[Cryptography] Security model of blockchains ?

[John Levine]

In article <b7b2bffa-47ae-3f50-fbf5-08a248c79680 at agner.io> you write:
>"Hijacking Bitcoin: Routing Attacks on Cryptocurrencies" ->
>https://btc-hijack.ethz.ch/

Thanks, that was the sort of thing I was looking for, assumptions in
the security model that may not be true, and bad things happen if they
are indeed not true.

It is my impression that blockchains generally assume a flat mostly
reliable connection among nodes.  This paper says that 13 ISPs host
30% of the whole Bitcoin network, and 60% of connections cross three
ISPs.  I wonder who and where those three are.

R's,
John