What is Certificate Authority?
Also known as: CA
A Certificate Authority (CA) is a trusted entity that issues digital certificates after verifying that the requester controls the domain or identity named in the certificate.
A Certificate Authority (CA) is an organization that issues digital certificates used in TLS/SSL, code signing, email encryption, and other public-key infrastructure (PKI) applications. The CA's core job is to verify the identity of the certificate requester before signing a certificate that binds a public key to that identity. This verification process follows rules defined in the CA's Certification Practice Statement (CPS) and typically complies with industry standards like the CA/Browser Forum Baseline Requirements.
The CA signs the certificate using its own private key, creating a chain of trust. Browsers and operating systems ship with a list of trusted root CA certificates. When a client receives a server's certificate during a TLS handshake, it checks the certificate's digital signature against the public key of the issuing CA. If the CA is trusted and the signature is valid, the client can trust that the certificate holder controls the domain. Intermediate CAs may sit between the root and the end-entity certificate to reduce the risk of root key compromise.
CAs are audited regularly by third parties (such as WebTrust) to ensure they follow strict operational security and verification procedures. Notable public CAs include Let's Encrypt, DigiCert, and GlobalSign. The security of the entire HTTPS ecosystem depends on CAs being honest and competent. Incidents like the DigiNotar breach (2011) show that a compromised CA can break trust for any domain.
Key facts
- A CA signs digital certificates after verifying domain control or organizational identity.
- Browser root stores define which CAs are trusted by default.
- Let's Encrypt is a free, automated CA that issues over 300 million certificates as of 2024.
- CAs are audited against WebTrust or ETSI standards for operational security.
- The CA/Browser Forum Baseline Requirements govern publicly trusted CAs.
How it works in practice
Related terms
References
More in TLS & PKI
ACME Protocol
ACME (Automated Certificate Management Environment) is a protocol that automates the issuance, renewal, and revocation of TLS certificates, defined in RFC 8555.
Certificate Chain
A certificate chain is an ordered list of certificates, starting with the server certificate and ending with a root CA, that a client validates to establish trust in the server's identity.
Certificate Pinning
A security technique where an application trusts only a specific, pre-selected certificate or public key for a given server, bypassing the standard chain of trust.
Cipher Suite
A cipher suite is a named set of cryptographic algorithms negotiated during a TLS handshake, specifying key exchange, authentication, encryption, and integrity protection for secure communications.
Domain Validation
Domain Validation (DV) is the lowest level of certificate validation used in TLS/SSL, where the certificate authority verifies only that the applicant controls the domain name, typically via an HTTP or DNS challenge.
ECH
Encrypted Client Hello (ECH) is a TLS extension that encrypts the Client Hello message, including the Server Name Indication (SNI), to prevent on-path observers from learning the target hostname during the handshake.
Extended Validation
Extended Validation (EV) is the highest level of TLS certificate assurance, requiring the certificate authority to perform rigorous, human-verified checks on the legal identity and operational existence of the requesting organization before issuance.
HSTS
HTTP Strict Transport Security (HSTS) is a web security policy mechanism that forces browsers to interact with a website only over HTTPS, preventing downgrade attacks and cookie hijacking.
Intermediate Certificate
An intermediate certificate is a subordinate CA certificate signed by a root CA, used to sign end-entity certificates and enable path validation while the root remains offline.
Let's Encrypt
Let's Encrypt is a free, automated, public certificate authority operated by the Internet Security Research Group (ISRG) that issues short-lived Domain Validation (DV) TLS certificates via the ACME protocol.