[chubot]

All the problems I've solved seem about equally hard, since I put my full effort into solving them :) I tend to go into new subfields where I don't have a background, so the hardness is probably just proportional to the length of time I spent on the project.

The problems I failed to solve seem are the ones that seem the hardest, of course. I tried to write a cluster manager / distributed OS by myself, starting almost from scratch, and that was too much. I spent upwards of 4 years on it, and had some success, but I'm starting to move on.

In particular, I learned that having a reasonable amount of security with reasonable amount of development effort in a distributed system is still an unsolved problem. It's basically a bottomless pit of work.