HTTP, TLS & SSH

HTTP & HTTPS

HTTP (HyperText Transfer Protocol) is the protocol used to transfer data between a client (browser) and a web server. It runs over TCP on port 80.

HTTPS is HTTP with encryption (TLS). Runs on port 443.

HTTP Request Structure

GET /index.html HTTP/1.1
Host: example.com
User-Agent: Mozilla/5.0
Accept: text/html

HTTP Response Structure

HTTP/1.1 200 OK
Content-Type: text/html
Content-Length: 1234

<html>...</html>

Common HTTP Methods

Method	Purpose
GET	Retrieve a resource
POST	Submit data / create a resource
PUT	Replace a resource
PATCH	Partially update a resource
DELETE	Delete a resource

HTTP Status Codes

Code Range	Category	Examples
1xx	Informational	100 Continue
2xx	Success	200 OK, 201 Created, 204 No Content
3xx	Redirect	301 Moved Permanently, 302 Found
4xx	Client Error	400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found
5xx	Server Error	500 Internal Server Error, 502 Bad Gateway, 503 Service Unavailable

HTTP/1.1 vs HTTP/2 vs HTTP/3

Version	Key Feature
HTTP/1.1	One request per connection (keep-alive helps)
HTTP/2	Multiplexing — multiple requests over one TCP conn
HTTP/3	Uses QUIC (UDP-based), faster connection setup

TLS/SSL — Securing the Connection

TLS (Transport Layer Security) is the cryptographic protocol that makes HTTPS secure. SSL is its older, deprecated predecessor — the terms are often used interchangeably.

What TLS Provides

Encryption — Data is encrypted; eavesdroppers can’t read it.
Authentication — Server proves its identity via a certificate.
Integrity — Data cannot be tampered with in transit (via message authentication codes).

TLS Handshake (Simplified)

Client                              Server
  │                                   │
  │──── ClientHello (TLS version, ──→ │
  │     supported ciphers)            │
  │                                   │
  │ ←── ServerHello + Certificate ─── │
  │     (public key inside)           │
  │                                   │
  │  Client verifies certificate      │
  │  against trusted CA               │
  │                                   │
  │──── Key exchange ───────────────→ │
  │                                   │
  │ ←──── Encrypted data ───────────→ │

Certificates and CA

A certificate contains the server’s public key and is signed by a Certificate Authority (CA) (e.g., Let’s Encrypt, DigiCert).
Your OS/browser has a list of trusted CAs. If the server’s cert is signed by one of them, the connection is trusted.
In DevOps: use Let’s Encrypt (free) with Certbot to auto-provision and renew TLS certificates.

SSH — Secure Shell

SSH is the protocol DevOps engineers use to securely connect to remote servers. It runs over TCP on port 22.

How SSH Authentication Works

Password authentication: Simple but less secure. Brute-force attacks are a risk.

Key-based authentication (recommended):

Your machine                        Remote Server
 ┌─────────────────┐                ┌───────────────────────┐
 │ Private Key     │                │ ~/.ssh/authorized_keys │
 │ (kept secret)   │                │ (your public key)      │
 └─────────────────┘                └───────────────────────┘
         │                                      │
         └──── SSH handshake proves you ────────┘
               hold the private key
               (without sending it)

Common SSH Commands

# Connect to a server
ssh user@server-ip

# Connect with a specific private key
ssh -i ~/.ssh/my-key.pem ubuntu@server-ip

# Copy files to/from server (SCP)
scp file.txt user@server:/home/user/
scp user@server:/path/to/file.txt ./local/

# Generate an SSH key pair
ssh-keygen -t ed25519 -C "your-email@example.com"

# SSH tunneling (port forwarding)
# Forward local port 8080 to remote port 80
ssh -L 8080:localhost:80 user@server-ip

SSH Config File (`~/.ssh/config`)

Avoid typing long commands by saving settings:

Host my-server
    HostName 34.100.200.50
    User ubuntu
    IdentityFile ~/.ssh/my-key.pem

Now just type: ssh my-server