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What is Certificate authority?

An organization that verifies the binding between a public key and its owner (a domain name or organization) and issues certificates in which it signs that statement with its own private key; also called a CA. The certificates of root CAs (root certificates) built into the trust store of the operating system or browser are the starting point of trust: a root CA issues certificates to intermediate CAs, and intermediate CAs issue them to servers and others. The receiving side verifies a certificate by following the signatures from the server certificate to the intermediate certificate and on to the root certificate (the certificate chain). When a certificate must be invalidated, for example because its private key has leaked, the CA revokes it.

Japanese name: 認証局

Certificate authority appears in 6 practice questions across 3 topics, including Applied Cryptography, Computer Networking Basics, Information Security Basics.

Field: Applied Cryptography

What certificate validation checks

Validation checks that the chain of signatures leads to a root certificate in the trust store, that each certificate is within its validity period, that the host name being connected to is listed in the certificate's SAN (subject alternative name), that the certificate is meant for this use, and that it has not been revoked. Skipping any one of these may let a fake server through.

Domain validation and kinds of certificates

Most web server certificates are DV certificates, which only confirm that the applicant controls the domain, and they can be issued and renewed automatically with a protocol called ACME. There are also OV and EV certificates, which also confirm that the organization exists, but the strength of the encryption is the same.

Certificate Transparency

To guard against certificates issued wrongly through CA mistakes or compromise, Certificate Transparency is used: issued certificates are recorded in public append-only logs. Domain owners can monitor the logs to check that no certificate they do not recognize has been issued for their domains.

Try it: questions about Certificate authority

Which of the following correctly describes the certificate chain when a browser validates a web server's certificate?

  1. AA server certificate must be signed directly by a root certificate authority, and certificates issued through intermediate certificate authorities are not trusted
  2. BThe browser uses the root certificate sent by the server as the starting point of trust as it is
  3. CThe browser follows the signatures from the server certificate to the intermediate CA's certificate and trusts it if they lead to a root certificate built into the OS or browser
Show answer and explanation

Answer: C. The browser follows the signatures from the server certificate to the intermediate CA's certificate and trusts it if they lead to a root certificate built into the OS or browser

Generally a server certificate is signed by an intermediate certificate authority, and the intermediate CA's certificate is signed by a root certificate authority. A root certificate is a certificate signed by itself; it is built into the trust store of the OS or browser in advance and serves as the starting point of trust. The server sends its own certificate and the intermediate certificate, and the browser follows the signatures in turn to check whether they lead to a root certificate in the trust store. Keeping the root private key offline under strict protection and doing day-to-day issuance with intermediate CAs means that, if a problem occurs, only the intermediate CA's certificate needs to be revoked.

FAQ

Who signs a root certificate?

A root certificate is a self-signed certificate, signed with its own private key. Its trustworthiness comes not from the signature but from the operating system or browser vendor having reviewed it and placed it in the trust store. That is why, if someone secretly adds a root certificate to a trust store, its owner can forge certificates for any site.

Why do root CAs not issue server certificates directly?

Because if a root private key leaked, the damage would continue until trust stores were updated. Root private keys are kept offline under strict protection, for example in HSMs, and day-to-day issuance is done by intermediate CAs. If an intermediate CA has a problem, only its certificate needs to be revoked.

Certificate authority in the explanations

Questions that mention Certificate authority (6)

  1. Which of the following correctly describes the certificate chain when a browser validates a web server's certificate?
  2. Which of the following does HTTPS (HTTP protected by TLS) protect?
  3. In TLS, why can a browser conclude that the server it connects to is not an impostor?
  4. It has been discovered that the private key corresponding to a web server's server certificate may have been leaked. The certificate has not yet expired. What is the most appropriate action for the operator?
  5. Which of the following correctly describes the mechanism of Certificate Transparency (CT)?
  6. Which of the following correctly describes the role of an HSM (hardware security module)?

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Related terms

TLS HTTP (Hypertext Transfer Protocol) Diffie–Hellman key exchange Hardware security module (HSM) FIPS 140-3 Trusted Platform Module (TPM) AES (Advanced Encryption Standard) Authenticated encryption (AEAD) Birthday attack Block cipher mode of operation

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