Unixcop published a tutorial showing you how to configure Apache2 to use SSL client authentication.

Configure Apache2 to use SSL Client Authentication

SSL Client Authentication is a process by which a server checks client provided certificate to make an SSL connection. If the client certificate is trusted, then a successful SSL connection is made with server. Else client is denied access to the website.

Configure Apache2 to use SSL Client Authentication - Unix / Linux the admins Tutorials