Obfuscation for Cryptographic Purposes

Loosely speaking, an obfuscation O of a function f should satisfy two requirements: firstly, using O, it should be possible to evaluate f; secondly, O should not reveal anything about f that cannot be learnt from oracle access to f alone. Several definitions for obfuscation exist. However, most of them are very hard to satisfy, even when focusing on specific applications such as obfuscating a point function (e.g., for authentication purposes). In this work, we propose and investigate two new variants of obfuscation definitions. Our definitions are simulation-based (i.e., require the existence of a simulator that can efficiently generate fake obfuscations) and demand only security on average (over the choice of the obfuscated function). We stress that our notions are not free from generic impossibilities: there exist natural classes of function families that cannot be securely obfuscated. Hence we cannot hope for a general-purpose obfuscator with respect to our definition. However, we prove that there also exist several natural classes of functions for which our definitions yield interesting results.     

Physicochemical effects of varying fuel composition on knock characteristics of natural gas mixtures

The physicochemical origins of how changes in fuel composition affect autoignition of the end gas, leading to engine knock, are analyzed for a natural gas engine. Experiments in a lean-burn, high-speed medium-BMEP gas engine are performed using a reference natural gas with systematically varied fractions of admixed ethane, propane and hydrogen. Thermodynamic analysis of the measured non-knocking pressure histories shows that, in addition to the expected changes arising from changes in the heat capacity of the mixture, changes in the combustion duration relative to the compression cycle (the combustion “phasing”) caused by variations in burning velocity dominate the effects of fuel composition on the temperature (and pressure) of the end gas. Thus, despite the increase in the heat capacity of the fuel–air mixture with addition of ethane and propane, the change in combustion phasing is actually seen to increase the maximum end-gas temperature slightly for these fuel components. By the same token, the substantial change in combustion duration upon hydrogen addition strongly increases the end-gas temperature, beyond that caused by the decrease in mixture heat capacity. The impact of these variations in in-cylinder conditions on the knock tendency of the fuel have been assessed using autoignition delay times computed using SENKIN and a detailed chemical mechanism for the end gas under the conditions extant in the engine. The results show that the ignition-promoting effect of hydrogen is mainly the result of the increase in end-gas temperature and pressure, while addition of ethane and propane promotes ignition primarily by changing the chemical autoignition behavior of the fuel itself. Comparison of the computed end-gas autoignition delay time, based on the complete measured pressure history of each gas, with the measured Knock-Limited Spark Timing shows that the computed delay time accurately reflects the measured knock tendency of the fuels.     

A flexible natural coordinates formulation (FNCF) for the efficient simulation of small‐deformation multibody systems

This paper introduces the novel flexible natural coordinates formulation to model small‐deformation multibody dynamics. The main contribution of this work is its resulting constant mass matrix and quadratic constraint equations devoid of any other nonlinearities. These properties are similar to those of a natural coordinates formulation for rigid multibody systems with the addition of constant damping and stiffness matrices to model the flexibility under the assumption of small deformations. As such, it is a straightforward extension to natural coordinates while maintaining its beneficial properties. The main concept of the current approach is to introduce ample redundancy in the set of generalized coordinates to simplify the kinematics ensuring the aforementioned properties and the similarity to a natural coordinates approach. This is not achievable by standard techniques that introduce redundancy. Not only does this offer a very simple equation structure but also interesting properties toward the development of system‐level model order reduction techniques for flexible multibody systems as well as a straightforward parameter gradient extraction. The formulation accuracy is validated with a floating frame of reference implementation.     

A Microwell-Based Intestinal Organoid-Macrophage Co-Culture System to Study Intestinal Inflammation

The mammalian intestinal epithelium contains more immune cells than any other tissue, and this is largely because of its constant exposure to pathogens. Macrophages are crucial for maintaining intestinal homeostasis, but they also play a central role in chronic pathologies of the digestive system. We developed a versatile microwell-based intestinal organoid-macrophage co-culture system that enables us to recapitulate features of intestinal inflammation. This microwell-based platform facilitates the controlled positioning of cells in different configurations, continuous in situ monitoring of cell interactions, and high-throughput downstream applications. Using this novel system, we compared the inflammatory response when intestinal organoids were co-cultured with macrophages versus when intestinal organoids were treated with the pro-inflammatory cytokine TNF-α. Furthermore, we demonstrated that the tissue-specific response differs according to the physical distance between the organoids and the macrophages and that the intestinal organoids show an immunomodulatory competence. Our novel microwell-based intestinal organoid model incorporating acellular and cellular components of the immune system can pave the way to unravel unknown mechanisms related to intestinal homeostasis and disorders.