From the 7.1 release notes
Major New Feature Enhancements for 7.1
ZCS 7.1 includes several new enhancements to improve localization support for APAC based customers and to implement the server side support for S/ MIME and smartcard authentication technologies.
Open ID Consumer Support. (Bug 17147)
Certificate authentication support (Bug 29625)
Public Key lookup for S/MIME via External LDAP (Bug 50947)
Public Key lookup for S/MIME via Contacts (Bug 53093)
Store and lookup Public Key certificates for S/MIME in Zimbra GAL (Bug 55649)
However, can't find any documentation on how these could be implemented. Anyone using these?