In addition to storage and computing power, cloud providers ensure confidentiality of user data through the use of various encryption technologies. The need to decrypt the data before it can be operated upon exposes a possible security hole which could be exploited by untrustworthy system administrators. Homomorphic encryption allows operations on encrypted data without the need to first decrypt it making it attractive for cloud computing. However, it incurs significant overhead of storage and computation and is therefore infeasible in practice. Somewhat homomorphic schemes have been proposed to handle specific applications - one such scheme, the Zhou Wornell Scheme, operates on vectors of integers. We demonstrate that this scheme is vulnerable to plaintext recovery attacks for a range of vector sizes. We explore the trade-offs between plaintext vector length, public key size and security. Increasing vector length increases security but at the cost of greatly increased public key size. We suggest a way of reducing the size of the public key by up to 90%. Finally, we propose a variant of this scheme which is secure against plaintext recovery attacks.

Leakage of information between two processes sharing the same processor cache has been exploited in many novel approaches targeting various cryptographic algorithms. The software implementation of AES is an specially attractive target since it makes extensive use of cache-resident table lookups. We consider two attack scenarios where either the plaintext or ciphertext is known. We employ a multi-threaded spy process and ensure that each time slice provided to the victim (running AES) is small enough so that it makes a very limited number of table accesses. We design and implement a suite of algorithms to deduce the 128-bit AES key using as input the set of (unordered) cache line numbers captured by the spy threads in an access-driven cache-based side channel attack. Our algorithms are expressed using simple relational algebraic operations and run in under a minute. Above all, our attack is highly efficient -- we demonstrate recovery of the full AES key given only about 6 -- 7 blocks of plaintext or ciphertext (theoretically even a single block would suffice). This is a substantial improvement over previous cache-based side channel attacks that require between 100 and a million encryptions. Moreover, our attack supports varying cache hit/miss observation granularities, does not need frequent interruptions of the victim and will work even if the victim makes up to 60 cache accesses before being interrupted. Finally, we develop analytic models to estimate the number of encryptions/decryptions required as a function of access granularity and compare model results with those obtained from our experiments.

International audience

Elliptic curve scalar multiplication ([k]P where k is an integer and P is a point on the elliptic curve) is widely used in encryption and signature generation. In this paper, we explore a factorization-based approach called Near-Factorization that can be used in conjunction with existing optimization techniques such as Window NAF (Non Adjacent Form). We present a performance model of Near-Factorization and validate model results with those from a simulation. We compare Near-Factorization with wNAF for a range of scalar sizes, window sizes, divisor lengths and Hamming weights of divisor. The use of Near-Factorization with wNAF results in a considerable reduction in the effective Hamming weight of the scalar and a reduction in overall computation cost for Koblitz curves.

Times series techniques have been extensively used for Sales forecasting. Research has established that a combination forecast works better than a single forecast. Our research attempts to design an Intelligent Forecasting Engine which will use a combination forecasting technique. This design is based on use of Genetic Algorithms, for selecting the best methods to combine for forecasting. Early results demonstrate that Genetic Algorithms have the potential to become a powerful tool for time series sales forecasting.

Dynamic self-organizing groups like wikipedia, and f/oss have special security requirements not addressed by typical access control mechanisms. An example is the ability to collaboratively modify access control policies based on the evolution of the group and trust and behavior levels. In this paper we propose a new framework for dynamic multi-level access control policies based on trust and reputation. The framework has interesting features wherein the group can switch between policies over time, influenced by the system's state or environment. Based on the behavior and trust level of peers in the group and the current group composition, it is possible for peers to collaboratively modify policies such as join, update and job allocation. We have modeled the framework using the declarative language Prolog. We also performed some simulations to illustrate the features of our framework.

Wormhole routing is an attractive routing technique offering low latency communication without the need to buffer an entire packet in a single node. A new queueing-theoretic model for obtaining throughput and latency of binary hypercubes supporting wormhole routing is developed here. The model is very accurate in predicting the performance of an actual multicomputer over a range of network sizes, packet lengths, and input port priority mappings. Utilizing the model, the performance of networks with identical topologies but different node architectures is estimated.>

The reliability resulting from two forms of redundancy, spatial and temporal, in multistage interconnection networks is examined. The extra-stage shuffle-exchange network (SEN+) which is an example of the former is investigated here. The SEN+ is decomposed into two subnetworks connected by two extreme stages. Given k random faults in one subnetwork, our problem reduces to estimating the maximum and minimum number of switches in the other subnetwork that must be operational for full access, i.e., connection between every input (processor) and output (memory) pair. This investigation results in obtaining analytical estimates for the upper and lower bounds on its reliability which are a substantial improvement over existing ones. We next examine the effect of wrap-around connections from each output node to its corresponding input node. This may provide a path between a source-destination pair under faults by diverting a given packet through one or more intermediate destinations. The reliability offered by this scheme that provides temporal redundancy is also estimated and compared with the scheme employing hardware redundancy.>

A tree-based interconnection architecture (called KYKLOS) for multicomputer systems is proposed. While the general form of the topology consists of multiple m-ary trees sharing a common set of leaf nodes, the focus is on the dual-tree case. One version of the dual-tree KYKLOS involves a bottom tree where the ordering of descendants of nodes at every level is an m-way shuffle. This architecture provides fault tolerance and vastly improved properties over the simple double tree while retaining the simplicity and low fan-out of the m-ary tree. Different routing strategies are presented and network properties as a function of routing strategy are investigated. The O(N/sup 2/) congestion at the root of the m-ary tree is reduced to O(N/sup 1.5/) while average normalized communication latencies are decreased. Applications of this architecture to facilitate parallel input/output (I/O) access and parallel processing of database operations such as the relational join are studied.>