Parse static challenge response in auth-pam plugin

Parse static challenge response in auth-pam plugin

If static challenge is in use, the password passed to the plugin by openvpn
is of the form "SCRV1:base64-pass:base64-response". Parse this string to
separate it into password and response and use them to respond to queries
in the pam conversation function.

On the plugin parameters line the substitution keyword for the static
challenge response is "OTP". For example, for pam config named "test" that
prompts for "user", "password" and "pin", use

plugin openvpn-auth-pam.so "test user USERNAME password PASSWORD pin OTP"

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1532486093-24793-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17307.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>