From the little I know about Knuth, while he is a bit of a joker, he doesn't seem like the type to make an announcement to hype people up. That either leads me to believe this will actually be something huge, or it's just a joke of some sort.
or perhaps he can break RSA in polynomial time or something like that
Pretty sure that's the same thing as proving P=NP.
No it's not. The factoring problem/RSA problem have never been proven to be NP-complete. Shor's algorithm shows that it's NP, but not NP-complete (or hard). I think it's also in co-NP.
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Yeah.... but like he said, RSA being in P does not imply that P=NP because RSA has not be shown (and isn't believed) to be NP-complete. If P=NP, then RSA would of course be in P.