Is Bitcoin Vulnerable On Asynchornous Networks?

Forget all the hubbub about who Satoshi Nakamoto is in person(s), something much more interesting has come up this week: a possible attack on the principle behind the technology underlying Bitcoin.

Nakamoto's blockchain is intended to enable consensus to be reached in a permissionless setting.
Although anyone can join or leave the protocol, the protocol should prevent “sybil attacks”.  To do this it relies upon solving a computational puzzles: the so called proof of work.  However, the assumption behind proving that this is sufficient to prevent attacks has always been that the network on which it operates is synchronous, something that is not quite true in the real world.  Likewise possible attacks have typically made the same assumption, such as those analysed by Yonatan Sompolinsky and Aviv Zohar.

Not surprising then that I was interested when a paper was published entitled Analysis of the Blockchain Protocol in Asynchronous Network.  Thankfully for all the Bitcoin fans it shows a degree of robustness in networks with limited delays, but outside of certain bounds it also demonstrates a simple attack which shows that the proof of work needs to be made harder.

No Honour Among Thieves (or Assassins)

The use of blockchain technology has increasingly focussed on uses other than cryptocurrencies.  One challenge being addressed is how do you deal with someone who you don't know, may never have met, and yet with whom you wish to exchange cryptocurrency for goods and services, especially as the transaction is ostensibly anonymous on both sides.  Well, the answer, many feel, is in the form of Smart Contracts, which can be supported by the blockchain itself.

However, as with so much in technology, smart contracts have a darker side.  A paper that popped up this week gives a very good summary and analysis of various scenarios in which smart contracts could be used between criminals.  The scenarios include everything up to an including hiring an assassin: how can you be sure that the assassin will do the job if you pay him or vice versa how can the assassin be sure of being paid if he kills the poor victim.

The paper, entitled "The Ring of Gyges: Investigating the Future of Criminal Smart Contracts" explores some ideas I had never thought of, but which are quite fascinating.  The types of criminal contract demonstrated in the paper are: