Towards Overcoming the Undercutting Problem

For Bitcoin and similar cryptocurrencies, their mining processes are currently incentivized with fixed block rewards and voluntary transaction fees. However, the block rewards are supposed to vanish gradually and the remaining incentive of transaction fees is optional and arbitrary. Under those circumstances, Carlsten et al.[CCS~2016] find that an interesting undercutting attack, where the attacker deliberately forks an existing chain by leaving wealthy transactions unclaimed to attract petty complaint miners to its fork, can become the equilibrium strategy for miners. Motivated by similar phenomenons in economics, we take a closer look at the undercutting analysis and find the result to be questionable: In [CCS~2016], fees are accumulated at a fixed rate and miners can collect all unclaimed fees regardless of block size limit, which is often not feasible in practice. Besides, ignoring a potentially large amount of fees unclaimable in a single block can inaccurately inflate the profitability of undercutting. In this work, we define a model that considers claimable fees based on available transactions that can be assembled into the block size limit and upgrades petty compliant miners to be rational where they decide whether to move to other chains subject to expected returns from different choices. In this new model, we first identify the conditions that are necessary to make undercutting profitable. Second, we propose a defense against undercutting by manipulating transactions selected into the new block to invalidate the above-identified conditions. Finally, we complement the above analytical results with an experimental analysis over Bitcoin and Monero. We demonstrate that our conditions for undercutting to be profitable are effective (an increase of 0.5-4.5% in Bitcoin and 8% in Monero) and the avoidance technique fulfills its purpose of allowing miners to earn around fair shares.Comment: 15 pages, 6 figure

Fine-grained Poisoning Attack to Local Differential Privacy Protocols for Mean and Variance Estimation

Although local differential privacy (LDP) protects individual users' data from inference by an untrusted data curator, recent studies show that an attacker can launch a data poisoning attack from the user side to inject carefully-crafted bogus data into the LDP protocols in order to maximally skew the final estimate by the data curator. In this work, we further advance this knowledge by proposing a new fine-grained attack, which allows the attacker to fine-tune and simultaneously manipulate mean and variance estimations that are popular analytical tasks for many real-world applications. To accomplish this goal, the attack leverages the characteristics of LDP to inject fake data into the output domain of the local LDP instance. We call our attack the output poisoning attack (OPA). We observe a security-privacy consistency where a small privacy loss enhances the security of LDP, which contradicts the known security-privacy trade-off from prior work. We further study the consistency and reveal a more holistic view of the threat landscape of data poisoning attacks on LDP. We comprehensively evaluate our attack against a baseline attack that intuitively provides false input to LDP. The experimental results show that OPA outperforms the baseline on three real-world datasets. We also propose a novel defense method that can recover the result accuracy from polluted data collection and offer insight into the secure LDP design

Research on the Measurement Error of MWIR Average Atmospheric Transmittance

The atmospheric transmittance in mid-wavelength infrared (MWIR) band reflects the characteristics of atmospheric particles, which is typically used in MWIR imaging system correction and data extraction. MWIR imager is wide spectral band instrument and its measuring output is usually affected by broadband atmospheric transmittance which is usually considered as average influence, therefore it is exactly of great importance to study the measurement of MWIR average atmospheric transmittance, which is valuable for MWIR images correction and application. A measurement error model of MWIR atmospheric transmittance was proposed according to the measuring method of the broadband average atmospheric transmittance. Because the transmittance measurement principle mentioned in this paper is indirect measurement, some direct input data are needed to be acquired through infrared imager, blackbody, etc. Finally, combining the measurement error theory with data of the experiments, the MWIR atmospheric transmittance and its measurement error are extracted. The results show that the MWIR atmospheric transmittance measured value is reasonable according to the empirical value in sunny day. And the measurement error objectively reflects most aspects of the test, exactly proving the validity of the measurement experiment

Iridescent Daytime Radiative Cooling with No Absorption Peaks in the Visible Range

Coatings for passive radiative cooling applications must be highly reflected in the solar spectrum, and thus can hardly support any coloration without losing their functionality. In this work, a colorful daytime radiative cooling surface based on structural coloration is reported. A designed radiative cooler with a bioinspired array of truncated SiO2 microcones is manufactured via a self-assembly method and reactive ion etching. Complemented with a silver reflector, the radiative cooler exhibits broadband iridescent coloration due to the scattering induced by the truncated microcone array while maintaining an average reflectance of 95% in the solar spectrum and a high thermal emissivity (ε) of 0.95, owing to the reduced impedance mismatch provided by the patterned surface at infrared wavelengths, reaching an estimated cooling power of ≈143 W m-2 at an ambient temperature of 25 °C and a measured average temperature drop of 7.1 °C under direct sunlight. This strong cooling performance is attributed to its bioinspired surface pattern, which promotes both the aesthetics and cooling capacity of the daytime radiative cooler