Understanding the TLS Handshake: Unlocking the Secrets Behind the Padlock 🔒
In this article, we will explore the intricate process that occurs between your web browser and the website you are visiting to secure your connection and earn that reassuring padlock symbol. Given the complexity of this subject, I’ll reference a helpful infographic throughout our discussion, which you can find here. It visually outlines the messages that flow between your browser (the Client) and the website server to initiate a Transport Layer Security (TLS) session.
For a better understanding, consider keeping the infographic open in a separate tab while you read through the details.
A Brief Introduction
Before delving into the steps of the TLS handshake, it’s crucial to emphasize that the primary objectives of SSL/TLS are twofold:
- ✅ To verify the Server’s identity
- ✅ To generate secure session keys for data protection
We’ll begin by clarifying two essential concepts:
Record vs. Packets
The term “Record” in the handshake context represents a complete unit of data exchanged during the handshake, which is distinct from a “Packet.” Multiple records can fit into a single packet, or a single record might be broken across several packets.
Key Cryptographic Concepts
Understanding the TLS handshake requires familiarity with a few cryptographic principles, including:
While we won’t dive deeply into these concepts here, I encourage you to explore the provided links if they are new to you. Our focus will remain squarely on the handshake itself.
The TLS Handshake Process
1️⃣ Client Hello
The handshake process commences when the Client (your browser) sends a Client Hello message that contains five critical fields:
- SSL Version
- Random Number
- Session ID
- Cipher Suites
- Extensions
Let’s break down these components:
1️⃣.1 SSL Version
The Client specifies the highest SSL/TLS version it supports (e.g., SSL 3.0, TLS 1.0, TLS 1.2,
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