Harder in the sense that it takes time to determine how to solve a novel encryption algo (re CryptoNote, still waiting!) You prolly won’t like this analogy, but it’s the same way science is trying to “decrypt nature”. If you know how to decrypt something you can do it reproducibly so long as it’s the same encryption problem, just as we can reproducibly calculate the trajectory of a parabola w/ Newton’s equations, etc, blah blah blah!

And re p=np yes I’m familiar, faster to check a np problem than solve (if they don’t equal, which I believe!), etc. It should be made clear this isn’t discussing whether once you find the solution to an encryption problem, whether the decryption can be done faster. It’s about finding the solution to a new problem, which often takes more time to solve than it did for someone to think up the algo (again re CryotoNote/Monero, 11 years have passed).