Understanding the TLS Handshake: Unlocking the Secrets Behind Secure Connections 🔒

In our increasingly digital world, we are constantly interacting over the internet, often without a second thought for the security that underpins these actions. One of the most significant aspects of secure communications online is the TLS (Transport Layer Security) handshake — the intricate process that ensures that the padlock symbol appears in your browser, indicating a secure connection.

In this post, we’ll explore the series of exchanges that occur between your web browser (the client) and the website you are visiting (the server) to establish a secure TLS session. Along the way, we’ll refer to a comprehensive infographic, which you can view here. Keeping this graphic handy will enhance your understanding of the handshake process as we break it down step by step.

The TLS Handshake: Overview

The primary objectives of SSL/TLS are twofold:
1. Authentication: Ensuring the server is who it claims to be.
2. Encryption: Establishing session keys that secure the data transferred during the session.

Before delving deeper, it’s crucial to clarify two key concepts:

Differentiating Records and Packets

In the context of the TLS handshake, the messages exchanged between the client and server are termed “records.” It’s essential to note that records and packets are not synonymous; multiple records may exist within a single packet, or conversely, a single record may require several packets for transmission.

The Role of Cryptography

A foundational understanding of cryptographic principles—such as hashing, MACs (Message Authentication Codes), and encryption—is beneficial for grasping how the TLS handshake operates. While we won’t explore these concepts in detail here, links to additional resources are provided should you wish to expand your knowledge.

Step 1: Client Hello

The TLS handshake commences with the Client Hello message sent from your browser. This message contains five essential elements:

• SSL Version: The highest supported SSL/TLS version is communicated.
• Random Number: A 32-byte random value is generated to contribute to session key creation.
• Session ID: A unique identifier for session resumption requests.
• Cipher Suites: A list of supported cipher suites from which the server can select.
• Extensions: Optional