5 ms·

Blockchains like Bitcoin are actually not tamper-proof. They can be attacked by 51% attacks where you can even rewrite history if you have enough hashpower. The

by foepys 5y ago

Blockchains like Bitcoin are actually not tamper-proof. They can be attacked by 51% attacks where you can even rewrite history if you have enough hashpower. The protocol is explicitly designed to always follow the longest chain thus the only defense is to hash faster than the attackers. This might vary for other blockchains but the biggest and most mentioned is particularly unsafe in that regard.

kdragon 5y ago
You can't rewrite the history of blocks that have already been distributed. You may fool SPV nodes but any node with a copy of the blockchain (even if pruned) will reject your version.
- deleted 5y ago
  [deleted]
- foepys 5y ago
  This is false. Please read the white paper. It clearly states in section 4: > The majority decision is represented by the longest chain, which has the greatest proof-of-work effort invested in it. If a majority of CPU power is controlled by honest nodes, the honest chain will grow the fastest and outpace any competing chains. If the majority of the hashpower (not nodes!) is dishonest, you can rewrite history. It's the reason why the current difficulty is part of a block. This has been done numerous times in the past for small but highly traded altcoins.
deleted 5y ago
[deleted]
PhilippGille 5y ago
There are wo things you skip over: 1. You can't rewrite everything. Given enough hash power to create a longer chain, you can create a block that a) removes any transactions from a block in the original chain and b) contains new valid transactions (must be signed by you so you must be the owner of the Bitcoins used in the tx), allowing you to double spend your coins, but you can't change other people's transactions. 2. With each new block changing a past one becomes harder, while you make it sound as if you could arbitrarily rewrite history. Merchants usually wait several blocks before accepting your on-chain payment. Exchanges wait 6 blocks as that's seen as infeasible to change a block that's buried under 5 other blocks for non-nation state actors. TL;DR: 1) Other people's transactions can at most be removed but not be changed and 2) data on the Bitcoin blockchain is tamper proof after x blocks.
- XorNot 5y ago
  This is ignoring that if you own 51% of the network, you are free to just rewind the chain back to whatever you think is suitable, and rebuild it from that point on. Sure, someone, somewhere will know the truth but who cares - it's all about information control, and you control 51% of the network. Now your opponents have to scramble with social countermeasures to try and discredit your chain. A politically savvy attacker will simply buy off the main social nodes favorably. EDIT: The problem with "oh but people will know" argument is that it assumes credibility outside the network, the thing which cryptocurrencies like to claim is unnecessary. In reality, if the network is 51% attacked, your entire recovery strategy is assuming enough people believe you about what's real or should be real. This isn't even theoretical: this is literally what happened to Ethereum.
  - sennight 5y ago
    > This is ignoring that if you own 51% of the network... Are you talking about some kind of node sybil attack? Because the only way I can make sense of what you've said is if a system had client implementations maintaining zero state - relying completely on unsecured network consensus. But in that scenario hashing power wouldn't enter the equation. I don't know of any cryptocurrency that operates that way, and doubt that one ever has outside of a home lab. Or are you talking about someone trying to hijack a cryptocurrency by creating brand confusion and convincing people to run modified code, like the failed bcash campaign against bitcoin? A 51% attack on a blockchain has no rewrite ability past the point that where an attacker establishes a longer chain. That is why it is a "chain"... where do you think the hashing function inputs come from that are being fed into the adversarial mining hardware? An earlier block - the one where the attack started, you can't go back further than that without having to throw out and rehash your entire attack fork.
    - zie 5y ago
      You are both mostly right, as I understand it. If I had >50% hashing power of BTC(for exactly 1 transaction), I can make the current chain say anything I want, like give me all the BTC, and it would become valid and "permanent". To re-write actual history takes a lot more work, and wouldn't be possible with 51% for 1 transaction.. If I was able to maintain 51% control for a long time, then I can do anything I want for as long as I have 51% control. Though I imagine after that very 1st transaction, the world of BTC would blow up and everyone would stop hashing BTC as there would be zero point as the current chain is now effectively useless. This is the real issue that I see, 1 transaction of 51% power is enough to permanently wipe out all of BTC's worth. As far as I'm aware every crypto currency basically has this same problem.

by foepys 5y ago

kdragon 5y ago

You can't rewrite the history of blocks that have already been distributed. You may fool SPV nodes but any node with a copy of the blockchain (even if pruned) will reject your version.

deleted 5y ago
[deleted]
foepys 5y ago
This is false. Please read the white paper. It clearly states in section 4: > The majority decision is represented by the longest chain, which has the greatest proof-of-work effort invested in it. If a majority of CPU power is controlled by honest nodes, the honest chain will grow the fastest and outpace any competing chains. If the majority of the hashpower (not nodes!) is dishonest, you can rewrite history. It's the reason why the current difficulty is part of a block. This has been done numerous times in the past for small but highly traded altcoins.

deleted 5y ago

[deleted]

PhilippGille 5y ago

There are wo things you skip over: 1. You can't rewrite everything. Given enough hash power to create a longer chain, you can create a block that a) removes any transactions from a block in the original chain and b) contains new valid transactions (must be signed by you so you must be the owner of the Bitcoins used in the tx), allowing you to double spend your coins, but you can't change other people's transactions. 2. With each new block changing a past one becomes harder, while you make it sound as if you could arbitrarily rewrite history. Merchants usually wait several blocks before accepting your on-chain payment. Exchanges wait 6 blocks as that's seen as infeasible to change a block that's buried under 5 other blocks for non-nation state actors. TL;DR: 1) Other people's transactions can at most be removed but not be changed and 2) data on the Bitcoin blockchain is tamper proof after x blocks.

XorNot 5y ago
This is ignoring that if you own 51% of the network, you are free to just rewind the chain back to whatever you think is suitable, and rebuild it from that point on. Sure, someone, somewhere will know the truth but who cares - it's all about information control, and you control 51% of the network. Now your opponents have to scramble with social countermeasures to try and discredit your chain. A politically savvy attacker will simply buy off the main social nodes favorably. EDIT: The problem with "oh but people will know" argument is that it assumes credibility outside the network, the thing which cryptocurrencies like to claim is unnecessary. In reality, if the network is 51% attacked, your entire recovery strategy is assuming enough people believe you about what's real or should be real. This isn't even theoretical: this is literally what happened to Ethereum.
- sennight 5y ago
  > This is ignoring that if you own 51% of the network... Are you talking about some kind of node sybil attack? Because the only way I can make sense of what you've said is if a system had client implementations maintaining zero state - relying completely on unsecured network consensus. But in that scenario hashing power wouldn't enter the equation. I don't know of any cryptocurrency that operates that way, and doubt that one ever has outside of a home lab. Or are you talking about someone trying to hijack a cryptocurrency by creating brand confusion and convincing people to run modified code, like the failed bcash campaign against bitcoin? A 51% attack on a blockchain has no rewrite ability past the point that where an attacker establishes a longer chain. That is why it is a "chain"... where do you think the hashing function inputs come from that are being fed into the adversarial mining hardware? An earlier block - the one where the attack started, you can't go back further than that without having to throw out and rehash your entire attack fork.
  - zie 5y ago
    You are both mostly right, as I understand it. If I had >50% hashing power of BTC(for exactly 1 transaction), I can make the current chain say anything I want, like give me all the BTC, and it would become valid and "permanent". To re-write actual history takes a lot more work, and wouldn't be possible with 51% for 1 transaction.. If I was able to maintain 51% control for a long time, then I can do anything I want for as long as I have 51% control. Though I imagine after that very 1st transaction, the world of BTC would blow up and everyone would stop hashing BTC as there would be zero point as the current chain is now effectively useless. This is the real issue that I see, 1 transaction of 51% power is enough to permanently wipe out all of BTC's worth. As far as I'm aware every crypto currency basically has this same problem.