The problem is, you're missing the point. What they would do if they came across illegal content is irrelevant. They shouldn't be looking at the content in the first place, so this is a non-issue.
Ever. I don't have the original source in front of me, but with enough bits, assuming there isn't some fundamental flaw in the encryption algorithm, you couldn't brute force a key before the heat death of the universe even if you recruited every particle in the visible universe for your computation.
Big "if".