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In Proof-of-Work, a block hash is costly to manipulate (say, wanting the hash to be even, or have the hash of the hash smaller than some number) since there is a lot of computing power put into selecting the right header to produce a valid hash.

I'm wondering - are the block hashes in the various Proof-of-Stake implementations easier to manipulate since the hashes aren't bound by PoW mining? Could a PoS miner spend some computational time to generate many valid blocks and pick one to use, or are there some other constrains preventing block hash fudging?

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Absolutely. ECDSA signatures at not unique, they can always be permuted by the signer at will by re-signing with a new nonce. The creator of the block can also choose to reorder or omit transactions to alter the block and then re-sign this to get a new block hash.

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  • Does that affect security of blocks/blockchain in PoS systems? Changing a particular block hash, would require recalculating the hashes of blocks on top of it - no?
    – Albert S
    Dec 24, 2016 at 1:29
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    This is called "stake grinding" and demolishes the security of PoS systems.
    – Claris
    Dec 27, 2016 at 21:20

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