*15:03*[Event][New] PETS'13: Privacy Enhancing Technologies Symposium

Submission: 19 February 2013

Notification: 30 March 2013

From July 10 to July 12

Location: Bloomington, Indiana, USA

More Information: http://petsymposium.org/2013/

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Submission: 19 February 2013

Notification: 30 March 2013

From July 10 to July 12

Location: Bloomington, Indiana, USA

More Information: http://petsymposium.org/2013/

Submission: 31 January 2013

Notification: 15 March 2013

From May 28 to May 30

Location: Heraklion, Greece

More Information: www.wistp.org

2013-01-01

This paper considers the transfer of digital data over {\\sl leaky and noisy} communication channels. We develop defensive strategies exploiting the fact that noise prevents the attacker from accurately measuring leakage.

The defense strategy described in this paper pairs each useful data element $k$ with a camouflage value $v$ and simultaneously transmits both $k$ and $v$ over the channel. This releases an emission $e(k,v)$. We wish to select the camouflage values $v(k)$ as a function of $k$ in a way that makes the quantities $e(k,v(k))$ as {\\sl indistinguishable} as possible from each other.

We model the problem and show that optimal camouflage values can be computed from side-channels under very weak physical assumptions. The proposed technique is hence applicable to a wide range of readily available technologies.

We propose algorithms for computing optimal camouflage values when the number of samples per trace is moderate (typically $\\leq 6$) and justify our models by a statistical analysis.

We also provide experimental results obtained using FPGAs.

The traditional notion of {\\em program obfuscation} requires that an obfuscation $\\tilde{f}$ of a program $f$ computes the exact same function as $f$, but beyond that, the code of $\\tilde{f}$ should not leak any information about $f$. This strong notion of {\\em virtual black-box} security was shown by Barak et al. (CRYPTO 2001) to be impossible to achieve, for certain {\\em unobfuscatable function families}. The same work raised the question of {\\em approximate obfuscation}, where the obfuscated $\\tilde{f}$ is only required to approximate $\\tilde{f}$; that is, $\\tilde{f}$ only agrees with $f$ on some input distribution.

We show that, assuming {\\em trapdoor permutations}, there exist families of {\\em robust unobfuscatable functions} for which even approximate obfuscation is impossible. That is, obfuscation is impossible even if the obfuscated $\\tilde{f}$ only agrees with $f$ with probability slightly more than $\\frac{1}{2}$, on a uniformly sampled input (below $\\frac{1}{2}$-agreement, the function obfuscated by $\\tilde{f}$ is not uniquely defined). Additionally, we show that, assuming only one-way functions, we can rule out approximate obfuscation where $\\tilde{f}$ is not allowed to err, but may refuse to compute $f$ with probability close to $1$.

We then demonstrate the power of robust unobfuscatable functions by exhibiting new implications to resettable protocols that so far have been out of our reach. Concretely, we obtain a new non-black-box simulation technique that reduces the assumptions required for resettably-sound zero-knowledge protocols to {\\em one-way functions}, as well as reduce round-complexity. We also present a new simplified construction of simultaneously resettable zero-knowledge protocols that does not rely on collision-resistent hashing. Finally, we construct a three-message simultaneously resettable $\\WI$ {\\em argument of knowledge} (with a non-black-box knowledge extractor). Our constructions are based on a special kind of ``resettable slots\" that are useful for a non-black-box simulator, but not for a resetting prover.

Wireless Sensor Networks (WSNs) pose a number of unique security challenges that demand innovation in several areas including the design of cryptographic primitives and protocols. Despite recent progress, the efficient implementation of Elliptic Curve Cryptography (ECC) for WSNs is still a very active research topic and techniques to further reduce the time and energy cost of ECC are eagerly sought. This paper presents an optimized ECC implementation that we developed from scratch to comply with the severe resource constraints of 8-bit sensor nodes such as the MICAz and IRIS motes. Our ECC software uses Optimal Prime Fields (OPFs) as underlying algebraic structure and supports two different families of elliptic curves, namely Weierstra{\\ss}-form and twisted Edwards-form curves. Due to the combination of efficient field arithmetic and fast group operations, we achieve an execution time of $5.9 \\cdot 10^6$ clock

cycles for a full 160-bit scalar multiplication on an 8-bit ATmega128

microcontroller, which is 2.78 times faster than the widely-used TinyECC library. Our implementation also shows that the energy cost of ephemeral ECDH key exchange between two MICAz (or IRIS) motes amounts to only 38.7 mJ per mote (including radio communication). A mote with a standard AA battery pack could theoretically perform up to 174,278 ECDH key exchanges before running out of energy.

In this work we consider generic algorithms to find near-collisions

for a hash function. If we consider only hash computations, it is

easy to compute a lower-bound for the complexity of near-collision

algorithms, and to build a matching algorithm. However, this

algorithm needs a lot of memory, and makes than 2^{n/2} memory

accesses. Recently, several algorithms have been proposed without

this memory requirement; they require more hash evaluations, but the

attack is actually more practical. They can be divided in two main

categories: some are based on truncation, and some are based on

covering codes.

In this paper, we give a new insight to the generic complexity of a

near-collision attack. First, we consider time-memory trade-offs for

truncation-based algorithms. For a practical implementation, it seems

reasonable to assume that some memory is available and we show that

taking advantage of this memory can significantly reduce the

complexity. Second, we show a new method combining truncation and

covering codes. The new algorithm is always at least as good as the

previous works, and often gives a significant improvement. We

illustrate our results by giving a 10-near collision for MD5: our

algorithm has a complexity of 2^45.4 using 1TB of memory while the

best previous

Non-interactive key exchange (NIKE) is a fundamental but much-overlooked cryptographic primitive. It appears as a major contribution in the ground-breaking paper of Diffie and Hellman, but NIKE has remained largely unstudied since then. In this paper, we provide different security models for this primitive and explore the relationships between them. We then give constructions for secure NIKE in the Random Oracle Model based on the hardness of factoring and in the standard model based on the hardness of a variant of the decisional Bilinear Diffie Hellman Problem for asymmetric pairings. We also study the relationship between NIKE and public key encryption (PKE), showing that a secure NIKE scheme can be generically converted into an IND-CCA secure PKE scheme. This conversion also illustrates the fundamental nature of NIKE in public key cryptography.

Functional encryption is a powerful primitive: given an encryption

$\\Enc(x)$ of a value $x$ and a secret key $\\sk_f$ corresponding to a

circuit $f$, it enables efficient computation of $f(x)$ without revealing any additional information about $x$. Constructing functional encryption schemes with succinct ciphertexts that guarantee security for even a single secret key (for a general function $f$) is an important open problem with far reaching applications, which this paper addresses.

Our main result is a functional encryption scheme \\textit{for any general function $f$ of depth $d$, with succinct ciphertexts} whose size grows with the depth $d$ rather than the size of the circuit for $f$. We prove the security of our construction based on the intractability of the learning with error (LWE) problem. More generally, we show how to construct a functional encryption scheme

from \\textit{any} public-index predicate encryption scheme and fully homomorphic encryption scheme.

Previously, the only known constructions of functional encryption were either for specific inner product predicates, or for a weak form of functional encryption where the ciphertext size grows

with the size of the circuit for $f$.

We demonstrate the power of this result, by using it to construct a

\\textit{reusable circuit garbling scheme with input and circuit privacy}: an open problem that was studied extensively by the cryptographic community during the past 30 years since Yao\'s introduction of a one-time circuit garbling method in the mid 80\'s. Our scheme also leads to a new paradigm for general function obfuscation which we call token-based obfuscation. Furthermore, we show applications of our scheme to homomorphic encryption for Turing machines where the evaluation runs in input-specific time rather than worst case time, and to publicly verifiable and secret delegation.

2012-12-28

Inspired by cold boot attacks, Heninger and Shacham (Crypto 2009) initiated the study of the problem of how to recover an RSA private key from a noisy version of that key. They gave an algorithm for the case where some bits of the private key are known with certainty. Their ideas were extended by Henecka, May and Meurer (Crypto 2010) to produce an algorithm that works when all the key bits are subject to error. In this paper, we bring a coding-theoretic viewpoint to bear on the problem of noisy RSA key recovery. This viewpoint allows us to cast the previous work as part of a more general framework. In turn, this enables us to explain why the previous algorithms do not solve the motivating cold boot problem, and to design a new algorithm that does (and more). In addition, we are able to use concepts and tools from coding theory -- channel capacity, list decoding algorithms, and random coding techniques -- to derive bounds on the performance of the previous algorithms and our new algorithm.

Recently, He et al. (Computers and Mathematics with Applications, 2012, 64(6): 1914-1926) proposed a new efficient certificateless two-party authenticated key agreement protocol. They claimed their protocol was provably secure in the extended Canetti-Krawczyk (eCK) model. In this paper, we will show that their protocol is insecure. A type I adversary, who obtains one party\'s ephemeral private key, can impersonate the party to cheat the other party and compute the shared session key successfully. For overcoming this weakness, we also propose a simple countermeasure.

This paper presents some proposals of protocols for two types of schemes such as verifiable delegation of computation and remote electronic voting, based on polynomial properties. Our protocols for verifiable delegation of computation are aimed to the efficient evaluation of polynomials, working on schemes where the polynomial and/or the input are kept secret to the server. Our proposal for remote electronic voting allows the verification of vote well-formation upon reception at the voting server, with little overhead of computations for the voter.