Do not rush to change all your passwords!
Let me qualify that statement. I have heard and seen many knee jerk reactions from "professionals" that state we should all rush out to change our banking and social media passwords. The first question we should be asking is, what, who, why. The second, do I need to make a change right away, if at all.
Simple. All systems that use the OpenSSL security certificate service.
This article describes the method for adding an email signature to your mail client under Mac OS X Mavericks (may be backward compatible with Lion too, untested).