Yubikey4 and gpg-agent
Using the Yubikey4 for ssh key generation and storage as well as one-time-password generation. There are a ton of guides to a ton of subtly different methods of doing basically this, I mean only to provide a simple explanation of what worked for me.
Use gpg-agent instead of ssh-agent
if test -f $XDG_RUNTIME_DIR/gpg-agent-info && kill -0 $(head -n 1 $XDG_RUNTIME_DIR/gpg-agent-info | cut -d: -f2) 2>/dev/null ; then eval $(< $XDG_RUNTIME_DIR/gpg-agent-info) else eval $(gpg-agent --daemon --enable-ssh-support --write-env-file $XDG_RUNTIME_DIR/gpg-agent-info) fi export GPG_AGENT_INFO export SSH_AUTH_SOCK
|Does not work in GnuPG 2.1+ due to this change.
Generate an authentication key
I used the method described in this blog post to generate the authentication subkey on the Yubikey itself:
$ gpg2 --edit-key YOURKEY gpg> addcardkey
Select authentication, provide expiry, create the key, and save.
gpg2 -K should show the new key, and
gpgkey2ssh AUTHKEY should provide a version for suitable authorized_keys. First access of the key will present a box for the PIN. After that PIN-less access and OTP generation should work until it is unplugged. I did not have to install any smart card related utilities for this to work as expected.
I found that despite being generated on the Yubikey the authentication private key remained on my local keychain. Removing it did not affect ssh behavior.