My organization has a Cisco ASA 5510 which I have made act as a firewall/gateway for one of our offices. Most resources a remote user would come looking for exist inside. I've implemented the usual deal - basic inside networks with outbound NAT, one primary outside interface with some secondary public IPs in the PAT pool for public-facing services, a couple site-to-site IPSec links to other branches, etc. - and I'm working now on VPN.

I have the WebVPN (clientless SSL VPN) working and even traversing the site-to-site links. At the moment I'm leaving a legacy OpenVPN AS in place for thick client VPN. What I would like to do is standardize on an authentication method for all VPN then switch to the Cisco's IPSec thick VPN server.

I'm trying to figure out what's really possible for authentication for these VPN users (thick client and clientless). My organization uses Google Apps and we already use dotnetopenauth to authenticate users for a couple internal services. I'd like to be able to do the same thing for thin and thick VPN.

Alternatively a signature-based solution using RSA public keypairs (ssh-keygen type) would be useful to identify user@hardware.

I'm trying to get away from legacy username/password auth especially if it's internal to the Cisco (just another password set to manage and for users to forget). I know I can map against an existing LDAP server but we have LDAP accounts created for only about 10% of the user base (mostly developers for Linux shell access).

I guess what I'm looking for is a piece of middleware which appears to the Cisco as an LDAP server but will interface with the user's existing OpenID identity. Nothing I've seen in the Cisco suggests it can do this natively. But RSA public keys would be a runner-up, and much much better than standalone or even LDAP auth. What's really practical here?