On Thu, Apr 10, 2014 at 7:32 PM, Pieter Wuille <pieter.wuille@gmail.com> wrote:

If you trust hashrate for determining which UTXO set is valid, a 51%
attack becomes worse in that you can be made to believe a version of
history which is in fact invalid.

If there are invalidation proofs, then this isn't strictly true.

If you are connected to 10 nodes and only 1 is honest, it can send you the proof that your main chain is invalid.

For bad scripts, it shows you the input transaction for the invalid input along with the merkle path to prove it is in a previous block.

For double spends, it could show the transaction which spent the output.

Double spends are pretty much the same as trying to spend non-existent outputs anyway.

If the UTXO set commit was actually a merkle tree, then all updates could be included.

Blocks could have extra data with the proofs that the UTXO set is being updated correctly.

To update the UTXO set, you need the paths for all spent inputs.

It puts a large load on miners to keep things working, since they have to run a full node.

If they commit the data to the chain, then SPV nodes can do local checking.

One of them will find invalid blocks eventually (even if one of the other miners don't).

--
Pieter

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