Forefront TMG Proxy Authentication (Client Certificate)

I was looking at the authentication methods that the TMG supports for forward proxy scenario.

http://technet.microsoft.com/en-us/library/cc441695.aspx

My scenario requires proxy to support client certificate auth and the clients connecting to proxy should present a client certificate for connecting to proxy.

Can someone please help if this is possible with TMG? Or can someone please recommend a 3rd party forward proxy software that supports client certificate authentication.