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2014-02-04
19:17 [Pub][ePrint]

We present a novel mechanism that allows a client

to securely outsource his private data to the cloud while at the same

time to delegate to a third party the right to run

certain algorithms on his data. The mechanism is

privacy-preserving, meaning that the third party only learns the result

of his algorithm on the client\'s data, while at the same time the access

pattern on the client\'s data is hidden from the cloud. To achieve this we

combine recent advances in the field of Oblivious RAM and Secure Two-Party

Computation: We develop an Oblivious RAM which is ran between the cloud and a

proxy server, and which does not need the data to be decrypted at any

point. The evaluation on the data is done by employing Yao\'s garbled

circuit solution for Secure Two-Party Computation.

19:17 [Pub][ePrint]

Anonymity and authenticity are both important yet often conflicting security goals in a wide range of applications. On the one hand for many applications (say for access control) it is crucial to be able to verify the identity of a given legitimate party (a.k.a. entity authentication). Alternatively an application might require that no one but a party can communicate on its behalf (a.k.a. message authentication). Yet, on the other hand privacy concerns also dictate that anonymity of a legitimate party should be preserved; that is no information concerning the identity of parties should be leaked to an outside entity eavesdropping on the communication. This conflict becomes even more acute when considering anonymity with respect to an active entity that may attempt to impersonate other parties in the system.

In this work we resolve this conflict in two steps. First we formalize what it means for a system to provide both authenticity and anonymity even in the presence of an active man-in-the-middle adversary for various specific applications such as message and entity authentication using the constructive cryptography framework of~\\cite{Mau11}. Our approach inherits the composability statement of constructive cryptography and can therefore be directly used in any higher-level context. Next we demonstrate several simple protocols for realizing these systems, at times relying on a new type of (probabilistic) Message Authentication Code (MAC) called \\emph{key indistinguishable} (KI) MACs. Similar to the key hiding encryption schemes of~\\cite{BellareBDP01} they guarantee that tags leak no discernible information about the keys used to generate them.

19:17 [Pub][ePrint]

A \\emph{key-homomorphic} pseudorandom function (PRF) family

$\\set{F_{s} \\colon D \\to R}$ allows one to efficiently compute the

value $F_{s+t}(x)$ given $F_{s}(x)$ and $F_{t}(x)$. Such functions

have many applications, such as distributing the operation of a

key-distribution center and updatable symmetric encryption. The only

known construction of key-homomorphic PRFs without random oracles, due

to Boneh \\etal (CRYPTO~2013), is based on the learning with errors

(\\lwe) problem and hence on worst-case lattice problems. However, the

security proof relies on a very strong \\lwe assumption (i.e., very

large approximation factors), and hence has quite inefficient

parameter sizes and runtimes.

In this work we give new constructions of key-homomorphic PRFs that

are based on much weaker \\lwe assumptions, are much more efficient in

time and space, and are still highly parallel. More specifically, we

improve the \\lwe approximation factor from exponential in the input

length to exponential in its \\emph{logarithm} (or less). For input

length~$\\lambda$ and~$2^{\\lambda}$ security against known lattice

algorithms, we improve the key size from~$\\lambda^{3}$ to~$\\lambda$

bits, the public parameters from~$\\lambda^{6}$ to~$\\lambda^{2}$ bits,

and the runtime from~$\\lambda^{7}$ to~$\\lambda^{\\omega+1}$ bit

operations (ignoring polylogarithmic factors in~$\\lambda$), where

$\\omega \\in [2,2.373]$ is the exponent of matrix multiplication. In

addition, we give even more efficient ring-\\lwe-based constructions

whose key sizes, public parameters, and \\emph{incremental} runtimes on

consecutive inputs are all \\emph{quasi-linear}~$\\Otil(\\lambda)$, which

is optimal up to polylogarithmic factors. To our knowledge, these are

the first \\emph{low-depth} PRFs (whether key homomorphic or not)

enjoying any of these efficiency measures together with nontrivial

proofs of~$2^{\\lambda}$ security under any conventional assumption.

19:17 [Pub][ePrint]

In the last few years the efficiency of secure multi-party computation (MPC) increased in several orders of magnitudes. However, this alone might not be enough if we want MPC protocols to be used in practice.

A crucial property that is needed in many applications is that everyone can check that a given (secure) computation was performed correctly -- even in the extreme case where all the parties involved in the computation are corrupted, and even if the party who wants to verify the result was not involved. An obvious example of this is electronic voting, but also in many types of auctions one may want independent verification of the result. Traditionally, this is achieved by using non-interactive zero-knowledge proofs.

A recent trend in MPC protocols is to have a more expensive preprocessing phase followed by a very efficient online phase, e.g., the recent so-called SPDZ protocol by Damgård et al. Applications such as voting and some auctions are perfect applications for these protocols, as the parties usually know well in advance when the computation will take place, and using those protocols allows us to use only cheap information theoretic primitives in the actual computation. Unfortunately no protocol of the SPDZ type supports an audit phase.

In this paper we formalize the concept of publicly auditable secure computation and provide an enhanced version of the SPDZ protocol where, even if all the servers are corrupted, anyone with access to the transcript of the protocol can check that the output is indeed correct. Most importantly, we do so without compromising the performance of SPDZ i.e., the cost of our online phase is the same as that of SPDZ, up to a small constant factor of about two.

19:17 [Pub][ePrint]

Bitcoin is a peer-to-peer (p2p) electronic cash system that

uses a distributed timestamp service to record transactions in a public ledger (called the Blockchain). A critical component of Bitcoin\'s success is the decentralized nature of its architecture, which does not require or even support the establishment of trusted authorities. Yet the absence of certification creates obstacles to its wider acceptance in e-commerce and official uses. We propose a certification system for Bitcoin that offers: a) an opt-in guarantee to send and receive bitcoins only to/ from certified users; b) control of creation of bitcoins addresses (certified users) by trusted authorities.

Our proposal may encourage the adoption of Bitcoin in different scenarios that require an officially recognized currency, such

as tax payments--often an integral part of e-commerce transactions.

19:17 [Pub][ePrint]

We propose Mixcoin, a protocol to facilitate anonymous payments using the Bitcoin currency system. We build on the emergent phenomenon of currency mixes, adding an accountability mechanism to expose theft. Unlike other proposals to improve anonymity in Bitcoin, our scheme can be deployed immediately with no changes to Bitcoin itself. We demonstrate that incentives of mixes and clients can be aligned to ensure that rational mixes will not steal from clients. We contrast mixing for financial anonymity with better-studied communication mixes, demonstrating important and subtle new attacks.

19:17 [Pub][ePrint]

Most lattice-based cryptographic schemes which enjoy a security proof suffer from huge key sizes and heavy computations. This is also true for the simpler case of identification protocols. Recent progress on ideal lattices has significantly improved the efficiency, and made it possible to implement practical lattice-based cryptography on constrained devices like FPGAs and smart phones. However, to the best of our knowledge, no previous attempts were made to implement lattice-based schemes on smart cards. In this paper, we report the results of our implementation of several state-of-the-art and highly-secure lattice-based identification protocols on smart cards and microcontrollers. Our results show that only a few of such protocols fit into the limitations of these devices. We also discuss the implementation challenges and techniques to perform lattice-based cryptography on constrained devices, which may be of independent interest.

19:17 [Pub][ePrint]

A recent trend in cryptography is to formally show the leakage resilience of cryptographic implementations in a given leakage model. One of the most prominent leakage models -- the so-called bounded leakage model -- assumes that the amount of leakage is a-priori bounded. Unfortunately, it has been pointed out that the assumption of bounded leakages is hard to verify in practice. A more realistic assumption is to assume that leakages are sufficiently noisy, following the engineering observation that real-world physical leakages are inherently noisy. While the noisy leakage assumption has first been studied in the seminal work of Chari et al. (CRYPTO 99), the recent work of Prouff and Rivain (Eurocrypt 2013) provides the first analysis of a full masking scheme under a physically motivated noise model. In particular, the authors show that a block-cipher implementation that uses an additive masking scheme is secure against noisy leakages. Unfortunately, the security analysis of Prouff and Rivain has three important shortcomings: (1) it requires leak-free gates, (2) it considers a restricted adversarial model (random message attacks), and (3) the security proof has limited application for cryptographic settings. In this work, we provide an alternative security proof in the same noisy model that overcomes these three challenges. We achieve this goal by a new reduction from noisy leakage to the important theoretical model of probing adversaries (Ishai et al~ -- CRYPTO 2003). Our work can be viewed as a next step of closing the gap between theory and practice in leakage resilient cryptography: while our security proofs heavily rely on concepts of theoretical cryptography, we solve problems in practically motivated leakage models.

16:17 [Pub][ePrint]

The article proposes one-pass authenticated key establishment protocol

in random oracles for Wireless Sensor Networks. Security of the protocol relies on Computational Diffie-Hellman Problem on Bilinear Pairings. In one-pass key establishment protocol, the initiator computes a session key and a related message. The key token is to be sent to the intended receiver using receiver\'s public key and sender

secret key. From the received key token the receiver compute the session key, which is the same as the one computed by the sender, using sender public key and receiver\'s secret key. Because of low communication overhead, the scheme is better suited for Wireless Sensor Networks(WSNs) than the traditional key establishment protocol to establish the session key between two adjacent nodes.

16:17 [Pub][ePrint]

In recent years, \\emph{lattice-based} cryptography has been recognized

for its many attractive properties, such as strong provable security

guarantees and apparent resistance to quantum attacks, flexibility for

realizing powerful tools like fully homomorphic encryption, and high

asymptotic efficiency. Indeed, several works have demonstrated that

for basic tasks like encryption and authentication, lattice-based

primitives can have performance competitive with (or even surpassing)

those based on classical mechanisms like RSA or Diffie-Hellman.

However, there still has been relatively little work on developing

lattice cryptography for deployment in \\emph{real-world} cryptosystems

and protocols.

In this work we take a step toward that goal, by giving efficient

and practical lattice-based protocols for key transport, encryption,

and authenticated key exchange that are suitable as drop-in\'\'

components for proposed Internet standards and other open protocols.

The security of all our proposals is provably based (sometimes in the

random-oracle model) on the well-studied learning with errors over

rings\'\' problem, and hence on the conjectured worst-case hardness of

problems on ideal lattices (against quantum algorithms).

One of our main technical innovations (which may be of independent

interest) is a simple, low-bandwidth \\emph{reconciliation} technique

that allows two parties who approximately agree\'\' on a secret value

to reach \\emph{exact} agreement, a setting common to essentially all

lattice-based encryption schemes. Our technique reduces the

ciphertext length of prior (already compact) encryption schemes nearly

twofold, at essentially no cost.% in security, key size, or runtime.

16:17 [Pub][ePrint]

In the last decade, pairing-based cryptography has been the most intensively studied subject in the cryptography field. Various optimization techniques have been developed to speed up the pairing computation. However, implementing a pairing-based cryptosystem in resource constrained devices has been less tried. Moreover, due to progress on solving the discrete logarithm problem, those implementations are no longer safe to use. In this paper, we report an implementation of a couple of pairing-based cryptosystems at a high security level on a 32-bit microcontroller in a USB token. It shows that USB tokens supporting secure pairing-based cryptosystems are viable.