Security Of Blockchain Protocol Against Long Delay Attack

In recent,years,more and more attention have been paid on cryptocurren-cies represented by bitcoin.Unlike conventional elect,ric payment systems,one of the main advantages of bitcoin is decentralization,which means the transaction ledger is maintained by public.Although cdecentralization can provide robust-ness and prevent single point of failure,it poses new challenges for security and efficiency.Since the introduction of bitcoin.there have been series of works dis-cussing the security of bitcoin protocol.Act,ually,at the core of bitcoin protocol is a consensus protocol called blockchain,which plays a central role in the secu-rity of bitcoin system.Pass.Seeman and shelat's recent work showed that the Nakamoto blockchain is secure in an asynchronous networ-k where me,ssages can be maliciously delayed by at most ??1/np.with n being the number of miners and p the mining hardness.However,the condition ??1/np cannot be guaran-teed in practice.For example,long delay among networks can be easily caused by equipment failures or malicious attacks.This brings up a interesting question:Is the blockchain protocol based on POW still secure in a real world asynchronous network,where long delay relative to the mining hardness is allowed?In order to solve this problem,we provide a new model of blockchain prot.ocol in long delay environments to capture the attacks of the adversary.We extend the defmitions of three basic properties of blockchain protocol:chain growt.common prefix and chain quality.Those new cdefinitions can capture the security requirements of blockchain in a more flexible way.In addition,we propose a special tree called TreeMC to recorcd the evolution of the main chains and reflect security properties of the blockchain.Using Treemc we prove that blockchain with proper parameters can still achieve the above three properties in asynchronous networks with long delay.where the limit,of delay ? is slightly smaller than 1/np.