man rust-crypto is a bit of a terrible.
hasn't been updated in 6 months.
heaps of fun compiler bugs.
here's the one that bit me:
https://github.com/DaGenix/rust-crypto/issues/390

luckily this is only me learning - not something for production.

It's concerning that the choices are:
rust-crypto - maintainer seems to have left the building
rust-openssl - openssl... 🤔
ring - limited algo support at present.