Over in the Cisco Learning Network forum, a thread-starter asked:
Can WPA/TKIP be cracked? Either someone in my guesthouse used up 80 gigs of data or someone close by cracked the password and used it. I suspect someone in the guest house because i find it hard to believe WPA/TKIP can be cracked and even if it can be cracked it would not be easy to do. How difficult if at all is it to crack WPA/TKIP? I want to change the password for them anyway, can i use - and _ and? characters?
An obviously very smart fellow named "Zach" made this posting which the thread-starter, here (and others, here, too, if they're interested), should read.
In particular, read from about two-thirds of the way down his posting, where he begins with the words "The solutions:".
I'm using my gateway's "WPA-PSK (TKIP)/WPA2-PSK (AES)" setting. In keeping with this of Zach's posting...
Change the name of your router to something unique. Your ESSID is used by your wlan supplicant as a cryptographic salt over the PMK. Changing this will eliminate pre-computation attacks.
...I've long used my own unique ESSID. Additionally, in keeping with his...
Make a unique password incorpating unique characters, numbers, capitals. multiple words, and lowercase letters. This is more important than length. Increasing the length of the password will only increase the passwords strength but it doesn't need to be obscenely
long. Strength lies in variance of potential. This will eliminate dictionary attacks and make brute-force impossible without a super-computer.
...mine is 25 characters which consist of letters, numbers, upper-and-lowercase, and special characters. No part of it spells anything.
I do several other things both which Zach does and doesn't enumerate there; but in addition to the above, and said other things, and in at least the spirit of what he wrote here...
Enable detailed logging and if possible forward that to your email.
...I long ago wrote a bit of scripting code that auto-launches with Windows startup, and just runs in the system tray; which code periodically, throughout the day, hard-refreshes and then parses the webpage in my gateway which lists all connected devices; and it then tells me both as a pop-up-with-triple-beep-through-the-motherboard-speaker (not the regular audio speakers, just in case they're muted or something) on my desktop-replacement-laptop computer's screen, and also via text to my phone (which is either in a pouch on my belt, or at least never more than five feet from me, 24/7/365), if anything new has shown-up.
For those without that skill, there are several "who's on my WI-FI" type apps out there, some of them free. A good and simple one is [this badboy]. Just auto-start it with Windows, let it sit in the system tray, and tell it to "beep on new device" and you'll have something similar to what my script does (except that it won't SMS you). However, using [a simple scripting tool] you can cause an SMS or email to be sent to your phone when a new device on the LAN makes the app beep.
Hope that helps.