What Is a Certificate Authority

A Certificate Authority (CA) is an organization that issues and manages digital certificates. When a website communicates over HTTPS, the browser needs to verify whether the site is genuinely operated by its legitimate owner. The certificate issued by a CA provides the basis for this verification.

A CA confirms that the certificate applicant is the rightful administrator of the domain, then digitally signs a certificate binding the public key to the domain name. Browsers contain a built-in list of trusted CAs (root store) and only accept certificates signed by CAs in that list.

Certificate Types - DV, OV, and EV

Certificates issued by CAs are classified into three types based on the rigor of identity verification.

Verifies only domain control. Validation completes through DNS record configuration or email verification, with issuance in minutes. This is the type Let's Encrypt provides for free. Suitable for personal sites and blogs.

Verifies domain control plus the organization's existence. Requires review through business registration and phone verification, taking several days. Used for corporate sites and business systems.

Issued after the most rigorous review. Verifies the organization's legal existence, physical location, and applicant authority. Previously displayed the organization name in green in the browser address bar, but this has been discontinued. Used by financial institutions and large enterprises.

Encryption strength does not differ between DV, OV, and EV. The difference lies in the level of verification of who manages the domain.

Chain of Trust

The CA system operates on a hierarchical structure called the chain of trust.

Root CA: The CA at the top of the trust hierarchy. It self-signs its own certificate (root certificate). Root certificates are pre-installed in operating systems and browsers, functioning as the trust anchor without users being aware.
Intermediate CA: A CA that received its certificate from the root CA. In most cases, intermediate CAs issue the actual server certificates. This design avoids the risk of directly using the root CA's private key.
Server Certificate: The certificate installed on a website, signed by an intermediate CA.

When a browser verifies a server certificate, it traces the chain from server certificate to intermediate CA certificate to root certificate. If it reaches a root certificate in the root store, the certificate is deemed trustworthy. If any link in the chain is broken, the browser displays a warning.

The intermediate CA design allows the root CA's private key to be stored in an offline HSM (Hardware Security Module), so even if an intermediate CA is compromised, the root certificate does not need to be revoked.

Impact of Let's Encrypt

Let's Encrypt, launched in 2015, fundamentally changed web security as a CA that issues DV certificates for free and automatically.

Accelerated HTTPS adoption: Before Let's Encrypt, SSL certificates cost thousands of yen annually, and HTTPS adoption was slow for personal sites and small services. Free certificates drove web-wide HTTPS adoption from about 40% in 2015 to over 95% by 2025.
Automation via ACME protocol: Standardized the ACME (Automatic Certificate Management Environment) protocol for automating certificate issuance and renewal. Using client tools like Certbot, certificate acquisition and web server configuration complete with a single command.
Short 90-day validity: While traditional certificates had 1-2 year validity periods, Let's Encrypt uses 90 days. Shorter validity reduces private key compromise risk and promotes automated renewal workflows.

Let's Encrypt is a nonprofit project operated by the Internet Security Research Group (ISRG), sponsored by Mozilla, Google, Cisco, and others.

The DigiNotar Incident and Certificate Transparency

In 2011, the Dutch CA DigiNotar was hacked, resulting in fraudulent certificates being issued for over 500 domains including google.com. Attackers reportedly used these fake certificates to intercept Gmail communications of Iranian users. DigiNotar was removed from all browser trust lists and effectively went out of business.

This incident led to the development of Certificate Transparency (CT) as a mechanism to ensure CA accountability.

CT Logs: All certificates issued by CAs are recorded in public, append-only log servers. Domain administrators can monitor CT logs to detect unauthorized certificates issued for their domains.
SCT (Signed Certificate Timestamp): Evidence of CT log recording. Servers present SCTs during the TLS handshake, and browsers verify them. Chrome has required SCTs since 2018.
Combined with HSTS: CT detects fraudulent certificates while HSTS prevents HTTP downgrade, achieving defense in depth against man-in-the-middle attacks.

CT represents a shift from a model of trusting CAs to one of verifying CAs.

Common Misconceptions

A padlock icon in the browser means the site is safe: The padlock only indicates that communication is encrypted; it does not guarantee the site operator is trustworthy. Phishing sites can also obtain DV certificates for HTTPS. Checking the domain name in the URL is essential.
EV certificates provide stronger encryption than DV certificates: Encryption strength does not depend on certificate type. DV and EV use the same TLS protocol and cipher suites. The difference is in the rigor of applicant identity verification, not encryption level.

Certificate Authority (CA)