Using GPG for SSH Authentication

It's a fairly simply process to have gpg-agent handle your SSH authentication. To start off, you'll need to have a private GnuPG key generated with an appropriate subkey for authentication. Once that's taken care of, open up ~/.gnupg/gpg-agent.conf

$ cat ~/.gnupg/gpg-agent.conf
    enable-ssh-support
    default-cache-ttl 60
    max-cache-ttl 120

Now you'll need to append the following to ~/.bashrc, or the appropriate rc file for your shell

$ cat ~/.bashrc
    export GPG_TTY="$(tty)"
    export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket)
    gpg-connect-agent updatestartuptty /bye > /dev/null

Once that's done, you'll need to let gpg-agent know which GnuPG subkey to use for SSH authentication. Run the following and copy the keygrip associated with the subkey you've generated specifically for authentication. Don't use my keygrip, however. The output here is just for an example. GnuPG computes the keygrips from the public key, so nothing here is sensitive or private.

$ gpg --with-keygrip -k ben@gbmor.dev
    pub   rsa4096/0xEAB272409CD12FF0 2018-11-25 [SC]
        Key fingerprint = 291A AFF7 A291 7DAB 0E01  6B9C EAB2 7240 9CD1 2FF0
        Keygrip = DE06FAA273017BBD8778F94639611CEF53AB9EBC
    uid                   [ultimate] Ben Morrison <ben@gbmor.dev>
    sub   rsa4096/0xF9C3B650612249D9 2018-11-25 [E]
        Keygrip = 751ADAC109736316B6ABEBB3F2BDF4612F8A630C
    sub   rsa4096/0x4969E5731CFEB507 2018-11-25 [A]
        Keygrip = 44D1BDC0C1931E2E018E7CE49CDE14BFB4EA11E3
    sub   rsa4096/0x8F192E4720BB0DAC 2018-11-25 [S]
        Keygrip = 240966CBF2791D8C34D0DA646925435FED49F9BF

Now, open ~/.gnupg/sshcontrol and paste the keygrip into that file. It's the keygrip just below the key marked [A] for authentication. Verify that the correct keygrip has been selected by running these two and comparing the output:

$ ssh-add -L
    ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAACAQCakJKfXUuX/ZDxJQySdxCeQfxTu0g
    KPCESGDyadvFAPDxtcTfOrxfqJLZx8CodkC7hzHT/QEy/xMgN18Q== cardno:000609861127

$ gpg --export-ssh-key <keyid>
    ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAACAQCakJKfXUuX/ZDxJQySdxCeQfxTu0g
    KPCESGDyadvFAPDxtcTfOrxfqJLZx8CodkC7hzHT/QEy/xMgN18Q== openpgp:0x1CFEB507

The ssh output should match the gpg output (except maybe the little trailing comment, like here). Also, I've removed most of the public key I'm using as an example for brevity's sake. It should be quite a bit longer than this. If ssh is correct, kill off gpg-agent

$ pkill gpg-agent

Then open up a new terminal and attempt to connect to a server!

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Using GPG for SSH Authentication

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