Using a Yubikey to Secure SSH on macOS (Minimalist Version)

SSH is critical in most people’s devops process, be it remote server logins or Git commits. After reading about one too many stories about companies getting hacked that way, I decided to use Yubikeys to store my private SSH keys.

You can either use the PIV- or OpenPGP module for this purpose. I decided to use the former because it’s better integrated and seems to be more reliable. There are a number of guides available online. They all required some tinkering and small adjustment for macOS. So here is my own complete guide.

Install Dependencies

Start by installing two required packages from Homebrew

$ brew install yubico-piv-tool opensc

Next you need to copy the OpenSC PKCS11 driver to a new location, so SSH-Agent can pick it up. By default Homebrew will symlink it, which does not work.

$ rm /usr/local/lib/opensc-pkcs11.so
$ cp $(brew list opensc | grep lib/opensc-pkcs11.so) /usr/local/lib/opensc-pkcs11.so

As a last setup step, add the following line to ~/.ssh/config, so SSH will pick it up when authenticating to a remote server. You can either add it at the top or below a Host example.com block to only apply to that host.

PKCS11Provider /usr/local/lib/opensc-pkcs11.so

Generate Private Keys and Store on Yubikey

You could generate the private key directly on the Yubikey and it will never leave the key. This is great for security but also means you can’t make a backup or copy it to a second Yubikey as backup. For that reason we will securely generate a private SSH key on a RAM disk and then copy it to two Yubikeys.

Start by creating a RAM disk and going into the mount point

$ diskutil erasevolume HFS+ RAMDisk `hdiutil attach -nomount ram://2048`
$ cd /Volumes/RAMDisk

Next generate a new private RSA key (only this specific format and length is supported) and a public key and certificate in the correct format.

$ ssh-keygen -m PEM -t rsa -b 2048 -o -a 100 -C yubikey -f yubikey
$ ssh-keygen -e -f ./yubikey.pub -m PKCS8 > yubikey.pub.pkcs8
$ yubico-piv-tool -a verify-pin -a selfsign-certificate -s 9a -S "/CN=SSH key/" --valid-days=3650 -i yubikey.pub.pkcs8 -o cert.pem

You should now see four files on your RAM disk. The commands below will copy the private key to a Yubikey and also add the self-signed certificate. The last step is mostly to comply with the PIV standard and not really related to the SSH login we want. You can repeat this step for every additional Yubikey you want to seed with this particular private SSH key.

You can customize the touch- and PIN policy to your linking. The command below requires a touch whenever the key is used.

$ yubico-piv-tool -s 9a --pin-policy=once --touch-policy=always -a import-key -i yubikey
$ yubico-piv-tool -a verify -a import-certificate -s 9a -i cert.pem

Using the Yubikey for SSH Logins

Now you are ready to log in to a remote server using the private SSH key stored on the Yubikey. To test the new setup, add the public key to ~/.ssh/authorized_keys or any other place appropriate for the service you are using. You can view the public key using either of those commands, even after you remove the RAM disk.

$ cat ./yubikey.pub  # public key saved on RAM disk
$ ssh-keygen -D /usr/local/lib/opensc-pkcs11.so  # dump directly from Yubikey

After adding the public key to a test server, log in like this:

$ ssh -v -I /usr/local/lib/opensc-pkcs11.so

If it works, you will see those lines and the Yubikey will start flashing to signal it’s waiting for a touch.

debug1: Offering public key: /usr/local/lib/opensc-pkcs11.so RSA SHA256:aeq9rAsbxxxxxxxFWG4 token agent
debug1: Server accepts key: /usr/local/lib/opensc-pkcs11.so RSA SHA256:aeq9rAsbxxxxxxFWG4 token agent

For convenience, you can link your hardware key with SSH-agent to avoid entering the PIN all the time. The first command will load the key, the second one will unload it. This will even survive prolonged hibernation. If someone removes the key or restarts the machine, a PIN will be required.

$ ssh-add -s /usr/local/lib/opensc-pkcs11.so  # add key
$ ssh-add -e /usr/local/lib/opensc-pkcs11.so  # remove key
$ ssh-add -L  # list available keys with public key

Now you should be ready to use the new, secure SSH key in production. Be sure to keep a backup on a second Yubikey in a save place and unmount the RAM disk after validating it works.

Here some usage ideas. You can use the key in any place that uses SSH.

  • SSH login to important production servers
  • Secure SSH proxy to a bastion inside a private network
  • Secure backups with BorgBase.com. You could set all server-keys as append-only and use the Yubikey for full access for pruning.
  • Login to a Git code repo. Be sure to use SSH, not HTTPS.

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