Ethereum Classic Double Spend Attack?

Yesterday, I warned that double spend attacks were cheap and particularly likely for smaller coins using standard hash algorithms. Coincidentally (?) later that day there was this:

We can confirm that there was a successful 51% attack on the Ethereum Classic (#ETC) network with multiple 100+ block reorganization. We recommend all services to closely monitored the chain and significantly increase required confirmations.

— Bitfly (@etherchain_org) January 7, 2019

It’s not entirely clear whether that is true or if there is an alternative explanation. Coinbase, however, says that approximately $500,000 was double spent. You can find a good discussion on Hacker News.  You can also find an interesting calculation of the cost of renting enough hashing power to 51% dominate various networks here. It’s cheap. The costs given are underestimates in one respect since they don’t include block rewards but overestimates in another as renting may not always be possible.

Here’s some back of the envelope calculations on the cost of the ETC attack. If I am reading the blockchain stats correctly, ETC has a block time of about 15 seconds and the chain was reorganized almost to a depth of 100 blocks or 1500 seconds, i.e. 25 minutes. The cost of dominating the ETC hasing power for an hour is around $5000. Thus, this attack could have been very profitable, even adding in substantial setup costs. Feel free to write in the comments if these numbers look wrong.

As I mentioned yesterday, it’s not surprising that this is happening now because with massive falls in prices in most cryptocurrencies there is an excess supply of computation. Expect more stress testing this year.

Hat tip: The excellent Jake Seliger.