Talking about PoS, how can you not understand Nothing At Stake?

PoS (Proof of Stake) is essentially a consensus agreement for obtaining a certain amount of currency to obtain mining qualifications and benefits. After the initial Proof of Stake was proposed in 2013, Nothing At Stake (no interest) issues and The Long-Range Attack issue is a challenge that is often discussed in the Proof of Stake consensus protocol. Want to learn more about PoS? Know what it is and you need to understand why.

Today we will first look at the Nothing At Stake problem.

Nothing At Stake, a situation where someone loses nothing when behaving badly, but stands to gain everything.

The essence of the Nothing At Stake problem is that “there is no cost, no more benefits”. Specifically, when there is a fork in the PoS consensus system, the block node can “out of any loss” and simultaneously block out multiple chains, thus making it possible to obtain “all the benefits”. ".

This is like having a window. When you have a queue, you can get the money. When there is only one window, everyone will line up and everyone will have fun.

But when the second window appears. As we all know, in the end, it is possible that only one of the two windows will receive "real money", and the other money will become waste paper. But you don't know which window will be the window for "real money". So what do you do? You may order the next errands to help you split up and queue up at another window.

However, when the actual PoS is out of the block node, there is no cost at this time, but the computer does an operation.

What kind of chaos will this cause?

Smart out-of-block nodes will have the power to generate new forks, support or initiate illegal transactions, and other profit-seeking nodes will queue up on multiple chains (windows) to support new forks. As time goes on, there are more and more forks, illegal transactions, and madness. The blockchain will no longer be the only chain, and there is no way for all the block nodes to reach a consensus.

In addition, the non-interest issue also makes double-flower attacks easier. Unlike PoW's 51% attack, PoS attack nodes only need a certain amount of computing power (sometimes only 1%) to be able to attack. For example, a total pledge accounts for only 30% of the currency, and the attack cost is easier to attack than the 60% pledge network.

How easy is it? As shown in the figure below, there are still three outbound nodes A, B, and C. If A is an attacking node, it creates two transactions when the fork is generated. Send one X credit to one of your wallet addresses, and send X coins to the exchange on the other fork. The B and C block nodes will be out of the two fork chains at the same time due to Nothing At Stake. When the transaction is confirmed by the exchange, A converts X coins for sale into a privacy currency and moves out of the exchange. After that, A increases the weight of the block by increasing the amount of pledge money, or creating a number of other out-of-block nodes, and continues to block only in the forked chain. At this point, the longest chain is obvious, and gradually widen the gap, which will eventually become the longest chain. A successfully doubles X coins.

Why is the Nothing At Stake issue just a challenge for PoS? Isn't PoW going to be there?

The PoW mechanism is born to avoid this problem. Because at the time of the block, the mine unions pay the opportunity cost – computing resources. If the fork appears, the miner needs to carefully choose which chain to make the block. Once the wrong choice is made, the cost of the calculation will not be profitable. Miners will not choose to divide the power on both chains, which will only reduce the probability of the original chain's block by half, which may not be worth the candle.

So how do you solve the Nothing At Stake problem?

The general strategy is post-punish, and if it is judged to be a malicious block, a part of the stake or margin will be used as a fine. However, all penalties and regulatory measures are just after the fact, not as invisible constraints that PoW needs to force out.

Let's see how Tezos uses a penalty mechanism to avoid the Nothing At Stake problem: participation in mining on the Tezos chain requires a margin. This margin is used to ensure that the baker is honest, if the baker tries Broadcasting the block on both branches, his margin will be confiscated. If the baker successfully creates and broadcasts a legitimate block, he will receive block rewards and transaction fees. This mechanism uses money rewards to avoid double mining problems, and the safety of the nodes is maintained by the baker.

In addition to post-penalties, the solution to date is to sacrifice a certain degree of performance, decentralization principles, and the design of contract mechanisms is more complicated.

Where is this complexity reflected?

In PoS, the roles and responsibilities of the nodes are numerous: in addition to the similar nodes (Poner) in the PoW, and the Validator is the block endorsement, the State Guardian catches the "bad guys".

The guardian hunter's stake has a certain lock-up period. Generally, it is necessary to obtain the pledge coins after X blocks, so that the guardian hunter can take enough time to seize the illegal node.

In addition, although the project party can select trusted nodes through the pre-node filtering, the problem of centralization is still inevitable:

  • If the network develops rapidly and the benefits exceed the reputation of the organization, then the selected real-name nodes are not necessarily trusted;
  • Whether the node has sufficient motivation or technical ability to guarantee its own anti-attack capability, thus ensuring network security;
  • The number of nodes is limited, the scalability is not enough, and it is still centralized.

Source: cobo wallet