A certificate authority (CA) is a trusted entity that issues digital certificates, which are data files used to cryptographically link an entity with a public key. Certificate authorities are a critical part of the internet's public key infrastructure (PKI) because they issue the Secure Sockets Layer (SSL) certificates that web browsers use to authenticate content sent from web servers.
All major web browsers use web servers' SSL certificates to maintain trust in content delivered online; they all must trust the certificate authorities to issue certificates reliably. SSL certificates are used with the Transport Layer Security (TLS) protocol to encrypt and authenticate data streams for the HTTPS protocol and are sometimes referred to as SSL/TLS certificates or, simply, TLS certificates.
Digital certificates contain data about the entity that issued the certificate along with cryptographic data that can be used to verify the identity of the entity linked to the digital certificate. Typically, a digital certificate will contain information about the entity to which it has been issued, including the entity's public key and expiration date for the certificate as well the entity's name, contact information and other information linked to the certified entity.
Web servers transmit this information when a web browser initiates a secure connection over HTTPS; the certificate is sent to the web browser, which authenticates the certificate against its own root certificate store. The major browser companies -- Microsoft, Google, Apple and Mozilla -- each maintain their own web browser root certificate stores, in which they post the root certificates of the certificate authorities the publisher has decided their browser will trust.
An entity or person who needs a digital certificate can request one from a certificate authority; once the certificate authority verifies the applicant's identity, it generates a digital certificate for the applicant and digitally signs that certificate with the certificate authority's private key. The digital certificate can then be authenticated (for example, by a web browser) using the certificate authority's public key.
The certificate authority's root certificate should never be used directly for signing digital certificates, but rather is used to generate intermediate certificates as needed; different intermediate certificates are generated for different purposes. For example, a CA provider may use an intermediate certificate to sign all digital certificates generated for different levels of trust, or a separate intermediate certificate to be used for all digital certificates generated for a particular customer organization.
Certificate authorities may accept requests from applicants directly, though they often delegate the task of authenticating applicants to registration authorities (RAs). A registration authority is often used for marketing and customer support: The RA collects and authenticates digital certificate requests, and then submits those requests to the certificate authority, which then issues the certificate to be passed through the RA to the applicant.
Uses of a certificate authority
The best-known use of certificate authorities is for issuing SSL certificates to entities that publish content on the web. Certificate authorities issue three levels of SSL certificate, corresponding to different levels of trust in those certificates. Certificates with higher levels of trust usually cost more because they require more work on the part of the certificate authority.
The three different levels of trusted certificates include:
- Extended Validation (EV) certificates provide the highest level of assurance that the certificate authority has validated the entity requesting the certificate. The Certification Authority Browser Forum (CA/Browser Forum) spells out detailed requirements for the process that certificate authorities must apply when verifying information provided by the applicant for an EV certificate. For example, an individual requesting an EV certificate must be validated through face-to-face interaction with the applicant as well as review of a personal statement, one primary form of identification such as passport, driver's license or military ID, as well as two secondary forms of identification.
- Organization Validated (OV) certificates provide the next highest level of assurance. Certificate authorities generally perform some level of vetting of the applicants, which may include telephone verification as well as use of external or third parties to confirm information submitted by the applicant. OV certificates can be issued if the applicant can demonstrate that it holds administrative control of the domain name for which the certificate is requested and that the organization can be shown to exist as a legal entity.
- Domain Validated (DV) certificates require only that the applicant demonstrate ownership of the domain for which the certificate is being requested. DV certificates can be acquired almost instantly and at a low -- or no -- cost. For example, Let's Encrypt is a free service that can be used to get SSL certificates at no cost.
In addition to SSL certificates linked to domain names and issued for authenticating and encrypting data sent to and from websites, certificate authorities issue other types of digital certificate for different purposes including:
- Code signing certificates are used by software publishers and developers to sign their software distributions. End users can then use them to authenticate and validate software downloads from the vendor or developer.
- Email certificates enable entities to sign, encrypt and authenticate email using the S/MIME (Secure Multipurpose Internet Mail Extension) protocol for secure email attachments.
- Device certificates can be issued to internet of things devices to enable secure administration and authentication of software or firmware updates.
- Object certificates can be used to sign and authenticate any type of software object.
- User or client certificates are used by individuals for various authentication purposes, and are sometimes known as signature verification certificates.
Certificate authorities in recent years increasingly have shifted their business focus from issuing SSL certificates for web domains to providing a wider range of certification services.
How a certificate authority works
While there is no technical obstacle preventing an individual or organization from creating their own certificate authority, publicly trusted certificate authorities usually participate in the CA/Browser Forum, sometimes called the CA/B Forum, which is the industry group governing how certificate authorities work with web browsers. Most members of the group are either certificate authorities or web browser vendors, but certificate consumer organizations also participate.
The CA/Browser Forum maintains guidelines for all aspects of the creation, distribution and use of digital certificates in the web, including policies for certificate expiration and certificate revocation.
Certificate authority activities start with a root certificate, which is used as the ultimate basis for trust in all certificates issued by the authority. The root certificate, along with the private key associated with that certificate, is usually treated with the highest level of security and is usually stored offline in a protected facility and may be stored on a device that is unpowered except when the certificate is needed.
The certificate authority will use that root certificate to create intermediate certificates, which are the certificates used to sign the digital certificates issued by the authority. This allows the public to trust the issued certificates, while at the same time protecting the root in instances where an intermediate certificate expires or is revoked.
Intermediate certificates may also be used for issuing digital certificates through registration authorities, entities to which a certificate authority may delegate some or all of the requirements to authenticate the organization and domain identity for an entity seeking a certificate. According to CA/Browser Forum rules, the certificate authority must contractually require the registration authority to comply and document their compliance with CA/Browser Forum rules. In addition, the certificate authority is required to limit the registration authority to registering certificates within the domain namespace assigned to the RA.
Failure to police its registration authorities was one of the behaviors cited against Symantec's certificate authority operations, which ultimately led the company to divest responsibility for that business to DigiCert in 2017.
Certificate authorities themselves are also subject to extensive rules requiring operational audits, and infractions can bring down additional required audits and other consequences for any infractions or activities which might undermine trust in their operations. Prior to Symantec's divestiture of its CA operations, the CA/Browser Forum had called for a number of different consequences which Symantec failed to complete satisfactorily.