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To: Ethan Heilman <eth3rs@gmail.com>
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Subject: Re: [bitcoin-dev] Improving SPV security with PoW fraud proofs
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Good morning Ethan,
> My above email contains an error. The SPV client needs to only
> download S+1, not S+1 and S+2.
>
> I agree with you that a weakness of this approach is a miner can make
> SPV clients do substantially more work. However:
>
> 1. Mining a block which will never be accepted is an expensive way to
> make SPV clients download, validate and discard ~2-4 megabytes of
> data. There are far less expensive ways of wasting the resources of
> SPV clients. Its unclear why someone would want to do this instead of
> just packeting full nodes or SPV servers like we saw with the recent
> DDoS attacks against electrum servers.
>
> 2. SPV clients may not even learn about these splits because it
> requires that someone relay the split to them. Honest full nodes
> should not relay such splits. To their bitcoin's worth the attacker
> must also connect to lots of SPV clients.
>
> 3. Having SPV clients slow down or become full nodes when a malicious
> miner with significant mining power is attempting to disrupt the
> network is probably a best case outcome. I would prefer this failure
> mode to the current SPV behavior which is to just go with the
> "longest" chain.
I understand.
It seems a reasonable point to do so.
As I understand it, this requires that UTXO commitments be mandatory.
In particular, if UTXO commitments were not mandatory, it would be trivial =
to force chainsplits at heights where a UTXO commitment was not made, and f=
orce an SPV node to download more blocks backwards until a block with a UTX=
O commitment is found.
More difficult is: how can an SPV node acquire the UTXO set at a particular=
block?
Fullnodes automatically update their UTXO set at each block they accept as =
tip.
Reversing the blocks to update the UTXO set at a particular past time would=
require a good amount of CPU and memory.
Thus any service that can provide the actual UTXO set at each block would p=
otentially be attackable by simply requesting enough past blocks.
Regards,
ZmnSCPxj
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