The PAM module can utilize the HMAC-SHA1 Challenge-Response mode found in YubiKeys starting with version 2.2 for *offline authentication*. This action creates the initial state information with the C/R to be issued at the next logon.
The utility currently outputs the state information to a file in the current user's home directory (_\~/.yubico/challenge-123456_ for a YubiKey with serial number API readout enabled, and _~/.yubico/challenge_ for one without).
The PAM module supports a system wide directory for these state files (in case the user's home directories are encrypted), but in a system wide directory, the 'challenge' part should be replaced with the username. Example : _/var/yubico/challenges/alice-123456_.
To use the system-wide mode, you currently have to move the generated state files manually and configure the PAM module accordingly.
== EXAMPLES
First, program a YubiKey for challenge response on Slot 2 :