RHEL 6 Subversion server configured for smart card (CAC) access?

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http://www.linuxquestions.org – Guys, Trying to get some info on setting up a Subversion server with smart card (CAC) authentication. We have some offsite people needing access to our SVN repo but they must be able to CAC in, and not use VPN. I know we would have to setup the webserver part, but I wasn't sure about authenticating with a CAC externally? We will be using RHEL 6 server. Do any of you guys have any insight or a better solution? TIA (HowTos)