# Assessing Blockchain Consensus and Security Mechanisms against the 51% Attack
## 1 Introduction  
Blockchain is an emerging technology that allows performing digital transactions within a short period [1]. It is a secure medium which works by sharing information over the peer-to-peer (P2P) network. A piece of shared information to a single entity reaches all participants within the network, without the data being tampered. In addition to crypto-transactions, it is also leveraged for various other purposes, such as insuring intellectual property, generating financial contracts, tracing food production, and tracking supply chains [2]. The rise of this technology has been obstructed from time to time by various pernicious attacks. Attacks such as the Sybil attack, Eclipse attack, border gateway protocol (BGP) hijacking, and 51% attack originated from an attempt to strike the blockchain network. Amongst these, the 51% attack has been neglected by security researchers mainly due to the immense attacking cost. However, recent attacks have proven that the 51% attacks can be performed on various modern cryptocurrencies. The proof-of-work (PoW) consensus protocol is an immediate threat of the 51% attack compared to other consensus protocols. Recent attacks were mainly targeting crypto-coins that rely on PoW.  
Bitcoin is the first cryptocurrency which was implemented in the blockchain network [3]. It is based on the PoW consensus protocol, and several cryptocurrencies inherited the consensus rules. After Bitcoin appeared, new cryptocurrencies evolved to take advantage of the blockchain network. The more this technology was adapted, the more it drew the attention of potential attackers.  
The low hashing coins are more prone to this attack technique due to the gigantic costs on high hashing coins. As a result, the recent attacks were mostly targeting low hashing coins, such as BitcoinGold (BTG), Verge (XVG) and Ethereum Classic (ETC). The dangerous part of a 51% attack is that there is no way to detect it until it is fully executed; hence, when an adversary is in possession of the required hashing power, the chances of success can be close to 100%.  
In summary, the major contributions in this paper are:  
* We discuss seven major security threats that jeopardize the blockchain network;
* We reveal the limitations of the consensus mechanisms by classifying them in several attacks;
* We define the majority hash rate problem and point out the consequences of the attack techniques;
* We analyze five security protection mechanisms specific to the 51% attack and demonstrate their limitations against the 51% attack exploitation.  
This paper is organized as follows. Section 2 is the background section, which defines the concept behind blockchain technology. Section 3 illustrates seven attacks which can affect the blockchain network significantly. Section 4 discusses the three widely adopted consensus protocols and their limitations. This section also classifies the consensus based on various attacks. Section 5 discusses the majority hash rate problem in depth and points out the major consequences of this attack. In Section 6, we present the defense mechanisms to mitigate the 51% attack. Section 7 involves our analysis of the defense mechanisms to determine their robustness against the 51% attack. Finally, we conclude the paper in Section 8, pointing out the main weaknesses which suggest new modifications to the existing consensus protocols.