Is there a 2-factor authentication plugin for Craft?
I've heard of one in private testing, but I'm not aware if they plan to release it publicly or not.
Until the official Craft plugin store becomes a thing, the most comprehensive list of Craft plugins available I know of is over on the awesome Straight Up Craft.
I want to build a system that someone on a dynamic IP could access, so static IP's are a bit harder.
O.K. What's the reasoning there, if you don't mind me asking?
I was also thinking if there was a way to add another field to the login page, which was a decryption key, then use one the twig decryption method. If you don't have it, you'd see the data, but it would be all garbled up.
Not currently, but we've discussed giving plugins the ability to control the HTML of the login page. Probably worth making an official feature request for that.
It is worth mentioning that there is an onBeforeLogin event a plugin can already listen to and apply their own custom password business logic (i.e. must be 200 characters long, have 40 emojis, 20 Egyptian hieroglyphs, be purple and only work on Tuesday).