I often see the 51% attack described by saying that if Bitcoin is presented with two conflicting chains, it will select the longest one. This would seem to work because difficulty only re-adjusts at certain block intervals, meaning more hash power makes mining faster possible, until difficulty adjusts, at least.

On PoS type coins, difficulty re-adjusts every block, so even if you bring a bigger gun in terms of more coin or coin-age, you should theoretically, even on a shorter scale mint the same numberof blocks as the 'main chain'. So if there is no 'longer chain', how doe PoS select a winning fork in the case that one occurs?

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Usually longest chain is selected as main chain.The other condition is if two users find two blocks at same time.The block which is approved by maximum nodes in the network will be considered and will be added to longest chain.And the other block is considered as orphan block and considered as forked chain.


I have concluded that it is the chain with the most 'work' which is in fact the same as PoW. 'Longest' chain is a misnomer that comes from the infrequent difficulty retarget in Bitcoin. What is actually meant is the chain with the highest Difficulty over the entire chain. So for example if you are at block 1000 and two chains occur, even though block 1001a and 1001b may occur, and even if there is a split in consensus over several blocks until you reach for example blocks 1005a and 1005b, the chain with the highest aggregate Difficulty since the genesis block will be the one that nodes will accept. The reason is that higher Difficulty implies more coin-age/more hashes-per-second, meaning most work was spent on building that branch.

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