TLS Handshake

Also Known As

TL;DR

Explanation

The TLS handshake authenticates the server (and optionally the client), negotiates the TLS version and cipher suite, and establishes session keys. TLS 1.3 completes in one round-trip vs TLS 1.2's two, significantly reducing connection latency. Understanding the handshake is essential for diagnosing certificate errors, choosing strong cipher suites, and implementing mutual TLS (mTLS) for service-to-service authentication.

Diagram

sequenceDiagram
    participant C as Client
    participant S as Server
    C->>S: ClientHello - TLS version, cipher suites
    S->>C: ServerHello - chosen cipher
    S->>C: Certificate with public key
    C->>C: Verify certificate chain
    C->>S: Key exchange encrypted with server public key
    C->>S: Finished encrypted
    S->>C: Finished encrypted
    Note over C,S: Symmetric encryption now active

Common Misconception

Why It Matters

Common Mistakes

• Leaving TLS 1.0/1.1 enabled — both are deprecated and vulnerable to known downgrade attacks.
• Not enabling OCSP stapling — clients make a separate round-trip to check certificate revocation.
• Weak cipher suites (RC4, 3DES, NULL) still listed as acceptable in server config.
• Not testing with ssllabs.com — misconfigurations are often invisible without external scanning.

Code Examples

# nginx — TLS misconfiguration:
ssl_protocols TLSv1 TLSv1.1 TLSv1.2;  # Should not include TLS 1.0/1.1
ssl_ciphers ALL;                          # Allows weak ciphers
# Missing: ssl_stapling on;
# Missing: ssl_session_cache

# nginx — secure TLS config:
ssl_protocols TLSv1.2 TLSv1.3;
ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256;
ssl_prefer_server_ciphers off;
ssl_stapling on;
ssl_stapling_verify on;
ssl_session_cache shared:SSL:10m;
ssl_session_timeout 1d;

References

• ↗ https://developer.mozilla.org/en-US/docs/Web/Security/Transport_Layer_Security
• ↗ https://wiki.mozilla.org/Security/Server_Side_TLS

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