*06:15* [PhD][New]
Edoardo Persichetti: Improving the Efficiency of Code-Based Cryptography
Name: Edoardo Persichetti

Topic: Improving the Efficiency of Code-Based Cryptography

Category: public-key cryptography

Description: Recent public-key cryptography is largely based on number theory problems, such as factoring or computing of discrete logarithm. These systems constitute an excellent choice in many applications, and their security is well defined and understood. One of the major drawbacks, though, is that they will be vulnerable once quantum computers of an appropriate size are available. There is then a strong need for alternative systems that would resist attackers equipped with quantum technology.

\r\n\r\nOne of the most well-known systems of this kind is the McEliece cryptosystem, introduced in 1978, that is based on algebraic coding theory. There are no known vulnerabilities against quantum computers, and it has a very fast and efficient encryption procedure. However, it has also one big flaw, the size of the public key, that makes it impractical for many applications.

\r\n\r\nThe first part of this thesis is dedicated to finding a way to significantly reduce the size of the public key.\r\nLatest publications achieve very good results by using codes with particular structures, obtaining keys as small as 4,096 bits. Unfortunately, almost all of the variants presented until now have been broken or proven to be insecure against the so-called *structural attacks*, i.e. attacks that aim to exploit the hidden structure in order to recover the private key. \r\nMy work is based on Generalized Srivastava codes and represents a generalization of the Quasi-Dyadic scheme proposed by Misoczki and Barreto, with two advantages: a better flexibility, and improved resistance to all the known attacks. An efficient implementation of the above scheme is also provided, as a result of a joint work with P.-L. Cayrel and G. Hoffmann.

\r\n\r\nIn the next chapters, other important aspects of code-based cryptography are investigated. These include the study of a higher security standard, called indistinguishability under a chosen ciphertext attack, in the standard model, and th[...]

*22:17* [Pub][ePrint]
Shielding circuits with groups, by Eric Miles and Emanuele Viola
We show how to efficiently compile any given circuit C into a leakage-resistant circuit C\' such that any function on the wires of C\' that leaks information during a computation C\'(x) yields advantage in computing the product of |C\'|^{Omega(1)} elements of the alternating group A_u. In combination with new compression bounds for A_u products, also obtained here, C\' withstands leakage from virtually any class of functions against which average-case lower bounds are known. This includes communication protocols, and AC^0 circuits augmented with few arbitrary symmetric gates. If NC^1 \\neq TC^0 then then the construction resists TC^0 leakage as well. In addition, we extend the construction to the multi-query setting by relying on a simple secure hardware component.We build on Barrington\'s theorem [JCSS \'89] and on the previous leakage-resistant constructions by Ishai et al. [Crypto \'03] and Faust et al. [Eurocrypt \'10]. Our construction exploits properties of A_u beyond what is sufficient for Barrington\'s theorem.

*22:17* [Pub][ePrint]
Generalized (Identity-Based) Hash Proof System and Its Applications , by Yu Chen and Zongyang Zhang and Dongdai Lin and Zhenfu Cao
In this work, we generalize the paradigm of hash proof system (HPS) proposed by Cramer and Shoup~\\cite{CS2002}. In the central of our generalization, we lift subset membership problem to distribution distinguish problem.

Our generalized HPS clarifies and encompass all the known public-key encryption (PKE) schemes

that essentially implement the idea of hash proof system.

Moreover, besides existing smoothness property, we introduce an additional property named anonymity for HPS.

As a natural application, we consider anonymity for PKE in the presence of key-leakage,

and provide a generic construction of leakage-resilient anonymous PKE from anonymous HPS.

We then extend our generalization to the identity-based setting.

Concretely, we generalize the paradigm of identity-based hash proof system (IB-HPS)

proposed by Boneh~\\textit{et al.}~\\cite{BGH2007} and Alwen~\\textit{et al.}~\\cite{Alwen2010},

and introduce anonymity for it. As an interesting application of anonymous IB-HPS,

we consider security for public-key encryption with keyword search (PEKS) in the presence of token-leakage,

and provide a generic construction of leakage-resilient secure PEKS from leakage-resilient anonymous IBE,

which in turn is based on anonymous IB-HPS.

*16:17* [Pub][ePrint]
Defensive Leakage Camouflage, by E. Brier and Q. Fortier and R. Korkikian and K. W. Magld and D. Naccache and G. Ozari de Almeida and A. Pommellet and A. H. Ragab and J. Vuillemin
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.

*16:17* [Pub][ePrint]
On the Impossibility of Approximate Obfuscation and Applications to Resettable Cryptography, by Nir Bitansky and Omer Paneth
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.

*16:17* [Pub][ePrint]
Twisted Edwards-Form Elliptic Curve Cryptography for 8-bit AVR-based Sensor Nodes, by Dalin Chu and Johann Gro{\\ss}sch{\\\"a}dl and Zhe Liu
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 $5.9 \\cdot 10^6$ clockcycles 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.