PKI / Chain of Trust

You visit bank.com for the first time. The server presents a certificate claiming that its public key belongs to bank.com, but an attacker could present a false claim too. Your browser needs a trusted authority to vouch for that claim. With millions of websites and certificates regularly renewed, approving each certificate by hand would be unmanageable. How can the browser verify a new certificate using trust it already has?

Operating systems and browsers trust a relatively small set of root authorities. Their vendors or local administrators explicitly choose these trusted roots as the starting points for certificate verification. The website typically sends its own certificate plus an intermediate CA certificate—two certificates in this example, or more if the chain has additional intermediates. The root certificate normally comes from the browser's or operating system's trusted store. The browser verifies the root's signature on the intermediate certificate, then the intermediate's signature on the website certificate. This lets it check new certificates automatically using roots it already trusts.

Trust can form a chain:
Trusted Root
     ↓
Intermediate CA
     ↓
bank.com

If each signature and constraint validates, the browser can establish trust from the known root to the server certificate.