hardening PGP using GnuPG and Yubikey

the reasons for encryption are very obvious, so moving beyond this we will talk about: subkeys: how to manage them, generate them, and parameters essential in their functionality that may require considerable planning. HSM: why do we have these, do we always need them, challenges and concerns in their implementation and maintenance. revocation: to revoke or not? how do we renew? why should keys expire? key length considerations: not all keys are created equal, why would we use weaker keys? stronger ones? a PIV implementation of OpenPGP: what is PIV? how does it keep us safe? how does the PIV model of security work with the understood principles of openpgp and gnupg in Linux? how to maintain a PIV implementation of gnupg generated subkeys and the challenges in maintaining PIV style cryptographic environments.