A Certificate Authority (CA) (or Certification Authority) is an entity that issues digital certificates.
The digital certificate certifies the ownership of a public key by the named subject of the certificate. This allows others (relying parties) to rely upon signatures or assertions made by the private key that corresponds to the public key that is certified.
In this model of trust relationships, a CA is a trusted third party that is trusted by both the subject (owner) of the certificate and the party relying upon the certificate.
In the context of a website, when we use the term digital certificate we often refer to SSL certificates. The CA is the authority responsible for issuing SSL certificates publicly trusted by web browsers.
Anyone can issue SSL certificates, but those certificates would not be trusted automatically by web browsers. Certificates such as these are called self-signed. The CA has the responsibility to validate the entity behind an SSL certificate request and, upon successful validation, the ability to issue publicly trusted SSL certificates that will be accepted by web browsers. Essentially, the browser vendors rely on CAs to validate the entity behind a web site.
Certificate Authorities, or Certificate Authorities / CAs, issue Digital Certificates. Digital Certificates are verifiable small data files that contain identity credentials to help websites, people, and devices represent their authentic online identity (authentic because the CA has verified the identity). CAs play a critical role in how the Internet operates and how transparent, trusted transactions can take place online. CAs issue millions of Digital Certificates each year, and these certificates are used to protect information, encrypt billions of transactions, and enable secure communication.
An SSL Certificate is a popular type of Digital Certificate that binds the ownership details of a web server (and website) to cryptographic keys. These keys are used in the SSL/TLS protocol to activate a secure session between a browser and the web server hosting the SSL Certificate. In order for a browser to trust an SSL Certificate, and establish an SSL/TLS session without security warnings, the SSL Certificate must contain the domain name of website using it, be issued by a trusted CA, and not have expired.
With all these SSL Certificates in use, who decides a CA can be trusted?
Browsers, operating systems, and mobile devices operate authorized CA "membership" programs where a CA must meet detailed criteria to be accepted as a member. Once accepted the CA can issue SSL Certificates that are transparently trusted by browsers, and subsequently, people and devices relying on the certificates. There are a relatively small number of authorized CAs, from private companies to governments, and typically the longer the CA has been operational, the more browsers and devices will trust the certificates the CA issues. For certificates to be transparently trusted, they must have significant backward compatibility with older browsers and especially older mobile devices ¡V this is known as ubiquity and is one the most important features a CA can offer its customers.
Prior to issuing a Digital Certificate, the CA will conduct a number of checks into the identity of the applicant. The checks relate to the class and type of certificate being applied for. For example, a domain validated SSL Certificate will have verified the ownership of the domain to be included within the Certificate, whereas an Extended Validation SSL will include additional information on the company, verified by the CA through many company checks.