International Association
for Cryptologic Research

This paper combines cryptographic voting and web page ranking and proves that it is possible to hold elections so as not to limit a voter by a list of candidates, to benefit from voter\'s personal experience in dealing with people, to make wide and proportional representation, and to achieve secrecy, including incoercibility, and verifiability of cryptographic voting systems.

One of the most well known micropayment scheme is the PayWord scheme. It is designed to be onevendor, so if we apply it for multiple vendors, it does not protect against double spending. We extended the PayWord scheme, it supports shopping at multiple vendors without an on-line broker or an on-line secure database. The proposed credit-based system uses one hash chain, hence besides the secret signature key only the seed and a random value should be securely stored. Our scheme is analyzed in applied pi calculus, we prove that it fulfills payment approval, secure payment authorization, secrecy of payment information and unreusability.

Side channel and fault attacks take advantage from the fact that the behavior of crypto implementations can be observed and provide hints that simplify revealing keys. These attacks are normally prepared by analyzing devices that are identical to the real target. Here we propose to individualize the design of cryptographic devices in order to prevent attacks that use identical devices. We implemented three different designs that provide exactly the same cryptographic function, i.e. an ECC kP multiplication. The synthesis and power simulation results show clear differences in the area consumed as well as in the power traces. We envision that this type of protection mechanism is relevant e.g. for wireless sensor networks from which devices can easily be stolen for further analysis in the lab.

We revisit the problem of finding small solutions to a collection of linear equations modulo an unknown divisor $p$ for a known composite integer $N$. In Asiacrypt\'08, Herrmann and May introduced a heuristic algorithm for this problem, and their algorithm has many interesting applications, such as factoring with known bits problem, fault attacks on RSA signatures, etc. In this paper, we consider two variants of Herrmann-May\'s equations, and propose some new techniques to solve them. Applying our algorithms, we obtain a few by far the best analytical/experimental results for RSA and its variants. Specifically,

\\begin{itemize}

\\item We improve May\'s results (PKC\'04) on small secret exponent attack on RSA variant with moduli $N = p^rq$ ($r\\geq 2$).

\\item We extend Nitaj\'s result (Africacrypt\'12) on weak encryption exponents of RSA and CRT-RSA.

\\end{itemize}

With sensitive data being increasingly stored on mobile devices and

laptops, hard disk encryption is more important than ever. In

particular, being able to plausibly deny that a hard disk contains

certain information is a very useful and interesting research

goal. However, it has been known for some time that existing

``hidden volume\'\' solutions, like TrueCrypt, fail in the face of an

adversary who is able to observe the contents of a disk on multiple,

separate occasions. In this work, we explore more robust

constructions for hidden volumes and present HIVE, which is

resistant to more powerful adversaries with multiple-snapshot

capabilities. In pursuit of this, we propose the first security

definitions for hidden volumes, and prove HIVE secure under these

definitions. At the core of HIVE, we design a new write-only

Oblivious RAM. We show that, when only hiding writes, it is

possible to achieve ORAM with optimal O(1) communication complexity

and only poly-logarithmic user memory. This is a significant

improvement over existing work and an independently interesting

result. We go on to show that our write-only ORAM is specially

equipped to provide hidden volume functionality with low overhead

and significantly increased security. Finally, we implement HIVE as

a Linux kernel block device to show both its practicality and

usefulness on existing platforms.

Enabling private database queries is an important and challenging research problem with many real-world applications. The goal is for the client to obtain the results of its queries without learning anything else about the database, while the outsourced server learns nothing about the queries or data, including access patterns. The secure-computation-over-ORAM architecture offers a promising approach to this problem, permitting sub-linear time processing of the queries (after pre-processing) without compromising security.

In this work we examine the feasibility of this approach, focusing specifically on secure-computation protocols based on somewhat-homomorphic encryption (SWHE). We devised and implemented secure two-party protocols in the semi-honest model for the path-ORAM protocol of Stefanov et al. This provides access by index or keyword, which we extend (via pre-processing) to limited conjunction queries and range queries. Some of our sub-protocols may be interesting in their own right, such as our new protocols for encrypted comparisons and blinded permutations.

We implemented our protocols on top of the HElib homomorphic encryption library. Our basic single-threaded implementation takes about 30 minutes to process a query on a database with $2^{22}$ records and 120-bit long keywords, providing a cause for optimism about the viability of this direction, and we expect a better optimized implementation to be much faster.

In this paper, we present a variant of Diem\'s $\\widetilde{O}(q)$ index calculus algorithm to attack the discrete logarithm problem (DLP) in Jacobians of genus 3 non-hyperelliptic curves over a finite field $\\mathbb{F}_q$. We implement this new variant in C++ and study the complexity in both theory and practice, making the logarithmic factors and constants hidden in the $\\widetilde{O}$-notation precise.

Our variant improves the computational complexity at the cost of a moderate increase in memory consumption, but we also improve the computational complexity even when we limit the memory usage to that of Diem\'s original algorithm. Finally, we examine how parallelization can help to reduce both the memory cost per computer and the running time for our algorithms.

A program obfuscator takes a program and outputs an \"scrambled\" version of it, where the goal is that the obfuscated program will not reveal much

about its structure beyond what is apparent from executing it. There are several ways of formalizing this goal. Specifically, in

indistinguishability obfuscation, first defined by Barak et al. (CRYPTO 2001), the requirement is that the results of obfuscating any two

functionally equivalent programs (circuits) will be computationally indistinguishable. Recently, a fascinating candidate construction for

indistinguishability obfuscation was proposed by Garg et al. (FOCS 2013). This has led to a flurry of discovery of intriguing constructions of

primitives and protocols whose existence was not previously known (for instance, fully deniable encryption by Sahai and Waters, STOC 2014). Most

of them explicitly rely on additional hardness assumptions, such as one-way functions.

Our goal is to get rid of this extra assumption. We cannot argue that indistinguishability obfuscation of all polynomial-time circuits implies

the existence of one-way functions, since if $P = NP$, then program obfuscation (under the indistinguishability notion) is possible. Instead, the

ultimate goal is to argue that if $P \\neq NP$ and program obfuscation is possible, then one-way functions exist.

Our main result is that if $NP \\not\\subseteq ioBPP$ and there is an efficient (even imperfect) indistinguishability obfuscator, then there are

one-way functions. In addition, we show that the existence of an indistinguishability obfuscator implies (unconditionally) the existence of SZK-

arguments for NP. This, in turn, provides an alternative version of our main result, based on the assumption of hard-on-the average NP problems.

To get some of our results we need obfuscators for simple programs such as CNF formulas.

We propose a simple cast-as-intended remote e-voting protocol where the security is based on the use of secure (and trusted) smart cards that incorporate incard numeric keyboards and LCD displays, and can perform a limited number of cryptographic operations (like encryption, signing, and random number generation). The protocol, while very simple, is significantly more secure (in the sense of ``cast-as-intended\'\') and convenient to use than the e-voting protocol currently used in Norway. The protocol is developed primarily with the idea of deploying it in Estonia within the next $3$ to $10$ years. Since in Estonia, a vast majority of the population already has ID-cards with digital signing and authentication functionality, and the use of ID-cards is a required prerequisite to participate in Estonian e-voting anyway, our assumption of every voter having a secure hardware token makes sense in this concrete context.

Bitcoin is the first digital currency to see widespread adoption. While payments are conducted between pseudonyms, Bitcoin cannot offer strong privacy guarantees: payment transactions are recorded in a public decentralized ledger, from which much information can be deduced. Zerocoin (Miers et al., IEEE S&P 2013) tackles some of these privacy issues by unlinking transactions from the payment\'s origin. Yet, it still reveals payments\' destinations and amounts, and is limited in functionality.

In this paper, we construct a full-fledged ledger-based digital currency with strong privacy guarantees. Our results leverage recent advances in zero-knowledge Succinct Non-interactive ARguments of Knowledge (zk-SNARKs).

First, we formulate and construct decentralized anonymous payment schemes (DAP schemes). A DAP scheme enables users to directly pay each other privately: the corresponding transaction hides the payment\'s origin, destination, and transferred amount. We provide formal definitions and proofs of the construction\'s security.

Second, we build Zerocash, a practical instantiation of our DAP scheme construction. In Zerocash, transactions are less than 1 kB and take under 6 ms to verify --- orders of magnitude more efficient than the less-anonymous Zerocoin and competitive with plain Bitcoin.

Smooth projective hash functions have been used as building block for various cryptographic applications, in particular for password-based authentication.

In this work we propose the extended concept of distributed smooth projective hash functions where the computation of the hash value is distributed across $n$ parties and show how to instantiate the underlying approach for languages consisting of Cramer-Shoup ciphertexts.

As an application of distributed smooth projective hashing we build a new framework for the design of two-server password authenticated key exchange protocols, which we believe can help to \"explain\" the design of earlier two-server password authenticated key exchange protocols.

Cryptography, Security & Privacy Research Group at Koç University has multiple openings for both M.Sc. and Ph.D. level applications. All accepted applicants will receive competitive scholarships including tuition waiver, housing, monthly stipend, computer, travel support, etc.

For more information about our group and projects, visit

http://crypto.ku.edu.tr

For questions, contact Asst. Prof. Alptekin Küpçü

http://home.ku.edu.tr/~akupcu

For applying online, and questions about the application-process, visit

http://gsse.ku.edu.tr

For summer internship opportunities, visit

http://kusrp.ku.edu.tr

The Ruhr-Universität Bochum (RUB) is one of Germany’s leading research universities with more than 50 scientists working in IT-security and cryptography. The Faculty of Mathematics invites applications for the position of a Junior Professor (W1) in Cryptography to start as soon as possible.

The future holder of the position will represent the subject in research and teaching.

We are seeking a candidate with an excellent research record in cryptography, in particular in theoretical cryptography, provable security, protocols, or secure multi-party computation.

The position is non-tenured with an initial appointment for 3 years, and renewable for another 3 years after a positive mid-term review.

Candidates for the professorship are expected to have strong leadership qualities, particularly

• excellent level of commitment in academic teaching

• willingness to participate in interdisciplinary research

• willingness and ability to attract externally funded research projects

• or to contribute to joint research projects of the department.

We present a novel and generic construction of a lossless compression algorithm for Schnorr-like signatures utilizing publicly accessible randomness. This strategy is from a mathematical and algorithmic point of view very interesting, since it is closely related to vector quantization techniques used for audio and video compression. Conceptually, exploiting public randomness in order to reduce the signature size has never been considered in cryptographic applications. This opens new directions for improving existing signature schemes. We illustrate the applicability of our compression algorithm using the examples of current-state-of-the-art signature schemes such as the efficient constructions due to Lyubashevsky et al. and the GPV signature scheme instantiated with the efficient trapdoor construction from Micciancio and Peikert. Both schemes benefit from increasing the main security parameter $n$, which is positively correlated with the compression rate measuring the amount of storage savings. For instance, GPV signatures admit improvement factors of approximately $\\lg n$ implying compression rates of about $65$\\% for practical parameters without suffering loss of information or decrease in security, meaning that the original signature can always be recovered from its compressed state. Similarly, for signatures generated according to the scheme due to G\\\"uneysu et al. we achieve compression rates of approximately $60$\\% and even $73$\\%, when combining with previous compression algorithms. As a further interesting result, we propose a generic unrestricted aggregate signature scheme.

The present public key encryption in this paper involves the use of two values and they are the shadows values of a base value, and the base value is derived from the two shadows values. Whenever two integer values (first shadow value and second shadow value) are multiplied producing a product value and the value of one is subtracted from the product value a first base value is derived and it is the first base value of the two shadows values. The derived first base value may be divided by any divisor that it may be divided with which produces a positive integer quotient result and zero for the remainder. All values that are used in the division and the quotient result are bases values for the chosen shadow value-pair. Then one of the base values is chosen along with the two chosen shadows values and they comprise a triplet values that represent the public key to encrypt a message and the private key to decrypt the encrypted message.

Increasingly, confidential medical records are being stored in data centers hosted by hospitals or large companies. As sophisticated algorithms for predictive analysis on medical data continue to be developed, it is likely that, in the future, more and more computation will be done on private patient data. While encryption provides a tool for assuring the privacy of medical information, it limits the functionality for operating on such data. Conventional

encryption methods used today provide only very restricted possibilities or none at all to operate on encrypted data without decrypting it first. Homomorphic encryption provides a tool for

handling such computations on encrypted data, without decrypting the data, and without even needing the decryption key.

In this paper, we discuss possible application scenarios for homomorphic encryption in order to ensure privacy of sensitive medical data. We describe how to privately conduct predictive analysis tasks on encrypted data using homomorphic encryption. As a proof of concept, we present a working implementation of a prediction service running in the cloud (hosted on Microsoft\'s Windows Azure), which takes as input private encrypted health data, and returns the probability of suffering cardiovascular disease in encrypted form. Since the cloud service uses homomorphic encryption, it makes this prediction while handling only encrypted data, learning nothing about

the submitted confidential medical data.

Several recent and high-profile incidents give cause to believe that randomness failures of various kinds are endemic in deployed cryptographic systems. In the face of this, it behoves cryptographic researchers to develop methods to immunise - to the extent that it is possible - cryptographic schemes against such failures. This paper considers the practically-motivated situation where an adversary is able to force a public key encryption scheme to reuse random values, and functions of those values, in encryption computations involving adversarially chosen public keys and messages. It presents a security model appropriate to this situation, along with variants of this model. It also provides necessary conditions on the set of functions used in order to attain this security notation, and demonstrates that these conditions are also sufficient in the Random Oracle Model. Further standard model constructions achieving weaker security notions are also given, with these constructions having interesting connections to other primitives including: pseudo-random functions that are secure in the related key attack setting; Correlated Input Secure hash functions; and public key encryption schemes that are secure in the auxiliary input setting (this being a special type of leakage resilience).

We present a ``universal\'\' Random Access Machine (RAM in short) for tamper and leakage resilient computation. The RAM has one CPU that accesses three storages (called disks in the following), two of them are secret, while the other one is public. The CPU has constant size for each fixed value of security parameter $k$. We construct a compiler for this architecture which transforms any keyed primitive into a RAM program where the key is encoded and stored on the two secret disks and the instructions for evaluating the functionality are stored on the public disk.

The compiled program tolerates arbitrary independent tampering of the disks. That is, the adversary can tamper with the intermediate values produced by the CPU, and the program code of the compiled primitive on the public disk. In addition, it tolerates bounded independent leakage from the disks and continuous leakage from the communication channels between the disks and the CPU.

Although it is required that the circuit of the CPU is tamper and leakage proof, its design is independent of the actual primitive being computed and its internal storage is non-persistent, i.e., all secret registers are reset between invocations. Hence, our result can be interpreted as reducing the problem of shielding arbitrary complex computations to protecting a single, simple and ``universal\'\' component. As a main ingredient of our construction we use continuous

non-malleable codes that satisfy certain additional properties.

This paper extends the certificateless public key infrastructure model that was proposed by Hassouna et al by proposing new digital signature scheme to provide true non-repudiation,

the proposed signature scheme is short and efficient, it is also has strength point that the KGC has no contribution in signature generation/verification process, therefore any compromise

of the KGC does not affect the non-repudiation service of the system. Furthermore, even the KGC cannot do signature forgery by (temporary) replacing the user\'s public key.