Re: [bitcoin-dev] Trusted merkle tree depth for safe tx inclusion proofs without a soft fork

[Sergio Demian Lerner <sergio.d.lerner@gmail.com>]

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Yo can fool a SPV wallet even if it requires a thousands confirmations
using this attack, and you don't need a Sybil attack, so yes, it impacts
SPV wallets also. The protections a SPV node should have to prevent this
attack are  different, so it must be considered separately.

It should be said that a SPV node can avoid accepting payments if any
Merkle node is at the same time a valid transaction, and that basically
almost eliminates the problem.

SPV Wallet would reject valid payments with a astonishingly low probability.



On Sat, Jun 9, 2018 at 2:45 PM Peter Todd <pete@petertodd.org> wrote:

> On Sat, Jun 09, 2018 at 02:21:17PM +0200, Sergio Demian Lerner wrote:
> > Also it must be noted that an attacker having only 1.3M USD that can
> > brute-force 72 bits (4 days of hashing on capable ASICs) can perform the
> > same attack, so the attack is entirely feasible and no person should
> accept
> > more than 1M USD using a SPV wallet.
>
> That doesn't make any sense. Against a SPV wallet you don't need that
> attack;
> with that kind of budget you can fool it by just creating a fake block at
> far
> less cost, along with a sybil attack. Sybils aren't difficult to pull off
> when
> you have the budget to be greating fake blocks.
>
> > Also the attack can be repeated: once you create the "extension point"
> > block, you can attack more and more parties without any additional
> > computation.
>
> That's technically incorrect: txouts can only be spent once, so you'll
> need to
> do 2^40 work each time you want to repeat the attack to grind the matching
> part
> of the prevout again.
>
> --
> https://petertodd.org 'peter'[:-1]@petertodd.org
>

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