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2013-02-20
19:17 [Pub][ePrint]

We propose a new notion of secure multiparty computation aided by a computationally-powerful but untrusted \"cloud\" server. In this notion that we call

on-the-fly multiparty computation (MPC), the cloud can non-interactively perform arbitrary, dynamically chosen computations on data belonging to arbitrary sets of users chosen on-the-fly. All user\'s input data and intermediate results are protected from snooping by the cloud as well as other users.

This extends the standard notion of fully homomorphic encryption (FHE), where users can only enlist the cloud\'s help in evaluating functions on their own encrypted data.

In on-the-fly MPC, each user is involved only when initially uploading his (encrypted) data to the cloud, and in a final output decryption phase when outputs are revealed; the complexity of both is independent of the function being computed and the total number of users in the system. When users upload their data, they need not decide in advance which function will be computed, nor who they will compute with; they need only retroactively approve the eventually-chosen functions and on whose data the functions were evaluated.

This notion is qualitatively the best possible in minimizing interaction, since the users\' interaction in the decryption stage is inevitable: we show that removing it would imply generic program obfuscation and is thus impossible.

Our contributions are two-fold:

1. We show how on-the-fly MPC can be achieved using a new type of encryption scheme that we call multikey FHE, which is capable of operating on inputs encrypted under multiple, unrelated keys. A ciphertext resulting from a multikey evaluation can be jointly decrypted using the secret keys of all the users involved in the computation.

2. We construct a multikey FHE scheme based on NTRU, a very efficient public-key encryption scheme proposed in the 1990s. It was previously not known how to make NTRU fully homomorphic even for a single party. We view the construction of (multikey) FHE from NTRU encryption as a main contribution of independent interest. Although the transformation to a fully homomorphic system deteriorates the efficiency of NTRU somewhat, we believe that this system is a leading candidate for a practical FHE scheme.

18:44 [Job][New]

We invite expressions of interest for the position of Canada Excellence Research Chair (CERC) in Security and Privacy for the New Digital Economy, to be held at the tenured full or associate professor level in the David R. Cheriton School of Computer Science at the University of Waterloo.

The CERC program awards world-class researchers up to $10 million over seven years to establish ambitious research programs at Canadian universities. Further details are offered at www.cerc.gc.ca. An overall package worth more than twice this amount will fund the CERC, additional faculty and staff, and their required infrastructure. The applicant will be an outstanding researcher, well-recognized as exceptional within the subfield of security and privacy. It will also be essential for the candidate to demonstrate remarkable promise in leadership and in the mobilization of talents to deliver successful outcomes. In particular, we are looking for an individual who is expert in security solutions for networked and mobile environments with a critical appreciation for linking privacy to the required solutions. To promote the adoption of novel technological solutions, the CERC must also have an aptitude in working well with public policy experts. To apply, send a cover letter and a curriculum vitae by e-mail at deanmath (at) uwaterloo.ca or by regular mail. Applications received by May 30, 2013 will receive full consideration. Selection of the candidate is subject to final oversight by the government\\\'s CERC Selection Committee. The University of Waterloo encourages applications from all qualified individuals, including women, members of visible minorities, native people and persons with disabilities. We are proud to offer organizations for Women in Computer Science (cs.uwaterloo.ca/~wics) and Women in Mathematics (women.math.uwaterloo.ca) as well as an AccessAbility Services Office for persons with disabiliti 13:17 [Pub][ePrint] Software attestation has become a popular and challenging research topic at many established security conferences. It aims for verifying the software integrity of (typically) resource-constrained embedded devices. However, for practical reasons, software attestation cannot rely on stored cryptographic secrets or dedicated trusted hardware. Instead, it exploits side-channel information, such as the time that the underlying device needs for a specific computation. Unfortunately, traditional cryptographic solutions and arguments are not applicable in this setting, making new approaches for the design and analysis necessary. This is certainly one of the main reasons why the security properties and assumptions of software attestation have been only vaguely discussed and have never been formally treated, as it is common sense in modern cryptography. Thus, despite its popularity and its expected impact for practice, a sound approach for designing secure software attestation schemes is still an important open problem. We introduce the first formal security framework for software attestation and formalize various system and design parameters. Moreover, we present a generic software attestation scheme that captures most existing schemes in the literature. Finally, we analyze its security within our framework, yielding sufficient conditions for provably secure software attestation schemes. We regard these results as a first step towards putting software attestation on a solid ground and as a starting point for further research. 13:17 [Pub][ePrint] The concept of quantum-secure readout of Physical Unclonable Functions (PUFs) has recently been realized experimentally in an optical PUF system. We analyze the security of this system under the strongest type of classical attack: the challenge estimation attack. The adversary performs a measurement on the challenge quantum state in order to learn as much about it as he can. Using this knowledge he then tries to reconstruct the challenge and to emulate the PUF. We consider quadrature measurements, which are the most informative practical measurements known to us. We prove that even under this attack the expected number of photons detected in the verification mechanism is approximately a factor$S+1$too low; here$S$is the Quantum Security Parameter, defined as the number of modes in the optical system divided by the number of photons in the challenge. The photon count allows for a reliable distinction between an authentic PUF and a challenge estimation attack. 13:17 [Pub][ePrint] We present a computationally secure MPC protocol for threshold adversaries which is parametrized by a value L. When L=2 we obtain a classical form of MPC protocol in which interaction is required for multiplications, as L increases interaction is reduced in that one requires interaction only after computing a higher degree function. When L approaches infinity one obtains the FHE based protocol of Gentry, which requires no interaction. Thus one can trade communication for computation in a simple way. Our protocol is based on an interactive protocol for bootstrapping\'\' a somewhat homomorphic encryption scheme. The key contribution is that our presented protocol is highly communication efficient enabling us to obtain reduced communication when compared to traditional MPC protocols for relatively small values of L. 13:17 [Pub][ePrint] Recent research results on \"bucketed\" ORAM reduce com- munication of N-capacity storage with blocks of length l bits down to poly-logarithmic complexity O(l · log^3 N ). The individual buckets, how- ever, are constructed using traditional ORAMs which have worst-case communication complexity being linear in their size. PIR protocols are able to provide better worst-case bounds, but have traditionally been less practical than ORAM due to the fact that they require O(N) computa- tion complexity on the server. This paper presents Path-PIR, a hybrid construction between PIR and ORAM that overcomes the individual weaknesses of each. Path-PIR\'s main idea is to replace the individual buckets in the ORAM construction by Shi et al. [15] with buckets backed by PIR. We show that this leads to substantially smaller data transfer costs for many databases of practical size and lower worst-case costs, O(l · log^2 (N) + log^3 (N)), than the existing construction. Additionally, the typically high computational cost of PIR is offset by the small size of the individual buckets. We also show that Path-PIR has very low latency, i.e., a low amount of data is required before a user receives the result of his data request (less than 10 times the block size). Using Amazon EC2, we demonstrate that monetary cost induced by the server\'s PIR computation are far outweighed by the savings in data transfer. 13:17 [Pub][ePrint] We present a square root algorithm in F_q which generalizes Atkins\'s square root algorithm for q=5(mod 8) and Kong et al.\'s algorithm for q=9(mod 16) Our algorithm precomputes a primitive 2^s-th root of unity where s is the largest positive integer satisfying 2^s| q-1, and is applicable for the cases when s is small. The proposed algorithm requires one exponentiation for square root computation and is favorably compared with the algorithms of Atkin, Muller and Kong et al. 10:17 [Pub][ePrint] Radio Frequency IDentification (RFID) technology is a wireless identification method in which security and privacy are important parameters for public acceptance and widespread use. In order to thwart such security and privacy problems, a wide variety of authentication protocols have been proposed in the literature. In 2010, Yeh et al\'s proposed a new RFID authentication protocol conforming to EPC Class 1 Generation 2 standard. They claimed that this protocol is secure against DoS attack, replay attack, DATA forgery attack, and provides untraceability and forward secrecy. In 2012, Yoon showed that this protocol does not provide forward secrecy and DATA integrity. He improved the protocol and tried to eliminate the weaknesses and claimd that the improved protocol does not have the weaknesses of the primary protocol. In this paper, we show that the improved protocol has some weaknesses including DoS attack, back-end server impersonation, tag impersonation and DATA forgery attack. We also show that it can not provide forward secrecy of the reader and untraceability. We improve the protocol, which offers a high level of security and provides mutual authentication, untraceability and forward secrecy as well as resistance to DATA forgery, replay and DoS attacks, while retaining a competitive communication cost. 10:17 [Pub][ePrint] We analyze the security of three-share hardware implementations against differential power analysis and advanced variants such as mutual information analysis. We present dedicated distinguishers that allow to recover secret key bits from any cryptographic primitive that is implemented as a sequence of quadratic functions. Starting from the analytical treatment of such distinguishers and information-theoretic arguments, we derive the success probability and required number of traces in the presence of algorithmic noise. We show that attacks on three-share hardware implementation require a number of traces that scales in the third power of the algorithmic noise variance. Finally, we apply and test our model on Keccak in a keyed mode. 10:17 [Pub][ePrint] Proving security of Hierarchical Identity-Based Encryption (HIBE) and Attribution Based Encryption scheme is a challenging problem. There are multiple well-known schemes in the literature where the best known (adaptive) security proofs degrade exponentially in the maximum hierarchy depth. However, we do not have a rigorous understanding of why better proofs are not known. (For ABE, the analog of hierarchy depth is the maximum number of attributes used in a ciphertext.) In this work, we define a certain commonly found checkability property on ciphertexts and private keys. Roughly the property states that any two different private keys that are both supposed to\'\' decrypt a ciphertext will decrypt it to the same message. We show that any simple black box reduction to a non-interactive assumption for a HIBE or ABE system that contains this property will suffer an exponential degradation of security. 10:17 [Pub][ePrint] The Short Integer Solution (SIS) and Learning With Errors (LWE) problems are the foundations for countless applications in lattice-based cryptography, and are provably as hard as approximate lattice problems in the worst case. A important question from both a practical and theoretical perspective is how small their parameters can be made, while preserving their hardness. We prove two main results on SIS and LWE with small parameters. For SIS, we show that the problem retains its hardness for moduli$q \\geq \\beta \\cdot n^{\\delta}$for any constant$\\delta > 0$, where$\\beta$is the bound on the Euclidean norm of the solution. This improves upon prior results which required$q \\geq \\beta \\cdot \\sqrt{n \\log n}$, and is essentially optimal since the problem is trivially easy for$q \\leq \\beta$. For LWE, we show that it remains hard even when the errors are small (e.g., uniformly random from$\\set{0,1}$), provided that the number of samples is small enough (e.g., linear in the dimension$n$of the LWE secret). Prior results required the errors to have magnitude at least$\\sqrt{n}\$ and to come from a Gaussian-like distribution.