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TLS Handshake

TLS establishes encryption and authenticates the server before HTTP exchange.

Verify in Practice​

openssl s_client -connect api.example.com:443 -servername api.example.com
curl -v https://api.example.com/health

Common Problems​

  • Certificate expired or missing intermediate CA.
  • Hostname mismatch between SNI and SAN/CN.
  • Unsupported protocol or cipher overlap.

TLS Version Evolution​

VersionYearKey ChangesStatus
TLS 1.01999Based on SSL 3.0❌ Deprecated (RFC 8996)
TLS 1.12006Security improvements❌ Deprecated (RFC 8996)
TLS 1.22008AEAD ciphers (AES-GCM), SHA-256✅ Widely used
TLS 1.320181-RTT handshake, PFS mandatory✅ Recommended

TLS 1.2 vs TLS 1.3 Handshake​

TLS 1.2 Handshake (2-RTT)​

Client                          Server
  │                               │
  │──── ClientHello ──────────────│
  │<─── ServerHello ─────────────│
  │<─── Certificate ─────────────│
  │<─── ServerKeyExchange ────────│
  │<─── ServerHelloDone ─────────│
  │                               │
  │──── ClientKeyExchange ───────│
  │──── ChangeCipherSpec ────────│
  │──── Finished ────────────────│
  │<─── ChangeCipherSpec ────────│
  │<─── Finished ────────────────│
  │                               │
  │==== Application Data =========│

TLS 1.3 Handshake (1-RTT)​

Client                          Server
  │                               │
  │──── ClientHello + Key Share ──│  ← Client sends key share immediately
  │<─── ServerHello + Key Share ─│  ← Server responds with key + cert
  │<─── Certificate ─────────────│
  │<─── Finished ────────────────│
  │──── Finished ────────────────│
  │                               │
  │==== Application Data =========│  ← Only 1 round trip!

TLS 1.3 Key Improvements:

  • 1-RTT handshake (down from 2-RTT in TLS 1.2)
  • 0-RTT resumption for returning connections
  • Perfect Forward Secrecy (PFS) is mandatory, not optional
  • Removed insecure algorithms: RSA key exchange, CBC-mode ciphers, MD5/SHA-1, RC4, DES
  • Simplified cipher suites: Only 5 approved (vs hundreds in TLS 1.2)

Certificate Chain Validation​

Root CA (self-signed, trusted by browser/OS)
  └── Intermediate CA
        └── End-entity Certificate (your domain)

Validation steps:

  1. Check certificate has not expired
  2. Verify signature chain back to trusted Root CA
  3. Check hostname matches certificate's Subject Alternative Name (SAN)
  4. Verify certificate is not revoked (OCSP or CRL)
  5. Check Certificate Transparency (CT) log inclusion

mTLS (Mutual TLS)​

In standard TLS, only the client verifies the server. In mTLS, the server also verifies the client:

  • Used in Service Mesh (Istio, Linkerd) for zero-trust networking
  • Each service has its own certificate
  • Eliminates need for shared secrets between services
  • Certificate rotation handled automatically by mesh control plane

Let's Encrypt & ACME​

Automated certificate management:

# Install certbot
apt install certbot

# Obtain certificate for nginx
certbot --nginx -d example.com -d www.example.com

# Auto-renewal (certbot installs a cron job)
certbot renew --dry-run
  • Free certificates via Let's Encrypt
  • ACME protocol automates domain validation and certificate issuance
  • Certificates are 90-day with automatic renewal
  • Widely adopted: >300M certificates issued