I've heard that government agencies et al. have not been able to crack Tor. Why have they been unable? What would one have to do to crack Tor? Are there any Tor "stress tests"?
Well, first of all - there's NO unbreakable things in this world, everything can be cracked - it's just a question of resources applied and the time it consume. So - if something is not cracked yet, it only means that nowdays it's too laborious and long to do this. To crack Tor an attacker need to crack in real time AES cryptography and then triple-use it to decode the traffic. To crack AES is very laborious and no algo doing so is made so far.