## IACR News

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#### 20 September 2021

Award
The deadline for nominating IACR members for the 2021 IACR Fellows class is November 15th.

The IACR Fellows Program recognizes outstanding IACR members for technical and professional contributions to the field of cryptology.

Information about nominating a Fellow is available here.
###### Andrea Caforio, Fatih Balli, Subhadeep Banik
ePrint Report
Public knowledge about the structure of a cryptographic system is a standard assumption in the literature and algorithms are expected to guarantee security in a setting where only the encryption key is kept secret. Nevertheless, undisclosed proprietary cryptographic algorithms still find widespread use in applications both in the civil and military domains. Even though side-channel-based reverse engineering attacks that recover the hidden components of custom cryptosystems have been demonstrated for a wide range of constructions, the complete and practical reverse engineering of AES-128-like ciphers remains unattempted. In this work, we close this gap and propose the first practical reverse engineering of AES-128-like custom ciphers, i.e., algorithms that deploy undisclosed SubBytes, ShiftRows and MixColumns functions. By performing a side-channel-assisted differential power analysis, we show that the amount of traces required to fully recover the undisclosed components are relatively small, hence the possibility of a side-channel attack remains as a practical threat. The results apply to both 8-bit and 32-bit architectures and were validated on two common microcontroller platforms.
###### Geoffroy Couteau, Helger Lipmaa, Roberto Parisella, Arne Tobias Ødegaard
ePrint Report
Significantly extending the framework of (Couteau and Hartmann, Crypto 2020), we propose a general methodology to construct NIZKs for showing that an encrypted vector $\vec{\chi}$ belongs to an algebraic set, i.e., is in the zero locus of an ideal $\mathscr{I}$ of a polynomial ring. In the case where $\mathscr{I}$ is principal, i.e., generated by a single polynomial $F$, we first construct a matrix that is a quasideterminantal representation'' of $F$ and then a NIZK argument to show that $F (\vec{\chi}) = 0$. This leads to compact NIZKs for general computational structures, such as polynomial-size algebraic branching programs. We extend the framework to the case where $\IDEAL$ is non-principal, obtaining efficient NIZKs for R1CS, arithmetic constraint satisfaction systems, and thus for $\mathsf{NP}$. As an independent result, we explicitly describe the corresponding language of ciphertexts as an algebraic language, with smaller parameters than in previous constructions that were based on the disjunction of algebraic languages. This results in an efficient GL-SPHF for algebraic branching programs.
###### Francesco Berti, Chun Guo, Thomas Peters, François-Xavier Standaert
ePrint Report
The security proofs of leakage-resilient MACs based on symmetric building blocks currently rely on idealized assumptions that hardly translate into interpretable guidelines for the cryptographic engineers implementing these schemes. In this paper, we first present a leakage-resilient MAC that is both efficient and secure under standard and easily interpretable black box and physical assumptions. It only requires a collision resistant hash function and a single call per message authentication to a Tweakable Block Cipher ($\mathsf{TBC}$) that is unpredictable with leakage. This construction leverages two design twists: large tweaks for the $\mathsf{TBC}$ and a verification process that checks the inverse $\mathsf{TBC}$ against a constant. It enjoys beyond birthday security bounds. We then discuss the cost of getting rid of these design twists. We show that security can be proven without them as well. Yet, a construction without large tweaks requires stronger (non idealized) assumptions and may incur performance overheads if specialized $\mathsf{TBC}$s with large tweaks can be exploited, and a construction without twisted verification requires even stronger assumptions (still non idealized) and leads to more involved bounds. The combination of these results makes a case for our first pragmatic construction and suggests the design of $\mathsf{TBC}$s with large tweaks and good properties for side-channel countermeasures as an interesting challenge.
###### Information Security Group, Royal Holloway, University of London
Job Posting

Full-Time, Permanent

The Information Security Group (ISG) at Royal Holloway is looking to appoint two excellent permanent members of academic staff to contribute to its research and teaching. The applicant should have a high-quality research profile that fits within the wide range of research undertaken by the ISG. Successful applicants must be able to demonstrate enthusiasm for research as well as teaching and communicating with diverse audiences.

The ISG was founded in 1990 and carries out research and teaching at both undergraduate and postgraduate level, with particularly high numbers of master’s students – we are one of the very few academic departments worldwide devoted solely to Information Security, enabling our staff to focus their teaching in this area. Our MSc in Information Security is one of the oldest programmes in the world, having started in 1992 and has a large alumni network with over 4,000 graduates. We have hosted, and continue to host, a series of Centres for Doctoral Training (CDT) in cyber security, which has enabling us to recruit 10 fully funded and first-rate PhD students every year, contributing to a large and vibrant PhD community.

We are involved in a range of inter/multidisciplinary research activities, spanning technology to psychology and social sciences. Our research strengths have continued to generate significant collaborative opportunities from industry and other leading universities.

We are now recruiting academic members of staff who can complement or strengthen our existing research and teaching in Information Security. We are also interested in candidates with interests in broader multidisciplinary research. The successful candidate must hold a PhD or equivalent, and will have a proven research record. Experience in attracting funding, engaging with industry, or contributing to outreach activities will also be valuable.

Closing date for applications:

Contact: Prof. Chris Mitchell c.mitchell@rhul.ac.uk or Prof. Martin Albrecht martin.albrecht@royalholloway.ac.uk

###### Wei Dai
ePrint Report
Decentralized finance (DeFi) refers to interoperable smart contracts running on distributed ledgers offering financial services beyond payments. Recently, there has been an explosion of DeFi applications centered on Ethereum, with close to a hundred billion USD in total assets deposited as of September 2021. These applications provide financial services such as asset management, trading, and lending. The wide adoption of DeFi has raised important concerns, and among them is the key issue of privacy---DeFi applications store account balances in the clear, exposing financial positions to public scrutiny.

In this work, we propose a framework of privacy-preserving and composable DeFi on public-state smart contract platforms. First, we define a cryptographic primitive called a flexible anonymous transaction (FLAX) system with two distinctive features: (1) transactions authenticate additional information known as associated data'' and (2) transactions can be applied flexibly via a parameter that is determined at processing time, e.g. during the execution time of smart contracts. Second, we design a privacy-preserving token standard (extending ERC20), which requires read access to the inter-contract call stack and admits composable} usage by other contracts. Third, we demonstrate how the FLAX token standard can realize privacy-preserving variants of the Ethereum DeFi ecosystem of today---we show contract designs for asset pools, decentralized exchanges, and lending, covering the largest DeFi projects to date including Curve, Uniswap, Dai stablecoin, Aave, Compound, and Yearn. Lastly, we provide formal security definitions for FLAX and describe instantiations from existing designs of anonymous payments such as Zerocash, RingCT, Quisquis, and Zether.
ePrint Report
###### Tim Beyne
ePrint Report
A new interpretation of linear cryptanalysis is proposed. This 'geometric approach' unifies all common variants of linear cryptanalysis, reveals links between various properties, and suggests additional generalizations. For example, new insights into invariants corresponding to non-real eigenvalues of correlation matrices and a generalization of the link between zero-correlation and integral attacks are obtained. Geometric intuition leads to a fixed-key motivation for the piling-up principle, which is illustrated by explaining and generalizing previous results relating invariants and linear approximations. Rank-one approximations are proposed to analyze cell-oriented ciphers, and used to resolve an open problem posed by Beierle, Canteaut and Leander at FSE 2019. In particular, it is shown how such approximations can be analyzed automatically using Riemannian optimization.
###### Myrto Arapinis, Nikolaos Lamprou, Thomas Zacharias
ePrint Report
In this work, we study the Time-Lock Encryption (TLE) cryptographic primitive. The concept of TLE involves a party initiating the encryption of a message that one can only decrypt after a certain amount of time has elapsed. Following the Universal Composability (UC) paradigm introduced by Canetti [IEEE FOCS 2001], we formally abstract the concept of TLE into an ideal functionality. In addition, we provide a standalone definition for secure TLE schemes in a game-based style and we devise a hybrid protocol that relies on such a secure TLE scheme. We show that if the underlying TLE scheme satisfies the standalone game-based security definition, then our hybrid protocol UC realises the TLE functionality in the random oracle model. Finally, we present Astrolabous, a TLE construction that satisfies our security definition, leading to the first UC realization of the TLE functionality. Interestingly, it is hard to prove UC secure any of the TLE construction proposed in the literature. The reason behind this difficulty relates to the UC framework itself. Intuitively, to capture semantic security, no information should be leaked regarding the plaintext in the ideal world, thus the ciphertext should not contain any information relating to the message. On the other hand, all ciphertexts will eventually open, resulting in a trivial distinction of the real from the ideal world in the standard model. We overcome this limitation by extending any secure TLE construction adopting the techniques of Nielsen [CRYPTO 2002] in the random oracle model. Specifically, the description of the extended TLE algorithms includes calls to the random oracle, allowing our simulator to equivocate. This extension can be applied to any TLE algorithm that satisfies our standalone game-based security definition, and in particular to Astrolabous.
###### Seetal Potluri, Shamik Kundu, Akash Kumar, Kanad Basu, Aydin Aysu
ePrint Report
Existing logic-locking attacks are known to successfully decrypt a functionally correct key of a locked combinational circuit. Extensions of these attacks to real-world Intellectual Properties (IPs, which are sequential circuits) have been demonstrated through the scan-chain by selectively initializing the combinational logic and analyzing the responses. In this paper, we propose SeqL+ to mitigate a broad class of such attacks. The key idea is to lock selective functional-input/scan-output pairs of flip-flops without feedback to cause attackers to decrypt an incorrect key, and to scramble flip-flops with feedback to increase key length without introducing further vulnerabilities. We conduct a formal study of the scan-locking and scan-scrambling problems and demonstrate automating our proposed defense on any given IP. This study reveals the first formulation and complexity analysis of Boolean Satisfiability (SAT)-based attack on scan-scrambling. We formulate the attack as a conjunctive normal form (CNF) using a worst-case O(n^3) reduction in terms of scramble-graph size n, making SAT-based attack applicable and show that scramble equivalence classes are equi-sized and of cardinality 1. In order to defeat SAT-based attack, we propose an iterative swapping-based scan-cell scrambling algorithm that has O(n) implementation time-complexity and O(2^&#8970;(&#945;.n+1)/3&#8971;) SAT-decryption time-complexity in terms of a user-configurable cost constraint &#945; (0 < &#945; &#8804; 1). We empirically validate that SeqL+ hides functionally correct keys from the attacker, thereby increasing the likelihood of the decrypted key being functionally incorrect. When tested on pipelined combinational benchmarks (ISCAS, MCNC), sequential benchmarks (ITC), and a fully-fledged RISC-V CPU, SeqL+ gave 100% resilience to a broad range of state-of-the-art attacks including SAT [1], Double-DIP [2], HackTest [3], SMT [4], FALL [5], Shift-and-Leak [6], Multi-cycle [7], Scan-flushing [8], and Removal [9] attacks.
###### Ming Li, Jian Weng∗, Member, IEEE, Yi Li, Yongdong Wu, Jiasi Weng, Dingcheng Li, Robert Deng, Fellow, IEEE
ePrint Report
Blockchain interoperability is essential for the long-envisioned cross-chain decentralized applications. Existing hardware-based approaches demand several Trusted Execution Environments (TEEs) and large storage on the storage-limited TEEs. This paper presents a TEE-based privacy-preserving blockchain interoperability framework, calls as IvyCross, which decreases the requirement of TEE numbers and TEE's storage sizes by enforcing honest behaviors of TEE hosts with economic incentives. Specifically, IvyCross runs privacy-preserving cross-chain smart contracts atop two distributed TEE-powered hosts, and utilizes a sequential game between rational hosts to guarantee the correctness of contracts execution. IvyCross enables arbitrarily complex smart contracts execution across heterogenous blockchains at low costs. We formally prove the security of IvyCross in the Universal Composability framework. We also implement a prototype of IvyCross atop Bitcoin, Ethereum, and FISCO BOCS. The experiments indicate that (i) IvyCross is able to support privacy-preserving and multiple-round smart contracts for cross-chain communication; (ii) IvyCross successfully decreases the off-chain costs on storage and communication of a TEE without using complex cryptographic primitives; and (iii) the on-chain transaction fees in cross-chain communication are relatively low.
###### Andre Esser, Emanuele Bellini
ePrint Report
The selection of secure parameter sets requires an estimation of the attack cost to break the respective cryptographic scheme instantiated under these parameters. The current NIST standardization process for post-quantum schemes makes this an urgent task, especially considering the announcement to select final candidates by the end of 2021. For code-based schemes, recent estimates seemed to contradict the claimed security of most proposals, leading to a certain doubt about the correctness of those estimates. Furthermore, none of the available estimates include most recent algorithmic improvements on decoding linear codes, which are based on information set decoding (ISD) in combination with nearest neighbor search. In this work we observe that all major ISD improvements are build on nearest neighbor search, explicitly or implicitly. This allows us to derive a framework from which we obtain practical variants of all relevant ISD algorithms including the most recent improvements. We derive formulas for the practical attack costs and make those online available in an easy to use estimator tool written in python and C. Eventually, we provide classical and quantum estimates for the bit security of all parameter sets of current code-based NIST proposals.
###### Benedikt Bünz, Yuncong Hu, Shin'ichiro Matsuo, Elaine Shi
ePrint Report
A recent work by Shi and Wu (Eurocrypt'21) sugested a new, non-interactive abstraction for anonymous routing, coined Non-Interactive Anonymous Router (\NIAR). They show how to construct a \NIAR scheme with succinct communication from bilinear groups. Unfortunately, the router needs to perform quadratic computation (in the number of senders/receivers) to perform each routing.

In this paper, we show that if one is willing to relax the security notion to $(\epsilon, \delta)$-differential privacy, henceforth also called $(\epsilon, \delta)$-differential anonymity, then, a non-interactive construction exists with subquadratic router computation, also assuming standard hardness assumptions in bilinear groups. Morever, even when $1-1/\poly\log n$ fraction of the senders are corrupt, we can attain strong privacy parameters where $\epsilon = O(1/\poly\log n)$ and $\delta = \negl(n)$.
###### Santi J. Vives
ePrint Report
A peer-to-peer, permissionless, and distributed cryptographic voting system that relies only on the existence of generic digital signatures and encryption.
###### Diego Aranha, Mathias Hall-Andersen, Anca Nitulescu, Elena Pagnin, Sophia Yakoubov
ePrint Report
Ring signatures enable a signer to sign a message on behalf of a group anonymously, without revealing her identity. Similarly, threshold ring signatures allow several signers to sign the same message on behalf of a group; while the combined signature reveals that some threshold $t$ of the group members signed the message, it does not leak anything else about the signers' identities.

Anonymity is a central feature in threshold ring signature applications, such as whistleblowing, e-voting and privacy-preserving cryptocurrencies: it is often crucial for signers to remain anonymous even from their fellow signers. When the generation of a signature requires interaction, this is difficult to achieve. There exist threshold ring signatures with non-interactive signing - where signers locally produce partial signatures which can then be aggregated - but a limitation of existing threshold ring signature constructions is that all of the signers must agree on the group on whose behalf they are signing, which implicitly assumes some coordination amongst them. The need to agree on a group before generating a signature also prevents others - from outside that group - from endorsing a message by adding their signature to the statement post-factum.

We overcome this limitation by introducing extendability for ring signatures, same-message linkable ring signatures, and threshold ring signatures. Extendability allows an untrusted third party to take a signature, and extend it by enlarging the anonymity set to a larger set. In the extendable threshold ring signature, two signatures on the same message which have been extended to the same anonymity set can then be combined into one signature with a higher threshold. This enhances signers' anonymity, and enables new signers to anonymously support a statement already made by others.

For each of those primitives, we formalize the syntax and provide a meaningful security model which includes different flavors of anonymous extendability. In addition, we present concrete realizations of each primitive and formally prove their security relying on signatures of knowledge and the hardness of the discrete logarithm problem. We also describe a generic transformation to obtain extendable threshold ring signatures from same-message-linkable extendable ring signatures. Finally, we implement and benchmark our constructions.
###### Xavier Bonnetain, Gaëtan Leurent, María Naya-Plasencia, André Schrottenloher
ePrint Report
Recent works have shown that quantum period-finding can be used to break many popular constructions (some block ciphers such as Even-Mansour, multiple MACs and AEs...) in the superposition query model. So far, all the constructions broken exhibited a strong algebraic structure, which enables to craft a periodic function of a single input block. Recovering the secret period allows to recover a key, distinguish, break the confidentiality or authenticity of these modes.

In this paper, we introduce the \emph{quantum linearization attack}, a new way of using Simon's algorithm to target MACs in the superposition query model. Specifically, we use inputs of multiple blocks as an interface to a function hiding a linear structure. Recovering this structure allows to perform forgeries.

We also present some variants of this attack that use other quantum algorithms, which are much less common in quantum symmetric cryptanalysis: Deutsch's, Bernstein-Vazirani's, and Shor's. To the best of our knowledge, this is the first time these algorithms have been used in quantum forgery or key-recovery attacks.

Our attack breaks many parallelizable MACs such as LightMac, PMAC, and numerous variants with (classical) beyond-birthday-bound security (LightMAC+, PMAC) or using tweakable block ciphers (ZMAC). More generally, it shows that constructing parallelizable quantum-secure PRFs might be a challenging task.
###### Marek Broll, Federico Canale, Gregor Leander, Antonio Flórez Gutiérrez, María Naya-Plasencia
ePrint Report
We propose a general technique to improve the key-guessing step of several attacks on block ciphers. This is achieved by defining and studying some new properties of the associated S-boxes and by representing them as a special type of decision trees that are crucial for finding fine-grained guessing strategies for various attack vectors. We have proposed and implemented the algorithm that efficiently finds such trees, and use it for providing several applications of this approach, which include the best known attacks on NOKEON, GIFT, and RECTANGLE.
###### Yu Chen, Qiang Tang, Yuyu Wang
ePrint Report
In this work, we introduce the notion of hierarchical integrated signature and encryption (HISE), wherein a single public key is used for both signature and encryption, and one can derive a secret key used only for decryption from the signing key, which enables secure delegation of decryption capability. HISE enjoys the benefit of key reuse, and admits individual key escrow. We present two generic constructions of HISE. One is from (constrained) identity-based encryption. The other is from uniform one-way function, public-key encryption, and general-purpose public-coin zero-knowledge proof of knowledge. To further attain global key escrow, we take a little detour to revisit global escrow PKE, an object both of independent interest and with many applications. We formalize the syntax and security model of global escrow PKE, and provide two generic constructions. The first embodies a generic approach to compile any PKE into one with global escrow property. The second establishes a connection between three-party non-interactive key exchange and global escrow PKE. Combining the results developed above, we obtain HISE schemes that support both individual and global key escrow.

We instantiate our generic constructions of (global escrow) HISE and implement all the resulting concrete schemes for 128-bit security. Our schemes have performance that is comparable to the best Cartesian product combined public-key scheme, and exhibit advantages in terms of richer functionality and public key reuse. As a byproduct, we obtain a new global escrow PKE scheme that is $12-30 \times$ faster than the best prior work, which might be of independent interest.
###### Pantea Kiaei with Tom Conroy with Patrick Schaumont
ePrint Report
The bitsliced programming model has shown to boost the throughput of software programs. However, on a standard architecture, it exerts a high pressure on register access, causing memory spills and restraining the full potential of bitslicing. In this work, we present architecture support for bitslicing in a System-on-Chip. Our hardware extensions are of two types; internal to the processor core, in the form of custom instructions, and external to the processor, in the form of direct memory access module with support for data transposition. We present a comprehensive performance evaluation of the proposed enhancements in the context of several RISC-V ISA definitions (RV32I, RV64I, RV32B, RV64B). The proposed 14 new custom instructions use 1.5x fewer registers compared to the equivalent functionality expressed using RISC-V instructions. The integration of those custom instructions in a 5-stage pipelined RISC-V RV32I core requires 4.96% overhead. The proposed bitslice transposition unit with DMA provides a further speedup, changing the quadratic increase in execution time of data transposition to linear. Finally, we demonstrate a comprehensive performance evaluation using a set of benchmarks of lightweight and masked ciphers.
###### Pantea Kiaei with Zhenyuan Liu with Ramazan Kaan Eren with Yuan Yao with Patrick Schaumont
ePrint Report
Predicting the level and exploitability of side-channel leakage from complex SoC design is a challenging task. We present Saidoyoki, a test platform that enables the assessment of side-channel leakage under two different settings. The first is pre-silicon side-channel leakage estimation in SoC, and it requires the use of fast side-channel leakage estimation from a high level design description. The second is post-silicon side-channel leakage measurement and analysis in SoC, and it requires a hardware prototype that reflects the design description. By designing an in-house SoC and next building a side-channel leakage analysis environment around it, we are able to evaluate design-time (pre-silicon) side-channel leakage estimates as well as prototype (post-silicon) side-channel leakage measurements. The Saidoyoki platform hosts two different SoC, one based on a 32-bit RISC-V processor and a second based on a SPARC V8 processor. In this contribution, we highlight our design decisions and design flow for side-channel leakage simulation and measurement, and we present preliminary results and analysis using the Saidoyoki platform. We highlight that, while the post-silicon setting provides more side-channel leakage detail than the pre-silicon setting, the latter provides significantly enhanced test resolution and root cause analysis support. We conclude that pre-silicon side-channel leakage assessment can be an important tool for the security analysis of modern Security SoC.