How HTTPS works: TLS in brief

Separate the problem of agreeing on a key from the problem of encrypting bulk data.

Model answer. HTTPS is HTTP sent over a TLS (Transport Layer Security) connection, typically on TCP port 443. TLS provides three guarantees:

• Confidentiality — eavesdroppers cannot read the content.
• Integrity — tampering in transit is detected.
• Authentication — the client can verify the server's identity (and optionally the server can verify the client's).

TLS 1.3 handshake (simplified, one round trip)

1. ClientHello: supported TLS versions and cipher suites, a random value, and a key share for an ephemeral (EC)Diffie–Hellman exchange. The Server Name Indication (SNI) extension tells the server which site is wanted.
2. ServerHello: the chosen cipher suite and the server's own key share. Both sides can now compute the same shared secret and derive symmetric session keys; everything after this point is encrypted.
3. The server sends its certificate, a CertificateVerify signature made with its private key (proving it owns the certificate), and a Finished message.
4. The client verifies all of this, sends its own Finished, and HTTP data flows encrypted. (TLS 1.2 needed two round trips for a full handshake.)

Why both kinds of cryptography?

• Asymmetric operations (Diffie–Hellman, RSA/ECDSA signatures) solve key agreement and identity but are relatively slow.
• Symmetric ciphers such as AES-GCM or ChaCha20-Poly1305 are very fast and encrypt the bulk data, also providing integrity.
• Using ephemeral key exchange gives forward secrecy: stealing the server's private key later does not decrypt previously recorded sessions.

How the browser trusts the server

• The certificate binds a domain name to a public key and is signed by a Certificate Authority (CA).
• The browser checks the chain up to a root CA in its trust store, that the certificate covers the requested hostname, that it is within its validity period, and that it has not been revoked.

Common mistake: saying the server's public key encrypts all traffic — it is used only in the handshake. Follow-up: "What can an observer still see with HTTPS?" The IP addresses, ports, timing and sizes, and usually the hostname via SNI or DNS — but not the URL path, headers or body.