[bitcoin-dev] Purge attacks (spin on sabotage attacks)

ha su hasu.research at gmail.com
Fri Jan 31 13:38:22 UTC 2020

Hi all,

I think I discovered an interesting form of sabotage attack (possible for
miners) that tries to create coordination disincentives among Bitcoin users
- named after the dystopian movie The Purge, where all crime is legal for
one night every year.

* An attacker replaces the most recent blocks full of transactions with
empty blocks.
* Previously confirmed txns return into the mempool, where anyone with a
minimum of technical knowledge or access to public tools can
opportunistically double-spend their txns back to themselves. (the process
is the same as double-spending regular zero-conf txns)

The attack seems useful to undermine trust in Bitcoin's assurances, e.g.
the future finality of transactions. It differs from other forms of
sabotage (e.g. DoS by mining only empty blocks) in that it specifically
disrupts the coordination process among users in response to the attack.

By giving some users a chance to benefit from the attack, the attacker
gives them a vested interest in staying on the attack chain. If enough
users accept the invitation to double-spend, it might become harder to come
to consensus on how to deal with the attack.

Purge attacks probably don’t constitute a bigger risk than other known
forms of sabotage attacks, but seem like an interesting spin where the
attacker specifically targets the pre-coordination of defenders.

You can find the full report, incl. some mitigations against sabotage
attacks, at

Your feedback is highly appreciated.

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