Authenticating with HMAC-SHA1 Challenge-Response through linux-pam

You need a pam aware userland (e.g. shadow for login, sudo, sshd, ..) and
yubikey-personalization to succeed.
We will write our key to the second slot, as the first slot comes with a
higher secure level key which shouldn't be overwritten.

Then proceed as follows:

CHANGE {$USER} to your username!

$ ykpersonalize -2 -ochal-resp -ochal-hmac -ohmac-lt64 -oserial-api-visible
$ sudo mkdir /var/yubico
$ sudo chown root:root /var/yubico
$ sudo chmod 700 /var/yubico
$ ykpamcfg -2 -v
$ sudo mv ~/.yubico/challenge-123456 /var/yubico/{$USER}-123456
$ sudo chown root:root /var/yubico/*
$ sudo chmod 600 /var/yubico/*

then, edit your relative pam files to contain this as the first auth paragraph:
auth        sufficient    pam_yubico.so mode=challenge-response
chalresp_path=/var/yubico

This way ('sufficient'), you can still authenticate with your normal password, in case you
brake your yubikey or you foobar'd.