One thing you can do is to ask a company to do a patent landscape for you. There a few around (can't remember their name) and they charge a few grand to do it for you. The results usually come in within a week or two.

The instructions for them would be to look at encryption in general with particular focus on the environment you're talking about.

Also, try freepatentsonline.com instead of the USPTO's website. I find it's much easier on the eyes and the search a bit more human-friendly.

Pierre