[turc1656]

This is some great info for the less technically knowledgeable about Tor (like me!). However, I think your math in #3 is wrong.

Assuming random assignment/selection of the guards, each time one is chosen it has a 98% chance of not being "caught" by choosing an adversary's guard. Going with 50 services as you said would be .98^50=.364, meaning the chance of getting caught is 1-.364=.635 - 63.5%. This is vastly different than being nearly 100%.

[ohmygodel]

Fair enough! I was using as a heuristic the expected number of compromised guards, which would be 0.02*50 = 1. Moreover, things degrade exponentially over time. If half the guards rotate every month, the chance of choosing a bad guard is after 2 months is >86%, after 4 months is >95%, after 6 months is >98%.