> How can you rule out someone computing a longer block chain, when doing so involves simply duplicating the work done by the rest of the parties in the system

> How can you rule out someone computing a longer block chain, when doing so involves simply duplicating the work done by the rest of the parties in the system? What actually stops someone from doing this? I responded to another comment from you elsewhere, and I hate to confuse things by having one conversation in two places, but this is a really good question, and there is a straightforward answer. To make a longer blockchain, you have to have enough computing power to "catch up" to all the other people mining bitcoins, and then mine faster than they are. So, you have to have > 50% of the total hashing power. If you look into the mining "business," I think you will become convinced that only a nation-state or maybe Google could perform this kind of attack right now, and once ASIC mining technology is widely diffused, even that would be difficult. The "6 confirmations" thing the grandparent said is kind of arbitrary, but that is the number of blocks that bitcoin devs widely agree is enough to wait before you can be "effectively certain" that your transaction is in the longest block chain, and always will be. So that's how long you wait before handing over the keys when somebody purchases your mega-yacht with bitcoin. By "kind of arbitrary" I mean, in the context grandparent used it. If you do the math (as is given somewhere near the end of the bitcoin white paper), that is where you start approaching a 0% chance of the transaction not ending up in the longest chain.