International Association
for Cryptologic Research

The purpose of this paper is to use a Central Limit approach to develop a statistical framework for analysing ciphertexts in Ring-LWE homomorphic encryption schemes. This statistical framework gives rise to Normal approximations for ciphertext random variables, and we show that this allows probabilities to be determined more accurately and hence enables better bounds for decryption failure probabilities than the widely used existing approach based on $\delta$-subgaussian random variables. To demonstrate the benefit of the Central Limit approach, we apply our framework and results to a homomorphic Ring-LWE cryptosystem of Lyubashevsky, Peikert and Regev (Eurocrypt 2013, full version).

This paper presents CONCRETE (Commit-Encrypt-Send-the-Key) a new Authenticated Encryption mode that offers CIML2 security, that is, ciphertext integrity in the presence of nonce misuse and side-channel leakages in both encryption and decryption.

CONCRETE improves on a recent line of works aiming at leveled implementations, which mix a strongly protected and energy demanding implementation of a single component, and other weakly protected and much cheaper components. Here, these components all implement a tweakable block cipher TBC.

CONCRETE requires the use of the strongly protected TBC only once while supporting the leakage of the full state of the weakly protected components -- it achieves CIML2 security in the so-called unbounded leakage model.

All previous works need to use the strongly protected implementation at least twice. As a result, for short messages whose encryption and decryption energy costs are dominated by the strongly protected component, we halve the cost of a leakage-resilient implementation. CONCRETE additionally provides security when unverified plaintexts are released, and confidentiality in the presence of simulatable leakages in encryption and decryption.

In this paper, we introduce two lightweight historical data based multi-factor authenticated key exchange (HMAKE) protocols in the random oracle model. Our HMAKE protocols use a symmetric secret key, as their first authentication factor, together with their second authentication factor, historical data exchanged between the two parties in the past, and the third authentication factor, a set of secret tags associated with the historical data, to establish a secure communication channel between the client and the server.

A remarkable security feature of HMAKE is bounded historical tag leakage resilience, which means that (informally speaking) if a small portion of the secret tags is leaked to an adversary, it will not affect the security of one HMAKE protocol with an overwhelming probability. Our first HMAKE protocol can provide static bounded leakage resilience, meaning that the secret tags are leaked at the beginning of the security game. To enhance its security, our second HMAKE protocol makes use of our first protocol as a compiler to transform any passively secure two-message key exchange protocol to an actively secure HMAKE protocol with perfect forward secrecy, and therefore it can be secure even if the historical tags are compromised adaptively by an attacker.

In addition to the strong security properties we achieved, our protocols can potentially have great impacts in practice: they are efficient in computation, and they are compatible with legacy devices in cyber-physical systems.

There have been many successes in constructing explicit non-malleable codes for various classes of tampering functions in recent years, and strong existential results are also known. In this work we ask the following question: "When can we rule out the existence of a non-malleable code for a tampering class $\mathcal{F}$?"

We show that non-malleable codes are impossible to construct for three different tampering classes: 1. Functions that change $d/2$ symbols, where $d$ is the distance of the code; 2. Functions where each input symbol affects only a single output symbol; 3. Functions where each of the $n$ output symbols is a function of $n-\log n$ input symbols.

We additionally rule out constructions of non-malleable codes for certain classes $\mathcal{F}$ via reductions to the assumption that a distributional problem is hard for $\mathcal{F}$, that make black-box use of the tampering functions in the proof. In particular, this yields concrete obstacles for the construction of efficient codes for $\mathsf{NC}$, even assuming average-case variants of $P\not\subseteq\mathsf{NC}$.

Secure multiparty computation (MPC) often relies on sources of correlated randomness for better efficiency and simplicity. This is particularly useful for MPC with no honest majority, where input-independent correlated randomness enables a lightweight “non-cryptographic” online phase once the inputs are known. However, since the amount of correlated randomness typically scales with the circuit size of the function being computed, securely generating correlated randomness forms an efficiency bottleneck, involving a large amount of communication and storage. A natural tool for addressing the above limitations is a pseudorandom correlation generator (PCG). A PCG allows two or more parties to securely generate long sources of useful correlated randomness via a local expansion of correlated short seeds and no interaction. PCGs enable MPC with silent preprocessing, where a small amount of interaction used for securely sampling the seeds is followed by silent local generation of correlated pseudorandomness. A concretely efficient PCG for Vector-OLE correlations was recently obtained by Boyle et al. (CCS 2018) based on variants of the learning parity with noise (LPN) assumption over large fields. In this work, we initiate a systematic study of PCGs and present concretely efficient constructions for several types of useful MPC correlations. We obtain the following main contributions: – PCG foundations. We give a general security definition for PCGs. Our definition suffices for any MPC protocol satisfying a stronger security requirement that is met by existing protocols. We prove that a stronger security requirement is indeed necessary, and justify our PCG definition by ruling out a stronger and more natural definition. – Silent OT extension. We present the first concretely efficient PCG for oblivious transfer correlations. Its security is based on a variant of the binary LPN assumption and any correlation-robust hash function. We expect it to provide a faster alternative to the IKNP OT extension protocol (Crypto ’03) when communication is the bottleneck. We present several applications, including protocols for non-interactive zero-knowledge with bounded-reusable preprocessing from binary LPN, and concretely efficient related-key oblivious pseudorandom functions. – PCGs for simple 2-party correlations. We obtain PCGs for several other types of useful 2-party correlations, including (authenticated) one-time truth-tables and Beaver triples. While the latter PCGs are slower than our PCG for OT, they are still practically feasible. These PCGs are based on a host of assumptions and techniques, including specialized homomorphic secret sharing schemes and pseudorandom generators tailored to their structure. – Multiparty correlations. We obtain PCGs for multiparty correlations that can be used to make the circuit-dependent communication of MPC protocols scale linearly (instead of quadratically) with the number of parties.

Conditional cube attack was proposed by Huang et al. at EUROCRYPT 2017 to attack Keccak keyed mode. Inspired by dynamic cube attack, they reduce the degree by appending key bit conditions on the initial value (IV). Recently, Li et al. proposed new conditional cube attacks on Keccak keyed mode with extremely small degrees of freedom. In this paper, we find a new property on Li et al.'s method, and modify the new conditional cube attack for lightweight encryption algorithms using a 8-2-2 pattern, and apply it on 5-round Ketje Jr, 6-round Xoodoo-AE and Xoodyak, where Ketje Jr is among the 3rd round CAESAR competition candidates and Xoodyak is a Round 1 submission of the ongoing NIST lightweight cryptography project. Then we give the updated conditional cube attack analysis. All our results are of practical time complexity with negligible memory cost and our test codes are given in this paper. Notably, it is the first third-party cryptanalysis result for Xoodyak.

Dynamic Searchable Symmetric Encryption ($\mathsf{DSSE}$), apart from providing support for search operation, allows a client to perform update operations on outsourced database efficiently. Two security properties, viz., forward privacy and backward privacy are desirable from a $\mathsf{DSSE}$ scheme. The former captures that the newly updated entries cannot be related to previous search queries and the latter ensures that search queries should not leak matching entries after they have been deleted. These security properties are formalized in terms of the information leakage that can be incurred by the respective constructions. Existing backward private constructions either have a non-optimal communication overhead or they make use of heavy cryptographic primitives. Our main contribution consists of three efficient backward private schemes that aim to achieve practical efficiency by using light weight symmetric cryptographic components only. In the process, we also revisit the existing definitions of information leakage for backward privacy [Bost et al. CCS'17] and propose alternative formulations. Our first construction $\Pi_\mathsf{BP}\text{-}\mathsf{prime}$ achieves a stronger notion of backward privacy whereas our next two constructions $\Pi_\mathsf{BP}$ and $\Pi_\mathsf{WBP}$ achieve optimal communication complexity at the cost of some additional leakage. The prototype implementations of our schemes depict the practicability of the proposed constructions and indicate that the cost of achieving backward privacy over forward privacy is substantially small.

We devise new techniques for design and analysis of efficient lattice-based zero-knowledge proofs (ZKP). First, we introduce one-shot proof techniques for non-linear polynomial relations of degree $k\ge 2$, where the protocol achieves a negligible soundness error in a single execution, and thus performs significantly better in both computation and communication compared to prior protocols requiring multiple repetitions. Such proofs with degree $k\ge 2$ have been crucial ingredients for important privacy-preserving protocols in the discrete logarithm setting, such as Bulletproofs (IEEE S&P '18) and arithmetic circuit arguments (EUROCRYPT '16). In contrast, one-shot proofs in lattice-based cryptography have previously only been shown for the linear case ($k=1$) and a very specific quadratic case ($k=2$), which are obtained as a special case of our technique.

Moreover, we introduce two speedup techniques for lattice-based ZKPs: a CRT-packing technique supporting ``inter-slot'' operations, and ``NTT-friendly'' tools that permit the use of fully-splitting rings. The former technique comes at almost no cost to the proof length, and the latter one barely increases it, which can be compensated for by tweaking the rejection sampling parameters while still having faster computation overall.

To illustrate the utility of our techniques, we show how to use them to build efficient relaxed proofs for important relations, namely proof of commitment to bits, one-out-of-many proof, range proof and set membership proof. Despite their relaxed nature, we further show how our proof systems can be used as building blocks for advanced cryptographic tools such as ring signatures.

Our ring signature achieves a dramatic improvement in length over all the existing proposals from lattices at the same security level. The computational evaluation also shows that our construction is highly likely to outperform all the relevant works in running times. Being efficient in both aspects, our ring signature is particularly suitable for both small-scale and large-scale applications such as cryptocurrencies and e-voting systems. No trusted setup is required for any of our proposals.

Key exchange protocols in the asymmetric-key setting are known to provide stronger security properties than protocols in symmetric-key cryptography. In particular, they can provide perfect forward secrecy, as illustrated by key exchange protocols based on the Diffie-Hellman scheme. However public-key algorithms are too heavy for low-resource devices, which can then not benefit from forward secrecy. In this paper, we describe a scheme that solves this issue. Using a nifty resynchronisation technique, we propose an authenticated key exchange protocol in the symmetric-key setting that guarantees perfect forward secrecy. We prove that the protocol is sound, and provide a formal security proof.

We study protocols that rely on a public ledger infrastructure, concentrating on protocols for zero-knowledge contingent payment, whose security properties combine diverse notions of fairness and privacy. We argue that rigorous models are required for capturing the ledger semantics, the protocol-ledger interaction, the cryptographic primitives and, ultimately, the security properties one would like to achieve.

Our focus is on a particular level of abstraction, where network messages are represented by a term algebra, protocol execution by state transition systems (e.g. multiset rewrite rules) and where the properties of interest can be analyzed with automated verification tools. We propose models for: (1) the rules guiding the ledger execution, taking the coin functionality of public ledgers such as Bitcoin as an example; (2) the security properties expected from ledger-based zero-knowledge contingent payment protocols; (3) two different security protocols that aim at achieving these properties relying on different ledger infrastructures; (4) reductions that allow simpler term algebras for homomorphic cryptographic schemes.

Altogether, these models allow us to derive a first automated verification for ledger-based zero-knowledge contingent payment using the Tamarin prover. Furthermore, our models help in clarifying certain underlying assumptions, security and efficiency tradeoffs that should be taken into account when deploying protocols on the blockchain.

At the Computer Science Department at the University of Twente, we are looking for highly motivated and enthusiastic Assistant/Associate/Full Professors (f/m) in several domains.

In the Security & Privacy domain, we are particularly looking for someone in the areas of \"Big Data and Security\" (which considers both \"Big Data for Security\" and \"Security for Big Data\") and \"Security and the Internet of Things\" (broadly conceived).

For more information, please check the link provided below.

Closing date for applications: 25 May 2019

More information: https://www.utwente.nl/en/organization/careers/!/121825/assistantassociatefull-professors-in-computer-science

For a 3-year collaborative research project on automotive security, Lund University (Sweden) and Nanyang Technological University (Singapore) are seeking candidates for two postdoc/research fellow positions (from fresh post-doc to senior research fellow, flexible contract duration) in the areas of symmetric key cryptography and/or hardware implementations. One position is available for Lund University and one position is available for Nanyang Technological University.

Salaries are competitive and are determined according to the successful applicants accomplishments, experience and qualifications. Interested applicants should send their detailed CVs, cover letter and references to Prof. Thomas Johansson (thomas.johansson (at) eit.lth.se) and Prof. Thomas Peyrin (thomas.peyrin (at) ntu.edu.sg).

Review of applications starts immediately and will continue until positions are filled.

Closing date for applications: 15 October 2019

Contact: thomas.johansson (at) eit.lth.se and thomas.peyrin (at) ntu.edu.sg

Postdoctoral research fellow openings are immediately available in the School of Computer Science and Engineering (SCSE) at Nanyang Technological University (NTU) in Singapore. The postdoc will work with Assistant Professor Jun ZHAO (biography below) on one of the following topics:

1. Differential privacy with applications to deep learning, federated learning, or machine learning in general,

2. Local differential privacy,

3. Adversarial machine learning and security in AI systems,

4. Blockchains,

5. Other areas in AI security/privacy or IoT security/privacy.

Interested candidates can contact Jun Zhao via email at JunZhao (at) ntu.edu.sg?JunZhao (at) alumni.cmu.edu?via WeChat by scanning the QR code at http://www.ntu.edu.sg/home/JunZhao/wechat.png

via Skype at live:junzhaocmu, or by calling Singapore phone number +65 8648 3534 (the first two numbers 65 represent the area code of Singapore). Thanks.

Jun Zhao’s homepage: http://ntu.edu.sg/home/JunZhao/

Biography: Jun Zhao received a PhD degree in Electrical and Computer Engineering from Carnegie Mellon University (CMU) in the USA (advisors: Virgil Gligor, Osman Yagan), affiliating with CMU CyLab Security & Privacy Institute. He is currently an Assistant Professor at Nanyang Technological University (NTU) in Singapore. His research interests include blockchains, security, and privacy with applications to deep learning, the Internet of Things, and social networks.

Closing date for applications: 1 November 2019

Contact: Interested candidates can contact Jun Zhao via email at JunZhao (at) ntu.edu.sg?JunZhao (at) alumni.cmu.edu?via WeChat by scanning the QR code at http://www.ntu.edu.sg/home/JunZhao/wechat.png

via Skype at live:junzhaocmu, or by calling Singapore phone number +65 8648 3534 (the first two numbers 65 represent the area code of Singapore). Thanks.

More information: http://www.ntu.edu.sg/home/JunZhao/HirePostdoc.htm

Candidates interested to pursuing a PhD in security/cryptography are invited to apply for this PhD scholarship in the Department of Computer Science at the University of Warwick. This scholarship covers full Home/EU fees and a stipend payable at current UK Research Council rates for 3.5 years. Outstanding overseas students are also encouraged to apply. It\'s possible to cover the gap in the tuition fee for the overseas student if the student is really excellent.

The research topic falls under the general theme of security and cryptography. We are very flexible with the specific topic. Our previous research has been largely driven by tackling real-world security problems. Some of our research outputs have been adopted by the industry at a large scale and have had a significant societal impact. We expect the student to pursue a research topic that really matters in the real world and that matches their interest and background.

The Computer Science Department at Warwick is a leading department in the UK. In the 2014 Research Evaluation Framework (REF) which all UK universities participated in, Warwick computer science was ranked the 1st in terms of research output, 2nd in terms of impact and 2nd overall. It is also highly regarded for its research culture, informal environment, excellent students, and beautiful campus.

Ideally, candidates should have an excellent degree in computer science, engineering or related disciplines, solid mathematical background, excellent programming skills and a desire to tackle real-world problems.

For informal inquiries about this studentship, please contact Professor Feng Hao, feng.hao (at) warwick.ac.uk, enclosing a CV and a short description of your relevant background and interests within the research subject. Formal application of this PhD scholarship needs to be made online at the Warwick CS department website: https://warwick.ac.uk/fac/sci/dcs/admissions/postgraduateresearch/

Closing date for applications: 31 May 2019

Contact: feng.hao (at) warwick.ac.uk

More information: https://www.jobs.ac.uk/job/BRS537/phd-studentship-in-security

Fetch.AI is a world-changing project, a “decentralised digital world” where autonomous software agents act on the behalf of their owners, or themselves, to get useful economic work done.

We are a dynamic, fast-growing international team of experts and forward-thinking technology enthusiasts working on the convergence of blockchain, AI and multi-agent systems. We are building technology for both today and tomorrow - a collective super-intelligence on top of decentralized economic internet built with a highly scalable next-generation distributed ledger technology. Combined with machine learning, this delivers the predictions and infrastructure to power the future economy.

Do you like challenges and want to work on cutting edge state-of-the-art technology that will define how we will interact? Come and join us.

Job description

The role involves the design and implementation of cryptography techniques to build, maintain and enrich the functionalities of Fetch’s decentralised smart-ledger technology. Interested candidates will be provided with multiple opportunities to work at the intersection of Artificial Intelligence/Machine Learning and cryptography/security.

We are working at the cutting edge of cryptography, artificial intelligence, distributed computation and economics, and are therefore looking for people with a desire to create novel solutions for complex problems.

Responsibilities

You will be responsible for the timely delivery of varied projects within the Cryptography Team and wider Fetch.AI Teams

Skills and experience

A good mathematical background is essential

Software engineering skills in Python or C/C++, Linux, Git

A BSc/MSc in Cyber Security/Computer Science/Mathematics or a related field with previous exposure to programming with cryptography

Demonstrable skills in one or more of the following: systems security/protocol design/distributed computing

Proven track record of independently and successfully driving projects

Closing date for applications: 30 May 2019

Contact: David Wood

david.wood (at) fetch.ai

More information: https://careers.fetch.ai/jobs/cryptography-engineer/

Event date: 18 August 2019
Submission deadline: 1 June 2019
Notification: 1 July 2019

Event date: 14 November to 15 November 2019
Submission deadline: 17 September 2019
Notification: 23 October 2019

Event date: 9 September to 13 September 2019

With the rapid development of quantum technologies, quantum-safe cryptography has found significant attention. Hash-based signature schemes have been in particular of interest because of (i) the importance of digital signature as the main source of trust on the Internet, (ii) the fact that the security of these signatures relies on existence of one-way functions, which is the minimal assumption for signature schemes, and (iii) they can be efficiently implemented. Basic hash-based signatures are for a single message, but have been extended for signing multiple messages. In this paper we design a Multi-message Signature Scheme (MSS) based on an existing One-Time Signature (OTS) that we refer to as KSN-OTS. KSN uses SWIFFT, an additive homomorphic lattice-based hash function family with provable one-wayness property, as the one-way-function and achieves a short signature. We prove security of our proposed signature scheme in a new strengthened security model (multi-target multi-function) of MSS, determine the system parameters for 512 bit classical (256 bit quantum) security, and compare parameter sizes of our scheme against XMSS, a widely studied hash based MSS that has been a candidate for NIST standardization of post-quantum signature scheme. We give an efficient implementation of our scheme using Intel SIMD (Single Instruction Multiple Data) instruction set. For this, we first implement SWIFFT computation using a SIMD parallelization of Number Theoretic Transform (NTT) of elements of the ring $\mathbb{Z}_p[X]/(X^\n+1)$, that can support different levels of parallelization. We compare efficiency of this implementation with a comparable (security level) implementation of XMSS and show its superior performance on a number of efficiency parameters.

Recent foundational work on leakage-based attacks on encrypted databases has broadened our understanding of what an adversary can accomplish with a standard leakage profile. Nevertheless, all known value reconstruction attacks succeed under strong assumptions that may not hold in the real world. The most prevalent assumption is that queries should be issued uniformly at random by the client. We present the first value reconstruction attacks for encrypted databases without any assumptions about the query or data distribution. Our approach uses the search pattern leakage, which exists in all known structured encryption schemes but has not been effectively utilized so far. At the core of our method lies a support size estimator, a technique that utilizes the repetition of search tokens with the same response to estimate distances between encrypted values without any assumptions about the underlying distribution. We develop distribution-agnostic reconstruction attacks for both range queries and k-nearest-neighbor (k-NN) queries based on information extracted from the search pattern leakage. Our new range attack follows a different algorithmic approach than state-of-the-art attacks, which are fine-tuned to succeed under the uniform queries. Instead, we reconstruct plaintext values under a variety of skewed query distributions and even outperform the accuracy of previous approaches under uniform query distribution. Our new k-NN attack succeeds with far fewer samples than a previously proposed attack and scales to much larger values of k. We demonstrate the effectiveness of our attacks by experimentally testing them on a wide range of query distributions and database densities, both unknown to the adversary.