## IACR News

Here you can see all recent updates to the IACR webpage. These updates are also available:

#### 21 February 2020

###### Hasso-Plattner-Institute, University of Potsdam, Germany
Job Posting

The Cybersecurity - Identity Management group at the Hasso-Plattner-Institute (HPI), University of Potsdam is seeking to fill several PhD and PostDoc positions in the area of cryptography and privacy.

• Development of provably secure cryptographic protocols for real-world problems. Topics of interest include (but are not limited to):
• Distributed identity management
• Privacy-enhancing technologies
• Foundations and solutions for real-world cryptography
• Publish and present results at top-tier international conferences
• Participate in teaching activities
• Master's degree (or PhD for postdoctoral position) in Computer Science, Mathematics, or a related area by the time of appointment
• Profound knowledge in the areas of cryptography and IT security (for postdoctoral candidates proven in the form of publications in these areas)
• Excellent English language skills
What we offer
• Collaborative work environment in a newly established research group
• Outstanding setting to conduct top academic research with strong connections to industry
• Working at the HPI campus in Potsdam-Babelsberg (S-Bahn station Griebnitzsee) near to Berlin

We look forward to your application including two references, motivation letter and copies of relevant diplomas and certificates. Please submit your application documents (only as PDF) via email.

Closing date for applications:

Contact: Anja Lehman, email: anja.lehmann (at) hpi.de

###### Telecom SudParis, RST Department CNRS UMR 5157, SAMOVAR/R3S and IRT SytemX
Job Posting
The cryptographic protocols mainly comprise functionalities such as public-key encryption, digital signatures, and key exchange. These cryptographic protocols are based on the difficulty of certain mathematical operations, such as finding the prime factors of a very large integer. However, recent advances in quantum computing mean that these difficult math problems might soon be solved efficiently, with a potentially serious impact upon our security. Realizing the availability of quantum computers from research labs to general purpose use in the near future, the preparation against such a threat has taken shape recently, and happening to be a fast growing topic of research referred as post-quantum cryptography (PQC). Nowadays, certain candidate families of post-quantum schemes have been realized including code-based, hash-based, multivariate, lattice-based and supersingular isogeny-based. Security of all these primitives are based on different mathematical problems which are believed to be hard even with respect to quantum computation and quantum communications. In this project, we aim to achieve the following goals: • To study the existing PQC schemes and demonstrate state of the art PQC following an extensive review. • To propose post-quantum schemes taking in account their possible application to IoT security or Blockchain.

Closing date for applications:

Contact: joaquin.garcia_alfaro@telecom-sudparis.eu and Kalpana.SINGH@irt-systemx.fr

###### University of Clermont-Auvergne, France
Job Posting
Constraint Programming for Cryptanalysis of Symmetric Encryption Schemes
• Location: LIMOS, Clermont-Ferrand, France
• Salary: 2000€
• Duration: 1 year
• Keywords: Cryptanalysis symmetric, constraint programming, SAT solving, ILP.
• Starting date: As soon as possible, when we have a good candidate; at least before October 2020.
• A PhD in Computer Science, Applied Mathematics, Cryptography or related field.
• Competitive research record in symmetric cryptography or in constraint programming.
• Commitment, team working and a critical mind.
• Good written and verbal communication skills in English are essential.

This post-doc is founded by the ANR project Decrypt started in January 2019.

Transforming a theoretical cryptanalysis into a SAT problem or into a set of linear constraints could be a hard and time-consuming task. Our aim is to use constraint programming (CP) to simplify the way the symmetric key attacks are modeled and thus to overpass existing cryptanalytic results. Preliminary studies are really encouraging.

Goal

The main goal is to identify schemes and attacks for which it is possible to use off-the-shelf CP, SAT or ILP approaches. To achieve this goal, the work will be divided into the following tasks.

1. Study symmetric encryption schemes and identify for several schemes the different components that are used in the scheme design.
2. Design CP, SAT, and ILP models for cryptanalytic problems on selected schemes. We will mainly focus on the following attacks: cube attacks, conditional cube attacks with division property, (related-key) differential and linear cryptanalysis, word-based division property / integral distinguisher.
3. Experimentally evaluate CP, SAT, and ILP solvers on the models designed in previous tasks, and compare these solvers with existing dedicated cryptanalysis approaches. Design of a tool to automate this task is one of the goals of the project.

Closing date for applications:

###### Université Jean Monnet, Saint-Etienne, France
Job Posting
A Ph.D. position is open in université Jean Monnet. This Ph.D project aims to develop novel techniques that can protect cryptographic implementations against side-channel attacks and fault attacks. The protection studied will be mainly algorithmic protection, with experimental evaluation of their effectiveness and efficiency. Various directions can be explored with respect to the candidate background.

Closing date for applications:

Contact: Vincent Grosso

Job Posting
PQSecure Technologies, LLC is a growing startup company active in the area of post-quantum cryptographic engineering with focus on embedded and IoT devices. PQSecure is currently funded by National Science Foundation SBIR Phase II fund and is an active participant of National Institute of Standards and technology (NIST) for post-quantum cryptography. We are looking for a postdoc/research scientist with post-quantum cryptography and implementations experience and relevant publication records. Candidates should hold a PhD in Computer science/engineering, or electrical engineering, or applied mathematics with excellent programming skills and cryptographic protocol design. PQSecure offers a competitive hiring package including, 401k, health insurance, stock option, 3 weeks of PTO, travel reimbursement for relevant conferences, and etc.). The position is for one year with possibility of renewal based on the performance of the candidate. The salary will be $87,000 per year. PQSecure is located in the beautiful city Boca Raton, FL. The deadline for applications is February 15, 2020. The position will be open until filled. Closing date for applications: Contact: Dr. Reza Azarderakhsh razarder[a/t]pqsecurity.com ###### Junichi Tomida, Nuttapong Attrapadung ePrint Report At Eurocrypt'19, Attrapadung presented several transformations that dynamically compose a set of attribute-based encryption (ABE) schemes for simpler predicates into a new ABE scheme for more expressive predicates. Due to the powerful unbounded and modular nature of his compositions, many new ABE schemes can be obtained in a systematic manner (including those that resolved some open problems at the time). However, his approach heavily relies on so-called q-type assumptions, which are not standard. Devising such powerful compositions from standard assumptions was left as an important open problem. In this paper, we present a new framework for constructing ABE schemes that allow unbounded and dynamic predicate compositions among them, and show that the adaptive security of these composed ABE will be preserved by relying only on the standard matrix Diffie-Hellman (MDDH) assumption. This thus resolves the open problem posed by Attrapadung. Our framework significantly expands an area of ABEs that are realizable from standard assumptions. This includes the following adaptively secure large-universe ABEs for Boolean formulae under MDDH. -- The first completely unbounded monotone key-policy (KP)/ciphertext-policy (CP) ABE. Previously, such ABE has been only recently proposed, but only for the KP and small-universe flavor (Kowalczyk and Wee, Eurocrypt'19). -- The first completely unbounded non-monotone KP/CP-ABE. Especially, our ABEs support a new type of non-monotonicity that subsumes previous two types of non-monotonicity, namely, by Ostrovsky et al. (CCS'07) and by Okamoto and Takashima (CRYPTO'10). -- The first non-monotone KP and CP-ABE with constant-size ciphertexts and secret keys, respectively. -- The first monotone KP and CP-ABE with constant-size secret keys and ciphertexts, respectively. ###### Changmin Lee, Alexandre Wallet ePrint Report In ASIACRYPT 2019, Genise et al. describe GGH+19 a new somewhat homomorphic encryption scheme. The security relies on an inhomogeneous and non-structured variant of the NTRU assumption that they call MiNTRU. To allow for meaningful homomorphic computations, they use overstretched parameters, but they do not provide an analysis of their new assumption against the state-of-the-art attack of Kirchner and Fouque KF17 for overstretched modulus. We show that the parameters of GGH+19 do not satisfy the desired security by actually conducting the known analysis. We also report a successful break of the smallest set of parameters in around 15 hours of computations while they are claimed to reach 100 bits of security. ###### Itai Dinur ePrint Report We consider a \emph{collision search problem} (CSP), where given a parameter$C$, the goal is to find$C$collision pairs in a random function$f:[N] \rightarrow [N]$(where$[N] = \{0,1,\ldots,N-1\})$using$S$bits of memory. Algorithms for CSP have numerous cryptanalytic applications such as space-efficient attacks on double and triple encryption. The best known algorithm for CSP is \emph{parallel collision search} (PCS) published by van Oorschot and Wiener, which achieves the time-space tradeoff$T^2 \cdot S = \tilde{O}(C^2 \cdot N)$. In this paper, we prove that any algorithm for CSP satisfies$T^2 \cdot S = \tilde{\Omega}(C^2 \cdot N)$, hence the best known time-space tradeoff is optimal (up to poly-logarithmic factors in$N$). On the other hand, we give strong evidence that proving similar unconditional time-space tradeoff lower bounds on CSP applications (such as breaking double and triple encryption) may be very difficult, and would imply a breakthrough in complexity theory. Hence, we propose a new restricted model of computation and prove that under this model, the best known time-space tradeoff attack on double encryption is optimal. ###### Shweta Agrawal, Shota Yamada ePrint Report Boneh, Waters and Zhandry (CRYPTO 2014) used multilinear maps to provide a solution to the long-standing problem of public-key broadcast encryption (BE) where all parameters in the system are small. In this work, we improve their result by providing a solution that uses only bilinear maps and Learning With Errors (LWE). Our scheme is fully collusion-resistant against any number of colluders, and can be generalized to an identity-based broadcast system with short parameters. Thus, we reclaim the problem of optimal broadcast encryption from the land of “Obfustopia”. Our main technical contribution is a ciphertext policy attribute based encryption (CP-ABE) scheme which achieves special efficiency properties – its ciphertext size, secret key size, and public key size are all independent of the size of the circuits supported by the scheme. We show that this special CP-ABE scheme implies BE with optimal parameters; but it may also be of independent interest. Our constructions rely on a novel interplay of bilinear maps and LWE, and are proven secure in the generic group model. ###### Yindong Chen, Limin Lin, Chuliang Wei ePrint Report Let$k \ge 2$be an integer, define $S_t^k:=\Bigg\{(a,b)\in \mathbb{Z}^2\ \Big| \ { 0 \le a,b \le 2^{k}-2,\ a+b\equiv t ~(\text{mod} \ 2^k-1),\ \w(a)+\w(b)\le{k-1}}\Bigg\},$ where$t \in \mathbb{Z}, 1 \le t \le 2^k-2$. This paper gives the upper bound of cardinality of$S_t^k$when$\w(t)\le 10$, proving that a conjecture proposed by Tu and Deng in the case. ###### Meher Krishna Duggirala . , . Ravi Duggirala . , . Krishna Subba Rao Pulugurtha ePrint Report In this paper, a multivariate polynomial and exponential mappings based password protocol is presented. The method can be utilized in public domains. The key generator generates a vector, intended to be used as a password by the authentication protocol subsequently, such that when the vector is substituted and evaluated in certain fixed multivariate polynomials -- that may be listed in a public domain -- the value$0$is found as a result of proper authentication. The public domain in this context could be internal to a large, and possibly distributed, system. The key generator can take hints from the owner of the password to generate the particular zero vector to suit the user. It may take into consideration biometric and any other user specific information at the time of key generation. The information collected by the key generator can be saved by the owner of the password for its possible retrieval upon requisition by the user, during the period of its validity, in case it is forgotten by the user. ###### Lior Rotem, Gil Segev, Ido Shahaf ePrint Report Despite the fundamental importance of delay functions, underlying both the classic notion of a time-lock puzzle and the more recent notion of a verifiable delay function, the only known delay function that offers both sufficient structure for realizing these two notions and a realistic level of practicality is the iterated squaring'' construction of Rivest, Shamir and Wagner. This construction, however, is based on rather strong assumptions in groups of hidden orders, such as the RSA group (which requires a trusted setup) or the class group of an imaginary quadratic number field (which is still somewhat insufficiently explored from the cryptographic perspective). For more than two decades, the challenge of constructing delay functions in groups of known orders, admitting a variety of well-studied instantiations, has eluded the cryptography community. In this work we prove that there are no constructions of generic-group delay functions in cyclic groups of known orders: We show that for any delay function that does not exploit any particular property of the representation of the underlying group, there exists an attacker that completely breaks the function's sequentiality when given the group's order. As any time-lock puzzle and verifiable delay function give rise to a delay function, our result holds for these two notions we well, and explains the lack of success in resolving the above-mentioned long-standing challenge. Moreover, our result holds even if the underlying group is equipped with a$d$-linear map, for any constant$d \geq 2$(and even for super-constant values of$d$under certain conditions). ###### Mihir Bellare, Igors Stepanovs ePrint Report At the core of Apple's iMessage is a signcryption scheme that involves symmetric encryption of a message under a key that is derived from the message itself. This motivates us to formalize a primitive we call Encryption under Message-Derived Keys (EMDK). We prove security of the EMDK scheme underlying iMessage. We use this to prove security of the signcryption scheme itself, with respect to definitions of signcryption we give that enhance prior ones to cover issues peculiar to messaging protocols. Our provable-security results are quantitative, and we discuss the practical implications for iMessage. ###### Shuichi Katsumata, Ryo Nishimaki, Shota Yamada, Takashi Yamakawa ePrint Report A non-interactive zero-knowledge (NIZK) protocol enables a prover to convince a verifier of the truth of a statement without leaking any other information by sending a single message. The main focus of this work is on exploring short pairing-based NIZKs for all NP languages based on standard assumptions. In this regime, the seminal work of Groth, Ostrovsky, and Sahai (J.ACM'12) (GOS-NIZK) is still considered to be the state-of-the-art. Although fairly efficient, one drawback of GOS-NIZK is that the proof size is multiplicative in the circuit size computing the NP relation. That is, the proof size grows by$O(|C|\lambda)$, where$C$is the circuit for the NP relation and$\lambda$is the security parameter. By now, there have been numerous follow-up works focusing on shortening the proof size of pairing-based NIZKs, however, thus far, all works come at the cost of relying either on a non-standard knowledge-type assumption or a non-static$q$-type assumption. Specifically, improving the proof size of the original GOS-NIZK under the same standard assumption has remained as an open problem. Our main result is a construction of a pairing-based NIZK for all of NP whose proof size is additive in$|C|$, that is, the proof size only grows by$|C| +\poly(\lambda)$, based on the decisional linear (DLIN) assumption. Since the DLIN assumption is the same assumption underlying GOS-NIZK, our NIZK is a strict improvement on their proof size. As by-products of our main result, we also obtain the following two results: (1) We construct a perfectly zero-knowledge NIZK (NIPZK) for NP relations computable in NC1 with proof size$|w| \cdot \poly(\lambda)$where$|w|$is the witness length based on the DLIN assumption. This is the first pairing-based NIPZK for a non-trivial class of NP languages whose proof size is independent of$|C|$based on a standard assumption. (2)~We construct a universally composable (UC) NIZK for NP relations computable in NC1 in the erasure-free adaptive setting whose proof size is$|w| \cdot \poly(\lambda)$from the DLIN assumption. This is an improvement over the recent result of Katsumata, Nishimaki, Yamada, and Yamakawa (CRYPTO'19), which gave a similar result based on a non-static$q$-type assumption. The main building block for all of our NIZKs is a constrained signature scheme with decomposable online-offline efficiency. This is a property which we newly introduce in this paper and construct from the DLIN assumption. We believe this construction is of an independent interest. ###### Dan Boneh, Saba Eskandarian, Sam Kim, Maurice Shih ePrint Report An updatable encryption scheme is a symmetric-key encryption scheme that supports key-rotation on ciphertexts. A server that hosts a user's encrypted data can use a user-provided update token to rotate the key under which the data is encrypted while not learning any information about the underlying data. This work's contributions are threefold. First, we introduce new definitions for updatable encryption (in the ciphertext-dependent setting) that capture desirable security properties not covered in prior work. Next, we construct two new updatable encryption schemes. The first construction relies only on symmetric cryptographic primitives but only supports a bounded number of key rotations. The second supports a (nearly) unbounded number of updates and relies on almost key-homomorphic PRFs. We construct an efficient almost key-homomorphic PRF from the Ring Learning with Errors (RLWE) assumption to concretely instantiate our second construction. Finally, we implement both constructions and compare their performance to prior work. Our RLWE-based construction outperforms an existing updatable encryption scheme based on the hardness of DDH in elliptic-curve groups by over 200x in speed. Our construction based only on symmetric primitives has the highest encryption throughput, approaching the performance of AES, and the highest decryption throughput on ciphertexts that were re-encrypted fewer than fifty times, at which point the RLWE construction dominates. ###### Fabrice Benhamouda, Huijia Lin ePrint Report Reducing interaction in Multiparty Computation (MPC) is a highly desirable goal in cryptography. It is known that 2-round MPC can be based on the minimal assumption of 2-round Oblivious Transfer (OT) [Benhamouda and Lin, Garg and Srinivasan, EC 2018], and 1-round MPC is impossible in general. In this work, we propose a natural hybrid'' model, called \textbf{multiparty reusable Non-Interactive Secure Computation Market (mrNISC)}. In this model, parties publish encodings of their private inputs$x_i$at the beginning, once and for all. Later, any subset$I$of them can compute \emph{on-the-fly} a function$f$on their inputs$\vec x_I = {\{x_i\}}_{i \in I}$by just sending a single message to a stateless evaluator, conveying the result$f(\vec x_I)\$ and nothing else. Importantly, the input encodings can be \emph{reused} in any number of on-the-fly computations, and the same classical simulation security guaranteed by multi-round MPC, is achieved. In short, mrNISC has minimal yet tractable'' interaction pattern.

We initiate the study of mrNISC on several fronts. First, we formalize the security of mrNISC protocols in both a UC definition and a game-based definition. Second, we construct mrNISC protocols in the plain model with semi-honest and semi-malicious security based on bilinear groups. Third, we demonstrate the power of mrNISC by showing two applications: non-interactive MPC (NIMPC) with reusable setup and a distributed version of program obfuscation. In addition, at the core of our construction of mrNISC is a witness encryption scheme for a special language that verifies Non-Interactive Zero-Knowledge (NIZK) proofs of the validity of computations over committed values, which we believe is of independent interest.
###### Florian Tramèr, Dan Boneh, Kenneth G. Paterson
ePrint Report
Privacy-focused crypto-currencies, such as Zcash or Monero, aim to provide strong cryptographic guarantees for transaction confidentiality and unlinkability. In this paper, we describe side-channel attacks that let remote adversaries bypass these protections. We present a general class of timing side-channel and traffic-analysis attacks on receiver privacy. These attacks enable an active remote adversary to identify the (secret) payee of any transaction in Zcash or Monero. The attacks violate the privacy goals of these crypto- currencies by exploiting side-channel information leaked by the implementation of different system components. Specifically, we show that a remote party can link all transactions that send funds to a user, by measuring the response time of that user’s P2P node to certain requests. The timing differences are large enough that the attacks can be mounted remotely over a WAN. We responsibly disclosed the issues to the affected projects, and they have patched the vulnerabilities. We further study the impact of timing side-channels on the zero-knowledge proof systems used in these crypto-currencies. We observe that in Zcash’s implementation, the time to generate a zero-knowledge proof depends on secret transaction data, and in particular on the amount of transacted funds. Hence, an adversary capable of measuring proof generation time could break transaction confidentiality, despite the proof system’s zero-knowledge property. Our attacks highlight the dangers of side-channel leakage in anonymous crypto-currencies, and the need to systematically protect them against such attacks.
###### Michele Ciampi, Luisa Siniscalchi, Hendrik Waldner
ePrint Report
In this work, we provide a compiler that transforms a single-input functional encryption scheme for the class of polynomially bounded circuits into a multi-client functional encryption (MCFE) scheme for the class of separable functions. An n-input function f is called separable if it can be described as a list of polynomially bounded circuits f^1, ... , f^n s.t. f(x_1, ... , x_n)= f^1(x_1)+ ... + f^n(x_n) for all x_1 ,... , x_n.

Our compiler extends the works of Brakerski et al. [Eurocrypt 2016] and of Komargodski et al. [Eurocrypt 2017] in which a generic compiler is proposed to obtain multi-input functional encryption (MIFE) from single-input functional encryption. Our construction achieves the stronger notion of MCFE but for the less generic class of separable functions. Prior to our work, a long line of results has been proposed in the setting of MCFE for the inner-product functionality, which is a special case of a separable function.

We also propose a modified version of the notion of decentralized MCFE introduced by Chotard et al. [Asiacrypt 2018] that we call outsourceable mulit-client functional encryption (OMCFE). Intuitively, the notion of OMCFE makes it possible to distribute the load of the decryption procedure among at most n different entities, which will return decryption shares that can be combined (e.g., additively) thus obtaining the output of the computation. This notion is especially useful in the case of a very resource consuming decryption procedure, while the combine algorithm is non-time consuming. We also show how to extend the presented MCFE protocol to obtain an OMCFE scheme for the same functionality class.