • Authors: Qian Guo, Andreas Johansson, Thomas Johansson
      Pages: 800 - 827
      Abstract: In this paper, we propose the first key-recovery side-channel attack on Classic McEliece, a KEM finalist in the NIST Post-quantum Cryptography Standardization Project. Our novel idea is to design an attack algorithm where we submit special ciphertexts to the decryption oracle that correspond to cases of single errors. Decoding of such ciphertexts involves only a single entry in a large secret permutation, which is part of the secret key. Through an identified leakage in the additive FFT step used to evaluate the error locator polynomial, a single entry of the secret permutation can be determined. Iterating this for other entries leads to full secret key recovery. The attack is described using power analysis both on the FPGA reference implementation and a software implementation running on an ARM Cortex-M4. We use a machine-learning-based classification algorithm to determine the error locator polynomial from a single trace. The attack is fully implemented and evaluated in the Chipwhisperer framework and is successful in practice. For the smallest parameter set, it is using about 300 traces for partial key recovery and less than 800 traces for full key recovery, in the FPGA case. A similar number of traces are required for a successful attack on the ARM software implementation.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.800-827
  • Exploring Feature Selection Scenarios for Deep Learning-based Side-channel

    • Authors: Guilherme Perin, Lichao Wu, Stjepan Picek
      Pages: 828 - 861
      Abstract: One of the main promoted advantages of deep learning in profiling sidechannel analysis is the possibility of skipping the feature engineering process. Despite that, most recent publications consider feature selection as the attacked interval from the side-channel measurements is pre-selected. This is similar to the worst-case security assumptions in security evaluations when the random secret shares (e.g., mask shares) are known during the profiling phase: an evaluator can identify points of
      interest locations and efficiently trim the trace interval. To broadly understand how feature selection impacts the performance of deep learning-based profiling attacks, this paper investigates three different feature selection scenarios that could be realistically used in practical security evaluations. The scenarios range from the minimum possible number of features (worst-case security assumptions) to the whole available traces. Our results emphasize that deep neural networks as profiling models show successful key recovery independently of explored feature selection scenarios against first-order masked software implementations of AES-128. First, we show that feature selection with the worst-case security assumptions results in optimal profiling models that are highly dependent on the number of features and signal-to-noise ratio levels. Second, we demonstrate that attacking raw side-channel measurements with small deep neural networks also provides optimal models, that shortens the gap between worst-case security evaluations and online (realistic) profiling attacks. In all explored feature selection scenarios, the hyperparameter search always indicates a successful model with up to eight hidden layers for MLPs and CNNs, suggesting that complex models are not required for the considered datasets. Our results demonstrate the key recovery with less than ten attack traces for all datasets for at least one of the feature selection scenarios. Additionally, in several cases, we can recover the target key with a single attack trace.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.828-861
  • On the application of Two-Photon Absorption for Laser Fault Injection

    • Authors: Bodo Selmke, Maximilian Pollanka, Andreas Duensing, Emanuele Strieder, Hayden Wen, Michael Mittermair, Reinhard, Georg Sigl
      Pages: 862 - 885
      Abstract: Laser Fault Injection (LFI) is considered to be the most powerful semiinvasive fault injection method for implementation attacks on security devices. In this work we discuss for the first time the application of the nonlinear Two-Photon Absorption (TPA) effect for the purpose of LFI. Though TPA is an established technique in other areas, e.g. fluorescence microscopy, so far it did not receive any attention in the field of physical attack methods on integrated circuits. We show that TPA has several superior properties over the regular linear LFI method. The TPA effect allows to work on non-thinned devices without increasing the induced energy and hence the stress on the device. In contrast to regular LFI, the nonlinearity of the TPA effect leads to increased precision due to the steeper descent in intensity and also a vertically restricted photoelectric effect. By practical experiments, we demonstrate the general applicability of the method for a specific device and that unlike a regular LFI setup, TPA-LFI is capable to inject faults without triggering a latch-up effect. In addition we discuss the possible implications of TPA-LFI on various sensor-based countermeasures.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.862-885
  • GE vs GM: Efficient side-channel security evaluations on full
           cryptographic keys

    • Authors: Anca Rădulescu, Pantelimon G. Popescu, Marios O. Choudary
      Pages: 886 - 905
      Abstract: Security evaluations for full cryptographic keys is a very important research topic since the past decade. An efficient rank estimation algorithm was proposed at FSE 2015 to approximate the empirical guessing entropy remaining after a side-channel attack on a full AES key, by combining information from attacks on each byte of he key independently. However, these could not easily scale to very large keys over 1024 bits. Hence, at CHES 2017, it was proposed a new approach for scalable security evaluations based on Massey’s guessing entropy, which was shown tight and scalable to very large keys, even beyond 8192 bits. Then, at CHES 2020, it was proposed a new method for estimating the empirical guessing entropy for the case of full-key evaluations, showing also important divergences between the empirical guessing entropy and Massey’s guessing entropy. However, there has been some confusion in recent publications of side-channel evaluation methods relying on these two variants of the guessing entropy. Furthermore, it remained an open problem to decide which of these methods should be used and in which context, particularly given the wide acceptance of the empirical guessing entropy in the side-channel community and the relatively little use of the other.
      In this paper, we tackle this open problem through several contributions. First of all, we provide an unitary presentation of both versions of the guessing entropy, allowing an easy comparison of the two metrics. Secondly, we compare the two metrics using a set of common and relevant indicators, as well as three different datasets for side-channel evaluations (simulated, AVR XMEGA 8-bit microcontroller and a 32-bit device). We used these indicators and datasets also to compare the three full-key evaluation methods from FSE 2015, CHES 2017 and CHES 2020, allowing us to provide a clear overview of the usefulness and limitations of each method. Furthermore, our analysis has enabled us to find a new method for verifying the soundness of a leakage model, by comparing both versions of the guessing entropy. This method can be easily extended to full-key evaluations, hence leading to a new useful method for side-channel evaluations.
      PubDate: 2022-08-31
      DOI: 10.46586/tches.v2022.i4.886-905
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