Monetary Policy and Excessive Bank Risk Taking

If monetary policy is to aim at financial stability, how would it change? To analyze this question, this paper develops a general-form model with endogenous bank risk profiles. Policy rates affect both bank incentives to search for yield and the cost of wholesale funding. Financial stability objectives are then shown to make a monetary authority more conservative and more aggressive. Conservative as it sets higher rates on average. And aggressive because, in reaction to negative shocks, cuts are deeper but shorter-lived than otherwise. Keeping cuts short is crucial as bank risk responds primarily to stable low rates. Within the short span, cuts then must be deep to achieve standard objectives.Monetary policy;Financial stability

Monetary Policy and Excessive Bank Risk Taking

If monetary policy is to aim at financial stability, how would it change? To analyze this question, this paper develops a general-form model with endogenous bank risk profiles. Policy rates affect both bank incentives to search for yield and the cost of wholesale funding. Financial stability objectives are then shown to make a monetary authority more conservative and more aggressive. Conservative as it sets higher rates on average. And aggressive because, in reaction to negative shocks, cuts are deeper but shorter-lived than otherwise. Keeping cuts short is crucial as bank risk responds primarily to stable low rates. Within the short span, cuts then must be deep to achieve standard objectives.

More is Less: Perfectly Secure Oblivious Algorithms in the Multi-Server Setting

The problem of Oblivious RAM (ORAM) has traditionally been studied in a single-server setting, but more recently the multi-server setting has also been considered. Yet it is still unclear whether the multi-server setting has any inherent advantages, e.g., whether the multi-server setting can be used to achieve stronger security goals or provably better efficiency than is possible in the single-server case. In this work, we construct a perfectly secure 3-server ORAM scheme that outperforms the best known single-server scheme by a logarithmic factor. In the process, we also show, for the first time, that there exist specific algorithms for which multiple servers can overcome known lower bounds in the single-server setting.Comment: 36 pages, Accepted in Asiacrypt 201

Perfectly Secure Oblivious RAM with Sublinear Bandwidth Overhead

Oblivious RAM (ORAM) has established itself as a fundamental cryptographic building block. Understanding which bandwidth overheads are possible under which assumptions has been the topic of a vast amount of previous works. In this work, we focus on perfectly secure ORAM and we present the first construction with sublinear bandwidth overhead in the worst-case. All prior constructions with perfect security require linear communication overhead in the worst-case and only achieve sublinear bandwidth overheads in the amortized sense. We present a fundamentally new approach for construction ORAM and our results significantly advance our understanding of what is possible with perfect security. Our main construction, Lookahead ORAM, is perfectly secure, has a worst-case bandwidth overhead of $\mathcal{O}(\sqrt{n})$, and a total storage cost of $\mathcal{O}(n)$ on the server-side, where $n$ is the maximum number of stored data elements. In terms of concrete server-side storage costs, our construction has the smallest storage overhead among all perfectly and statistically secure ORAMs and is only a factor 3 worse than the most storage efficient computationally secure ORAM. Assuming a client-side position map, our construction is the first, among all ORAMs with worst-case sublinear overhead, that allows for a $\mathcal{O}(1)$ online bandwidth overhead without server-side computation. Along the way, we construct a conceptually extremely simple statistically secure ORAM with a worst-case bandwidth overhead of $\mathcal{O}(\sqrt{n}\frac{\log{n}}{\log{\log{n}}})$, which may be of independent interest

Lower Bounds for Encrypted Multi-Maps and Searchable Encryption in the Leakage Cell Probe Model

Encrypted multi-maps (EMMs) enable clients to outsource the storage of a multi-map to a potentially untrusted server while maintaining the ability to perform operations in a privacy-preserving manner. EMMs are an important primitive as they are an integral building block for many practical applications such as searchable encryption and encrypted databases. In this work, we formally examine the tradeoffs between privacy and efficiency for EMMs. Currently, all known dynamic EMMs with constant overhead reveal if two operations are performed on the same key or not that we denote as the $\mathit{global\ key\text{-}equality\ pattern}$. In our main result, we present strong evidence that the leakage of the global key-equality pattern is inherent for any dynamic EMM construction with $O(1)$ efficiency. In particular, we consider the slightly smaller leakage of $\mathit{decoupled\ key\text{-}equality\ pattern}$ where leakage of key-equality between update and query operations is decoupled and the adversary only learns whether two operations of the $\mathit{same\ type}$ are performed on the same key or not. We show that any EMM with at most decoupled key-equality pattern leakage incurs $\Omega(\log n)$ overhead in the $\mathit{leakage\ cell\ probe\ model}$. This is tight as there exist ORAM-based constructions of EMMs with logarithmic slowdown that leak no more than the decoupled key-equality pattern (and actually, much less). Furthermore, we present stronger lower bounds that encrypted multi-maps leaking at most the decoupled key-equality pattern but are able to perform one of either the update or query operations in the plaintext still require $\Omega(\log n)$ overhead. Finally, we extend our lower bounds to show that dynamic, $\mathit{response\text{-}hiding}$ searchable encryption schemes must also incur $\Omega(\log n)$ overhead even when one of either the document updates or searches may be performed in the plaintext

Evaluation of a proposed minimum path impotence routing policy in wireless sensor networks

Routing in wireless sensor networks is a challenging task due to the energy hole problem, which negatively affects the network's operation. Taking into account path impotence, i.e., a metric based on the transmission distance and the energy left at the nodes’ batteries, an adjustable routing policy is proposed here that allows nodes to choose different parent nodes for forwarding their data packets toward the sink node. A major difference from the conventional approaches is that path impotence is determined by the most impotent node, instead of adding up the metric along the path. Due to this difference, the proposed policy propagates path impotence values throughout the network efficiently, with reduced extra control messages (e.g., no need to continuously reconstruct routing trees). The number of messages sent by the particular policy (i.e., overhead) is analytically investigated here and, in addition, it is analytically shown that no deadlocks are possible. Simulation results are used for evaluating the proposed policy against other eight similar policies that appear in the literature. It is demonstrated that when the introduced overhead is taken into account (e.g., energy is consumed when transmitting messages similarly to data packet transmissions), the proposed policy outperforms the other policies under certain conditions, related to the size of the message compared to the size of data packets. © 201

Acquisition of competence in colonoscopy: The learning curve of trainees

Background and Study Aims: Most official training programs in colonoscopy recommend that trainees should carry out a minimum of 100 procedures, but limited data exist on the technical progress of trainees. The aim of our study was to estimate the number of supervised procedures required for obtaining competence in colonoscopy. Materials and Methods: Between 1990 and 1997 we have prospectively evaluated the performance of eight consecutive trainees in colonoscopy. The extent of intubated colon was recorded after each endoscopic procedure. Regression analysis was used to study the effect on the trainees' success rates on the number of colonoscopies which they had done. Results: Out of 2255 colonoscopies carried out over an 8-year period 1408 were suitable for evaluation, fulfilling the training criteria for diagnostic colonoscopy. Senior staff (n = 2) did 430 colonoscopies with a success rate of 91%, which was significantly reduced to 79% (344/434 colonoscopies; χ2 = 20.67, df = 1, P < 0.001), when taking over colonoscopies which trainees had failed to complete. Regression analysis of trainees' data (y = 88.97 - (2185/x), r = 0.74, F1,21 = 23.43, P < 0.001) showed success rates of 67% (95% CI, 59 to 75%) and 77% (95% CI, 66 to 88%), when 100 and 180 procedures, respectively, had been done. These figures had been attained by the end of the second and third year of training in colonoscopy. Conclusions: Our regression analysis model shows the technical progress of trainees over a 3-year period as they learned how to carry out colonoscopy. Depending on individual skill, between 100-180 procedures, done over a 2-3-year period, are required before trainees can be considered competent in colonoscopy

Detection of somatostatin receptors in human osteosarcoma

Background: The location of osteosarcoma in the metaphysis as well as the age of the patients during the most rapid tumour growth suggest that factors related to skeletal growth are involved in the pathogenesis of this tumour. In this aspect this study aims to detect somatostatin receptors in human osteosarcomas and correlate this finding with the clinical outcome of the tumour. Patients and methods: Immunohistochemical staining for the presence of somatostatin receptors as well as overall survival and disease free survival rates were retrospectively studied in twenty-nine osteosarcoma patients. Results: Four osteosarcomas with several aggressive biologic behaviour expressed somatostatin receptors. In these four young patients the event free rate was 0% and the overall survival rate was 50% at 4, 3 years. In contrast the event free survival rate of the twenty-five patients with negative somatostatin receptor status was 72% with an overall survival rate of 76% at 4,3 years. Conclusion: The present study demonstrates the existence of somatostatin receptors in human osteosarcoma. Tumours expressing somatostatin receptors seemed to be aggressive with a very low disease free and overall survival rate compared to osteosarcoma with negative receptor status. © 2008 Ioannou et al; licensee BioMed Central Ltd