Selfish mining network with gamma = 0 is broken

[pkel]

The selfish mining network as described in the following blog post is broken. Gamma=0 results in a devide-by-zero and hence unbounded message delays from attacker to defender. These message delays weaken the attacker.

https://pkel.github.io/cpr/blog/generalizing-selfish-minings-gamma/#constructing-a-network

---

As a hotfix, I've decreased the epsilon to 1e-9. This enables gamma=0.01 without risking significant message delays.

e7521ad#diff-d63bdccebde9f15fb0baea8999c8565dfe15c0cb0dbcf37ef3a896770550c290

---

It would be better to sample the message delays from a different interval. Maybe fix the interval length, then move it up or down to meet the given gamma. I recall that we had something like this before. Could be worth browsing the history of the network implementation.