🔒 SSL/TLS

SSL (older protocol family):
  Original protocol from the early web era
  SSL 1.0, 2.0, 3.0
  Legacy today (SSL 2.0/3.0 are no longer recommended)

TLS (Transport Layer Security):
  Successor to SSL
  TLS 1.0, 1.1, 1.2, 1.3
  TLS 1.2 is widely used; TLS 1.3 is the newest widely deployed version

"SSL" is often used colloquially to mean TLS.
When someone says "SSL certificate," they mean TLS.

1. CONFIDENTIALITY
   Data is encrypted
  Passive eavesdroppers shouldn't be able to read it

2. INTEGRITY
   If data is modified in transit,
   tampering is detected

3. AUTHENTICATION
  The server proves its identity
  (for HTTPS: via a valid certificate for the hostname)

Without TLS:
  Username: alice
  Password: (your password)
  → Someone on the network path may be able to see or modify this

With TLS:
  Encrypted blob: 4f8a2c9e7d3b...
  → Looks like noise to anyone without the keys

Before data can flow, client and server do a "handshake":

1. Client Hello
   "Hi server! I support TLS 1.3, 1.2..."

2. Server Hello
   "Let's use TLS 1.3. Here's my certificate."

3. Certificate Validation
   Client checks: Is this certificate legit?

4. Key Exchange
   Both sides derive shared session keys
   (without revealing them on the wire)

5. Encrypted Connection Established
   Application data is now encrypted and integrity-protected

(In modern TLS, the handshake uses asymmetric cryptography to agree on shared keys; bulk data is then protected with symmetric encryption.)

Client                              Server
  │                                    │
  │──── Client Hello ────────────────→ │
  │                                    │
  │←─── Server Hello + Certificate ─── │
  │                                    │
  │     (Validate certificate)         │
  │                                    │
  │──── Key Exchange ────────────────→ │
  │←─── Key Exchange ───────────────── │
  │                                    │
  │     [Session keys derived]         │
  │                                    │
  │════ Encrypted traffic ════════════ │

Certificate contains:
  - Domain name (example.com)
  - Organization name
  - Public key
  - Issuer (Certificate Authority)
  - Validity period (start date - end date)
  - Digital signature

Root CA (trusted by browsers)
    ↓ signs
Intermediate CA
    ↓ signs
Your site's certificate

Browsers trust your cert because they trust the chain!
If the chain validates to a trusted root and the certificate matches the hostname, the browser treats the connection as authenticated.

Trusted CAs:
  - Let's Encrypt (free!)
  - DigiCert
  - Sectigo (formerly Comodo CA)

CAs verify you own the domain before issuing.

Faster handshake:
  Full TLS 1.2 handshake: typically 2 round trips
  Full TLS 1.3 handshake: typically 1 round trip
  (Resumed sessions can be fewer; TLS 1.3 also supports 0-RTT data in some cases)

Stronger security:
  Removed many legacy algorithms
  Forward secrecy by default

Proves: You control the domain
Validation: Automated (email, DNS record)
Cost: Free (Let's Encrypt) to $$
Visual: 🔒 in browser

Proves: Organization identity verified
Validation: Manual verification
Cost: $$
Visual: 🔒 + organization name visible

Proves: Extensive organization verification
Validation: Thorough business verification
Cost: $$$
Visual: Modern browsers largely de-emphasize EV UI indicators

1. Install certbot (or similar tool)
2. Run: certbot --nginx -d example.com
3. Certificate issued in minutes!
4. Auto-renewal on a regular schedule

Free and automated.

1. Generate CSR (Certificate Signing Request)
2. Submit to CA with payment
3. Complete validation
4. Receive certificate
5. Install on server

More support, longer validity, warranty.

Browser: "Your connection is not private"
User: Scared away!

Solution: Auto-renewal (certbot renew)

Certificate for: www.example.com
Accessing: example.com

Browser: "Certificate mismatch!"

Solution: Include all domains in certificate (SAN)

You signed your own certificate.
Browsers don't trust it.
Warning displayed.

OK for: Development, internal tools
Not OK for: Public websites

Disable older versions.
TLS 1.0 and 1.1 are widely disabled on modern clients.

HTTP Strict Transport Security
Tells browsers: "Prefer HTTPS for this site"
Prevents downgrade attacks.

Note: HSTS is learned after a successful HTTPS visit (unless you use HSTS preload).

http://example.com → https://example.com

Don't let users accidentally use HTTP.

Expired cert = site errors
Set up auto-renewal or calendar reminders.

HTTPS page loading HTTP resources (images, scripts).
Browser may block or warn.
Use HTTPS for all page resources.

verify=False in HTTP clients = BAD
You're accepting any certificate, even fake ones.
Avoid disabling certificate validation in production; for development/internal systems, use a proper trust store or pin/trust a specific internal CA.

Some old ciphers are known to be weak.
Configure server to use modern ciphers only.
Use tools like Mozilla SSL Config Generator.