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2014-02-15
20:12 [Job][New]

The group for Cryptography and Computer Algebra at the Technische Universität Darmstadt chaired by Prof. Dr. Johannes Buchmann is looking for a Ph.D. student. Candidates should have a superior Master\\\'s degree in mathematics, computer science, or related disciplines.

The opportunity to work in the area of cryptography --- more precisely, in lattice-based cryptography --- is given to the prospective Ph.D. student. Any prior experience in lattice-based cryprography is certainly an asset.

2014-02-14
19:17 [Pub][ePrint]

In 2013, Borghoff \\emph{et al}. introduced a slender-set linear

cryptanalysis on PRESENT-like ciphers with key-dependent secret

S-boxes. In this paper, we propose an improved slender-set linear

attack to PRESENT-like ciphers with secret S-boxes. We investigate

three new cryptanalytic techniques, and use them to recover the

secret S-boxes efficiently. Our first new idea is that we propose a

new technique to support consistency of partitions of the input to

the secret S-boxes. Our second new technique is that we present a

more efficient method to recover the coordinate functions of secret

The third new technique is that we propose a method of constructing

all correct coordinate function of secret S-boxes by pruning search

algorithm. In particular, we implemented a successful linear attack

on the full round Maya in practice. In our experiments, the correct

S-box can be recovered with $2^{36}$ known plaintexts, $2^{18.9}$

time complexity and negligible memory complexity at a success rate

of 87.5\\%. Our attack is the improvement and sequel of Borghoff\'s

work on PRESENT-like cipher with secret S-boxes.

19:17 [Pub][ePrint]

We extend the Tiny-OT two party protocol of Nielsen et al (CRYPTO 2012) to the case of $n$ parties in the dishonest majority setting. This is done by presenting a novel way of transferring pairwise authentications into global authentications. As a by product we obtain a more efficient manner of producing globally authenticated shares, which in turn leads to a more efficient two party protocol than that of Nielsen et al.

19:17 [Pub][ePrint]

We propose the first general framework for designing actively secure private function evaluation (PFE), not based on universal circuits. Our framework is naturally divided into pre-processing and online stages and can be instantiated using any generic actively secure multiparty computation (MPC) protocol.

Our framework helps address the main open questions about efficiency of actively secure PFE. On the theoretical side, our framework yields the first actively secure PFE with linear complexity in the circuit size. On the practical side, we obtain the first actively secure PFE for arithmetic circuits with $O(g \\cdot \\log g)$ complexity where $g$ is the circuit size. The best previous construction (of practical interest) is based on an arithmetic universal circuit and has complexity $O(g^5)$.

We also introduce the first linear Zero-Knowledge proof of correctness of extended permutation\" of ciphertexts (a generalization of ZK proof of correct shuffles) which maybe of independent interest.

19:17 [Pub][ePrint]

In this paper, we describe the families of block ciphers named SHipher. We show a symmetric encryption framework based on the SubSet-Sum problem. This framework can provide families of secure, flexible, and any-size block ciphers. We have extensively cryptanalyzed our encryption framework. We can easily control the computational cost by a key selection. Also, this framework offers excellent performance and it is flexible and general enough to admit a variety of implementations on different non-Abelian groups. In this paper, we provide one implementation using a group of matrices whose determinants are 1. This implementation accepts any block size satisfying 3l-1. If l=21, the block size is 62 bits, which suits for full spectrum of lightweight applications. While if $l=341$, the block size is 1022, which provides high security level up to resistant 2^{684} differential-attack effort and 2^{1022} brute-force attack effort.

19:17 [Pub][ePrint]

The problem raised by incremental encryption is the overhead due to the larger storage space required by the provision of random blocks together with the ciphered versions of a given document. Besides,

permitting variable-length modifications on the ciphertext leads to privacy preservation issues. In this paper we present incremental encryption schemes which are space-efficient, byte-wise incremental and which preserve perfect privacy in the sense that they hide the fact that an update operation has been performed on a ciphered document. For each scheme, the run time of updates performed turns out to be very efficient and we discuss the statistically adjustable trade-off between computational cost and storage space required by the produced ciphertexts.

19:17 [Pub][ePrint]

We present a secure multi-party computation (MPC) protocol in the honest-majority setting, which aims to reduce the communication costs in the situation where there are a large number of parties (as in a cloud scenario). Our goal is to reduce the usage of point-to-point channels, so as to enable the cloud to be used for multiple different protocol executions. We assume that the number of adversarially controlled parties is relatively small, and that an adversary is unable to target the proactive corruption of a subset of the parties (technically we assume a static corruption model for simplicity). As well as enabling a cloud provider to run multiple MPC protocols, our protocol also has highly efficient theoretical communication costs as a general MPC protocol when compared with other protocols in the literature; in particular the communication cost, for circuits of a suitably large depth, is $\\Order(|\\Circuit| \\cdot \\kappa^7)$, for security parameter $\\kappa$~and circuit size $|\\Circuit|$.

19:17 [Pub][ePrint]

HElib is a software library that implements homomorphic encryption (HE), specifically the Brakerski-Gentry-Vaikuntanathan (BGV) scheme, focusing on effective use of the Smart-Vercauteren ciphertext packing techniques and the Gentry-Halevi-Smart optimizations. The underlying cryptosystem serves as the equivalent of a \"hardware platform\" for HElib, in that it defines a set of operations that can be applied homomorphically, and specifies their cost. This \"platform\" is a SIMD environment (somewhat similar Intel SSE and the like), but with a unique cost metrics and parameters. In this report we describe some of the algorithms and optimization techniques that are used in HElib for data movement and simple linear algebra over this \"platform.\"

16:17 [Pub][ePrint]

We construct a structure-preserving signature scheme that is selectively randomizable and works in all types of bilinear groups. We give matching lower bounds showing that our structure-preserving signature scheme is optimal with respect to both signature size and public verification key size.

State of the art structure-preserving signatures in the asymmetric setting consist of 3 group elements, which is known to be optimal. Our construction preserves the signature size of 3 group elements and also at the same time minimizes the verification key size to 1 group element.

Depending on the application, it is sometimes desirable to have strong unforgeability and in other situations desirable to have randomizable signatures. To get the best of both worlds, we introduce the notion of selective randomizability where the signer may for specific signatures provide randomization tokens that enable randomization.

Our structure-preserving signature scheme unifies the different pairing-based settings since it can be instantiated in both symmetric and asymmetric groups. Since previously optimal structure-preserving signatures had only been constructed in asymmetric bilinear groups this closes an important gap in our knowledge. Having a unified signature scheme that works in all types of bilinear groups is not just conceptually nice but also gives a hedge against future cryptanalytic attacks. An instantiation of our signature scheme in an asymmetric bilinear group may remain secure even if cryptanalysts later discover an efficiently computable homomorphism between the source groups.

16:17 [Pub][ePrint]

Multiple encryption---the practice of composing a blockcipher several

times with itself under independent keys---has received considerable

attention of late from the standpoint of provable security. Despite

these efforts proving definitive security bounds (i.e., with matching

attacks) has remained elusive even for the special case of triple

encryption. In this paper we close the gap by improving both the best

known attacks and best known provable security, so that both bounds

match. Our results apply for arbitrary number of rounds and show that

the security of $\\ell$-round multiple encryption is precisely

$\\exp(\\kappa + \\min\\{\\kappa (\\ell\'-2)/2), n (\\ell\'-2)/\\ell\'\\})$ where

$\\exp(t) = 2^t$ and where $\\ell\' = 2\\lceil \\ell/2\\rceil$ is the even

integer closest to $\\ell$ and greater than or equal to $\\ell$, for all

$\\ell \\geq 1$. Our technique is based on Patarin\'s H-coefficient

method and reuses a combinatorial result of Chen and Steinberger

originally required in the context of key-alternating ciphers.

16:17 [Pub][ePrint]

We propose a new and simple framework for constructing fully homomorphic encryption (FHE) which is completely different from the previous work. We use finite non-commutative (a.k.a., non-abelian) groups which are \"highly non-commutative\" (e.g., the special linear groups of size two) as the underlying structure. We show that, on such groups, the AND and NOT operations on plaintext bits (which are sufficient to realize an arbitrary operation by composing them) can be emulated by a \"randomized commutator\" (which essentially requires the non-commutativity) and division operations on ciphertext elements, respectively. Then we aim at concealing the \"core structure\" of ciphertexts by taking conjugation by a secret element (where the non-commutativity is again essential), rather than adding noise to the ciphertext as in the previous FHE schemes. The \"noise-freeness\" of our framework yields the fully-homomorphic property directly, without the bootstrapping technique used in the previous schemes to remove the noise amplified by the homomorphic operations. This makes the overall structure of the FHE schemes significantly simpler and easier to understand. Although a secure instantiation based on the framework has not been found, we hope that the proposed framework itself is of theoretical value, and that the framework is flexible enough to allow a secure instantiation in the future.