IANAL, but from a EU-centric perspective on copyright (which is the only one I can reliably talk about) the idea of a proprietary encryption key is bogus. A creative work can be copyrighted if it has sufficient originality (or under some other very specific conditions). Smaller parts of such a work are not copyrighted if they don't meet that criteria on their own. The encryption key (which is very probably randomly generated and definitely not a creative work) thus can't be copyrighted on it's own. At least in the EU, there should be no argument against sharing said key (at least in respect to copyright).
I honestly can't talk about other jurisdictions (maybe someone else here can) but I imagine it should be similar to this in many other countries.
I wrote about sharing but even profiting from it should be legally permissible.
Of course not. There are laws against that. Laws that are not copyright laws.
Oh, sorry, I thought this was about legality. If we want to talk about the morality of evading copyright we should also about the morality of copyright itself, how it historically came to be and whose interests it was supposed to serve (it wasn't made to support creatives). Actually there is surprisingly little evidence that the introduction of copyright increased the incentives for creatives to publish or made them wealthier (except a select few). I think there is a better case to be made for the morality of sharing creative works unlawfully than for limiting the sharing of those works for a century after their creation.