I feel like "global passive adversary" is kind of like a True Scotsman. There doesn't seem to be a fixed definition; rather we work backwards from random attacks and determine whether we think they were global adversary worthy or not, and if so then that makes the perpetrator a global adversary.
Could Lizard Squad have executed this attack? (I assume anybody with a botnet could start new Tor relays, so yes.) Is Lizard Squad a global adversary?
A global passive adversary is anyone who can execute a Sybil attack, basically. So the required size scales with the size of the network. A global passive adversary for a network with only ten nodes would only have to have five nodes itself.
The "global" adjective is just used, I think, because cryptographers presume a production deployment of the cryptosystems they discuss would be something like the Web: large enough (millions of nodes) to require globe-spanning resources (millions of other nodes owned by a single group) to execute the attack successfully.
Seen under that lens, neither Tor nor Bitcoin nor any other modern cryptosystem needs a "global" passive adversary to break it. Just a regular "passive adversary."
That's really interesting. So if there were multiple "global passive adversaries" then the network would become stronger and stronger? At least until one gives up and removes all their nodes at once.
Imagine if China (used for population reasons) managed to send 300 million spies to the US to socially-engineer their way into all US citizens' personal lives. Now imagine India (again, for population reasons) simultaneously trying the same thing: now, one half of the time, the Chinese are just spying on "American citizens" who are really Indian spies, the Indians are just spying on Chinese spies, and one half of Americans go unmonitored.
It's sort of the same game-theoretic advantage you get from participating in a battle royale competition over participating in a 1v1 competition: for each new adversary you face, that adversary is also dragged down by all the other adversaries and becomes that much easier to deal with.
This really only applies specifically to Sybil attacks, though.
Could Lizard Squad have executed this attack? (I assume anybody with a botnet could start new Tor relays, so yes.) Is Lizard Squad a global adversary?