Found and fixed the bug in oscrypto:
https://github.com/wbond/oscrypto/pull/61
@DNSB: You were correct, there was a bug in how oscrypto was loading the legacy provider, resulting in it not getting loaded at all.
I'll wait for a couple days and see if the maintainer sees my PR and merges it into a new release - if not, I will include a patched version of oscrypto in my plugin until this is merged.