Date: Fri, 19 Apr 2019 04:48:23 +0000
To: Ethan Heilman <eth3rs@gmail.com>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Improving SPV security with PoW fraud proofs
Precedence: list

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