We'll use SSH for most secure communications (shell, git, etc.). So we'll have to generate a new SSH key for each new Mac system.
SSH client and server are already installed on Mac OS X.
The key generation process is interactive, requiring you to answer a couple questions, and to enter a passphrase.
ssh-keygen -C "$HOSTNAME"
Once you generate an SSH key pair, you'll need to copy the public key to any system that we need to log into.
Here's a list of sites and systems to copy the public key to: