Discussions of secure PHP coding. Security in software is important, so don't be afraid to ask. And when answering: be anal. Nitpick. No security vulnerability is too small.
What's this for exactly? There was a fairly long thread here about challenge response login systems possibly a year ago. It's not the same thing but I'm wondering if it could be of use to you.
As tcsoft says. PGP or GPG (GnuPG) would be a good way to go.