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Über dieses Buch

This book constitutes the proceedings of the 19th International Conference on Cryptographic Hardware and Embedded Systems, CHES 2017, held in Taipei, Taiwan, in September 2017.
The 33 full papers presented in this volume were carefully reviewed and selected from 130 submissions.
The annual CHES conference highlights new results in the design and analysis of cryptographic hardware and soft- ware implementations. The workshop builds a valuable bridge between the research and cryptographic engineering communities and attracts participants from industry, academia, and government organizations.



Side Channel Analysis I


A Side-Channel Assisted Cryptanalytic Attack Against QcBits

QcBits is a code-based public key algorithm based on a problem thought to be resistant to quantum computer attacks. It is a constant-time implementation for a quasi-cyclic moderate density parity check (QC-MDPC) Niederreiter encryption scheme, and has excellent performance and small key sizes. In this paper, we present a key recovery attack against QcBits. We first used differential power analysis (DPA) against the syndrome computation of the decoding algorithm to recover partial information about one half of the private key. We then used the recovered information to set up a system of noisy binary linear equations. Solving this system of equations gave us the entire key. Finally, we propose a simple but effective countermeasure against the power analysis used during the syndrome calculation.

Mélissa Rossi, Mike Hamburg, Michael Hutter, Mark E. Marson

Improved Blind Side-Channel Analysis by Exploitation of Joint Distributions of Leakages

Classical side-channel analysis include statistical attacks which require the knowledge of either the plaintext or the ciphertext to predict some internal value to be correlated to the observed leakages.In this paper we revisit a blind (i.e. leakage-only) attack from Linge et al. that exploits joint distributions of leakages. We show – both by simulations and concrete experiments on a real device – that the maximum likelihood (ML) approach is more efficient than Linge’s distance-based comparison of distributions, and demonstrate that this method can be easily adapted to deal with implementations protected by first-order Boolean masking. We give example applications of different variants of this approach, and propose countermeasures that could prevent them.Interestingly, we also observe that, when the inputs are known, the ML criterion is more efficient than correlation power analysis.

Christophe Clavier, Léo Reynaud

Convolutional Neural Networks with Data Augmentation Against Jitter-Based Countermeasures

Profiling Attacks Without Pre-processing

In the context of the security evaluation of cryptographic implementations, profiling attacks (aka Template Attacks) play a fundamental role. Nowadays the most popular Template Attack strategy consists in approximating the information leakages by Gaussian distributions. Nevertheless this approach suffers from the difficulty to deal with both the traces misalignment and the high dimensionality of the data. This forces the attacker to perform critical preprocessing phases, such as the selection of the points of interest and the realignment of measurements. Some software and hardware countermeasures have been conceived exactly to create such a misalignment. In this paper we propose an end-to-end profiling attack strategy based on the Convolutional Neural Networks: this strategy greatly facilitates the attack roadmap, since it does not require a previous trace realignment nor a precise selection of points of interest. To significantly increase the performances of the CNN, we moreover propose to equip it with the data augmentation technique that is classical in other applications of Machine Learning. As a validation, we present several experiments against traces misaligned by different kinds of countermeasures, including the augmentation of the clock jitter effect in a secure hardware implementation over a modern chip. The excellent results achieved in these experiments prove that Convolutional Neural Networks approach combined with data augmentation gives a very efficient alternative to the state-of-the-art profiling attacks.

Eleonora Cagli, Cécile Dumas, Emmanuel Prouff

CacheZoom: How SGX Amplifies the Power of Cache Attacks

In modern computing environments, hardware resources are commonly shared, and parallel computation is widely used. Parallel tasks can cause privacy and security problems if proper isolation is not enforced. Intel proposed SGX to create a trusted execution environment within the processor. SGX relies on the hardware, and claims runtime protection even if the OS and other software components are malicious. However, SGX disregards side-channel attacks. We introduce a powerful cache side-channel attack that provides system adversaries a high resolution channel. Our attack tool named CacheZoom is able to virtually track all memory accesses of SGX enclaves with high spatial and temporal precision. As proof of concept, we demonstrate AES key recovery attacks on commonly used implementations including those that were believed to be resistant in previous scenarios. Our results show that SGX cannot protect critical data sensitive computations, and efficient AES key recovery is possible in a practical environment. In contrast to previous works which require hundreds of measurements, this is the first cache side-channel attack on a real system that can recover AES keys with a minimal number of measurements. We can successfully recover AES keys from T-Table based implementations with as few as ten measurements.

Ahmad Moghimi, Gorka Irazoqui, Thomas Eisenbarth

Higher Order Countermeasures


High-Order Conversion from Boolean to Arithmetic Masking

Masking with random values is an effective countermeasure against side-channel attacks. For cryptographic algorithms combining arithmetic and Boolean masking, it is necessary to switch from arithmetic to Boolean masking and vice versa. Following a recent approach by Hutter and Tunstall, we describe a high-order Boolean to arithmetic conversion algorithm whose complexity is independent of the register size k. Our new algorithm is proven secure in the Ishai, Sahai and Wagner (ISW) framework for private circuits. In practice, for small orders, our new countermeasure is one order of magnitude faster than previous work.We also describe a 3rd-order attack against the 3rd-order Hutter-Tunstall algorithm, and a constant, 4th-order attack against the t-th order Hutter-Tunstall algorithms, for any $$t \ge 4$$ .

Jean-Sébastien Coron

Reconciling Masking in Hardware and Software

The continually growing number of security-related autonomous devices requires efficient mechanisms to counteract low-cost side-channel analysis (SCA) attacks. Masking provides high resistance against SCA at an adjustable level of security. A high level of SCA resistance, however, goes hand in hand with an increasing demand for fresh randomness which drastically increases the implementation costs. Since hardware based masking schemes have other security requirements than software masking schemes, the research in these two fields has been conducted quite independently over the last ten years. One important practical difference is that recently published software schemes achieve a lower randomness footprint than hardware masking schemes. In this work we combine existing software and hardware masking schemes into a unified masking algorithm. We demonstrate how to protect software and hardware implementations using the same masking algorithm, and for lower randomness costs than the separate schemes. Especially for hardware implementations the randomness costs can in some cases be halved over the state of the art. Theoretical considerations as well as practical implementation results are then used for a comparison with existing schemes from different perspectives and at different levels of security.

Hannes Gross, Stefan Mangard

Changing of the Guards: A Simple and Efficient Method for Achieving Uniformity in Threshold Sharing

Since they were first proposed as a countermeasure against differential power analysis (DPA) and differential electromagnetic analysis (DEMA) in 2006, threshold schemes have attracted a lot of attention from the community concentrating on cryptographic implementations. What makes threshold schemes so attractive from an academic point of view is that they come with an information-theoretic proof of resistance against a specific subset of side-channel attacks: first-order DPA. From an industrial point of view they are attractive as a careful threshold implementation forces adversaries to DPA of higher order, with all its problems such as noise amplification. A threshold scheme that offers the mentioned provable security must exhibit three properties: correctness, incompleteness and uniformity. A threshold scheme becomes more expensive with the number of shares that must be implemented and the required number of shares is lower bound by the algebraic degree of the function being shared plus 1. Defining a correct and incomplete sharing of a function of degree d in $$d+1$$ shares is straightforward. However, up to now there is no generic method to achieve uniformity and finding uniform sharings of degree-d functions with $$d+1$$ shares has been an active research area. In this paper we present a generic, simple and potentially cheap method to find a correct, incomplete and uniform $$d+1$$-share threshold scheme of any S-box layer consisting of degree-d invertible S-boxes. The uniformity is not implemented in the sharings of the individual S-boxes but rather at the S-box layer level by the use of feedforward and some expansion of shares. When applied to the Keccak-$$p$$ nonlinear step $$\chi $$, its cost is very small.

Joan Daemen

Generalized Polynomial Decomposition for S-boxes with Application to Side-Channel Countermeasures

Masking is a widespread countermeasure to protect implementations of block-ciphers against side-channel attacks. Several masking schemes have been proposed in the literature that rely on the efficient decomposition of the underlying s-box(es). We propose a generalized decomposition method for s-boxes that encompasses several previously proposed methods while providing new trade-offs. It allows to evaluate $$n\lambda $$ -bit to $$m\lambda $$ -bit s-boxes for any integers $$n,m,\lambda \ge 1$$ by seeing it a sequence of mn-variate polynomials over $$\mathbb {F}_{2^{\lambda }}$$ and by trying to minimize the number of multiplications over $$\mathbb {F}_{2^{\lambda }}$$ .

Dahmun Goudarzi, Matthieu Rivain, Damien Vergnaud, Srinivas Vivek

Emerging Attacks I


Nanofocused X-Ray Beam to Reprogram Secure Circuits

Synchrotron-based X-ray nanobeams are investigated as a tool to perturb microcontroller circuits. An intense hard X-ray focused beam of a few tens of nanometers is used to target the flash, EEPROM and RAM memory of a circuit. The obtained results show that it is possible to corrupt a single transistor in a semi-permanent state. A simple heat treatment can remove the induced effect, thus making the corruption reversible. An attack on a code stored in flash demonstrates unambiguously that this new technique can be a threat to the security of integrated circuits.

Stéphanie Anceau, Pierre Bleuet, Jessy Clédière, Laurent Maingault, Jean-luc Rainard, Rémi Tucoulou

Novel Bypass Attack and BDD-based Tradeoff Analysis Against All Known Logic Locking Attacks

Logic locking has emerged as a promising technique for protecting gate-level semiconductor intellectual property. However, recent work has shown that such gate-level locking techniques are vulnerable to Boolean satisfiability (SAT) attacks. In order to thwart such attacks, several SAT-resistant logic locking techniques have been proposed, which minimize the discriminating ability of input patterns to rule out incorrect keys. In this work, we show that such SAT-resistant logic locking techniques have their own set of unique vulnerabilities. In particular, we propose a novel “bypass attack” that ensures the locked circuit works even when an incorrect key is applied. Such a technique makes it possible for an adversary to be oblivious to the type of SAT-resistant protection applied on the circuit, and still be able to restore the circuit to its correct functionality. We show that such a bypass attack is feasible on a wide range of benchmarks and SAT-resistant techniques, while incurring minimal run-time and area/delay overhead. Binary decision diagrams (BDDs) are utilized to analyze the proposed bypass attack and assess tradeoffs in security vs overhead of various countermeasures.

Xiaolin Xu, Bicky Shakya, Mark M. Tehranipoor, Domenic Forte

Post Quantum Implementations


McBits Revisited

This paper presents a constant-time fast implementation for a high-security code-based encryption system. The implementation is based on the “McBits” paper by Bernstein, Chou, and Schwabe in 2013: we use the same FFT algorithms for root finding and syndrome computation, similar algorithms for secret permutation, and bitslicing for low-level operations. As opposed to McBits, where a high decryption throughput is achieved by running many decryption operations in parallel, we take a different approach to exploit the internal parallelism in one decryption operation for the use of more applications. As the result, we manage to achieve a slightly better decryption throughput at a much higher security level than McBits. As a minor contribution, we also present a constant-time implementation for encryption and key-pair generation, with similar techniques used for decryption.

Tung Chou

High-Speed Key Encapsulation from NTRU

This paper presents software demonstrating that the 20-year-old NTRU cryptosystem is competitive with more recent lattice-based cryptosystems in terms of speed, key size, and ciphertext size. We present a slightly simplified version of textbook NTRU, select parameters for this encryption scheme that target the 128-bit post-quantum security level, construct a KEM that is CCA2-secure in the quantum random oracle model, and present highly optimized software targeting Intel CPUs with the AVX2 vector instruction set. This software takes only 307 914 cycles for the generation of a keypair, 48 646 for encapsulation, and 67 338 for decapsulation. It is, to the best of our knowledge, the first NTRU software with full protection against timing attacks.

Andreas Hülsing, Joost Rijneveld, John Schanck, Peter Schwabe

FPGA-based Key Generator for the Niederreiter Cryptosystem Using Binary Goppa Codes

This paper presents a post-quantum secure, efficient, and tunable FPGA implementation of the key-generation algorithm for the Niederreiter cryptosystem using binary Goppa codes. Our key-generator implementation requires as few as 896,052 cycles to produce both public and private portions of a key, and can achieve an estimated frequency Fmax of over 240 MHz when synthesized for Stratix V FPGAs. To the best of our knowledge, this work is the first hardware-based implementation that works with parameters equivalent to, or exceeding, the recommended 128-bit “post-quantum security” level. The key generator can produce a key pair for parameters $$m=13$$, $$t=119$$, and $$n=6960$$ in only 3.7 ms when no systemization failure occurs, and in $$3.5 \cdot 3.7$$ ms on average. To achieve such performance, we implemented an optimized and parameterized Gaussian systemizer for matrix systemization, which works for any large-sized matrix over any binary field $$\text {GF}(2^m)$$. Our work also presents an FPGA-based implementation of the Gao-Mateer additive FFT, which only takes about 1000 clock cycles to finish the evaluation of a degree-119 polynomial at $$2^{13}$$ data points. The Verilog HDL code of our key generator is parameterized and partly code-generated using Python and Sage. It can be synthesized for different parameters, not just the ones shown in this paper. We tested the design using a Sage reference implementation, iVerilog simulation, and on real FPGA hardware.

Wen Wang, Jakub Szefer, Ruben Niederhagen

Cipher & Protocol Design

Blockcipher-Based Authenticated Encryption: How Small Can We Go?

This paper presents a design of authenticated encryption (AE) focusing on minimizing the implementation size, i.e., hardware gates or working memory on software. The scheme is called $$\textsf {COFB}$$, for COmbined FeedBack. $$\textsf {COFB}$$ uses an n-bit blockcipher as the underlying primitive, and relies on the use of a nonce for security. In addition to the state required for executing the underlying blockcipher, $$\textsf {COFB}$$ needs only n / 2 bits state as a mask. Till date, for all existing constructions in which masks have been applied, at least n bit masks have been used. Thus, we have shown the possibility of reducing the size of a mask without degrading the security level much. Moreover, it requires one blockcipher call to process one input block. We show $$\textsf {COFB}$$ is provably secure up to $$O(2^{n/2}/n)$$ queries which is almost up to the standard birthday bound. We also present our hardware implementation results. Experimental implementation results suggest that our proposal has a good performance and the smallest footprint among all known blockcipher-based AE.

Avik Chakraborti, Tetsu Iwata, Kazuhiko Minematsu, Mridul Nandi

Gimli : A Cross-Platform Permutation

This paper presents Gimli, a 384-bit permutation designed to achieve high security with high performance across a broad range of platforms, including 64-bit Intel/AMD server CPUs, 64-bit and 32-bit ARM smartphone CPUs, 32-bit ARM microcontrollers, 8-bit AVR microcontrollers, FPGAs, ASICs without side-channel protection, and ASICs with side-channel protection.

Daniel J. Bernstein, Stefan Kölbl, Stefan Lucks, Pedro Maat Costa Massolino, Florian Mendel, Kashif Nawaz, Tobias Schneider, Peter Schwabe, François-Xavier Standaert, Yosuke Todo, Benoît Viguier

GIFT: A Small Present

Towards Reaching the Limit of Lightweight Encryption

In this article, we revisit the design strategy of PRESENT, leveraging all the advances provided by the research community in construction and cryptanalysis since its publication, to push the design up to its limits. We obtain an improved version, named GIFT, that provides a much increased efficiency in all domains (smaller and faster), while correcting the well-known weakness of PRESENT with regards to linear hulls.GIFT is a very simple and clean design that outperforms even SIMON or SKINNY for round-based implementations, making it one of the most energy efficient ciphers as of today. It reaches a point where almost the entire implementation area is taken by the storage and the Sboxes, where any cheaper choice of Sbox would lead to a very weak proposal. In essence, GIFT is composed of only Sbox and bit-wiring, but its natural bitslice data flow ensures excellent performances in all scenarios, from area-optimised hardware implementations to very fast software implementation on high-end platforms.We conducted a thorough analysis of our design with regards to state-of-the-art cryptanalysis, and we provide strong bounds with regards to differential/linear attacks.

Subhadeep Banik, Sumit Kumar Pandey, Thomas Peyrin, Yu Sasaki, Siang Meng Sim, Yosuke Todo

Making Password Authenticated Key Exchange Suitable for Resource-Constrained Industrial Control Devices

Connectivity becomes increasingly important also for small embedded systems such as typically found in industrial control installations. More and more use-cases require secure remote user access increasingly incorporating handheld based human machine interfaces, using wireless links such as Bluetooth. Correspondingly secure operator authentication becomes of utmost importance. Unfortunately, often passwords with all their well-known pitfalls remain the only practical mechanism.We present an assessment of the security requirements for the industrial setting, illustrating that offline attacks on passwords-based authentication protocols should be considered a significant threat. Correspondingly use of a Password Authenticated Key Exchange protocol becomes desirable. We review the significant challenges faced for implementations on resource-constrained devices.We explore the design space and shown how we succeeded in tailoring a particular variant of the Password Authenticated Connection Establishment (PACE) protocol, such that acceptable user interface responsiveness was reached even for the constrained setting of an ARM Cortex-M0+ based Bluetooth low-energy transceiver running from a power budget of 1.5 mW without notable energy buffers for covering power peak transients.

Björn Haase, Benoît Labrique

Security Evaluation


Back to Massey: Impressively Fast, Scalable and Tight Security Evaluation Tools

None of the existing rank estimation algorithms can scale to large cryptographic keys, such as 4096-bit (512 bytes) RSA keys. In this paper, we present the first solution to estimate the guessing entropy of arbitrarily large keys, based on mathematical bounds, resulting in the fastest and most scalable security evaluation tool to date. Our bounds can be computed within a fraction of a second, with no memory overhead, and provide a margin of only a few bits for a full 128-bit AES key.

Marios O. Choudary, P. G. Popescu

Fast Leakage Assessment

We describe a fast technique for performing the computationally heavy part of leakage assessment, in any statistical moment (or other property) of the leakage samples distributions. The proposed technique outperforms by orders of magnitude the approach presented at CHES 2015 by Schneider and Moradi. We can carry out evaluations that before took 90 CPU-days in 4 CPU-hours (about a 500-fold speed-up). As a bonus, we can work with exact arithmetic, we can apply kernel-based density estimation methods, we can employ arbitrary pre-processing functions such as absolute value to power traces, and we can perform information-theoretic leakage assessment. Our trick is simple and elegant, and lends itself to an easy and compact implementation. We fit a prototype implementation in about 130 lines of C code.

Oscar Reparaz, Benedikt Gierlichs, Ingrid Verbauwhede

FPGA Security


Your Rails Cannot Hide from Localized EM: How Dual-Rail Logic Fails on FPGAs

Protecting cryptographic implementations against side-channel attacks is a must to prevent leakage of processed secrets. As a cell-level countermeasure, so called DPA-resistant logic styles have been proposed to prevent a data-dependent power consumption.As most of the DPA-resistant logic is based on dual-rails, properly implementing them is a challenging task on FPGAs which is due to their fixed architecture and missing freedom in the design tools.While previous works show a significant security gain when using such logic on FPGAs, we demonstrate this only holds for power-analysis. In contrast, our attack using high-resolution electromagnetic analysis is able to exploit local characteristics of the placement and routing such that only a marginal security gain remains, therefore creating a severe threat.To further analyze the properties of both attack and implementation, we develop a custom placer to improve the default placement of the analyzed AES S-box. Different cost functions for the placement are tested and evaluated w.r.t. the resulting side-channel resistance on a Spartan-6 FPGA. As a result, we are able to more than double the resistance of the design compared to cases not benefiting from the custom placement.

Vincent Immler, Robert Specht, Florian Unterstein

How to Break Secure Boot on FPGA SoCs Through Malicious Hardware

Embedded IoT devices are often built upon large system on chip computing platforms running a significant stack of software. For certain computation-intensive operations such as signal processing or encryption and authentication of large data, chips with integrated FPGAs, FPGA SoCs, which provide high performance through configurable hardware designs, are used. In this contribution, we demonstrate how an FPGA hardware design can compromise the important secure boot process of the main software system to boot from a malicious network source instead of an authentic signed kernel image. This significant and new threat arises from the fact that the CPU and FPGA are connected to the same memory bus, so that FPGA hardware designs can interfere with secure boot routines on FPGA SoCs that are without any interruption on regular SoCs. An enabling factor is that integrated hardware designs are likely bought from external partners and there is a realistic lack of security review at the system integrators. This facilitates flaws or even unwanted functionality in such hardware designs. We perform a proof of concept on a Xilinx Zynq-7000 FPGA SoC, and the threat can be generalized to other devices. We also present as effective mitigation, an easy-to-review and re-usable wrapper module which prevents any unauthorized memory access by included hardware designs.

Nisha Jacob, Johann Heyszl, Andreas Zankl, Carsten Rolfes, Georg Sigl

Emerging Attacks II


Illusion and Dazzle: Adversarial Optical Channel Exploits Against Lidars for Automotive Applications

With the advancement in computing, sensing, and vehicle electronics, autonomous vehicles are being realized. For autonomous driving, environment perception sensors such as radars, lidars, and vision sensors play core roles as the eyes of a vehicle; therefore, their reliability cannot be compromised. In this work, we present a spoofing by relaying attack, which can not only induce illusions in the lidar output but can also cause the illusions to appear closer than the location of a spoofing device. In a recent work, the former attack is shown to be effective, but the latter one was never shown. Additionally, we present a novel saturation attack against lidars, which can completely incapacitate a lidar from sensing a certain direction. The effectiveness of both the approaches is experimentally verified against Velodyne’s VLP-16.

Hocheol Shin, Dohyun Kim, Yujin Kwon, Yongdae Kim

Hacking in the Blind: (Almost) Invisible Runtime User Interface Attacks

We describe novel, adaptive user interface attacks, where the adversary attaches a small device to the interface that connects user input peripherals to the target system. The device executes the attack when the authorized user is performing safety-, or security-critical operations, by modifying or blocking user input, or injecting new events. Although the adversary fully controls the user input channel, to succeed he needs to overcome a number of challenges, including the inability to directly observe the state of the user interface and avoiding being detected by the legitimate user. We present new techniques that allow the adversary to do user interface state estimation and fingerprinting, and thus attack a new range of scenarios that previous UI attacks do not apply to. We evaluate our attacks on two different types of platforms: e-banking on general-purpose PCs, and dedicated medical terminals. Our evaluation shows that such attacks can be implemented efficiently, are hard for the users to detect, and would lead to serious violations of input integrity.

Luka Malisa, Kari Kostiainen, Thomas Knell, David Sommer, Srdjan Capkun

On the Security of Carrier Phase-Based Ranging

Multicarrier phase-based ranging is fast emerging as a cost-optimized solution for a wide variety of proximity-based applications due to its low power requirement, low hardware complexity and compatibility with existing standards such as ZigBee and 6LoWPAN. Given potentially critical nature of the applications in which phase-based ranging can be deployed (e.g., access control, asset tracking), it is important to evaluate its security guarantees. Therefore, in this work, we investigate the security of multicarrier phase-based ranging systems and specifically focus on distance decreasing relay attacks that have proven detrimental to the security of proximity-based access control systems (e.g., vehicular passive keyless entry and start systems). We show that phase-based ranging, as well as its implementations, are vulnerable to a variety of distance reduction attacks. We describe different attack realizations and verify their feasibility by simulations and experiments on a commercial ranging system. Specifically, we successfully reduced the estimated range to less than $$3\, \mathrm {m}$$ even though the devices were more than 50 m apart. We discuss possible countermeasures against such attacks and illustrate their limitations, therefore demonstrating that phase-based ranging cannot be fully secured against distance decreasing attacks.

Hildur Ólafsdóttir, Aanjhan Ranganathan, Srdjan Capkun

Side Channel Analysis II


Single-Trace Side-Channel Attacks on Masked Lattice-Based Encryption

Although lattice-based cryptography has proven to be a particularly efficient approach to post-quantum cryptography, its security against side-channel attacks is still a very open topic. There already exist some first works that use masking to achieve DPA security. However, for public-key primitives SPA attacks that use just a single trace are also highly relevant. For lattice-based cryptography this implementation-security aspect is still unexplored.In this work, we present the first single-trace attack on lattice-based encryption. As only a single side-channel observation is needed for full key recovery, it can also be used to attack masked implementations. We use leakage coming from the Number Theoretic Transform, which is at the heart of almost all efficient lattice-based implementations. This means that our attack can be adapted to a large range of other lattice-based constructions and their respective implementations.Our attack consists of 3 main steps. First, we perform a template matching on all modular operations in the decryption process. Second, we efficiently combine all this side-channel information using belief propagation. And third, we perform a lattice-decoding to recover the private key. We show that the attack allows full key recovery not only in a generic noisy Hamming-weight setting, but also based on real traces measured on an ARM Cortex-M4F microcontroller.

Robert Primas, Peter Pessl, Stefan Mangard

A Systematic Approach to the Side-Channel Analysis of ECC Implementations with Worst-Case Horizontal Attacks

The wide number and variety of side-channel attacks against scalar multiplication algorithms makes their security evaluations complex, in particular in case of time constraints making exhaustive analyses impossible. In this paper, we present a systematic way to evaluate the security of such implementations against horizontal attacks. As horizontal attacks allow extracting most of the information in the leakage traces of scalar multiplications, they are suitable to avoid risks of overestimated security levels. For this purpose, we additionally propose to use linear regression in order to accurately characterize the leakage function and therefore approach worst-case security evaluations. We then show how to apply our tools in the contexts of ECDSA and ECDH implementations, and validate them against two targets: a Cortex-M4 and a Cortex-A8 micro-controllers.

Romain Poussier, Yuanyuan Zhou, François-Xavier Standaert

Sliding Right into Disaster: Left-to-Right Sliding Windows Leak

It is well known that constant-time implementations of modular exponentiation cannot use sliding windows. However, software libraries such as Libgcrypt, used by GnuPG, continue to use sliding windows. It is widely believed that, even if the complete pattern of squarings and multiplications is observed through a side-channel attack, the number of exponent bits leaked is not sufficient to carry out a full key-recovery attack against RSA. Specifically, 4-bit sliding windows leak only 40% of the bits, and 5-bit sliding windows leak only 33% of the bits.In this paper we demonstrate a complete break of RSA-1024 as implemented in Libgcrypt. Our attack makes essential use of the fact that Libgcrypt uses the left-to-right method for computing the sliding-window expansion. We show for the first time that the direction of the encoding matters: the pattern of squarings and multiplications in left-to-right sliding windows leaks significantly more information about the exponent than right-to-left. We show how to extend the Heninger-Shacham algorithm for partial key reconstruction to make use of this information and obtain a very efficient full key recovery for RSA-1024. For RSA-2048 our attack is efficient for 13% of keys.

Daniel J. Bernstein, Joachim Breitner, Daniel Genkin, Leon Groot Bruinderink, Nadia Heninger, Tanja Lange, Christine van Vredendaal, Yuval Yarom

Encoding Techniques


Faster Homomorphic Function Evaluation Using Non-integral Base Encoding

In this paper we present an encoding method for real numbers tailored for homomorphic function evaluation. The choice of the degree of the polynomial modulus used in all popular somewhat homomorphic encryption schemes is dominated by security considerations, while with the current encoding techniques the correctness requirement allows for much smaller values. We introduce a generic encoding method using expansions with respect to a non-integral base, which exploits this large degree at the benefit of reducing the growth of the coefficients when performing homomorphic operations. This allows one to choose a smaller plaintext coefficient modulus which results in a significant reduction of the running time. We illustrate our approach by applying this encoding in the setting of homomorphic electricity load forecasting for the smart grid which results in a speed-up by a factor 13 compared to previous work, where encoding was done using balanced ternary expansions.

Charlotte Bonte, Carl Bootland, Joppe W. Bos, Wouter Castryck, Ilia Iliashenko, Frederik Vercauteren

Hiding Secrecy Leakage in Leaky Helper Data

PUFs provide cryptographic keys for embedded systems without dedicated secure memory. Practical PUF implementations often show a bias in the PUF responses, which leads to secrecy leakage in many key derivation constructions. However, previously proposed mitigation techniques remove the bias at the expense of discarding large numbers of PUF response bits. Instead of removing the bias from the input sequence, this work reduces the secrecy leakage through the helper data. We apply the concept of wiretap coset coding to add randomness to the helper data such that an attacker cannot isolate significant information about the key anymore.Examples demonstrate the effectiveness of coset coding for different bias parameters by computing the exact leakage for short code lengths and applying upper bounds for larger code lengths. In our case study, we compare a secrecy leakage mitigation design with coset coding and Differential Sequence Coding (DSC). It reduces the number of required PUF response bits by $$60\%$$ compared to state-of-the-art debiasing approaches.

Matthias Hiller, Aysun Gurur Önalan

Efficient Implementations


Very High Order Masking: Efficient Implementation and Security Evaluation

In this paper, we study the performances and security of recent masking algorithms specialized to parallel implementations in a 32-bit embedded software platform, for the standard AES Rijndael and the bitslice cipher Fantomas. By exploiting the excellent features of these algorithms for bitslice implementations, we first extend the recent speed records of Goudarzi and Rivain (presented at Eurocrypt 2017) and report realistic timings for masked implementations with 32 shares. We then observe that the security level provided by such implementations is uneasy to quantify with current evaluation tools. We therefore propose a new “multi-model” evaluation methodology which takes advantage of different (more or less abstract) security models introduced in the literature. This methodology allows us to both bound the security level of our implementations in a principled manner and to assess the risks of overstated security based on well understood parameters. Concretely, it leads us to conclude that these implementations withstand worst-case adversaries with $$>\!2^{64}$$ measurements under falsifiable assumptions.

Anthony Journault, François-Xavier Standaert


Efficient and Secure Implementation in Software

The PRESENT block cipher was one of the first hardware-oriented proposals for implementation in extremely resource-constrained environments. Its design is based on 4-bit S-boxes and a 64-bit permutation, a far from optimal choice to achieve good performance in software. As a result, most software implementations require large lookup tables in order to meet efficiency goals. In this paper, we describe a new portable and efficient software implementation of PRESENT, fully protected against timing attacks. Our implementation uses a novel decomposition of the permutation layer, and bitsliced computation of the S-boxes using optimized Boolean formulas, not requiring lookup tables. The implementations are evaluated in embedded ARM CPUs ranging from microcontrollers to full-featured processors equipped with vector instructions. Timings for our software implementation show a significant performance improvement compared to the numbers from the FELICS benchmarking framework. In particular, encrypting 128 bits using CTR mode takes about 2100 cycles on a Cortex-M3, improving on the best Assembly implementation in FELICS by a factor of 8. Additionally, we present the fastest masked implementation of PRESENT for protection against timing and other side-channel attacks in the scenario we consider, improving on related work by 15%. Hence, we conclude that PRESENT can be remarkably efficient in software if implemented with our techniques, and even compete with a software implementation of AES in terms of latency while offering a much smaller code footprint.

Tiago B. S. Reis, Diego F. Aranha, Julio López

Four on Embedded Devices with Strong Countermeasures Against Side-Channel Attacks

This work deals with the energy-efficient, high-speed and high-security implementation of elliptic curve scalar multiplication and elliptic curve Diffie-Hellman (ECDH) key exchange on embedded devices using Four$$\mathbb {Q}$$ and incorporating strong countermeasures to thwart a wide variety of side-channel attacks. First, we set new speed records for constant-time curve-based scalar multiplication and DH key exchange at the 128-bit security level with implementations targeting 8, 16 and 32-bit microcontrollers. For example, our software computes a static ECDH shared secret in $$\sim $$6.9 million cycles (or 0.86 s @8 MHz) on a low-power 8-bit AVR microcontroller which, compared to the fastest Curve25519 and genus-2 Kummer implementations on the same platform, offers 2$$\times $$ and 1.4$$\times $$ speedups, respectively. Similarly, it computes the same operation in $$\sim $$496 thousand cycles on a 32-bit ARM Cortex-M4 microcontroller, achieving a factor-2.9 speedup when compared to the fastest Curve25519 implementation targeting the same platform. Second, we engineer a set of side-channel countermeasures taking advantage of Four$$\mathbb {Q}$$’s rich arithmetic and propose a secure implementation that offers protection against a wide range of sophisticated side-channel attacks. Finally, we perform a differential power analysis evaluation of our software running on an ARM Cortex-M4, and report that no leakage was detected with up to 10 million traces. These results demonstrate the potential of deploying Four$$\mathbb {Q}$$ on low-power applications such as protocols for IoT.

Zhe Liu, Patrick Longa, Geovandro C. C. F. Pereira, Oscar Reparaz, Hwajeong Seo

Bit-Sliding: A Generic Technique for Bit-Serial Implementations of SPN-based Primitives

Applications to AES, PRESENT and SKINNY

Area minimization is one of the main efficiency criterion for lightweight encryption primitives. While reducing the implementation data path is a natural strategy for achieving this goal, Substitution-Permutation Network (SPN) ciphers are usually hard to implement in a bit-serial way (1-bit data path). More generally, this is hard for any data path smaller than its Sbox size, since many scan flip-flops would be required for storage, which are more area-expensive than regular flip-flops.In this article, we propose the first strategy to obtain extremely small bit-serial ASIC implementations of SPN primitives. Our technique, which we call bit-sliding, is generic and offers many new interesting implementation trade-offs. It manages to minimize the area by reducing the data path to a single bit, while avoiding the use of many scan flip-flops.Following this general architecture, we could obtain the first bit-serial and the smallest implementation of AES-128 to date (1560 GE for encryption only, and 1738 GE for encryption and decryption with IBM 130 nm standard-cell library), greatly improving over the smallest known implementations (about 30% decrease), making AES-128 competitive to many ciphers specifically designed for lightweight cryptography. To exhibit the generality of our strategy, we also applied it to the PRESENT and SKINNY block ciphers, again offering the smallest implementations of these ciphers thus far, reaching an area as low as 1065 GE for a 64-bit block 128-bit key cipher. It is also to be noted that our bit-sliding seems to obtain very good power consumption figures, which makes this implementation strategy a good candidate for passive RFID tags.

Jérémy Jean, Amir Moradi, Thomas Peyrin, Pascal Sasdrich


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