# HTTPS / TLS, Explained

You've seen the little padlock in the address bar a thousand times. Maybe someone told you it means "the site is safe," and you half-believed them. Then one day a page threw a full-screen red warning about a certificate, and you had to decide - on the spot - whether to click through or back away. That's exactly where this guide helps. By the end, you'll know what the padlock *actually* promises (and the dangerous thing it does **not** promise), how your browser and a server agree on a secret without ever mailing it to each other, and how to read a certificate error calmly instead of guessing.

This is an intermediate guide - it assumes you know roughly what HTTP is (a browser asking a server for a page). If that's fuzzy, read [HTTP Explained](/guides/http-explained) first, then come back.

## How to read this
- **Want the one idea to carry around?** Read [Phase 1: What HTTPS Protects (and Doesn't)](01-what-https-protects.md). It installs the mental model that fixes the most common and most dangerous misunderstanding about the padlock.
- **Want it to truly click?** Read all three in order. Phase 2 shows *how* the secret gets agreed on, and Phase 3 shows *who* decides the server is who it claims to be - each answers a question the last one opened.

## The phases
1. **[What HTTPS Protects (and Doesn't)](01-what-https-protects.md)** - TLS adds three things to HTTP: *encryption* (eavesdroppers can't read it), *integrity* (nobody can tamper with it undetected), and *authentication* (you're talking to the real server). And the big gotcha: the padlock means "encrypted to whoever holds the certificate," **not** "this site is trustworthy."
2. **[The Handshake & Keys](02-the-handshake-and-keys.md)** - how two strangers agree on a shared secret over an open wire. The trick: slow *asymmetric* keys to bootstrap, then fast *symmetric* encryption for the actual data. An annotated ASCII handshake.
3. **[Certificates & Trust](03-certificates-and-trust.md)** - a *certificate* binds a domain to a public key, signed by a *Certificate Authority* your browser already trusts. The chain of trust, why Let's Encrypt changed everything, and how to read "expired," "name mismatch," and "self-signed" errors without panic.

> This guide deliberately stays at "what is happening, and why." The exact byte-level mechanics of TLS 1.3, cipher suites, and certificate revocation live in deeper material; the difference between proving *who you are* and proving *what you're allowed to do* lives in [Authentication vs. Authorization](/guides/auth-vs-authz). Read this guide first - then those, when you want to go deeper.