Applied Cryptography
Which of the following correctly describes the certificate chain when a browser validates a web server's certificate?
- AA server certificate must be signed directly by a root certificate authority, and certificates issued through intermediate certificate authorities are not trusted
- BThe browser uses the root certificate sent by the server as the starting point of trust as it is
- 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.
- A. Going through an intermediate CA is normal, and the certificate is trusted if the chain leads to a root.
- B. If a root certificate sent by the server were the starting point of trust, an attacker could impersonate with a self-made root. The starting point is one in the trust store.
- C. Correct. The chain of signatures is followed up to a root certificate in the trust store.