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

Peter Todd pete at petertodd.org
Sat Jun 9 12:45:16 UTC 2018

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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