Install things

We’ll install a bunch of stuff with Homebrew:

• git to get a newer version than what Apple ships

• ykman to view and tweak Yubikey configuration

• pinentry (to pop up a window to enter your Yubikey PIN when required)

• GnuPG 2 to take care of doing all the cryptography

brew install \
    git \
    ykman \
    gnupg \
    pinentry \
    pinentry-mac 

Setup Yubikey

A modern Yubikey probably supports a lot of different interfaces: OTP, PIV, OpenPGP, FIDO U2F, FIDO2, OATH. Many have one or more PIN that help to prevent unauthorised usage. If you haven’t already, you should configure the various codes for all interfaces configured on your Yubikey.

We’re focussed on setting up OpenPGP so let’s just take care of that.

You can use ykman to check the current policy for OpenPGP on your Yubikey:

$ ykman openpgp info
OpenPGP version:            3.4
Application version:        5.4.3
PIN tries remaining:        3
Reset code tries remaining: 0
Admin PIN tries remaining:  3
Require PIN for signature:  Once
Touch policies:
  Signature key:      Off
  Encryption key:     Off
  Authentication key: Off
  Attestation key:    Off

You can use gpg --edit-card to modify the various passwords. When started, it’ll output the current card configuration and prompt for commands. Use admin to enter administration mode, then passwd to control the card passwords.

If you have a brand new card will have the following details:

• PIN: 123456
• Admin PIN: 12345678
• Reset Code: NONE
• Retries: 3

$ gpg --edit-card

...

gpg/card> admin
Admin commands are allowed

gpg/card> passwd
gpg: OpenPGP card no. D2760001240100000006196516380000 detected

1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit

You can change the number of attempts allowed for the PIN, reset code, and admin PIN:

$ ykman openpgp access set-retries 10 10 10

Create new keys

The general process looks something like:

$ gpg --gen-key
$ gpg --expert --edit-key KEY_ID
gpg> addkey
# Select whichever "set your own capabilities" option you like; it's probably a
# good idea to use the same key type as your main key.
#
# Enable Authenticate, disable the other actions (Sign and Encrypt)
#
# Set the key expiry to the same as the first key. In my case it was 3y

Export your key so that you can keep a backup off-line somewhere. Make sure that it is safe, secure, and offline. Print it out or write it on a CD or something and keep it with your important papers.

Then you can move the new keys to your Yubikey:

$ gpg --edit-key KEY_ID
# Switch to viewing private keys:
gpg> toggle
# First, move the primary key to the Yubikey. The key list will show usage of SC
# denoting a signing key, so move it to the "Signature key" slot.
gpg> keytocard
# Then select the first sub-key.
gpg> key 1
# The key list will show usage of E denoting encryption, so move it to the
# "Encryption key" slot in the Yubikey.
gpg> keytocard
# Finally, deselect the first sub-key and select the second sub-key:
gpg> key 1
gpg> key 2
# This sub-key will have usage "A", so move it to the "Authentication key" slot.
gpg> keytocard

At various points in this process, GnuPG will ask for the private key passphrase (if you set one when generating the key) and the Yubikey Admin PIN.

Setup GnuPG

echo "pinentry-program $(which pinentry-mac)" >> ~/.gnupg/gpg-agent.conf
killall gpg-agent

Test signing

You can verify that GnuPG is configured and working correctly by signing a sample message:

$ echo "Hello world" | gpg --clearsign
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

Hello world
-----BEGIN PGP SIGNATURE-----
...
-----END PGP SIGNATURE-----

Depending on the settings for your key, you may need to enter your Yubikey PIN (usually only the first time you’ve used it this session) and/or touch the key (see the ykman settings above to control this).

Configure git

You need to tell git which key to use when signing things and might like to set a few other options which control when things are signed and when signatures are displayed:

$ git config --global user.signingkey KEY_ID

Some settings which may help git to run GnuPG correctly:

$ git config --global gpg.program gpg

Some settings that control when git will sign things and display signatures:

$ git config --global log.showSignature true
$ git config --global commit.gpgSign true
$ git config --global tag.gpgSign true

GitHub configuration

Export your public key (note the PUBLIC KEY in the output below) and copy the output.

$ gpg --export --armor KEY_ID

-----BEGIN PGP PUBLIC KEY BLOCK-----
....
-----END PGP PUBLIC KEY BLOCK-----

Go to the “SSH and GPG keys” page of your GitHub settings and click add New GPG key. Paste the key into the text area, add a useful comment to help identify the key, and click the save button.

When you use the GitHub.com web-site to commit changes to your code, GitHub signs your commits with an internal key. To validate these commits, you’ll need to import and sign the key:

$ curl https://github.com/web-flow.gpg | gpg --import
$ gpg --lsign-key noreply@github.com

References

• Yubikey PGP Card edit

• Yubikey PGP Importing keys

• Yubikey PGP Git signing

• ykman openpgp manual

• Trusting GitHub’s key

• YubiKey Guide

• git-commit(1) man page

• git-tag(1) man page

• git-log(1) man page