How Does a Certificate Authority Work?
NIST defines a CA as a trusted entity that issues and revokes public key certificates. RFC 5280 says a certificate binds a public key value to a subject, and the CA’s digital signature asserts that binding.
Clients start from a limited number of CA public keys they trust. RFC 5280 describes a certification path: the website’s certificate is signed by one CA, plus zero or more CA certificates signed by other CAs, leading back to a trusted key. The CA at the top of that chain, whose key the client trusts, is usually called the root, and the CAs between it and the website’s certificate are called intermediates.
Why Does a Certificate Authority Matter?
Clients accept whatever a trusted CA signs. In October 2026 eight intermediate CA keys of eMudhra were reported as recoverable, because their private values were tiny integers below 256 (Mozilla bug 2083449). The eMudhra CA key compromise incident page has the dated record. Anyone with such a key could issue a certificate for any website. Combined with network interception, that enables an adversary-in-the-middle attack. For a November 2026 case where a CA exposed archived customer contact data, see what DigiCert customers should check after the breach.
Trust also depends on software checking the chain correctly. Canonical says LXD compared every certificate a client sent with its trust store, although only the first one proves the client holds the private key. That gap became the LXD authentication bypass CVE-2026-112816.
How Do You Defend Against a CA Failure?
- Keep the operating system and the CA bundles of your tools up to date, including those inside container images.
- Use revocation checking where your client supports it. RFC 5280 describes the certificate revocation list, a signed, time-stamped list of revoked certificates.
- Know which tools carry their own CA bundle, since removing a root from the operating system does not change them.
- Watch for certificates issued for your own names that you did not request.
Also Called
- Certification authority
Frequently Asked Questions
What is the difference between a root CA and an intermediate CA?
RFC 5280 describes a certification path. An end certificate is signed by one CA, and that CA's certificate is signed by other CAs up to a key the client trusts. The trusted top of the chain is the root; the CAs in between are intermediates.
What happens when a CA is distrusted?
Clients stop accepting certificates that chain to it once their trust lists are updated. For eMudhra, Mozilla decided to remove the emSign ECC Root CA - G3, and software with its own CA bundle stays exposed until updated.
How is a certificate revoked?
The CA lists it on a certificate revocation list, a time-stamped list of revoked certificates signed by the CA, which RFC 5280 describes. eMudhra revoked its eight affected intermediates on 18 October 2026.
