As far as cryptography is concerned, I think one of the best options these days is to teach the libsodium API. It's very rationally structured and well document

As far as cryptography is concerned, I think one of the best options these days is to teach the libsodium API. It's very rationally structured and well documented, and also available on every platform and in every language. Most importantly of all, there's nothing in there that you shouldn't use, which is one of the biggest problems with real world cryptography.