The article is a bit confusing (to me as a layman), basically as I understand it's saying that the reason we can't form cryptosystems out of arbitrary NP-complete problems is because there exist randomized algorithms that work for 99% of inputs (average vs. worst-case complexity, e.g. why the simplex algorithm works in practice). But there are some things missing:
* We know that randomized algorithms exist for NP-complete problems. But couldn't we have some problem in NP that while reducible to e.g. 3SAT always reduce to "very hard" instances?
* Conversely, there's no guarantee that there doesn't exist a randomized algorithm for something that's NP-hard (and not in NP), right?
The argument along the lines of NP-complete vs NP-hard seems to be a non-sequitur, since the actual distinction is average vs. worst-case hardness right? It may just so happen that all currently known problems in NP are easy "on average", but I don't think that's formally guaranteed.
* We know that randomized algorithms exist for NP-complete problems. But couldn't we have some problem in NP that while reducible to e.g. 3SAT always reduce to "very hard" instances?
* Conversely, there's no guarantee that there doesn't exist a randomized algorithm for something that's NP-hard (and not in NP), right?
The argument along the lines of NP-complete vs NP-hard seems to be a non-sequitur, since the actual distinction is average vs. worst-case hardness right? It may just so happen that all currently known problems in NP are easy "on average", but I don't think that's formally guaranteed.
Edit: https://theory.stanford.edu/~trevisan/average/slides.pdf seems to imply that (1) is an open question.