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Murch
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YesGenerally, all confirmations are relativistic: any chaintip could still be replaced with a heavier chain! However, reorganizations become increasingly costly and unlikely the more proof of work is piled on a block.

You are right, an attacker exerting a can censor or rollback any transactions. Additionally, this means that they canand thus perform a to return a payment they previously made to another party to themselves (also see What can an attacker with 51% of hash power do?). Generally, requiring more confirmations before considering a transaction reliable makes reorganization attacks more expensive. If If you are receiving payments that are orders of magnitudes larger than block rewards, you should haveconsider additional precautions in place (e.g. KYC, collateral, waiting for more confirmations). The occurrence of a sustained majority attack on Bitcoin could be considered a fatal scenario for Bitcoin: no transaction could be trusted anymore at any number of confirmations.

You may have read that Bitcoin requires the majority of the hashrate to be "honest" to work. Honest in this context means "not collaborating to attack the network" or "independently creating good-faith blocks". Fortunately, it is rather expensive to control the majority of the hashrate. Each block reward is currently worth about $70,000, so a six block majority attack would likely cost $350,000+ in electricity cost alone. Additionally, it would require access to thea majority of highly coveted and expensive hardware. Industrial Bitcoin miners have made enormous investments into application specific integrated circuits (ASICs), which can be used for nothing else than SHA-256d mining. The most likely way for such ana majority attack to come to pass on Bitcoin were ifwould be for someone tookto take over the mining controllers of multiple of the largest mining pools. I would hope that this would be discovered and remedied quickly.

That is not to say that your majority attack scenario hasn't come to pass before. Especially minority forks of larger networks, or other coins that use the same hashing algorithm than bigger networks have been majority attacked in the past. For example Bitcoin Gold was attacked with a 1,300 block reorganization in January 2020.

Yes, an attacker exerting a can censor or rollback any transactions. Additionally, this means that they can perform a to return a payment they previously made to another party to themselves (also see What can an attacker with 51% of hash power do?). Generally, requiring more confirmations before considering a transaction reliable makes reorganization attacks more expensive. If you are receiving payments that are orders of magnitudes larger than block rewards, you should have additional precautions in place (e.g. KYC, waiting for more confirmations). The occurrence of a sustained majority attack on Bitcoin could be considered a fatal scenario for Bitcoin: no transaction could be trusted anymore at any number of confirmations.

You may have read that Bitcoin requires the majority of the hashrate to be "honest" to work. Honest in this context means "not collaborating to attack the network" or "independently creating good-faith blocks". Fortunately, it is rather expensive to control the majority of the hashrate. Each block reward is currently worth about $70,000, so a six block majority attack would likely cost $350,000+ in electricity cost alone. Additionally, it would require access to the majority of highly coveted and expensive hardware. Industrial Bitcoin miners have made enormous investments into application specific integrated circuits (ASICs), which can be used for nothing else than SHA-256d mining. The most likely way for such an attack to come to pass on Bitcoin were if someone took over the mining controllers of multiple of the largest mining pools. I would hope that this would be discovered and remedied quickly.

That is not to say that your scenario hasn't come to pass before. Especially minority forks, or coins that use the same hashing algorithm than bigger networks have been majority attacked in the past. For example Bitcoin Gold was attacked with a 1,300 block reorganization in January 2020.

Generally, all confirmations are relativistic: any chaintip could still be replaced with a heavier chain! However, reorganizations become increasingly costly and unlikely the more proof of work is piled on a block.

You are right, an attacker exerting a can censor or rollback any transactions, and thus perform a to return a payment they previously made to another party to themselves (also see What can an attacker with 51% of hash power do?). If you are receiving payments that are orders of magnitudes larger than block rewards, you should consider additional precautions (e.g. KYC, collateral, waiting for more confirmations). The occurrence of a sustained majority attack on Bitcoin could be considered a fatal scenario for Bitcoin: no transaction could be trusted anymore at any number of confirmations.

You may have read that Bitcoin requires the majority of the hashrate to be "honest" to work. Honest in this context means "not collaborating to attack the network" or "independently creating good-faith blocks". Fortunately, it is rather expensive to control the majority of the hashrate. Each block reward is currently worth about $70,000, so a six block majority attack would likely cost $350,000+ in electricity cost alone. Additionally, it would require access to a majority of highly coveted and expensive hardware. Industrial Bitcoin miners have made enormous investments into application specific integrated circuits (ASICs), which can be used for nothing else than SHA-256d mining. The most likely way for a majority attack to come to pass on Bitcoin would be for someone to take over the mining controllers of multiple of the largest mining pools. I would hope that this would be discovered and remedied quickly.

That is not to say that your majority attack scenario hasn't come to pass before. Especially minority forks of larger networks, or other coins that use the same hashing algorithm than bigger networks have been majority attacked in the past. For example Bitcoin Gold was attacked with a 1,300 block reorganization in January 2020.

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Murch
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Yes, an attacker exerting a can censor or rollback any transactions. Additionally, this means that they can perform a to return a payment they previously made to another party to themselves (also see What can an attacker with 51% of hash power do?). Generally, requiring more confirmations before considering a transaction reliable makes reorganization attacks more expensive. If you are receiving payments that are orders of magnitudes larger than block rewards, you should have additional precautions in place (e.g. KYC, waiting for more confirmations). The occurrence of a sustained majority attack on Bitcoin could be considered a fatal scenario for Bitcoin: no transaction could be trusted anymore at any number of confirmations.

You may have read that Bitcoin requires the majority of the hashrate to be "honest" to work. Honest in this context means "not collaborating to attack the network" or "independently creating good-faith blocks". Fortunately, it is rather expensive to control the majority of the hashrate. Each block reward is currently worth about $70,000, so a six block majority attack would likely cost $350,000+ in electricity cost alone. Additionally, it would require access to the majority of highly coveted and expensive hardware. Industrial Bitcoin miners have made enormous investments into application specific integrated circuits (ASICs), which can be used for nothing else than SHA-256d mining. The most likely way for such an attack to come to pass on Bitcoin were if someone took over the mining controllers of multiple of the largest mining pools. I would hope that this would be discovered and remedied quickly.

That is not to say that your scenario hasn't come to pass before. Especially minority forks, or coins that use the same hashing algorithm than bigger networks have been majority attacked in the past. For example Bitcoin Gold was attacked with a 1,300 block reorganization in January 2020.