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So, I just ran through the same thought process and the problem is this: How does a new entrant to the network know which is the valid chain? They didn't see

So, I just ran through the same thought process and the problem is this: How does a new entrant to the network know which is the valid chain? They didn't see the previous 20 confirmations, so the only rule they can use is picking the longest chain. Another way to do it is to have a separate voting system to attest to the "valid" chain, but then you could use an ordinary botnet to outvote the real chain. In short, confirmations are there for vendors to be sure a few blocks weren't mined at the same time, but they do nothing against a 51% attack.

thaumasiotes 8y ago
I don't follow this. A new entrant to the network knows which is the valid chain because the chain the network uses defines "the network". Consider the case[1] of Ethereum Classic (ETC) versus Ethereum (ETH). They are exactly the same, except for the chain each group of miners follows. The ETC blockchain is a valid ETH blockchain and the ETH blockchain is a valid ETC blockchain, but miners adhere to one or the other for political reasons. As a new entrant to the ETC network, how do you know which blockchain to use? Well, you use the ETC blockchain, because that's the network you're entering. [1] I'm describing my understanding of the ETH/ETC split. If they've diverged more than I was aware of, that doesn't affect the argument; just substitute other names for ETH and ETC.
- Pyxl101 8y ago
  What you're missing is the process by which new blocks are added onto an existing chain. Let's say that you take the "true" Bitcoin or ETC blockchain, if you want to think of it that way, and now multiple actors are proposing to add blocks onto it, potentially multiple blocks. Which one of those additions wins? The one that's the longest and has the most work behind it - that one wins. If you propose an addition that is one block long, and I propose an addition that's 2 blocks long, then my chain wins. (These all have to be valid blocks that solve the mining challenge). Miners select the chain with the most work. That's the consensus mechanism by which the blockchain is established (among nodes operating all with the same rule sets). At any given instantaneous point in time, there may be multiple instantaneous forks of the main blockchain - people who have mined out valid next blocks and are fighting to get them accepted as the primary chain. Imagine that two different miners by coincidence mine out the next block. One of those chains has to win. The chain that wins is the one that propagates through the network most quickly such that other miners begin building upon it and extending the chain - the one with the most work wins. The other chain(s) die and the transactions on it have to be resubmitted. When multiple branches exist on the blockchain, some consensus process has to reconcile which of them wins. That process is that miners select the longest chain. Don't act as if one single block is magically appended to the chain and everyone knows which block that is; a consensus process operates to select that block (or sequence of blocks), and it operates by selecting the longest chain. When two or more blocks are mined out simultaneously by different miners, the one that wins is likely to be the chain that propagates more quickly to other miners. Since miners build on the longest chain, then the miner who distributes their chain more widely among other miners is likely to win, since those miners will begin building upon it (seeing it as the longest chain thus far). To continue the story, let's say that you and I both simultaneously mine out the next block of Bitcoin. You don't tell anyone about your block, and keep it to yourself. I distribute my block to the entire network. Thus, other miners start building on my block, and it becomes part of the main bitcoin blockchain. Once that happens, it doesn't matter if you start telling other people about your block -- it's not part of bitcoin any more because my chain exists which is much longer (my block + whatever other blocks people have mined out on top of it).
  - thaumasiotes 8y ago
    > When multiple branches exist on the blockchain, some consensus process has to reconcile which of them wins. That process is that miners select the longest chain. I've been arguing that this is a design mistake, and that the following selection algorithm would be superior: - Instead of selecting the longest chain, select the longest chain which does not alter any blocks which I, personally, consider permanent. Robin_Message correctly identifies that this algorithm doesn't do anything for a new entrant, because a new entrant doesn't have the exposure to history that an established miner has. But I don't see why this is an issue, as to enter a particular network, you must already copy that network's existing blockchain, and this step -- which you already have to take -- will give you the same exposure to history as the miner you copy it from. Imagine that two blockchains, ETH and ETC, diverge for political reasons. The ETH blockchain is more beloved among the people (the laity, not the miners), and for this reason a greater number of transactions occur in ETH than ETC over time. This guarantees that the ETH blockchain will be longer than the ETC blockchain. By the selection algorithm you identify, the ETC blockchain will be wiped out, because the ETH blockchain can always be re-proposed and will always dominate it. But in reality, the ETC miners will reject the ETH blockchain, because rejecting the ETH blockchain is the whole point of ETC.
    - nearbuy 8y ago
      With a hard fork, you change the code the miners are using so that blocks on the opposing fork are considered invalid. In a 51% attack, the malicious actor creates new blocks that look equally valid. The two lineages aren't labelled ETC and NewETC. How does a new entrant (or anyone who wasn't connected for the few hours it took to launch the attack) distinguish between the chains?
    - Robin_Message 8y ago
      > guarantees that the ETH blockchain will be longer than the ETC blockchain. Chain length is number of blocks, which is controlled by miners, not "laity". I think your system might prevent double spends; unfortunately it would do so by rendering the network unusable for any new users, who wouldn't have a way to know which is the good network to join. I suppose a central authority could be used to identify the good chain, but most coins prefer not to have one of those. The advantage of the simple chain length rule is that it is simple, requires no central authority, is self-correcting and the only weakness is a 51% attack.

thaumasiotes 8y ago

I don't follow this. A new entrant to the network knows which is the valid chain because the chain the network uses defines "the network". Consider the case[1] of Ethereum Classic (ETC) versus Ethereum (ETH). They are exactly the same, except for the chain each group of miners follows. The ETC blockchain is a valid ETH blockchain and the ETH blockchain is a valid ETC blockchain, but miners adhere to one or the other for political reasons. As a new entrant to the ETC network, how do you know which blockchain to use? Well, you use the ETC blockchain, because that's the network you're entering. [1] I'm describing my understanding of the ETH/ETC split. If they've diverged more than I was aware of, that doesn't affect the argument; just substitute other names for ETH and ETC.

Pyxl101 8y ago
What you're missing is the process by which new blocks are added onto an existing chain. Let's say that you take the "true" Bitcoin or ETC blockchain, if you want to think of it that way, and now multiple actors are proposing to add blocks onto it, potentially multiple blocks. Which one of those additions wins? The one that's the longest and has the most work behind it - that one wins. If you propose an addition that is one block long, and I propose an addition that's 2 blocks long, then my chain wins. (These all have to be valid blocks that solve the mining challenge). Miners select the chain with the most work. That's the consensus mechanism by which the blockchain is established (among nodes operating all with the same rule sets). At any given instantaneous point in time, there may be multiple instantaneous forks of the main blockchain - people who have mined out valid next blocks and are fighting to get them accepted as the primary chain. Imagine that two different miners by coincidence mine out the next block. One of those chains has to win. The chain that wins is the one that propagates through the network most quickly such that other miners begin building upon it and extending the chain - the one with the most work wins. The other chain(s) die and the transactions on it have to be resubmitted. When multiple branches exist on the blockchain, some consensus process has to reconcile which of them wins. That process is that miners select the longest chain. Don't act as if one single block is magically appended to the chain and everyone knows which block that is; a consensus process operates to select that block (or sequence of blocks), and it operates by selecting the longest chain. When two or more blocks are mined out simultaneously by different miners, the one that wins is likely to be the chain that propagates more quickly to other miners. Since miners build on the longest chain, then the miner who distributes their chain more widely among other miners is likely to win, since those miners will begin building upon it (seeing it as the longest chain thus far). To continue the story, let's say that you and I both simultaneously mine out the next block of Bitcoin. You don't tell anyone about your block, and keep it to yourself. I distribute my block to the entire network. Thus, other miners start building on my block, and it becomes part of the main bitcoin blockchain. Once that happens, it doesn't matter if you start telling other people about your block -- it's not part of bitcoin any more because my chain exists which is much longer (my block + whatever other blocks people have mined out on top of it).
- thaumasiotes 8y ago
  > When multiple branches exist on the blockchain, some consensus process has to reconcile which of them wins. That process is that miners select the longest chain. I've been arguing that this is a design mistake, and that the following selection algorithm would be superior: - Instead of selecting the longest chain, select the longest chain which does not alter any blocks which I, personally, consider permanent. Robin_Message correctly identifies that this algorithm doesn't do anything for a new entrant, because a new entrant doesn't have the exposure to history that an established miner has. But I don't see why this is an issue, as to enter a particular network, you must already copy that network's existing blockchain, and this step -- which you already have to take -- will give you the same exposure to history as the miner you copy it from. Imagine that two blockchains, ETH and ETC, diverge for political reasons. The ETH blockchain is more beloved among the people (the laity, not the miners), and for this reason a greater number of transactions occur in ETH than ETC over time. This guarantees that the ETH blockchain will be longer than the ETC blockchain. By the selection algorithm you identify, the ETC blockchain will be wiped out, because the ETH blockchain can always be re-proposed and will always dominate it. But in reality, the ETC miners will reject the ETH blockchain, because rejecting the ETH blockchain is the whole point of ETC.
  - nearbuy 8y ago
    With a hard fork, you change the code the miners are using so that blocks on the opposing fork are considered invalid. In a 51% attack, the malicious actor creates new blocks that look equally valid. The two lineages aren't labelled ETC and NewETC. How does a new entrant (or anyone who wasn't connected for the few hours it took to launch the attack) distinguish between the chains?
  - Robin_Message 8y ago
    > guarantees that the ETH blockchain will be longer than the ETC blockchain. Chain length is number of blocks, which is controlled by miners, not "laity". I think your system might prevent double spends; unfortunately it would do so by rendering the network unusable for any new users, who wouldn't have a way to know which is the good network to join. I suppose a central authority could be used to identify the good chain, but most coins prefer not to have one of those. The advantage of the simple chain length rule is that it is simple, requires no central authority, is self-correcting and the only weakness is a 51% attack.