All other guides I've seen (https://github.com/drduh/YubiKey-Guide being the most prolific) tell you to use the Yubikey's smartcard (PKCS#11) features with GnuPG via gpg-agent.

STOP THE MADNESS!

OpenSSH has supported OpenSC since version 5.4. This means that all you need to do is install the OpenSC library and tell SSH to use that library as your identity.

Ensure you install the cask version of OpenSC, not the formula. The cask version is a .pkg which will install the shared library to a location acceptable by ssh-agent. The formula does not, as Homebrew installs each version into its own location and it won't allow an unknown path to be used as a PKCS#11 library.

brew cask install opensc
brew install ykman   

sudo apt-add-repository ppa:yubico/stable
sudo apt update
sudo apt install opensc
sudo apt install yubikey-manager

The ykman tool can generate a new management key for you. For the PIN and PUK you'll need to provide your own values (6-8 digits).

ykman piv change-management-key --touch --generate
ykman piv change-pin -P 123456
ykman piv change-puk -P 12345678

Make sure you save the generated password somewhere secure such as a password manager. The management key is needed any time you generate a keypair or import a certificate.

The PUK should also be kept somewhere safe. This is used if the PIN is entered incorrectly too many times.

I did this all on macOS 10.14. Linux distributions should work in a similar way. This is based on Yubico's instructions but uses the newer ykman utility instead of the older yubico-piv-tool. The older tool doesn't seem to support generating PIV certificates and gives misleading errors.

1. Ensure CCID mode is enabled on the Yubikey

ykman mode

If CCID is not in the list, enable it by adding CCID to the list, e.g.

ykman mode OTP+FIDO+CCID

(This assumes you had OTP+FIDO previously, and still want them enabled.)

2. Generate a PIV key and output the public key

ykman piv generate-key 9a pubkey.pem

Alternatively, you can require that you have to touch the Yubikey every time the slot is accessed:

ykman piv generate-key --touch-policy always 9a pubkey.pem

This is an RSA 2048-bit key by default. Depending which Yubikey you have, you can change it using -a / --algorithm.

(9a is the PIV authentication slot.)

3. Generate a self-signed X.509 certificate

ykman piv generate-certificate -s "SSH key" 9a pubkey.pem

4. Export your SSH public key from the Yubikey

ssh-keygen -D /usr/local/lib/opensc-pkcs11.so

And that's all the hard stuff done.

Now just add the public key to your authorized_keys file on a remote host and try to use it:

ssh -I /usr/local/lib/opensc-pkcs11.so -i /usr/local/lib/opensc-pkcs11.so -o IdentitiesOnly=yes server.example.com

You should be prompted for your Yubikey's PIV PIN.

You can add the PKCS11 library to ssh-agent.

ssh-add -s /usr/local/lib/opensc-pkcs11.so

Once more you will be prompted for your PIN, and from there SSH authentication will happen as usual.

To configure ssh to use the Yubikey's SSH key, use the PKCS11Provider config option instead of IdentityFile, e.g.:

Host foo
  PKCS11Provider /usr/local/lib/opensc-pkcs11.so
  IdentitiesOnly yes

• When SSHing, you may get prompted with the key's subject name, like Enter PIN for 'SSH key':. But if you add the key to the agent, you'll get a prompt like Enter passphrase for PKCS#11:. These are the same PIN (your PIV PIN).

• If you remove the key from ssh-agent using ssh-add -d or ssh-add -D, you'll have to either remove and re-add the PKCS library to the agent or restart the agent.

  • To re-add the library run

    ssh-add -e /usr/local/lib/opensc-pkcs11.so
    ssh-add -s /usr/local/lib/opensc-pkcs11.so
    

  • On macOS, you can restart the agent with

    launchctl stop com.openssh.ssh-agent
    launchctl start com.openssh.ssh-agent