Understanding the TLS Handshake: Unlocking the Secrets Behind That Coveted Padlock 🔒

In the realm of internet security, the presence of a padlock icon next to a website’s URL signifies a safe connection. However, the process that leads to this assurance is intricate and often misunderstood. Today, we’ll take a detailed look at the TLS Handshake—the series of events that transpire between your device and the server to secure your data.

To accompany our discussion, I’ve referenced a helpful infographic that outlines these processes visually. It may be beneficial to have it open in a separate tab for reference as we delve into the details.

(Image linked from a Twitter thread.)

Introduction to TLS

Before we dive into the steps of the TLS Handshake, it’s essential to grasp the primary objectives of SSL/TLS protocols:

1. Authentication: Verifying the identity of the server to ensure it is who it claims to be.
2. Confidentiality: Establishing session keys to secure data during transmission.

As we navigate the handshake procedure, we’ll clarify two key concepts to enhance your understanding:

Distinguishing Records from Packets

While each section of the infographic represents a “Record” exchanged during the TLS handshake, it’s important to note that these Records do not equate to Packets. A Packet can contain multiple Records, and conversely, a single Record may span multiple Packets.

Cryptographic Foundations

To appreciate the nuances of the TLS Handshake, some familiarity with basic cryptographic principles is beneficial. These include:

• Hashing
• MACs and HMACs
• Encryption

While we won’t delve deeply into these topics, I encourage you to explore the links for a better grasp.

With those preliminaries out of the way, let’s break down the TLS Handshake step by step.

Step 1: Client Hello

The handshake commences when your browser (the Client) sends a Client Hello message. This packet includes crucial elements such as:

• SSL Version: Specifies the highest version of SSL that