I guess I always assumed that UTXO set commitments were an alternative security model (between SPV and full-node), not that they would cause the existing security model to be deprecated.
On Fri, Sep 18, 2015 at 3:43 PM, Patrick Strateman via bitcoin-dev < bitcoin-dev@lists.linuxfoundation.org> wrote: > Full nodes using UTXO set commitments is a change to the bitcoin > security model. > > Currently an attacker with >50% of the network hashrate can rewrite > history. > > If full nodes rely on UTXO set commitments such an attacker could create > an infinite number of bitcoins (as in many times more than the current > 21 million bitcoin limit). > > Before we consider mechanisms for UTXO set commitments, we should > seriously discuss whether the security model reduction is reasonable. > > On 09/18/2015 12:05 PM, Rune Kjær Svendsen via bitcoin-dev wrote: > > Currently, when a new node wants to join the network, it needs to > retrieve the entire blockchain history, starting from January 2009 and up > until now, in order to derive a UTXO set that it can verify new > blocks/transactions against. With a blockchain size of 40GB and a UTXO size > of around 1GB, the extra bandwidth required is significant, and will keep > increasing indefinitely. If a newly mined block were to include the UTXO > set hash of the chain up until the previous block — the hash of the UTXO > set on top of which this block builds — then new nodes, who want to know > whether a transaction is valid, would be able to acquire the UTXO set in a > trustless manner, by only verifying proof-of-work headers, and knowing that > a block with an invalid UTXO set hash would be rejected. > > > > I’m not talking about calculating a complicated tree structure from the > UTXO set, which would put further burden on already burdened Bitcoin Core > nodes. We simply include the hash of the current UTXO set in a newly > created block, such that the transactions in the new block build *on top* > of the UTXO set whose hash is specified. This actually alleviates Bitcoin > Core nodes, as it will now become possible for nodes without the entire > blockchain to answer SPV queries (by retrieving the UTXO set trustlessly > and using this to answer queries). It also saves bandwidth for Bitcore Core > nodes, who only need to send roughly 1GB of data, in order to synchronise a > node, rather than 40GB+. I will continue to run a full Bitcoin Core node, > saving the entire blockchain history, but it shouldn’t be a requirement to > hold the entire transaction history in order to start verifying new > transactions. > > > > As far as I can see, this also forces miners to actually maintain an > UTXO set, rather than just build on top of the chain with the most > proof-of-work. Producing a UTXO set and verifying a block against a chain > is the same thing, so by including the hash of the UTXO set we force miners > to verify the block that they want to build on top of. > > > > Am I missing something obvious, because as far as I can see, this solves > the problem of quadratic time complexity for initial sync: > http://www.youtube.com/watch?v=TgjrS-BPWDQ&t=2h02m12s > > > > The only added step to verifying a block is to hash the UTXO set. So it > does require additional computation, but most modern CPUs have a SHA256 > throughput of around 500 MB/s, which means it takes only two seconds to > hash the UTXO set. And this can be improved further (GPUs can do 2-3 GB/s). > A small sacrifice for the added ease of initial syncing, in my opinion. > > > > /Rune > > _______________________________________________ > > bitcoin-dev mailing list > > bitcoin-dev@lists.linuxfoundation.org > > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev > > > _______________________________________________ > bitcoin-dev mailing list > bitcoin-dev@lists.linuxfoundation.org > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev >
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