Private information retrieval (PIR) allows a user to retrieve a desired message out of K possible messages from N databases without revealing the identity of the desired message. There has been significant recent progress on understanding fundamental information theoretic limits of PIR, and in particular the download cost of PIR for several variations. Majority of existing works however, assume the presence of replicated databases, each storing all the K messages. In this work, we consider the problem of PIR from storage constrained databases. Each database has a storage capacity of µKL bits, where K is the number of messages, L is the size of each message in bits, and µ ∈ [1/N, 1] is the normalized storage.In the storage constrained PIR problem, there are two key design questions: a) how to store content across each database under storage constraints; and b) construction of schemes that allow efficient PIR through storage constrained databases. The main contribution of this work is a general achievable scheme for PIR from storage constrained databases for any value of storage. In particular, for any (N, K), with normalized storage µ = t/N , where the parameter t can take integer values t ∈ {1, 2, . . . , N }, we show that our proposed PIR scheme achieves a download cost of 1The extreme case when µ = 1 (i.e., t = N ) corresponds to the setting of replicated databases with full storage. For this extremal setting, our scheme recovers the information-theoretically optimal download cost characterized by Sun and Jafar as 1For the other extreme, when µ = 1/N (i.e., t = 1), the proposed scheme achieves a download cost of K. The most interesting aspect of the result is that for intermediate values of storage, i.e., 1/N < µ < 1, the proposed scheme can strictly outperform memory-sharing between extreme values of storage.

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Over time, the use of wireless technologies has significantly increased due to bandwidth improvements, cost-effectiveness, and ease of deployment. Owing to the ease of access to the communication medium, wireless communications and technologies are inherently vulnerable to attacks. These attacks include brute force attacks such as jamming attacks and those that target the communication protocol (Wi-Fi and Bluetooth protocols). Thus, there is a need to make wireless communication resilient and secure against attacks. Existing wireless protocols and applications have attempted to address the need to improve systems security as well as privacy. They have been highly effective in addressing privacy issues, but ineffective in addressing security threats like jamming and session hijacking attacks and other types of Denial of Service Attacks. In this paper, we present an "architecture for resilient wireless communications" based on the concept of Moving Target Defense. To increase the difficulty of launching successful attacks and achieve resilient operation, we changed the runtime characteristics of wireless links, such as the modulation type, network address, packet size, and channel operating frequency. The architecture reduces the overhead resulting from changing channel configurations using two communication channels, in which one is used for communication, while the other acts as a standby channel. A prototype was built using Software Defined Radio to test the performance of the architecture. Experimental evaluations showed that the approach was resilient against jamming attacks. We also present a mathematical analysis to demonstrate the difficulty of performing a successful attack against our proposed architecture.