The TLS Handshake#

Every HTTPS connection starts with a TLS handshake that establishes encryption parameters and verifies the server’s identity. The simplified flow for TLS 1.2:

Client                              Server
  |── ClientHello ──────────────────>|   (supported versions, cipher suites, random)
  |<────────────────── ServerHello ──|   (chosen version, cipher suite, random)
  |<──────────────── Certificate  ──|   (server's certificate chain)
  |<───────────── ServerKeyExchange ─|   (key exchange parameters)
  |<───────────── ServerHelloDone  ──|
  |── ClientKeyExchange ───────────>|   (client's key exchange contribution)
  |── ChangeCipherSpec ────────────>|   (switching to encrypted communication)
  |── Finished ────────────────────>|   (encrypted verification)
  |<──────────── ChangeCipherSpec ──|
  |<──────────────────── Finished ──|
  |<═══════ Encrypted traffic ═════>|

TLS 1.3 simplifies this significantly. The client sends its key share in the ClientHello, allowing the handshake to complete in a single round trip. TLS 1.3 also removed insecure cipher suites and compression, eliminating entire classes of vulnerabilities (BEAST, CRIME, POODLE).