Date: Sat, 9 Jun 2018 08:45:16 -0400
From: Peter Todd <pete@petertodd.org>
To: Sergio Demian Lerner <sergio.d.lerner@gmail.com>
Message-ID: <20180609124516.6ms6t7r5t7ikved6@petertodd.org>
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Cc: bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Trusted merkle tree depth for safe tx inclusion
 proofs without a soft fork
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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 acce=
pt
> more than 1M USD using a SPV wallet.

That doesn't make any sense. Against a SPV wallet you don't need that attac=
k;
with that kind of budget you can fool it by just creating a fake block at f=
ar
less cost, along with a sybil attack. Sybils aren't difficult to pull off w=
hen
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.

--=20
https://petertodd.org 'peter'[:-1]@petertodd.org

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