When connecting to a website via a secuire connection the visitor’s browser decides whether or not to trust the SSL certificate based on which Certification Authority has issued it. To determine this, the browser looks at its list of trusted issuing authorities – represented by a collection of Trusted Root CA certificates added into the browser by the browser vendor (such as Microsoft and Netscape).
Most SSL certificates are issued by CAs who own and use their own Trusted Root CA certificates, such as those issued by GeoTrust and RapidSSL.com. As GeoTrust and RapidSSL.com is known to browser vendors as a trusted issuing authority, its Trusted Root CA certificate has already been added to all popular browsers, and hence is already trusted. These SSL certificates are known as “single root” SSL certificates. RapidSSL.com, a subsidiary of GeoTrust, owns the Equifax root used to issue its certificates.
Some Certification Authorities do not have a Trusted Root CA certificate present in browsers, or do not use the root they do own, and use a “chained root” in order for their SSL certificates to be trusted – essentially a CA with a Trusted Root CA certificate issues a “chained” certificate which “inherits” the browser recognition of the Trusted Root CA. These SSL certificates are known as “chained root” SSL certificates.
Installation of chained root certificates are more complex and some web servers and applications are not compatible with chained root certificates.
For a Certification Authority to have and use its own Trusted Root CA certificate already present in browsers is a clear sign that they are long-time, stable and credible organizations who have long term relationships with the browser vendors (such as Microsoft and Google) for the inclusion of their Trusted Root CA certificates. For this reason, such CAs are seen as being considerably more credible and stable than chained root certificate providers who do not have a direct relationship with the browser vendors, or do not use their own root certificates to issue SSL certificates.
Chained root certificates require additional effort to install as the webserver must also have the chained root installed. This is not necessary for single root certificates