Definition of Certificate Authority
A Certificate Authority is a trusted third party, which certifies Public Key's to truly belong to their claimed owners. It is a key part of any Public Key Infrastructure, since it allows users to Trust that a given Public Key is the one they wish to use, either to send a private message to its owner or to verify the signature on a message sent by that owner.
Popular Certification Authorities, whose Public Key's are built into most World-Wide Web browsers, include:
