[bitcoin-dev] Hash of UTXO set as consensus-critical

Fri Sep 18 20:17:40 UTC 2015

There are a couple of points I’d like to address.

Firstly, yes, >50% attacks are a problem for Bitcoin. Bitcoin does not function if the majority of mining power is dishonest. There is no way around that. It’s how proof-of-work functions. And if we lose proof-of-work, we lose Bitcoin.

Secondly, I’m not suggesting that UTXO set hashes *replace* block hashes, or even that it should be in the block header (probably in the coinbase somewhere). I suggest it as an *addition* to the existing consensus rules. Full nodes can still verify the chain with the added step of hashing the UTXO set for every block. Of course, this can easily be deferred to after proof-of-work has been verified already, such that no work is wasted. Unless a 51% attack is in effect. But I argue that this is a moot point, since Bitcoin is useless anyway under such circumstances.

Lastly, I’m not suggesting miners discard the blockchain history. A miner has an incentive to be absolutely sure that the chain he’s building on is the right one. If he’s wrong, he loses money/income. There’s simply no reason for a professional miner *not* to do the full initial sync, which only needs to be done once. Non-miners, who just want to check the balance of their wallet, however, really don’t need to retrieve information about Hal Finney sending bitcoins to Satoshi in 2010. In any case, this practice isn’t sustainable.

In the end, it isn’t possible to control whether a miner verifies the entire blockchain anyway (anyone can send the UTXO set over the wire). Not letting the proof-of-work cover the UTXO hash doesn’t solve this problem, it only makes it impossible to know whether a given UTXO set is the one that the majority is mining on without retrieving the entire blockchain, and doing the verification yourself. People can choose to skip that regardless of what we do.

Furthermore, all nodes have the option of deciding which level of security they want. We’re not lessening security of the protocol, we’re strengthening the security of something that’s already possible to do (build on top of an unverified blockchain), but we’d rather want that people not do.

/Rune

> On 18 Sep 2015, at 21:43, Patrick Strateman via bitcoin-dev <bitcoin-dev at 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
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