International Association
for Cryptologic Research

Many advanced lattice cryptography applications require efficient algorithms for inverting the so-called "gadget" matrices, which are used to formally describe a digit decomposition problem that produces an output with specific (statistical) properties. The common gadget inversion problems are the classical (often binary) digit decomposition, subgaussian decomposition, Learning with Errors (LWE) decoding, and discrete Gaussian sampling. In this work, we build and implement an efficient lattice gadget toolkit that provides a general treatment of gadget matrices and algorithms for their inversion/sampling. The main contribution of our work is a set of new gadget matrices and algorithms for efficient subgaussian sampling that have a number of major theoretical and practical advantages over previously known algorithms. Another contribution deals with efficient algorithms for LWE decoding and discrete Gaussian sampling in the Residue Number System (RNS) representation.

We implement the gadget toolkit in PALISADE and evaluate the performance of our algorithms both in terms of runtime and noise growth. We illustrate the improvements due to our algorithms by implementing a concrete complex application, key-policy attribute-based encryption (KP-ABE), which was previously considered impractical for CPU systems (except for a very small number of attributes). Our runtime improvements for the main bottleneck operation based on subgaussian sampling range from 18x (for 2 attributes) to 289x (for 16 attributes; the maximum number supported by a previous implementation). Our results are applicable to a wide range of other advanced applications in lattice cryptography, such as GSW-based homomorphic encryption schemes, leveled fully homomorphic signatures, key-hiding PRFs and other forms of ABE, some program obfuscation constructions, and more.

Motivated by cryptographic applications such as predicate encryption, we consider the problem of representing an arbitrary predicate as the inner product predicate on two vectors. Concretely, fix a Boolean function $P$ and some modulus $q$. We are interested in encoding $x$ to $\vec x$ and $y$ to $\vec y$ so that $$P(x,y) = 1 \Longleftrightarrow \langle\vec x,\vec y\rangle= 0 \bmod q,$$ where the vectors should be as short as possible. This problem can also be viewed as a generalization of matching vector families, which corresponds to the equality predicate. Matching vector families have been used in the constructions of Ramsey graphs, private information retrieval (PIR) protocols, and more recently, secret sharing.

Our main result is a simple lower bound that allows us to show that known encodings for many predicates considered in the cryptographic literature such as greater than and threshold are essentially optimal for prime modulus $q$. Using this approach, we also prove lower bounds on encodings for composite $q$, and then show tight upper bounds for such predicates as greater than, index and disjointness.

Event date: 28 August to 30 August 2019
Submission deadline: 15 March 2019
Notification: 15 May 2019

Event date: 8 May to 10 May 2019
Submission deadline: 24 November 2018
Notification: 12 January 2019

Data-Independent Memory-hard functions (iMHFs) are a key cryptographic primitive underlying the design of moderately expensive password hashing algorithms and egalitarian proofs of work that are resistant to side-channel attacks. Several goals for MHFs have been proposed including bandwidth hardness, space-time (ST) complexity, amortized area-time (aAT) complexity and sustained space complexity. An iMHF can be specified using a directed acyclic graph (DAG) $G$ with $N=2^n$ nodes and low indegree, and the cost (aAT, ST etc...) to evaluate the iMHF can be analyzed using pebbling games. In particular, given a parameter $N$ (e.g., maximum acceptable running time) we would like to design the DAG $G$ to have maximum possible pebbling cost i.e., to ensure that the iMHF is as expensive as possible for an attacker to compute. Recently, Alwen et al. (CCS'17) gave a randomized DAG construction called DRSample and proved that the aAT cost to pebble the graph was $\Omega\left( N^2/\log N\right)$. In an asymptotic sense the DRSample outperformed all prior constructions including Argon2i, the winner of the password hashing competition, which can be pebbled with aAT cost at most $\mathcal{O}\left(N^{1.767}\right)$. In this work we first prove a matching upper bound on the pebbling cost of DRSample by analyzing the greedy pebbling attack of Boneh et al. (ASIACRYPT'16). This sequential attack on DRSample is simple, easy to implement and has good concrete performance. In fact, our results show that, for practical values of $N\leq 2^{24}$, Argon2i provides stronger resistance to known pebbling attacks than DRSample reversing a finding of Alwen et al. (CCS'17). We then develop a new iMHF candidate by extending DRSample with the bit-reversal graph, and show that the iMHF resists all known attacks in practice and has optimal asymptotic performance under every MHF metric. In particular, we prove that (1) any (nearly) sequential pebbling attack (including the greedy pebbling attack) has aAT cost $\Omega\left( N^2\right)$, (2) any parallel attacker has aAT cost at least $\Omega\left(N^2/\log N\right)$ and at least $\Omega\left(N^2 \log \log N/\log N\right)$ unless one can find new depth-reducing attacks against DRSample which significantly improve upon the state of the art, (3) the graph has high bandwidth-complexity, and (4) any pebbling either has aAT cost $\omega(N^2)$ or requires at least $\Omega(N)$ steps with $\Omega(N/\log N)$ pebbles on the DAG. This makes our construction the first practical iMHF with strong guarantees on the sustained space-complexity. We also observe that the Argon2i round function can (trivially) be evaluated in parallel, which would allow an attacker to reduce aAT costs by (nearly) an order of magnitude, and we develop an inherently sequential version of the Argon2i round function that prevents this attack. We implement our new iMHF candidate (with and without the sequential round function) and show that evaluation speed is nearly identical to Argon2i. Finally, we provide a pebbling reduction which proves that in the parallel random oracle model (PROM) the cost of evaluating an iMHF like Argon2i or DRSample+BRG is given by the pebbling cost of the underlying DAG.

A universal circuit (UC) is a general-purpose circuit that can simulate arbitrary circuits (up to a certain size $n$). At STOC 1976 Valiant presented a graph theoretic approach to the construction of UCs, where a UC is represented by an edge universal graph (EUG) and is recursively constructed using a dedicated graph object (referred to as supernode). As a main end result, Valiant constructed a 4-way supernode of size 19 and an EUG of size $4.75n\log n$ (omitting smaller terms), which remained the most size-efficient even to this day (after more than 4 decades).

Motivated by the emerging applications of UCs in various privacy preserving computation scenarios, we revisit Valiant's universal circuits, and propose a size-optimal 4-way supernode of size 18, and an EUG of size $4.5n\log n$. As a practical consequence, we reduce the size of universal circuits (and the number of AND gates) by more than 5\% in general (rather than just for small-size circuits in particular), and thus improve upon the efficiency of UC-based cryptographic applications accordingly. Our approach to the design of optimal supernodes is computer aided (rather than by hand as in previous works), which might be of independent interests. As a complement, we give lower bounds on the size of EUGs and UCs in Valiant's framework, which significantly improves upon the generic lower bound on UC size and therefore reduces the gap between theory and practice of universal circuits.

Fairness in Secure Multiparty Computation (MPC) is known to be impossible to achieve in the presence of a dishonest majority. Previous works have proposed combining MPC protocols with Cryptocurrencies in order to financially punish aborting adversaries, providing an incentive for parties to honestly follow the protocol. This approach also yields privacy-preserving Smart Contracts, where private inputs can be processed with MPC in order to determine the distribution of funds given to the contract. Unfortunately, the focus of existing work is on proving that this approach is possible and they present monolithic and mostly inefficient constructions. In this work, we put forth the first modular construction of ``Insured MPC'', where the result of the private computation of parties either yields an output describing how to distribute funds or a proof that a set of parties has misbehaved, allowing for financial punishments. Moreover, both the output and the proof of cheating are publicly verifiable, allowing third parties to independently validate an execution.

We present a highly efficient protocol which allows public verification of cheating behavior during the output stage. This scheme is constructed using a publicly verifiable homomorphic commitment scheme, for which we propose an efficient construction. Furthermore, we construct a compiler that uses any such scheme together with a Smart Contract to implement Insured MPC. This compiler requires a standard (non-private) Smart Contract. Our results are proven in the Universal Composability framework using a Global Random Oracle as the setup assumption. From a theoretical perspective, our general results provide the first characterization of sufficient properties that MPC protocols must achieve in order to be efficiently combined with Cryptocurrencies, as well as insights on publicly verifiable protocols. On the other hand, all our constructions and protocols are highly efficient and allow for a fast implementation.

This tutorial demonstrates how cryptographic security notions, constructions, and game-based security proofs can be formalized using the CryptHOL framework. As a running example, we formalize a variant of the hash-based ElGamal encryption scheme and its IND-CPA security in the random oracle model. This tutorial assumes familiarity with Isabelle/HOL basics and standard cryptographic terminology.

We consider the problem of Non-Interactive Secure Computation (NISC), a 2-message ``Sender-Receiver'' secure computation protocol that retains its security even when both parties can be malicious. While such protocols are easy to construct using garbled circuits and general non-interactive zero-knowledge proofs, this approach inherently makes a non-black-box use of the underlying cryptographic primitives and is infeasible in practice.

Ishai et al. (Eurocrypt 2011) showed how to construct NISC protocols that only use parallel calls to an ideal oblivious transfer (OT) oracle, and additionally make only a black-box use of any pseudorandom generator. Combined with the efficient 2-message OT protocol of Peikert et al. (Crypto 2008), this leads to a practical approach to NISC that has been implemented in subsequent works. However, a major limitation of all known OT-based NISC protocols is that they are subject to selective failure attacks that allows a malicious sender to entirely compromise the security of the protocol when the receiver's first message is reused.

Motivated by the failure of the OT-based approach, we consider the problem of basing \emph{reusable} NISC on parallel invocations of a standard arithmetic generalization of OT known as oblivious linear-function evaluation (OLE). We obtain the following results:

- We construct an information-theoretically secure reusable NISC protocol for arithmetic branching programs and general zero-knowledge functionalities in the OLE-hybrid model. Our zero-knowledge protocol only makes an absolute constant number of OLE calls per gate in an arithmetic circuit whose satisfiability is being proved. As a corollary, we get reusable NISC/OLE for general Boolean circuits using any one-way function. - We complement this by a negative result, showing that reusable NISC/OT is impossible to achieve, and a more restricted negative result for the case of the zero-knowledge functionality. This provides a formal justification for the need to replace OT by OLE. - We build a universally composable 2-message OLE protocol in the CRS model that can be based on the security of Paillier encryption and requires only a constant number of modular exponentiations. This provides the first arithmetic analogue of the 2-message OT protocols of Peikert et al. (Crypto 2008). - By combining our NISC/OLE protocol and the 2-message OLE protocol, we get protocols with new attractive asymptotic and concrete efficiency features. In particular, we get the first (designated-verifier) NIZK protocols where following a statement-independent preprocessing, both proving and verifying are entirely ``non-cryptographic'' and involve only a constant computational overhead.

We propose a proof of work protocol that computes the discrete logarithm of an element in a cyclic group. Individual provers generating proofs of work perform a distributed version of the Pollard rho algorithm. Such a protocol could capture the computational power expended to construct proof-of-work-based blockchains for a more useful purpose, as well as incentivize advances in hardware, software, or algorithms for an important cryptographic problem. We describe our proposed construction and elaborate on challenges and potential trade-offs that arise in designing a practical proof of work.

Recently Grubbs et al. [GLR17] initiated the formal study of message franking protocols. This new type of service launched by Facebook, allows the receiver in a secure messaging application to verifiably report to a third party an abusive message some sender has sent. A novel cryptographic primitive: committing AEAD has been initiated, whose functionality apart from confidentiality and authenticity asks for a compact commitment over the message, which is delivered to the receiver as part of the ciphertext. A new construction CEP (Committing Encrypt and PRF) has then been proposed, which is multi-opening secure and reduces the computational costs for the sender and the receiver.

Despite the merits of the message franking protocols [GLR17], our observation which launched this work, is that all the designs be it compositional or the CEP construction, leak too much when the receiver needs to open the abusive message to the third party. Namely, the receiver opens the entire message along with the opening key to the third party, thus confidentiality of the message is entirely broken. Moreover, the opening of the entire message increases the communication cost of the protocol and in cases of big messages being exchanged (attachments, videos, multimedia files, etc.) it might be unnecessary. We provide to the best of our knowledge the first formal treatment of message franking protocols with minimum leakage whereby only the abusive blocks are opened, while the rest non-abusive blocks of the message remain private.

First we give a new definition for multi-opening indistinguishability with partial opening (MO-IND-PO), which forces an adversary to distinguish encryptions of abusive blocks. We then design and analyze two protocols CEP-AOP1 (Committing Encrypt and PRF with After Opening Privacy) and CEP-AOP2, which adhere to the new privacy definition. As a side contribution we show a multi-opening secure CEP-AOP2 construction using only one PRF evaluation over the message, in a weaker but meaningful security model, relying only on standard assumptions of the underlying symmetric primitives.

We present an improved search strategy for a template attack on the secret DES key of a widely-used smart card, which is based on a Common-Criteria certified chip. We use the logarithm of the probability function as returned by the template attack itself, averaged over all 28 template positions along the rings representing the C and D Registers of the DES key schedule, as the sorting criteria for the key candidates. For weak keys - which in this attack model have a minimal rest entropy of only two bits - we find that on average only 37.75 bits need to be recovered by brute force when using only a single trace in the Exploitation Phase. This effort goes down to just a few bits for a single DES key when using only a few traces in Exploitation Phase.

Event date: 16 June to 20 June 2018
Submission deadline: 11 November 2018
Notification: 3 December 2018

Event date: 7 December to 9 December 2018
Submission deadline: 1 November 2018

As a Senior Cryptography Engineer - Cloud Engineer, you will:

Design, implement and deploy cryptographic algorithms tailored for a cloud environment.

Conduct research and development in differential privacy, secret sharing, multi-party secure computation and fully homomorphic encryption.

Perform security assessments of crypto-primitives, cryptosystems and cloud security solutions at the theoretical and implementation level.

Work closely with the other teams in the organization to design and deploy safe cloud-based solutions .

Be involved in the integration of developed cryptosystems within DarkMatter products.

Enjoy all the cultural, educational and travel opportunities Abu Dhabi offers

To bring your dream to life, you’ll need:

PhD degree in Cryptography, Applied Cryptography, Information Theory and Mathematics or Computer Science.

Extensive experience developing in various programming languages.

A desire to innovate in the UAE

 

Closing date for applications: 3 March 2019

Contact: Sheila Morjaria

Mehdi Messaoudi

More information: https://grnh.se/d694fd601

As a Cryptography Embedded Systems Engineer, you will:

• Design, implement and deploy cryptographic algorithms tailored for resource-constrained devices.

• Conduct research and development in lightweight cryptography.

• Perform security assessments of crypto-primitives and cryptosystems suitable for resource-constrained devices at the theoretical and implementation level.

• Work closely with the other teams in the organization to deploy secure embedded systems.

• Be involved in the integration of developed cryptosystems within DarkMatter products.

• Enjoy all the cultural, educational and travel opportunities Abu Dhabi offers.

To bring your dream to life, you’ll need:

• MS or PhD degree in Computer Science, Computer Engineering, Electrical Engineering, Cryptography or related field.

• Development experience within embedded systems, RFID and sensor networks.

• Knowledge of Unix/Linux environments and kernel development.

• Knowledge of one or more of the following: Microcontrollers, SoC, TrustZone, ARM processors, performance optimization, bootloading, firmware, x86 assembly, system BIOS or hardware/software integration.

• Knowledge of side-channel attacks and countermeasures.

• Experience coding in C/C++.

• A desire to innovate in the UAE

Closing date for applications: 3 April 2019

Contact: Sheila Morjaria

Mehdi Messaoudi

More information: https://grnh.se/fb5c073f1

About the Department

Cloudflare’s Technology team is working on building the future of Cloudflare by tackling strategic projects that have a large impact on the way Cloudflare systems, and the Internet at large, work. Engineers in the Technology team are expected to research new ideas and technologies, dive into existing codebases to make meaningful changes, work independently on greenfield projects, and collaborate closely with the engineering organization to achieve common goals.

The Cryptography team is a sub-team of the Technology team focused on solving difficult problems in security, performance, and privacy at scale using cryptographic tools. This involves systems engineering, open source software development, protocol design, the implementation of cryptographic primitives, contributions to cutting-edge research in collaboration with academia, participation in Internet standards organizations like the IETF, and more.

Closing date for applications: 1 July 2019

Contact: Nick Sullivan

More information: https://www.cloudflare.com/careers/departments/technology-research/

DTU Compute\'s Section for Cyber Security, invites applications for an appointment as Associate Professor/Assistant Professor within cryptology. The position is available from May 1, 2019 or according to mutual agreement.

The department, DTU Compute, is an internationally unique academic environment spanning the science disciplines mathematics, statistics and computer science. At the same time, we are an engineering department covering informatics and communication technologies (ICT) in their broadest sense. Finally, we play a major role in addressing the societal challenges of the digital society where ICT is a part of every industry, service, and human endeavor.

Responsibilities and tasks

Through the position, the University seeks to strengthen the research within cyber security. The cyber security section at DTU has experts in cryptology, in particular the design and analysis of ciphers and hash functions, but wishes to further strengthen its research within cryptology.

Topics of particular interest include but are not limited to:

• symmetric cryptology

• lightweight and resource-efficient cryptography

• post-quantum cryptology

• provable security of cryptographic primitives

• side-channel attacks and physical cryptanalysis

• analysis and protection of cryptographic implementations

• algorithmic aspects of cryptology

• efficient implementation of cryptographic primitives

Candidates with strong expertise in any other area of cryptology are also encouraged to apply.

Application procedure

To apply, please read the full job advertisement at www.career.dtu.dk

Please submit your online application no later than 1 December 2018.

Closing date for applications: 31 December 2018

We are looking for research fellows with expertise on blockchain and PKC, and offer a competitive salary. If you are interested, please send your CV to Prof. Jianying Zhou . Only short-listed candidates will be contacted for interview.

Closing date for applications: 8 January 2019

Contact: Prof. Jianying Zhou

Email: jianying_zhou (at) sutd.edu.sg

More information: http://jianying.space/

At the Graz University of Technology / Faculty of Computer Science and Biomedical Engineering the position of

University Professor of Information Security

is to be filled at the institute of Applied Information Processing and Communications (IAIK) as a full time permanent position according to section 98 of the Austrian Universities Act (§98 UG). IAIK is an internationally visible research center at TU Graz where more than 60 researchers work on a multitude of topics in information security.

We are seeking a candidate with proven scientific expertise who will represent the field of Information Security in research and teaching. The successful candidate will complement existing strengths at the institute and be an engaged teacher in the Computer Science programs at the bachelor, master, and PhD level.

Closing date for applications: 3 December 2018

Contact: Stefan Mangard, Email: Stefan.Mangard (at) iaik.tugraz.at

More information: https://www.tugraz.at/fakultaeten/infbio/news/vacancies/professor-of-information-security/