## IACR News

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#### 30 September 2020

###### Shoei Nashimoto, Daisuke Suzuki, Rei Ueno, Naofumi Homma
ePrint Report
RISC-V is equipped with physical memory protection (PMP) to prevent malicious software from accessing protected memory regions. One of the main objectives of PMP is to provide a trusted execution environment (TEE) that isolates secure and insecure applications. In this study, we propose a fault injection attack to bypass the isolation based on PMP. The proposed attack scheme involves extracting successful glitch parameters for fault injection under the assumption of a black-box environment. We implement a proof-of-concept TEE compatible with PMP in RISC-V, and we verify the feasibility and effectiveness of the proposed attack through some experiments conducted in the TEE. The results show that an attacker can bypass the isolation of the TEE and read data from the protected memory region.
###### Yuan Yao, Tarun Kathuria, Baris Ege, Patrick Schaumont
ePrint Report
Power-based side-channel leakage is a known problem in the design of security-centric electronic systems. As the complexity of modern systems rapidly increases through the use of System-on-Chip (SoC) integration, it becomes difficult to determine the precise source of the side-channel leakage. Designers of secure SoC must therefore proactively apply expensive countermeasures to protect entire subsystems such as encryption modules, and this increases the design cost of the chip. We propose a methodology to determine, at design time, the source of side-channel leakage with much greater accuracy, at the granularity of a single cell. Our methodology, Architecture Correlation Analysis, uses a leakage model, well known from differential side-channel analysis techniques, to rank the cells within a netlist according to their contribution to the side-channel leakage. With this analysis result, the designer can selectively apply countermeasures where they are most effective. We demonstrate Architecture Correlation Analysis (ACA) on an AES coprocessor in an SoC design, and we determine the sources of side-channel leakage at the gate-level within the AES module as well as within the overall SoC. We validate ACA by demonstrating its use in an optimized hiding countermeasure.
###### Mark Zhandry
ePrint Report
We explore the problem of traitor tracing where the pirate decoder can contain a quantum state. Our main results include: - We show how to overcome numerous definitional challenges to give a meaningful notion of tracing for quantum decoders - We give negative results, demonstrating barriers to adapting classical tracing algorithms to the quantum decoder setting. - On the other hand, we show how to trace quantum decoders in the setting of (public key) private linear broadcast encryption, capturing a common approach to traitor tracing.
###### Cecilia Boschini, Jan Camenisch, Max Ovsiankin, Nicholas Spooner
ePrint Report
In this paper we give efficient statistical zero-knowledge proofs (SNARKs) for Module/Ring LWE and Module/Ring SIS relations, providing the remaining ingredient for building efficient cryptographic protocols from lattice-based hardness assumptions. We achieve our results by exploiting the linear-algebraic nature of the statements supported by the Aurora proof system (Ben-Sasson et al.), which allows us to easily and efficiently encode the linear-algebraic statements that arise in lattice schemes and to side-step the issue of "relaxed extractors", meaning extractors that only recover a witness for a larger relation than the one for which completeness is guaranteed. We apply our approach to the example use case of partially dynamic group signatures and obtain a lattice-based group signature that protects users against corrupted issuers, and that produces signatures smaller than the state of the art, with signature sizes of less than 300 KB for the comparably secure version of the scheme. To obtain our argument size estimates for proof of knowledge of RLWE secret, we implemented the NIZK using libiop.
###### Karim Baghery, Alonso González, Zaira Pindado, Carla Ràfols
ePrint Report
This paper constructs unbounded simulation sound proofs for boolean circuit satisfiability under standard assumptions with proof size O(n+d) bilinear group elements, where d is the depth and n is the input size of the circuit. Our technical contribution is to add unbounded simulation soundness to a recent NIZK of González and Ràfols (ASIACRYPT'19) with very small overhead. We give two different constructions: the first one is more efficient but not tight, and the second one is tight. Our new scheme can be used to construct Signatures of Knowledge based on standard assumptions that also can be composed universally with other cryptographic protocols/primitives. As an independent contribution we also detail a simple formula to encode Boolean circuits as Quadratic Arithmetic Programs.
###### Navid Alamati, Luca De Feo, Hart Montgomery, Sikhar Patranabis
ePrint Report
Isogeny-based assumptions have emerged as a viable option for quantum-secure cryptography. Recent works have shown how to build efficient (public-key) primitives from isogeny-based assumptions such as CSIDH and CSI-FiSh. However, in its present form, the landscape of isogenies does not seem very amenable to realizing new cryptographic applications. Isogeny-based assumptions often have unique efficiency and security properties, which makes building new cryptographic applications from them a potentially tedious and time-consuming task.

In this work, we propose a new framework based on group actions that enables the easy usage of a variety of isogeny-based assumptions. Our framework generalizes the works of Brassard and Yung (Crypto’90) and Couveignes (Eprint’06). We provide new definitions for group actions endowed with natural hardness assumptions that model isogeny-based constructions amenable to group actions such as CSIDH and CSI-FiSh.

We demonstrate the utility of our new framework by leveraging it to construct several primitives that were not previously known from isogeny-based assumptions. These include smooth projective hashing, dual-mode PKE, two-message statistically sender-private OT, and Naor-Reingold style PRF. These primitives are useful building blocks for a wide range of cryptographic applications.

We introduce a new assumption over group actions called Linear Hidden Shift (LHS) assumption. We then present some discussions on the security of the LHS assumption and we show that it implies symmetric KDM-secure encryption, which in turn enables many other primitives that were not previously known from isogeny-based assumptions.
###### David Lanzenberger, Ueli Maurer
ePrint Report
This paper makes three contributions. First, we present a simple theory of random systems. The main idea is to think of a probabilistic system as an equivalence class of distributions over deterministic systems. Second, we demonstrate how in this new theory, the optimal information-theoretic distinguishing advantage between two systems can be characterized merely in terms of the statistical distance of probability distributions, providing a more elementary understanding of the distance of systems. In particular, two systems that are $\epsilon$-close in terms of the best distinguishing advantage can be understood as being equal with probability 1-$\epsilon$, a property that holds statically, without even considering a distinguisher, let alone its interaction with the systems. Finally, we exploit this new characterization of the distinguishing advantage to prove that any threshold combiner is an amplifier for indistinguishability in the information-theoretic setting, generalizing and simplifying results from Maurer, Pietrzak, and Renner (CRYPTO 2007).
###### Zvika Brakerski, Pedro Branco, Nico Döttling, Sanjam Garg, Giulio Malavolta
ePrint Report
Non-committing encryption (NCE) is a type of public key encryption which comes with the ability to equivocate ciphertexts to encryptions of arbitrary messages, i.e., it allows one to find coins for key generation and encryption which explain'' a given ciphertext as an encryption of any message. NCE is the cornerstone to construct adaptively secure multiparty computation [Canetti et al. STOC'96] and can be seen as the quintessential notion of security for public key encryption to realize ideal communication channels. A large body of literature investigates what is the best message-to-ciphertext ratio (i.e., the rate) that one can hope to achieve for NCE. In this work we propose a near complete resolution to this question and we show how to construct NCE with constant rate in the plain model from a variety of assumptions, such as the hardness of the learning with errors (LWE), the decisional Diffie-Hellman (DDH), or the quadratic residuosity (QR) problem. Prior to our work, constructing NCE with constant rate required a trusted setup and indistinguishability obfuscation [Canetti et al. ASIACRYPT'17].
###### Zvika Brakerski, Nico Döttling
ePrint Report
The hardness of the Ring Learning with Errors problem (RLWE) is a central building block for efficiency-oriented lattice-based cryptography. Many applications use an entropic'' variant of the problem where the so-called secret'' is not distributed uniformly as prescribed but instead comes from some distribution with sufficient min-entropy. However, the hardness of the entropic variant has not been substantiated thus far.

For standard LWE (not over rings) entropic results are known, using a lossiness approach'' but it was not known how to adapt this approach to the ring setting. In this work we present the first such results, where entropic security is established either under RLWE or under the Decisional Small Polynomial Ratio (DSPR) assumption which is a mild variant of the NTRU assumption.

In the context of general entropic distributions, our results in the ring setting essentially match the known lower bounds (Bolboceanu et al., Asiacrypt 2019; Brakerski and DÃ¶ttling, Eurocrypt 2020).
###### Robert Ransom
ePrint Report
In most post-quantum signature protocols, the verification procedure leaks information about which signature is being verified, and/or which public key is being used to verify the signature, to timing and other side-channel attacks. In some applications, this information leak is a breach of user privacy or system security.

One class of signature protocols, based on the parallel composition of many runs of one or more interactive cut-and-choose protocols, can be modified to enable constant-time verification at low cost by fixing the multiset of challenges which will be chosen at the cut-and-choose step and randomizing only their order based on the hash of the input message. As a side benefit, this technique naturally makes the size and structure of signatures a fixed system parameter, even if the underlying cut-and-choose protocol has different response sizes for each possible challenge at the cut-and-choose step.

When applied to a 5-pass “$q2$” interactive protocol, this technique requires essentially no extra rounds due to how fixed-weight binary vectors interact with the Kales--Zaverucha structural attack. Alternatively, when the data which must be transmitted for one of the two possible challenge values is significantly shorter than the other, or can be made so using standard and/or specialized compression techniques, a longer, lower-weight challenge vector can be used to obtain shorter signatures at the cost of more rounds of the underlying interactive protocol, with a much shallower computation-vs.-size tradeoff than the precomputation tree approach used in Picnic2, MUDFISH, and SUSHSYFISH.

As an example, these techniques reduce MQDSS signatures to under 15 kB and PKP-DSS signatures to under 14 kB with NIST Category 1 security against both secret key recovery and signature forgery. Further improvements in design and parameters allow PKP-DSS signatures under 10 kB with a security level and performance acceptable for almost all interactive authentication.

The asymptotic ROM proof of security published with MQDSS remains applicable to the optimized system, but the QROM proofs by Don et al. turn out to be invalid even for unmodified MQDSS.
###### Vadim Lyubashevsky, Ngoc Khanh Nguyen, Gregor Seiler
ePrint Report
We present a novel lattice-based zero-knowledge proof system for showing that (arbitrary-sized) committed integers satisfy additive and multiplicative relationships. The proof sizes of our schemes are between two to three orders of magnitude smaller than in the lattice proof system of Libert et al. (CRYPTO 2018) for the same relations. Because the proof sizes of our protocols grow linearly in the integer length, our proofs will eventually be longer than those produced by quantum-safe succinct proof systems for general circuits (e.g. Ligero, Aurora, etc.). But for relations between reasonably-sized integers (e.g. $512$-bit), our proofs still result in the smallest zero-knowledge proof system based on a quantum-safe assumption. Of equal importance, the run-time of our proof system is at least an order of magnitude faster than any other quantum-safe scheme.
###### Amos Beimel, Iftach Haitner, Kobbi Nissim, Uri Stemmer
ePrint Report
The shuffle model of differential privacy [Bittau et al. SOSP 2017; Erlingsson et al. SODA 2019; Cheu et al. EUROCRYPT 2019] was proposed as a viable model for performing distributed differentially private computations. Informally, the model consists of an untrusted analyzer that receives messages sent by participating parties via a shuffle functionality, the latter potentially disassociates messages from their senders. Prior work focused on one-round differentially private shuffle model protocols, demonstrating that functionalities such as addition and histograms can be performed in this model with accuracy levels similar to that of the curator model of differential privacy, where the computation is performed by a fully trusted party. A model closely related to the shuffle model was presented in the seminal work of Ishai et al. on establishing cryptography from anonymous communication [FOCS 2006].

Focusing on the round complexity of the shuffle model, we ask in this work what can be computed in the shuffle model of differential privacy with two rounds. Ishai et al. showed how to use one round of the shuffle to establish secret keys between every two parties. Using this primitive to simulate a general secure multi-party protocol increases its round complexity by one. We show how two parties can use one round of the shuffle to send secret messages without having to first establish a secret key, hence retaining round complexity. Combining this primitive with the two-round semi-honest protocol of Applebaum, Brakerski, and Tsabary [TCC 2018], we obtain that every randomized functionality can be computed in the shuffle model with an honest majority, in merely two rounds. This includes any differentially private computation.

We hence move to examine differentially private computations in the shuffle model that (i) do not require the assumption of an honest majority, or (ii) do not admit one-round protocols, even with an honest majority. For that, we introduce two computational tasks: common element, and nested common element with parameter $\alpha$. For the common element problem we show that for large enough input domains, no one-round differentially private shuffle protocol exists with constant message complexity and negligible $\delta$, whereas a two-round protocol exists where every party sends a single message in every round. For the nested common element we show that no one-round differentially private protocol exists for this problem with adversarial coalition size $\alpha n$. However, we show that it can be privately computed in two rounds against coalitions of size $cn$ for every $c < 1$. This yields a separation between one-round and two-round protocols. We further show a one-round protocol for the nested common element problem that is differentially private with coalitions of size smaller than $c n$ for all $0 < c < \alpha < 1 / 2$.
###### Siam Hussain, Baiyu Li, Farinaz Koushanfar, Rosario Cammarota
ePrint Report
We present TinyGarble2 – a C++ framework for privacy-preserving computation through the Yao’s Garbled Circuit (GC) protocol in both the honest-but-curious and the malicious security models. TinyGarble2 provides a rich library with arithmetic and logic building blocks for developing GC-based secure applications. The framework offers abstractions among three layers: the C++ program, the GC back-end and the Boolean logic representation of the function being computed. TinyGarble2 thus allowing the most optimized versions of all pertinent components. These abstractions, coupled with secure share transfer among the functions make TinyGarble2 the fastest and most memory-efficient GC framework. In addition, the framework provides a library for Convolutional Neural Networks (CNN). Our evaluations show that TinyGarble2 is the fastest among the current end-to-end GC frameworks while also being scalable in terms of memory footprint. Moreover, it performs 18× faster on the CNN LeNet-5 compared to the existing scalable frameworks.
###### Ricardo Moura, David R. Matos, Miguel Pardal, Miguel Correia
ePrint Report
TLS ensures confidentiality, integrity, and authenticity of communications. However, design, implementation, and cryptographic vulnerabilities can make TLS communication channels insecure. We need mechanisms that allow the channels to be kept secure even when a new vulnerability is discovered. We present MultiTLS, a middleware based on diversity and tunneling mechanisms that allows keeping communication channels secure even when new vulnerabilities are discovered. MultiTLS creates a secure communication channel through the encapsulation of k TLS channels, where each one uses a different cipher suite. We evaluated the performance of MultiTLS and concluded that it has the advantage of being easy to use and maintain since it does not modify any of its dependencies.
###### Shweta Agrawal, Daniel Wichs, Shota Yamada
ePrint Report
Broadcast Encryption with optimal parameters was a long-standing problem, whose first solution was provided in an elegant work by Boneh, Waters and Zhandry [BWZ14]. However, this work relied on multilinear maps of logarithmic degree, which is not considered a standard assumption. Recently, Agrawal and Yamada [AY20] improved this state of affairs by providing the first construction of optimal broadcast encryption from Bilinear Maps and Learning With Errors (LWE). However, their proof of security was in the generic bilinear group model. In this work, we improve upon their result by providing a new construction and proof in the standard model. In more detail, we rely on the Learning With Errors (LWE) assumption and the Knowledge of OrthogonALity Assumption (KOALA) [BW19] on bilinear groups.

Our construction combines three building blocks: a (computational) nearly linear secret sharing scheme with compact shares which we construct from LWE, an inner-product functional encryption scheme with special properties which is constructed from the bilinear Matrix Decision Diffie Hellman (MDDH) assumption, and a certain form of hyperplane obfuscation, which is constructed using the KOALA assumption. While similar to that of Agrawal and Yamada, our construction provides a new understanding of how to decompose the construction into simpler, modular building blocks with concrete and easy-to-understand security requirements for each one. We believe this sheds new light on the requirements for optimal broadcast encryption, which may lead to new constructions in the future.
###### Tomoki Kawashima, Katsuyuki Takashima, Yusuke Aikawa, Tsuyoshi Takagi
ePrint Report
SIDH and CSIDH are key exchange protocols based on isogenies and conjectured to be quantum-resistant. Since their protocols are similar to the classical Diffie–Hellman, they are vulnerable to the man-in-the-middle attack. A key exchange which is resistant to such an attack is called an authenticated key exchange (AKE), and many isogeny-based AKEs have been proposed. However, none of them are efficient in that they all have relatively large security losses. This is partially because the random self-reducibility of isogeny-based decisional problems has not been proved yet. In this paper, we show that the computational problem and the gap problem of CSIDH are random self-reducible. A gap problem is a computational problem given access to the corresponding decision oracle. Moreover, we propose a CSIDH-based AKE with small security loss, following the construction of Cohn-Gordon et al. at CRYPTO 2019, as an application of the random self-reducibility of the gap problem of CSIDH. Our AKE is proved to be the fastest CSIDH-based AKE when we aim at 110-bit security level.
###### Hao Guo, Siwei Sun, Danping Shi, Ling Sun, Yao Sun, Lei Hu, Meiqin Wang
ePrint Report
CRAFT is a lightweight tweakable block cipher proposed at FSE 2019, which allows countermeasures against Differential Fault Attacks to be integrated into the cipher at the algorithmic level with ease. CRAFT employs a lightweight and involutory S-box and linear layer, such that the encryption function can be turned into decryption at a low cost. Besides, the tweakey schedule algorithm of CRAFT is extremely simple, where four 64-bit round tweakeys are generated and repeatedly used. Due to a combination of these features which makes CRAFT exceedingly lightweight, we find that some input difference at a particular position can be preserved through any number of rounds if the input pair follows certain truncated differential trails. Interestingly, in contrast to traditional differential analysis, the validity of this invariant property is affected by the positions where the constant additions take place. We use this property to construct weak-tweakey'' truncated differential distinguishes of CRAFT in the single-key model. Subsequently, we show how the tweak additions allow us to convert these weak-tweakey distinguishers into ordinary secret-key distinguishers based on which key-recovery attacks can be performed. Moreover, we show how to construct MILP models to search for truncated differential distinguishers exploiting this invariant property. As a result, we find a 15-round truncated differential distinguisher of CRAFT and extend it to a 19-round key-recovery attack with $2^{60.99}$ data, $2^{68}$ memory, $2^{94.59}$ time complexity, and success probability 80.66%. Also, we find a 14-round distinguisher with probability $2^{-43}$ (experimentally verified), a 16-round distinguisher with probability $2^{-55}$, and a 20-round weak-key distinguisher ($2^{118}$ weak keys) with probability $2^{-63}$. Experiments on round-reduced versions of the distinguishers show that the experimental probabilities are sometimes higher than predicted. Finally, we note that our result is far from threatening the security of the full CRAFT.

#### 27 September 2020

###### University of St. Gallen, Switzerland
Job Posting
The University of St. Gallen in Switzerland and the chair of Cyber Security invites applications from PhD holders in the area of cryptography and information security. The researcher will join a group of researchers focusing in applied and theoretical cryptography, network and information security and privacy-preservation led by Prof. Katerina Mitrokotsa. We are affiliated to the Department of Computer Science (DCS) and the Institute of Computer Science. More precisely, the student shall be working on investigating efficient authentication and verifiable delegation of computation mechanisms that provide: i) provable security guarantees, and ii) rigorous privacy guarantees. The overall aim of the PhD position will be to design and evaluate provably secure cryptographic protocols for privacy-preserving authentication and verifiable delegation of computation protocols. The research shall also consider the case where multiple clients outsource jointly computations to untrusted cloud servers.
Research area: Research areas include but are not limited to:
• Verifiable computation
• Secure Multi Party Computation
• Privacy-preserving authentication
• Cryptographic primitives
• A MsC degree in Computer Science, Applied Mathematics or a relevant field;
• Strong mathematical and algorithmic CS background;
• Good skills in programming is beneficial;
• Excellent written and verbal communication skills in English
Starting date: Fall 2020 or by mutual agreement

Closing date for applications:

Contact: Katerina Mitrokotsa

###### IMDEA Software Institute, Madrid, Spain
Job Posting

The IMDEA Software Institute invites applications for tenure-track (Assistant Professor) positions. We are primarily interested in excellent candidates in Systems, including Distributed Systems, Embedded Systems, etc.; Data Science, including machine learning; Security and Privacy; Software Engineering>; and Cyber-Physical Systems. Exceptional candidates in other topics within the research areas of the Institute will also be considered. Tenured-level (Associate and Full Professor) applications are also welcome.

The primary mission of the IMDEA Software Institute is to perform research of excellence at the highest international level in the area of software development technologies. It is one of the highest ranked institutions worldwide in its main topic areas.

All positions require a doctoral degree in CS or closely related area, earned by the expected start date. Candidates for tenure-track positions will have shown exceptional promise in research and ability to work independently as well as collaboratively. Candidates for tenured positions must have an outstanding research record, recognized international stature, and demonstrated leadership. Experience in graduate student supervision is also valued at this level.

For full consideration, complete applications must be received by December 1, 2020 but will continue to be accepted until the positions are filled.

The institute is located in the vibrant area of Madrid, Spain. It offers an ideal working environment, combining the best aspects of a research center and a university department. The institute offers institutional funding and also encourages participation in national and international research projects. The working language at the institute is English.

Salaries at the Institute are internationally competitive, established on an individual basis, and include social security provisions, and in particular access to an excellent public health care system.

COVID Note: The Institute continues working and hiring, while strictly adopting all recommended hea

Closing date for applications:

Contact: hiring@software.imdea.org