return a private key generated using rsa or ecdsa.

returns 'public part' of the passed asymmetric encryption algo key 'priv'.

read and parse private key from 'keyFile', return the 'priv' key in the form of either *rsa.PrivateKey or *ecdsa.PrivateKey.

write private key 'priv' into 'keyFile' in ASN.1 DER format type of 'priv' can be *rsa.PrivateKey or *ecdsa.PrivateKey.