ona password – one machine

I set up a system for publishing audio news based on wordpress. I modified some plugins so only authenticated users are able to listen or download the real audio files.
So, everything is working fine, but my client asks me for a way to have increased security with their users accounts.

For another website (not wp based) another developer gaves my client a solution using a little Java program that is downloaded by the browser and that somehow identifies the machine where the user account was activated. I would like to set up similar system for my wp-based website, but don’t know where or exactly what to search. Can you point me in the right direction? Any links or ideas?

Thanks in advance