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Certificate Authorities (CA)

A Certificate Authority (CA) is an entity which signs and issues digital identity certificates claiming trust of the identified party. In this sense, it's a trusted third party between two transactions.

In x.509 systems, CAs are hierarchical in nature, with CAs being certified by CAs, until you reach a "root authority." The hierarchy of certificates is always a top-down, with a root authority's certificate at the top. A root authority is a CA that's trusted by enough or all of the interested parties, so that it doesn't need to be authenticated by yet another trusted third party.

A CA can be a company that, for a fee, signs and issues a public key certificate which states that the CA attests that the public key contained in the certificate belongs to its owner, as recorded in the certificate. In a sense, CA is a "digital notary public." One applies to the CA for a certificate by providing identity and contact information, as well as the public key. A CA must check an applicant's identity, so that users can trust certificates issued by that CA to belong to the identified applicant.