@kalle (1) Nitpick: the attack isn't quite free if he succeeds because Joe must include Td in his block where he could've instead included someone else's transaction that paid him a fee.
(2) By implication, the closer typical fees are to the amount of the attack, the less profitable a multi-transaction attack will be. E.g., if fee/tx is $1, then the $100 Lego attack only nets $99 compared to honest mining.
Besides that nitpick, the article's logic and conclusions seem accurate to me.
@harding Thanks! Good points, especially (2) might turn out really significant. I'll update the article tonight.
I haven't heard much about this kind of attack. Have you seen anything on this elsewhere?
@kalle Double spending lots of smaller transactions? I think it's been regularly mentioned. E.g., here's an excerpt from a StackExchange answer I wrote in 2014. https://bitcoin.stackexchange.com/questions/32562/when-to-worry-about-1-confirmation-payments/32564#32564
Also, the actual BetCoin Dice attack that stole ~$100,000 USD involved the double spending of hundreds of smallish transactions, so it's not just theory. (Though those double spends were against 0-conf transactions, but still with miner assistance.) https://bitcointalk.org/index.php?topic=327767.0
@harding Updated with your comments. Thanks a lot for your feedback. https://popeller.io/index.php/2020/11/22/1000-lego-sets/
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