Explain what the lock in the address bar actually guarantees, well enough to say what an attacker on the network can still see, what a certificate does and does not prove, and why each part of the handshake exists

How a Message Stays Private

Encryption in transit is usually described as a lock. This course takes the handshake apart: agreeing a secret with somebody you have never met, proving who they are, and what remains visible to anyone watching.

8 lessons, written and corrected before you arrived. Reading them here needs no account. The first reads the whole way through; the others open and then stop, because a page nobody owns cannot tell who is reading it. Starting the course gives you your own copy, where every idea has problems standing under it and you can ask about any sentence.

Start reading

01Everything Is Encrypted and Somebody on the Wire Can Still Tell Which Page You Opened and Roughly What You TypedBefore any of the machinery, the honest baseline: what a person watching the connection sees when every part of the encryption is working exactly as designed.
02Two Strangers Shout Numbers at Each Other Across a Crowded Room and Walk Away Sharing a Secret Nobody Else Heardopening onlyThe central trick of the whole subject, worked by hand with numbers small enough to check. Two parties agree a secret while an eavesdropper records every message they send.
03Hiding the Message Is the Easy Half, and a System That Only Hides It Can Be Taken Apart by an Attacker Who Never Reads a Wordopening onlyWhat actually happens to the shared secret: how it becomes several keys, what the cipher does with them, and why detecting tampering has to be built in rather than bolted on.
04You Have a Perfect Unbreakable Secret Channel to Somebody, and Nothing So Far Has Said a Single Word About Whoopening onlyKey agreement works beautifully and identifies nobody. This lesson builds the attack that exploits the omission, step by step, and shows why no amount of better cryptography fixes it.
05A Certificate Says One Narrow Thing, and Almost Every Belief People Hold About It Is a Guess Bolted On to That One Thingopening onlyWhat a certificate actually is, how the chain to a root is checked, and the precise claim it makes. The claim is smaller than the padlock implies and smaller than most designs assume.
06Five Lessons of Separate Machinery Turn Out to Be One Conversation That Takes Two Messages and Is Over Before the Page Is Requestedopening onlyThe complete exchange, in order, with each message traced back to the lesson it came from. Then the cost in round trips, and the two mechanisms that get it down to one or zero.
07Somebody Is Recording Your Traffic Today in the Hope of Stealing a Key in 2031, and the Fix Is to Throw the Key Away Before They Get Thereopening onlyWhat a stolen server key does to conversations that happened years earlier, why the old design handed over everything, and the one change that limits the damage to a single session.
08The Handshake Was Correct, the Cipher Is Sound, the Certificate Checked Out, and Your Data Is on Sale by Fridayopening onlyA complete and correct encrypted channel leaves the metadata, the two endpoints, and everything the far end chooses to do with what you sent. Here is the honest list, and the whole course in one page.