1% Shard Attack Explained – Ethereum Sharding (Contd..)

By Shawn Dexter / March 11, 2018

In a previous post I discuss why Ethereum prefers PoS over PoW to combat the 1% Shard Attack.  In this post I will go into a little bit more detail.

Fair warning:  This post may get a little technical. I am going to be intentionally redundant to drive my point through.

Recall that in PoW computational puzzles are solved in order to come to “agreement” on the ledger. This “agreement” is what forms the “consensus” of the blockchain. These computational puzzles require a lot of processing power. Hence, network participants have to consume electricity to come to agreement on the ledger.

This is key to note:
In PoW, participants have to put up an extrinsic cost (electricity & hardware ) to propose an agreement on a new state of the ledger

PoW: Is A 51% Attack A Feasible?

The extrinsic cost of a participant – electricity – is referred to as “Hash Power

A “51% Attack” can be executed when an attacker has a majority of the total hash power of the network. This attack will allow a participant to double spend, claim all rewards, censor transactions etc.

Note: I will break down 51% Attack in another post. It’s not what people usually think it is.

However, having 51% (or more) of the hash power of the ENTIRE network would require a lot of electricity & hardware. Currently the cost of a 51% on the Bitcoin network is:

  • $8 Billion US in hardware costs & $12.8 Million US  Per Day in electricity costs (source)

As you can see, it’s far too expensive/infeasible for an attacker to achieve this. (Furthermore, even if a participant achieves this, he’d be far more profitable if he performs the role of a “good actor”. More on this in my 51% Attack post)

PoW Sharded: Is a 1% Attack Feasible?

Alright, so now we understand how a 51% attack is infeasible in Proof Of Work (if not, don’t hesitate to email me. I understand that this can get confusing). But what if the network is split into groups/pieces (as is done in Sharding)?  

Let’s consider a blockchain network that is split into a hundred pieces (Hundred shards)

If the entire network has 100% of the hash power
then each of the 100 shards will have 1% of the hash power  

Q. But what if an attacker has even 1% control of the total network hash power? 

He can essentially concentrate his hash power on a single shard. Since each shard is responsible for 1% of the network, the attacker attains 100% control of the shard (Shard = 1% of network. Attacker = 1% of network)


PoW vs PoW Sharded

  • In an unsharded PoW system, a bad-actor  would need 51% of the hash-power to win majority vote and attack the network.
  • In a sharded PoW system of 100 shards, he only needs 1% of the hash-power to attack the network.

If an attacker achieves 1% of the entire networks hash power, he can effectively achieve 100% of the hash power of a single shard.  With that he can completely control a single shard.

Proof Of Stake allows Ethereum to easily & effectively (by using random-sampling) take away the attacker’s ability to concentrate his hash power on the shard of his choosing. Thus eliminating the 1% Attack vulnerability.

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