Capstone: a secure channel from parts

HTTPS looks like one padlock, but the guarantee behind it is four promises kept at once: nobody read the message, nobody altered it, nobody replayed an old one, and it really came from the peer you meant to talk to. This capstone has you keep all four yourself, assembling parts you built across P52 and P53 into a tiny working secure channel: Diffie–Hellman key agreement for the shared secret, an HMAC tag for integrity and authenticity, a monotonic counter — one that only ever increases — to defeat replays, and a chain-validated certificate to pin down the sender's identity. The hidden suite then attacks your channel with a tampered message, a replayed counter, and an untrusted sender, and the receiver you build must reject every one. When it does, "how HTTPS actually works" stops being a diagram — it ran in your hands.