brew install gpg # cli
brew cask install gpg-suite # macos pref pane + keychain integration
gpg --key-gen # make key
# produces this:
# /Users/cbnz/.gnupg/pubring.kbx
# ------------------------------
# pub rsa2048 2018-05-08 [SC] [expires: 2020-05-07]
# E2B0DD890A5926E124F8468A563E9ADC0A394F28
# uid [ultimate] Carl Bennett <carlbennettnz@gmail.com>
# sub rsa2048 2018-05-08 [E] [expires: 2020-05-07]
# using the last 8 chars of the id/hash/whatever:
git config --global user.signingkey 0A394F28 # use key with git
git config --global commit.gpgsign true # or per repo, or use `git commit -S` per commit