Hi,

After studying several Proof of Stake implementations I think it's not only an eco-friendly (and more ethical) alternative to Proof of Work, but correctly implemented could be 100% secure against all 51% history rewrite attacks. Over a "standard" PoS protocol like PoS v3.0, only 2 extra improvements are required:

- Hardcoded checkpoints: each Bitcoin Core release (each few months) should include a hardcoded checkpoint with the hash of the current block height in that moment. This simple measure protects the blockchain up to the last checkpoint, and prevents any Long-Range attack.

- Moving checkpoints: the nodes only allow chain reorgs not deeper than N blocks. If N is 10 blocks, then the nodes ignore any hard fork starting at any block under nodeBlockHeight - N. This fully protects nodes that are online and updated. Nodes that are not fully updated need some extra rule to be protected between the last hardcoded checkpoint and the current blockchain height. This extra rule could be connecting to a block explorer to download the hash of the current block height, or ask some trusted source like a friend and enter the hash manually. After being fully updated, the user can always check that he is in the correct chain checking with a block explorer.

Someone could have 99% of the coins and still would be unable to use the coins to do any history rewrite attack. The attacker could only slow down the network not creating his blocks, or censor transactions in his blocks.

What do you think? :)

Regards