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.