Crypto-noob here: let's say they broke SHA-256. Even then, they'd need to know the hash in advance, which they don't, right? It's really hard for me to follow, I'm getting old :(

EDIT: Looks like no, they don't need to know the hash. Rasulek wrote a great overview here: https://news.ycombinator.com/item?id=39818305

I'm still confused about the leading zeros thing. I thought the article author was using a leaky abstraction. It sounds suspicious/notgood that you have a significant amount of info up front. But I'm just a noob free-associating.