Generate public key from private key

[thewh1teagle]

Hi. Thanks for awesome library.
I wonder if it's possible to generate the public key from the private key?

[berkkirtay]

I have been using python-rsa for a while and I think it's not been implemented yet. It should be so easy to get public key from a private key since both of them depend on the number e, but to validate it, we also need to know prime numbers since e can't be bigger than the totient. Apologies if it's a feature already implemented.

[thewh1teagle]

@berkkirtay
I found the solution for that
with help from https://github.com/hvuhsg

pubkey = PublicKey(privateKey.n, privateKey.e)

[bliepp]

While this solution is totally fine I would consider it more error prone and less pythonic than a dedicated method or function. A syntax like

pubkey = privkey.public_key()

(see PR #187) encapsulates/black boxes the behavior into the PrivateKey class. In my eyes this makes totally sense since the public key is a direct derivation of the private key.