After a year I made a bugfix of my version of the original by redvers (more info see installation.txt in .rar file):
Extra features compared to "redvers" version:
-simple website to read/set the wifi password and remaining lifetime
-authentication between webserver and router is by using SSH with plain password (connection settings see ssh.php)
-automatic decreasing lifetime of used password
-non-used password will last forever until viewed by the webpage
-on the Router > System Log > General Log you can see important "guestwifi" events
Initial idea was by user "redvers76 or redvers" from
https://github.com/RMerl/asuswrt-merlin/wiki/Setting-a-random-password-for-guest-wifi
Additional ideas/features were made by user "RoutNew36" from
http://www.snbforums.com/threads/setting-a-random-password-for-guest-wifi.22700/
Functionality wise I stayed as close as "RoutNew36" version. I changed the Android app (.apk) in a simple website so every client can be used.
Furthermore I simplified all scripts and made them more readable, debuggable and configurable.
Download link:
You can always find the latest here:
http://i872272.iris.fhict.nl/downloads/guestwifi/guestwifi.rar
Previous versions:
http://i872272.iris.fhict.nl/downloads/guestwifi/guestwifi_v1.01.rar
Changelog:
v1.00 - 06 aug 2015 : initial version
v1.01 - 07 aug 2015 : [fix] changed relative path to absolute path in CheckGuestPassword.sh because it's called from cronjob from an unknown path
v1.10 - 10 aug 2016(!): [fix] generated password was saved in /tmp but this directory is emptied when router is rebooted