On May 16th I had a discussion with Melvin Carvalho about client side certificates on https://gitter.im/linkeddata/chat . I was not sure what they were, so I dug up some papers to try to understand it. I found "Using RDF Metadata to Enable Access Control on the Social Semantic Web"  and "FOAF+SSL: RESTful Authentication for the Social Web" . I was trying to understand how Node SOLID Server  was set up because I was getting authentication an authorization errors.
Fortunately, I just found a document called "Client Side certificates"  by Tim Berners-Lee and a spec called "WebID-TLS"  .