I might write this up as a full tutorial someday, but there's already a few of those out there. That said, here's a short outline of my current usage of PGP, aided by modern GPG and the OpenPGP smartcard functionality of a Yubikey 4.
- Use GnuPG 2.1. Private keys are stored in a
.ddirectory, can act as an ssh key agent, and you can forward your local
gpg-agentto a remote server. Oh, and it supports OpenGPG smartcards.
- Use OpenSSH > 6.7. Makes
gpg-agentforwarding much easier.
- Use contemporary GnuPG configuration settings Riseup used to have a good guide for this, but it's sadly vanished behind HSTS. But this YubiKey-Guide has the settings, if not the explanation. It's also a fairly comprehensive step-by-step set of instructions for the entire process.
- Keep backups of your master keypair and revocation certificates. Pretty straightforward, not only will you need this to, say, load a new Yubikey or change a subkey, you'll also need this to sign anyone else's keys. I keep three copies at all times, with one always in a bank safe deposit box.
- Generate your master key offline. A Raspberry Pi not plugged into any network is a great tool for this, although you'll most likely have to bang your hands on the keys quite a bit to generate enough entropy for key generation.
- Use an imense passphrase on your offline key. This is very easy, since you'll only need to actually use this to a) update any subkeys; b) sign anyone else's key; c) push your subkeys into your Yubikey. And the Yubikey will be protected by a) being a physical thing that; b) must be plugged in; c) unlocked with a six digit PIN; d) and touched to actually do anything. Speaking of....
- Use the yubitouch utility to require touch. You can find that here. I use the mode where you have to touch the Yubikey for all three subkey usage, and fix it so that the setting can't be changed without re-loading the key material. This can be slightly paranoid, and I do wish it had a mode to "require a touch if it hasn't required a touch in the last N seconds". But I do like knowing that every use of my Yubikey requries me to physically touch it.
- Make a 'transfer' copy of your GNUPGHOME to load subkeys onto your Yubikey The process of loading your subkeys into a Yubikey replaces the secret key material with a pointer that says "this subkey is in OpenPGP card with the serial number...", and traps it in the Yubikey (by design).
- Use git to track your offline keys This has saved me from at least one blunder, and it gives me a history of what I've been doing to the keys over time.
- Set your key expiration to a fixed date, and update every few months You can set a key to expire in, say, two years, and then three months later, move the expiration date forward three months, etc. This has got a couple useful side effects. One, if for some reason you lose control of your key, it at least will go away sometime. Two, it forces you to touch your master key at least semi-occasionally. In my setup, I touch all three copies of my master key once every three months, so I'll be able to recover if one of the USB thumb drives decides to give up the ghost. Much better than leaving a drive in a drawer and five years later learning that it's unreadable.
- CHANGE BOTH PINS ... but after you've done all the card setup. Many of the things above will require you to enter the unlock and/or admin PIN, and it's much easier to type '123456' or '12345678' for all of this. Make a good PIN, don't make it something easily guessable, etc. etc. In used three 2d10 rolls to make mine.
- Entering the PINs too many times doesn't brick the card. We had some confusion about this at work, and thought we'd bricked a card. It turns out that entering the regular PIN enough times just makes it so that it won't do anything other than allow you to use the admin PIN to reset the regular PIN. And if you enter the admin PIN wrong three times, it just wipes the key material from the key and resets it to factory defaults. In fact, I'm fairly certain that the script
- Other info from Yubico
- All of this owes a great deal of debt to Alex Cabal's Generating the Perfect GPG Keypair, which got me thinking all about this in the first place a few years ago.