Using a Yubikey for SSH on macOS

May 18th, 2020

SSH 8.2 introduced support for using any U2F key in place of a private key file. Using it on macOS with full support for ssh-agent is a bit more complex.

Generating the keys

1. You must choose between ed25519-sk and ecdsa-sk. Try ed25519-sk (Options 1 or 3) first. If it does not work due to device incompatibilities, fall back on ecdsa-sk (Options 2 or 4)

2. You must choose if you want to store the key handle as a resident key on the device. If you want to, use options 1 or 2. If not, use options 3 or 4.

   A U2F attestation requires a key handle to be sent to the device. When generating the key, ssh-keygen will create private and public key files that look similar to normal ssh key. The private key file is actually a key handle that cannot be used without the hardware token, however, the hardware token can also not be used without the key handle.

   A resident key solves this problem by storing the key handle on the device. However, your key may or may not support it and only a limited number of resident keys may be stored on a device. Additionally, it may reduce the security of your ssh key as they could use it if they steal the hardware device. For this reason, a good pin is important.

   It is your choice whether to use a resident key. If you do, you can load it directly to the ssh-agent using ssh-add -K, or write the key handle and public key to disk using ssh-keygen -K

ssh-keygen -t ed25519-sk -O resident # 1
ssh-keygen -t ecdsa-sk -O resident   # 2
ssh-keygen -t ed25519-sk             # 3
ssh-keygen -t ecdsa-sk               # 4

Updating SSH

SSH v8.2 is required to use a security key. Install it with brew.

brew install openssh

You can specifiy the path to the private key handle in your ssh config. Otherwise, you can configure the ssh-agent.

ssh-agent on macOS

To be used with a security key, the ssh-agent must be on v8.2, which the system default is not.

First, disable the macOS default ssh-agent for your user.

launchctl disable user/$UID/com.openssh.ssh-agent

Next, add a new launchd service for your ssh-agent. Add the following file to ~/Library/LaunchAgents/com.zerowidth.launched.ssh_agent.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Label</key>
	<string>com.zerowidth.launched.ssh_agent</string>
	<key>ProgramArguments</key>
	<array>
		<string>sh</string>
		<string>-c</string>
		<string>/usr/local/bin/ssh-agent -D -a ~/.ssh/agent</string>
	</array>
	<key>RunAtLoad</key>
	<true/>
</dict>
</plist>

And load it with launchctl load -w ~/Library/LaunchAgents/com.zerowidth.launched.ssh_agent.plist.

In your .bashrc or .zshrc, set SSH_AUTH_SOCK="~/.ssh/agent"

This plist was created using the launchd plist generator over at zerowidth. It runs the command /usr/local/bin/ssh-agent -D -a ~/.ssh/agent. -D prevents ssh-agent from forking, and -a ~/.ssh/agent directs the agent to create a socket file at that location that is referenced in $SSH_AUTH_SOCK.

Storing keys in the keyring

The following stanza can be adapted and placed in ~/.ssh/config. It removes the need to manually ssh-add keys with nonstandard names and stores key passwords if set in the macOS keyring.

Host *
  IgnoreUnknown UseKeychain
  UseKeychain yes
  AddKeysToAgent yes
  IdentityFile ~/.ssh/id_ecdsa_sk
  IdentityFile ~/.ssh/id_ed25519_sk

The first two lines direct ssh to use the macOS keychain to store passwords. The third automatically adds keys that are used to the agent and the last two specify additional keys to use. All of this can also be configured on a per host basis.