International Association
for Cryptologic Research

The Engineering Cryptographic Protocols Group at TU Darmstadt is looking for a doctoral student in Engineering Cryptographic Protocols for Cloud Computing.

Our group is involved in the two main research centers for IT security in Darmstadt, the Center for Advanced Security Research Darmstadt (CASED) and the European Center for Security and Privacy by Design (EC SPRIDE). We develop new methods and tools to optimize and automatically generate cryptographic protocols. See http://encrypto.de for details.

The candidate will work in the EU FP 7 research project PRACTICE (Privacy-Preserving Computation in the Cloud), http://www.practice-project.eu, with the goal of developing, optimizing, and automatically generating secure computation protocols for cloud computing.

The candidate is expected to have a completed Master (or equivalent) degree with excellent grades in IT security, computer science, electrical engineering, mathematics, or a closely related field. Solid knowledge in IT security, applied cryptography, and programming skills is required. Additional knowledge in cryptographic protocols, parallel computing, compiler construction, programming languages, and software engineering is a plus.

Review of applications starts immediately until the position is filled.

Please consult the webpage given below for more details and how to apply.

Submission: 1 June 2014
Notification: 11 July 2014
From September 1 to September 2
Location: Istanbul, Turkey
More Information: www.light-sec.org/

Submission: 20 May 2014
Notification: 10 July 2014
From September 10 to September 11
Location: Wroclaw, Poland
More Information: http://wls.orange-labs.fr/smartcondevsp2014/

The ZigBee specification is an emerging wireless technology designed to address the specific needs of low-cost, low-power wireless sensor networks and is built upon the physical and medium access control layers defined in IEEE 802.15.4 standard for wireless personal area networks (WPANs). A key component for the wide-spread success and applicability of ZigBee-based networking solutions will be its ability to provide enhanced security mechanisms that can scale to hundreds of nodes. Currently, however, an area of concern is the ZigBee key management scheme, which uses a centralized approach that introduces well-known issues of limited scalability and a single point of vulnerability. Moreover, ZigBee key management uses a public key infrastructure. Due to these limitations, we suggest replacing ZigBee key management with a better candidate scheme that is decentralized, symmetric, and scalable while addressing security requirements. In this work, we investigate the feasibility of implementing Localized Encryption and Authentication Protocol (LEAP+), a distributed symmetric based key management. LEAP+ is designed to support multiple types of keys based on the message type that is being exchanged. In this paper, we first conduct a qualitative security analysis of LEAP+ and the current ZigBee key management scheme. Using the QualNet 5.0.2 simulator, we implement LEAP+ on the ZigBee platform for the very first time. Experimental results show that a distributed key management scheme such as LEAP+ provides improved security and offers good scalability.

An isogeny graph is a graph whose vertices are principally polarized

abelian varieties and whose edges are isogenies between these varieties. In

his thesis, Kohel described the structure of isogeny graphs for elliptic

curves and showed that one may compute the endomorphism ring of an elliptic

curve defined over a finite field by using a depth first search algorithm

in the graph. In dimension 2, the structure of isogeny graphs is less understood and existing algorithms for computing endomorphism rings are very expensive.

Our setting considers genus 2 jacobians with complex multiplication,

with the assumptions that the real multiplication subring is maximal and

has class number one. We fully describe the isogeny graphs in that

case.

Over finite fields, we derive a depth first search algorithm for computing endomorphism rings locally at prime numbers, if the real multiplication is maximal. To the best of our knowledge, this is the first DFS-based algorithm in genus 2.

Cloud storage is very popular since it has many advantages, but there is a new threat to cloud storage that was not considered before. {\\it Self-updatable encryption} that updates a past ciphertext to a future ciphertext by using a public key is a new cryptographic primitive introduced by Lee, Choi, Lee, Park, and Yung (Asiacrypt 2013) to defeat this threat such that an adversary who obtained a past-time private key can still decrypt a (previously unread) past-time ciphertext stored in cloud storage. Additionally, an SUE scheme can be combined with an attribute-based encryption (ABE) scheme to construct a powerful revocable-storage ABE (RS-ABE) scheme introduced by Sahai, Seyalioglu, and Waters (Crypto 2012) that provides the key revocation and ciphertext updating functionality for cloud storage.

In this paper, we propose an efficient SUE scheme and its extended schemes. First, we propose an SUE scheme with short public parameters in prime-order bilinear groups and prove its security under a $q$-type assumption. Next, we extend our SUE scheme to a time-interval SUE (TI-SUE) scheme that supports a time interval in ciphertexts. Our TI-SUE scheme has short public parameters and also secure under the $q$-type assumption. Finally, we propose the first large universe RS-ABE scheme with short public parameters in prime-order bilinear groups and prove its security in the selective revocation list model under a $q$-type assumption.

We present a private information retrieval (PIR) scheme based on a somewhat homomorphic encryption (SWHE). In particular, we customize an NTRU-based SWHE scheme in order to evaluate a specific class of fixed depth circuits relevant for PIR implementation, thus achieving a more practical implementation. In practice, a SWHE that can evaluate a depth 5 circuit is sufficient to construct a PIR capable of retrieving data from a database containing 4 billion rows. We leverage this property in order to produce a more practical PIR scheme. Com- pared to previous results, our implementation achieves a significantly lower bandwidth cost (more than 1000 times smaller). The computational cost of our implementation is higher than previous proposals for databases containing a small number of bits in each row. However, this cost is amortized as database rows become wider.

We present the homomorphic evaluation of the Prince block cipher. Our leveled implementation is based on a generalization of NTRU. We are motivated by the drastic bandwidth savings that may be achieved by scheme conversion. To unlock this advantage we turn to lightweight ciphers such as Prince. These ciphers were designed from scratch to yield fast and compact implementations on resource constrained embedded platforms. We show that some of these ciphers have the potential to enable near practical homomorphic evaluation of block ciphers. Indeed, our analysis shows that Prince can be implemented using only a 24 level deep circuit. Using an NTRU based implementation we achieve an evaluation time of 3.3 seconds per Prince block - one and two orders of magnitude improvement over homomorphic AES implementations achieved using NTRU, and BGV-style homomorphic encryption libraries, respectively.

Oblivious RAM (ORAM) is an established technique to hide the access pattern to an untrusted storage system.

With ORAM, a curious adversary cannot tell what data address the user is accessing when observing the bits moving between the user and the storage system.

All existing ORAM schemes achieve obliviousness by adding redundancy to the storage system, i.e., each access is turned into multiple random accesses.

Such redundancy incurs a large performance overhead.

Though traditional data prefetching techniques successfully hide memory latency in DRAM based systems, it turns out that they do not work well for ORAM.

In this paper, we exploit ORAM locality by taking advantage of the ORAM internal structures.

Though it might seem apparent that obliviousness and locality are two contradictory concepts, we challenge this intuition by exploiting data locality in ORAM without sacrificing provable security.

In particular, we propose an ORAM prefetching technique called dynamic super block scheme and comprehensively explore its design space.

The dynamic super block scheme detects data locality in the program\'s working set at runtime, and exploits the locality in a data-independent way.

% based on the key observation that position map ORAMs have better locality than the data ORAM.

Our simulation results show that with dynamic super block scheme, ORAM performance without super blocks can be significantly improved. After adding timing protection to ORAM, the average performance gain is 25.5\\% (up to 49.4\\%) over the baseline ORAM and 16.6\\% (up to 30.1\\%) over the best ORAM prefetching technique proposed previously.

We study the problem of efficient (sub-linear) fuzzy search on encrypted outsourced data, in the symmetric-key setting. In particular, a user who stores encrypted data on a remote untrusted server forms queries that enable the server to efficiently locate the records containing the requested keywords, even though the user may misspell keywords or provide noisy data in the query. We define an appropriate primitive for a general \\emph{closeness} function on the message space that we call \\emph{efficiently fuzzy-searchable encryption} (\\emph{EFSE}).

Next we identify an optimal security notion for EFSE. We demonstrate that existing schemes do not meet our security definition and propose a new scheme that we prove secure under basic assumptions. Unfortunately, the scheme requires large ciphertext length, but we show that, in a sense, this space-inefficiency is unavoidable for a general, optimally-secure scheme.

Seeking the right balance between efficiency and security, we then show how to construct schemes that are more efficient and satisfy a weaker security notion that we propose. To illustrate, we present and analyze a more space-efficient scheme for supporting fuzzy search on biometric data that achieves the weaker notion.

Submission: 28 April 2014
Notification: 16 June 2014
From September 1 to September 5
Location: Paris, France
More Information: http://2014.qcrypt.net/

Submission: 15 December 2014
Notification: 1 February 2015
From April 2 to April 3
Location: Gaithersburg, USA
More Information: http://www.nist.gov/itl/csd/ct/post-quantum-crypto-workshop-2015.cfm

Zorro is a 128-bit lightweight block cipher supporting 128-bit keys, presented at CHES~2013 by G\\\'erard et al. One of the main design goals of the cipher was to allow efficient masking according to the Rivain and Prouff Scheme, which lead to a very unconventional design, using partial non-linear layers. Despite the security claims of the designers, the cipher was recently broken by differential and linear attacks due to Wang et al., recovering its 128-bit key with complexity of about $2^{108}$. These attacks are based on high-probability iterative characteristics that are made possible due a special property of the linear layer of Zorro, which is shown to be devastating in combination with its partial non-linear layer.

In this paper, we analyze the security of Zorro-like ciphers with partial non-linear layers by devising differential and linear characteristic search algorithms and key recovery algorithms. These algorithms exploit in a generic way the small number of Sboxes in a Zorro-like round, and are independent of any specific property of its linear layer (such as the one exploited by Wang et al.), or its Sbox implementation. When applied to the Zorro block cipher itself, we were able to find \\emph{the highest} probability characteristics for the full cipher and devise significantly improved attacks. Our differential attack has a time complexity of about $2^{45}$, requiring about $2^{44}$ chosen plaintexts, and our linear attack has a time complexity of about $2^{45}$, requiring about $2^{45}$ known plaintexts.

Independently of our results, the recently published paper by Rasoolzadeh et al. found similar iterative characteristics for Zorro by exploiting in a different way the devastating property of its linear layer, described by Wang et al. However, our improved key recovery techniques result in differential and linear attacks which are at least $2^{11}$ times faster. More significantly, the surprisingly large number of Zorro-like rounds analyzed by some of our generic techniques raises questions over the general design strategy of Zorro, namely, the use of partial non-linear layers.

Mission critical applications on homogenous mo- bile wireless sensor networks (HMWSNs) mandate new sets of security appliances to be friendly with existing resource constrained hardware platforms. To deliver a promising security, particularly in military deployments, mechanisms have to build upon an efficient key management that compensates HMWSNs constraints. Cluster-based key establishment is being the prime focus among the recent works in key establishment due to its significant improvement on network efficiency, security, scal- ability and flexibility. Therefore, we propose a Cluster-based framework to support pre-distribution key establishment schemes for HMWSNs. The proposed framework is compatible with most of pre-distribution schemes, and two instantiations are provided in this work to support our claim that the proposed framework improves security and scalability of the adopted schemes. We develop analytical models and conduct extensive simulations to evaluate the security and performance of the proposed frame- work, and the network connectivity under different scenarios.

Zorro is a 128-bit lightweight block cipher supporting 128-bit keys, presented at CHES~2013 by G\\\'erard et al. One of the main design goals of the cipher was to allow efficient masking according to the Rivain and Prouff Scheme, which lead to a very unconventional design, using partial non-linear layers. Despite the security claims of the designers, the cipher was recently broken by differential and linear attacks due to Wang et al., recovering its 128-bit key with complexity of about $2^{108}$. These attacks are based on high-probability iterative characteristics that are made possible due a special property of the linear layer of Zorro, which is shown to be devastating in combination with its partial non-linear layer.

In this paper, we analyze the security of Zorro-like ciphers with partial non-linear layers by devising differential and linear characteristic search algorithms and key recovery algorithms. These algorithms exploit in a generic way the small number of Sboxes in a Zorro-like round, and are independent of any specific property of its linear layer (such as the one exploited by Wang et al.), or its Sbox implementation. When applied to the Zorro block cipher itself, we were able to find \\emph{the highest} probability characteristics for the full cipher and devise significantly improved attacks. Our differential attack has a time complexity of about $2^{45}$, requiring about $2^{44}$ chosen plaintexts, and our linear attack has a time complexity of about $2^{45}$, requiring about $2^{45}$ known plaintexts.

Independently of our results, the recently published paper by Rasoolzadeh et al. found similar iterative characteristics for Zorro by exploiting in a different way the devastating property of its linear layer, described by Wang et al. However, our improved key recovery techniques result in differential and linear attacks which are at least $2^{11}$ times faster. More significantly, the surprisingly large number of Zorro-like rounds analyzed by some of our generic techniques raises questions over the general design strategy of Zorro, namely, the use of partial non-linear layers.

We evaluate the security of the recently proposed authenticated encryption scheme POET with regard to weak keys when its universal hash functions are instantiated with finite field multiplications. We give explicit constructions for weak key classes not covered by POET\'s

weak key testing strategy, and demonstrate how to leverage them to obtain universal forgeries.

In this paper, we present Functional Encryption (FE) schemes for finite languages from standard static assumption, viz., \\textit{Decisional Linear} (DLIN) assumption. These finite languages are described by Deterministic Finite Automatas (DFAs). Our first scheme is ciphertext-policy functional encryption (CP-FE), where a key $\\sk_w$ is labeled with a string $w$ over a fixed alphabet $\\Sigma$ and a ciphertext $\\cipher_\\amn$ is associated with a DFA $\\amn$ over the same alphabet $\\Sigma$. The key $\\sk_w$ can extract the message from the ciphertext $\\cipher_\\amn$ if the DFA $\\amn$ accepts the string $w$. This CP-FE scheme is constructed based on attribute-based encryption (ABE) structure of Okamoto-Takashima in Asiacrypt, 2012. To achieve the adaptive security, we put bounds on number of occurrences of any symbol in a string and in the set of transition tuples of a DFA. Due to this restriction, the size of key space (where the keys are indexed with strings) is reduced to finite. Hence, the functional scope of any DFA in our system can capture only finite language. Similarly, we obtain our second adaptively secure FE scheme in key-policy flavor from DLIN assumption. Both the schemes are shown to be secure in the standard model.

In this paper, we investigate the properties of iterative non-injective functions and the security of primitives where they are used. First, we introduce the Collision Probability Spectrum (CPS) parameter to quantify how far from a permutation a function is. In particular, we show that the output size decreases linearly with the number of iterations whereas the collision trees grow quadratically.

Secondly, we investigate the T-sponge construction and show how certain CPS and rate values lead to an improved preimage attack on long messages. As an example, we find collisions for the GLUON-64 internal function, approximate its CPS, and show an attack that violates the security claims. For instance, if a message ends with a sequence of 1~Mb (respectively 1~Gb) of zeros, then our preimage search takes time $2^{115.3}$ (respectively $2^{105.3}$) instead of $2^{128}$.

Garbled circuits offer a powerful primitive for computation on a user\'s personal data while keeping that data private. Despite recent improvements, constructing and evaluating circuits of any useful size remains expensive on the limited hardware resources of a smartphone, the primary computational device available to most users around the world. In this work, we develop a new technique for securely outsourcing the generation of garbled circuits to a Cloud provider. By outsourcing the circuit generation, we are able to eliminate the most costly operations from the mobile device, including oblivious transfers. After proving the security of our techniques in the malicious model, we experimentally demonstrate that our new protocol, built on this role reversal, decreases execution time by 98% and reduces network costs by as much as 92% compared to previous outsourcing protocols. In so doing, we demonstrate that the use of garbled circuits on mobile devices can be made nearly as practical as it is becoming for server-class machines.

Design and develop security solutions utilizing vetted cryptographic primitives. Application areas include Internet of Things (IoT), sensors, cyber-physical systems, and cloud and cognitive computing. Architectures must meet security and privacy requirements that involve, in particular, device and/or user authentication/authorization under various constraints on connectivity, communications bandwidth, processing complexity, and power consumption.

An important aspect of this position entails porting [our] existing algorithms and code into ARM TrustZone and/or hardware so as to conform to design principles of conventional standardized APIs. 3+ years of coding experience in C/C++ is required. Proficiency in programming FPGAs is a definite plus, as is experience in JNI.

This is a 3-month position with flexible start/end dates during May - September 2014 time frame.