Understanding the TLS Handshake: The Journey to Secure Connections

When you access a website, that reassuring padlock symbol 🔒 on the browser is more than just a graphic; it represents a crucial series of events that ensure your online communications are secure. In this post, we’ll explore the intricacies of the TLS Handshake, which establishes a safe and encrypted connection between you and the servers hosting the websites you visit.

For a more visual representation of the process, it might be beneficial to keep an infographic handy while reading through the explanations provided here.

An Overview of TLS

At its core, the primary functions of SSL/TLS protocols are straightforward:

• Authentication: Verifying that the server you’re connecting to is indeed who they claim to be.
• Confidentiality: Establishing session keys that enable the secure transfer of data.

Key Concepts Before We Begin

Before diving into the handshake process, it’s important to clarify two concepts:

Records vs. Packets

In the realm of TLS, each step in the handshake corresponds to what is termed a “Record.” These records differ from network “Packets.” Sometimes, multiple records are contained within a single packet, while, at other times, several packets are needed to deliver a single record.

A Brief Introduction to Cryptography

Familiarity with basic cryptographic concepts is beneficial for understanding the TLS Handshake. Terms such as Hashing, MACs, HMACs, and Encryption are foundational. While this article won’t delve deeply into these concepts, I encourage you to explore relevant resources if you find them unfamiliar.

With that groundwork laid, let’s break down the TLS handshake step by step.

Step 1: Client Hello

The handshake begins with the Client Hello message sent from your web browser to the server. This message includes several essential fields:

• SSL Version: The highest version of SSL/TLS supported by the client.
• Random Number: A 32-byte random value that contributes to generating session keys.
• Session ID: Used for resuming previously established sessions.
• Cipher Suites: A list of encryption algorithms supported by the client.
• Extensions: Additional features that enhance SSL/TLS functionality.

Step 1.1: Setting the SSL Version

The client conveys the highest SSL/TLS version it supports, and in response, the server does the same. They then settle on the highest version