FairBlock: Preventing Blockchain Front-running with Minimal Overheads

While blockchain systems are quickly gaining popularity, front-running remains a major obstacle to fair exchange. Front-running is a family of strategies in which a malicious party manipulates the order of transactions such that a transaction tx_2 which is broadcasted in time t_2 executes before the transaction of victim tx_1 which is broadcasted earlier in time t_1 (t_1 < t_2). In this thesis, we show how to apply Identity-Based Encryption (IBE) to prevent front-running with minimal bandwidth overheads. In our approach, to decrypt a block of N transactions, the number of messages sent across the network only grows linearly with the size of decrypting committees, S. That is, to decrypt a set of N transactions sequenced at a specific block, a committee only needs to exchange S decryption shares (independent of N). In comparison, previous solutions are based on threshold decryption schemes, where each transaction in a block must be decrypted separately by the committee, resulting in bandwidth overhead of N*S. Along the way, we present a model for fair block processing, explore technical challenges, and build prototype implementations. We show that on a sample of 1000 messages with 1000 validators our work saves 42.53 MB of bandwidth which is 99.6% less compared with the standard threshold decryption paradigm

High-Power Ion Thruster Technology

Performance data are presented for the NASA/Hughes 30-cm-diam 'common' thruster operated over the power range from 600 W to 4.6 kW. At the 4.6-kW power level, the thruster produces 172 mN of thrust at a specific impulse of just under 4000 s. Xenon pressure and temperature measurements are presented for a 6.4-mm-diam hollow cathode operated at emission currents ranging from 5 to 30 A and flow rates of 4 sccm and 8 sccm. Highly reproducible results show that the cathode temperature is a linear function of emission current, ranging from approx. 1000 C to 1150 C over this same current range. Laser-induced fluorescence (LIF) measurements obtained from a 30-cm-diam thruster are presented, suggesting that LIF could be a valuable diagnostic for real-time assessment of accelerator-arid erosion. Calibration results of laminar-thin-film (LTF) erosion badges with bulk molybdenum are presented for 300-eV xenon, krypton, and argon sputtering ions. Facility-pressure effects on the charge-exchange ion current collected by 8-cm-diam and 30-cm-diam thrusters operated on xenon propellant are presented to show that accel current is nearly independent of facility pressure at low pressures, but increases rapidly under high-background-pressure conditions

Characterization of the Near Plume Region of Hexaboride and Barium Oxide Hollow Cathodes operating on Xenon and Iodine

The use of electric propulsion for spacecraft primary propulsion, attitude control and station-keeping is ever-increasing as the technology matures and is qualified for flight. In addition, alternative propellants are under investigation, which have the potential to offer systems-level benefits that can enable particular classes of missions. Condensable propellants, particularly iodine, have the potential to significantly reduce the propellant storage system volume and mass. Some of the most widely used electric thrusters are electrostatic thrusters, which require a thermionic hollow cathode electron source to ionize the propellant for the main discharge and for beam neutralization. Failure of the hollow cathode, which often needs to operate for thousands of hours, is one of the main life-limiting factors of an electrostatic propulsion system. Common failure modes for hollow cathodes include poisoning or evaporation of the thermionic emitter material and erosion of electrodes due to sputtering. The mechanism responsible for the high energy ion production resulting in sputtering is not well understood, nor is the compatibility of traditional thermionic hollow cathodes with alternative propellants such as iodine. This work uses both an emissive probe and Langmuir probe to characterize the near-plume of several hollow cathodes operating on both xenon and iodine by measuring the plasma potential, plasma density, electron temperature and electron energy distribution function (EEDF). Using the EEDF the reaction rate coefficients for relevant collisional processes are calculated. A low current (\u3c 5 A discharge current) hollow cathode with two different hexaboride emitters, lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6), was operated on xenon propellant. The plasma potential, plasma density, electron temperature, EEDF and reaction rate coefficients were measured for both hexaboride emitter materials at a single cathode orifice diameter. The time-resolved plasma potential measurements showed low frequency oscillations (\u3c100 kHz) of the plasma potential at low cathode flow rates (\u3c4 SCCM) and spot mode operation between approximately 5 SCCM and 7 SCCM. The CeB6 and LaB6 emitters behave similarly in terms of discharge power (keeper and anode voltage) and plasma potential, based on results from a cathode with a 0.020�-diameter. Both emitters show almost identical operating conditions corresponding to the spot mode regime, reaction rates, as well as mean and RMS plasma potentials for the 0.020� orifice diameter at a flow rate of 6 SCCM and the same discharge current. The near-keeper region plasma was also characterized for several cathode orifice diameters using the CeB6 emitter over a range of propellant flow rates. The spot-plume mode transition appears to occur at lower flow rates as orifice size is increased, but has a minimum flow rate for stable operation. For two orifice diameters, the EEDF was measured in the near-plume region and reaction rate coefficients calculated for several electron- driven collisional processes. For the cathode with the larger orifice diameter (0.040�), the EEDFs show higher electron temperatures and drift velocities. The data for these cathodes also show lower reaction rate coefficients for specific electron transitions and ionization. To investigate the compatibility of a traditional thermionic emitter with iodine propellant, a low-power barium oxide (BaO) cathode was operated on xenon and iodine propellants. This required the construction and demonstration of a low flow rate iodine feed system. The cathode operating conditions are reported for both propellants. The emitter surface was inspected using a scanning electron microscope after various exposures to xenon and iodine propellants. The results of the inspection of the emitter surface are presented. Another low current (\u3c 5 A), BaO hollow cathode was operated on xenon and iodine propellants. Its discharge current and voltage, and plume properties are reported for xenon and iodine with the cathode at similar operating conditions for each. The overall performance of the BaO cathode on iodine was comparable to xenon. The cathode operating on iodine required slightly higher power for ignition and discharge maintenance compared to xenon, as evident by the higher keeper and anode potentials. Plasma properties in the near- plume region were measured using an emissive probe and single Langmuir probe. For both propellants, the plasma density, electron energy distribution function (EEDF), electron temperature, select reaction rate coefficients and time-resolved plasma potentials are reported. For both propellants the cathode operated the same keeper (0.25 A) and discharge current (3.1 A), but the keeper and anode potentials were higher with iodine; 27 V and 51 V for xenon, and 30 V and 65 V for iodine, respectively. For xenon, the mean electron energy and electron temperature were 7.5 eV and 0.7 eV, with bulk drift energy of 6.6 eV. For iodine, the mean electron energy and electron temperature were 6.3 eV and 1.3 eV, with a bulk drift energy of 4.2 eV. A literature review of relevant collisional processes and associated cross sections for an iodine plasma is also presented

The Adoption and Enforcement of a Technological Regime: The Case of the first IT Regime

In this paper we explore the process of adoption and enforcement of a number of new information processing technologies, such as the typewriter, calculators, tabulation gears and book-keeping machines, starting from the 1880s in the United States. We show that their innovation and diffusion was inexorably coupled to the economic development in the USA in the late 19th century. It is a complex and contradictory consequence of underlying socio-economic processes that led to the formation of modern organisational structures in large scale manufacturing which required systematic and efficient information processing. The typewriter and all the complementary office automation devices that entered the scene shortly after were part of a socio-technical regime that started being established: the office work regime or as we prefer to call it the first IT regime, as for the first time a technology was set up to process information on large scale. The logic of large scale manufacturing to produce standardised products in large series and to apply labour saving techniques was cast into the organisation of administration. This required a convergence of technical practices. The lock-in to the inferior QWERTY-keyboard is hence the outcome of the diffusion and hardening of the First IT Regime.Technological regimes; adaption and enforcement of technologies; information technology; QWERTY

Digital cooperation in the Baltic Sea region: a case of networked multi-level governance

The rapid advancement and implementation of Information and Communication Technologies (ICTs) has made the ‘digital world’ an inseparable part of contemporary societies. We have e-commerce, e-democracy, e-administration and e-‘just about anything’. Underlying these digital solutions is the understanding of a borderless and networked world with more and more decentralized states. Globalization pressures thus have the states thinking how to harness the potential of ICTs while upholding their core values. The answer seems to lie in learning how to collectively construct the information society – through horizontal and vertical, transnational and sub-national cooperation. As such, societies are increasingly moving towards polycentric forms of governance that span across state borders and help to accommodate the complexity of modern challenges. This thesis explores how globalization pressures facilitate the diffusion of power in the example of digital cooperation in the Baltic Sea Region (BSR), exploring how it has emerged and what it is like in this day and age. For this, a theoretical synthesis is developed between the concepts of regionalism, digitization and multi-level governance, serving as an analytical framework for advancing the central case study. Through a combination of methods including process-tracing and expert interviewing, the thesis will explore digital cooperation networks in the BSR so to arrive at a wider understanding of the emerging multi-level governance model in the regional digital agenda.http://www.ester.ee/record=b4500907*es

The Rise of Decentralized Autonomous Organizations: Coordination and Growth within Cryptocurrencies

The rise of cryptocurrencies such as Bitcoin is driving a paradigm shift in organization design. Their underlying blockchain technology enables a novel form of organizing, which I call the “decentralized autonomous organization” (DAO). This study explores how tasks are coordinated within DAOs that provide decentralized and open payment systems that do not rely on centralized intermediaries (e.g., banks). Guided by a Bitcoin pilot case study followed by a three-stage research design that uses both qualitative and quantitative data, this inductive study examines twenty DAOs in the cryptocurrency industry to address the following question: How are DAOs coordinated to enable growth? Results from the pilot study suggest that task coordination within DAOs is enabled by distributed consensus mechanisms at various levels. Further, findings from interview data reveal that DAOs coordinate tasks through “machine consensus” and “social consensus” mechanisms that operate at varying degrees of decentralization. Subsequent fuzzy-set qualitative comparative analyses (fsQCA), explaining when DAOs grow or decline, show that social consensus mechanisms can partially substitute machine consensus mechanisms in less decentralized DAOs. Taken together, the results unpack how DAO growth relies on the interplay between machine consensus, social consensus, and decentralization mechanisms. To conclude, I formulate three propositions to outline a theory of DAO coordination and discuss how this novel form of organizing calls for a revision of our conventional understanding of task coordination and organizational growth

Ion Velocity in the Discharge Channel and Near-Field of the HERMeS Hall Thruster

NASA is continuing the development of a 12.5-kW Hall thruster system, which is baselined in a phased exploration concept to expand human presence to cis-lunar space and eventually to Mars. The development team is transitioning knowledge gained from the testing of the government-built Technology Development Unit (TDU) to the contractor-built Engineering Development Unit (EDU). A new laser-induced fluorescence diagnostic that is compatible with the testing of engineering hardware was developed to obtain data for thruster model validation in the lowest background pressure achievable. Prior to performing the test on the EDU, the team performed a functional checkout test of this new diagnostic using the TDU. In addition to providing a checkout of the diagnostic, this test provided data that can be correlated to electron mobility for comparison to the EDU at a later date. A number of technical challenges related to large test facilities and interfacing with engineering hardware were overcome while implementing the new laser diagnostic system. The initial data set was in good agreement with prior optical and non-optical diagnostics data collected on the TDU thrusters. This data set also revealed the spatial origin of high angle ions that have been of concern for spacecraft integration

Analysis of electric propulsion electrical power conditioning component technology. Volume 1 - Data bank Final report

Analysis of electric propulsion electric power conditioning component technology - data revie