The security model of Bitcoin withstands the test of halving block rewards; both the market and technology prove that defending against 51% attacks does not require breaking the upper limit of 21 million.

Bitcoin's security model remains strong despite halving block rewards. Both the market and technology demonstrate that defending against 51% attacks does not necessitate surpassing the 21 million limit.

Recently, several prominent individuals claimed that as the Bitcoin block continues to halve, when the block rewards become too low, miners will receive fewer rewards. If the price of Bitcoin does not double, it will lead to the failure of the Bitcoin security model.

The Proof of Work (PoW) mechanism operates on a cooperative win-win model when the block rewards are sufficient. Miners adhere to the Bitcoin packaging rules, which are more beneficial to their own interests.

However, when the block rewards become too low (without a corresponding increase in price), miners will instead attack the network for profit.

Because of this, they propose to change the Bitcoin block reward halving mechanism at the appropriate time, to stop the halving and prevent miners from attacking the network.

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This reasoning is typical of bookish behavior.

The market has repeatedly proven that when miner rewards are insufficient, it does not truly threaten the security of Bitcoin.

The operations of BCH and BSV have provided sufficient evidence.

BTC, BCH, and BSV all use the same mining algorithm SHA256. Compared to BTC, the prices of BCH and BSV are too low. Currently, for the entire SHA256 mining machine, the miners mining BCH and BSV are the ones saying that the block rewards are too low.

The current BCH hash power is less than 1% of BTC, and the BSV hash power is less than 1/1000 of BTC.

It can be said that the current BCH, after several rounds of halving, has a block reward of only 1% of the current BTC, equivalent to the reward after 6 rounds of halving (6.25/2^6=0.0977).

But now BCH is very secure and does not need to overissue block rewards to ensure system security.

BSV has experienced a 51% attack event with a reorganization of 500 blocks, but the security model of Bitcoin’s PoW is not afraid of 51% attacks. BSV has endured similar attacks for several rounds and can still operate normally.

The harm caused by miners launching a 51% attack on the Bitcoin network (BTC, BCH, BSV) mainly consists of two aspects:

1. Normal transactions during the attack cannot receive block confirmations.

When I send you Bitcoin, the final security of the Bitcoin transaction to your address is probabilistic. The higher the block confirmation received by the transaction, the more secure it is because reversing the block (i.e., a 51% attack) incurs costs. The more blocks that need to be reversed, the higher the cost. When the block count reaches a certain number, no one can bear such a high cost.

When mining rewards are too low, the amount of mining power participating may be minimal. When there are many idle SHA256 mining machines, the cost of launching a 51% attack using these machines is very low.

This is why BSV has been attacked by 51% multiple times because there are too few normal SHA256 mining machines mining BSV, and it only takes a few rented machines to launch a 51% attack.

However, a 51% attack can only prevent a user’s transaction from being effectively confirmed, which means that under normal circumstances, it takes 10 minutes to get a confirmation. In a 51% attack, it may take 1 day or even 10 days to get a confirmation.

But a 51% attack cannot steal a user’s coins, nor can miners create fake coins. A 51% attack can only prevent transactions initiated by users from being effectively confirmed.

As long as the 51% attack stops, the user’s transactions can be effectively confirmed.

In fact, after multiple rounds of 51% attacks, it has been proven that BSV has not experienced any cases of transactions being stolen by miners, and miners have not created fake BSV coins.

2. A 51% attack can initiate double spending attacks on exchanges.

The principle of deposit and withdrawal on exchanges is as follows:

Users deposit Bitcoin into the exchange. When the number of confirmations for the deposit transaction reaches 1, the exchange credits the user’s account, and the user can sell the BTC.

When the number of confirmations for the deposit transaction reaches 6, the exchange allows the user to withdraw the deposited coins (or convert them into USDT, etc.).

In the event of a 51% attack, if the attacker deposits 10,000 BTC into the exchange, the attacker can immediately sell them for ETH after the exchange credits the account. Then, when the number of confirmations for the deposit transaction reaches 6, the attacker can withdraw all the sold ETH.

While initiating the deposit transaction, the attacker also launches a 51% attack on the BTC network, starting from the previous block that confirmed the deposit transaction and mining continuously for 7 blocks (assuming the normal chain only mines 6 blocks). Then, the attacker broadcasts these 7 blocks all at once, creating a longer chain with one more block than the previous 6 blocks. At this point, all other miners will abandon the 6-block chain and consider the attacker’s 7-block chain as the valid chain.

In this process, the attacker replaces the original 10,000 bitcoins in the deposit transaction with another transaction sent to their own address within the 7 blocks.

At this point, the 10,000 bitcoins that were originally credited to the exchange’s account automatically disappear, and the exchange also loses the ETH that the attacker withdrew.

This is a double spending attack, and only the exchange is affected.

Double spending attacks can also target businesses that accept Bitcoin and deliver goods to buyers after a certain number of confirmations.

However, exchange deposit confirmation systems now have sufficient capability to defend against double spending attacks, and KYC is the most effective method. Under KYC, if you dare to engage in double spending on an exchange, you will be caught in minutes.

Currently, the main focus of double spending attacks is on cross-chain bridges. This is why most cross-chain bridges do not support PoW chains much, because PoW can be subjected to 51% attack for double spending, but PoS mechanism has deterministic block confirmations that cannot be reversed after reaching a certain number of blocks.

But even if BSV has indeed been subject to double-spending attacks on some exchanges, there was a time when exchanges only credited transactions after 144 blocks of confirmation, and withdrawals were only allowed after 500 blocks of confirmation.

In fact, it has been proven that the double-spending attacks on individual exchanges did not break down the security system of BSV, and BSV remains intact. Only individual exchanges were affected.

Furthermore, technically speaking, it is very simple to prevent a 51% attack by adding a dynamic checkpoint on the blocks.

A checkpoint is essentially a hash value that includes the data of this block and all preceding blocks. Blocks that include a checkpoint cannot be reorganized.

In the first version of Bitcoin released by Satoshi Nakamoto, there was a checkpoint every 500 blocks. That’s why Bitcoin cannot be reorganized beyond 500 blocks. It can only be reorganized up to 500 blocks, no more.

This is also why BSV has only experienced attacks that reorganized up to 500 blocks.

The current block confirmation, such as PoS, is essentially using this checkpoint technology. It has been thoroughly validated as safe and feasible.

Those who claim that Bitcoin needs to break the 21 million limit may have ulterior motives.

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