6 research outputs found
A Deep Dive into Blockchain Selfish Mining
This paper studies a fundamental problem regarding the security of blockchain
on how the existence of multiple misbehaving pools influences the profitability
of selfish mining. Each selfish miner maintains a private chain and makes it
public opportunistically for the purpose of acquiring more rewards
incommensurate to his Hashrate. We establish a novel Markov chain model to
characterize all the state transitions of public and private chains. The
minimum requirement of Hashrate together with the minimum delay of being
profitable is derived in close-form. The former reduces to 21.48% with the
symmetric selfish miners, while their competition with asymmetric Hashrates
puts forward a higher requirement of the profitable threshold. The profitable
delay increases with the decrease of the Hashrate of selfish miners, making the
mining pools more cautious on performing selfish mining.Comment: 6 pages, 13 figure
Blockchain Mining with Multiple Selfish Miners
This paper studies a fundamental problem regarding the security of blockchain
PoW consensus on how the existence of multiple misbehaving miners influences
the profitability of selfish mining. Each selfish miner (or attacker
interchangeably) maintains a private chain and makes it public
opportunistically for acquiring more rewards incommensurate to his Hash power.
We first establish a general Markov chain model to characterize the state
transition of public and private chains for Basic Selfish Mining (BSM), and
derive the stationary profitable threshold of Hash power in closed-form. It
reduces from 25% for a single attacker to below 21.48% for two symmetric
attackers theoretically, and further reduces to around 10% with eight symmetric
attackers experimentally. We next explore the profitable threshold when one of
the attackers performs strategic mining based on Partially Observable Markov
Decision Process (POMDP) that only half of the attributes pertinent to a mining
state are observable to him. An online algorithm is presented to compute the
nearly optimal policy efficiently despite the large state space and high
dimensional belief space. The strategic attacker mines selfishly and more
agilely than BSM attacker when his Hash power is relatively high, and mines
honestly otherwise, thus leading to a much lower profitable threshold. Last, we
formulate a simple model of absolute mining revenue that yields an interesting
observation: selfish mining is never profitable at the first difficulty
adjustment period, but replying on the reimbursement of stationary selfish
mining gains in the future periods. The delay till being profitable of an
attacker increases with the decrease of his Hash power, making blockchain
miners more cautious on performing selfish mining.Comment: arXiv admin note: substantial text overlap with arXiv:1811.0826
Experiments and Numerical Simulation of Performances and Internal Flow for High-Speed Rescue Pump with Variable Speeds
The model pump is a high-speed, high-power pump designed to achieve rapid mine flooding rescue. This study conducted experiments to investigate pump performance curves, including head, efficiency, and power for the following six different rotation speeds: 3000, 3600, 4200, 4800, 5400, and 6000 rpm. Then, the numerical simulation method based on computational fluid dynamics commercial code Ansys was used to present the internal flow of the pump for the six different rotation speeds through steady and unsteady analyses. Results show that the numerical results agree well with experimental data. The designs of outlet and inlet angles of the impeller match each other well at high rotation speeds. The pressure pulsation coefficient Cp in the impeller and the diffuser channel remain constant at the same monitor point under different rotation speed conditions. The varying trend of the pressure-augmented coefficient ΔP indicates that, with the increase in rotation speed, the effect on pressure rise induced by the back part of the impeller channel is more evident than that by the front part. Also, the main frequency components of ΔP are concentrated on the region with low frequency. Moreover, the rotation speed has no significant effect on ΔP in the diffuser region. This study provides effective guidance and valuable reference for the design of high-speed, high-power pumps